4.1.1 Fundamentals of Concrete Technology

Concrete is made from cement, aggregates, chemical and mineral admixtures, and water. The active constituent of concrete is cement paste, the nature of which largely determines the performance of the concrete produced.

Cement is made by heating calcareous materials such as limestone or chalk and a source of silica and aluminum oxide such as shale, clay or slate in a rotary kiln to a temperature of about 1300–1450°C. The resultant clinker is cooled and ground with about 2–5% gypsum and/or anhydrite to a specified degree of fineness. The surface area of cement typically ranges between 2500 and 6000 cm² g⁻¹.

The major constituents of Portland cement are the four clinker phases:

Upon contact with water, the phases hydrate in an exothermic reaction, and a hardened cementitious matrix mainly consisting of calcium silicate hydrate (C-S-H), is formed. C-S-H forms tiny needles which are 600 nm to 2 µm in length and 5–10 nm thick (Figure 3). The needles intertwine as in a zip fastener, and this gives hardened cement its considerable compressive strength. A comprehensive presentation of cement chemistry and technology has been provided by Taylor (1988).

Aggregates consist of fine or coarse silica sand and gravel. A mix of cement, water and aggregates of particle size <4 mm is called a mortar. Concrete also contains coarse aggregates with particle sizes up to 32 mm.

Mineral admixtures such as ground or granulated blast-furnace slag, fly ash, silica fume and others are incorporated into concrete to improve its quality.

Chemical admixtures confer beneficial effects such as flowability, water reduction, retardation or acceleration, air-entrainment and anti-settling properties.

The performance of concrete depends on the quality of the ingredients, their proportions, placement, and exposure conditions. For example, the hydraulic activity of the clinker, the fineness and particle size distribution of the cement and the amount of mixing water influence the physico-chemical behavior of the hardened cement paste. As a general rule, a fine cement and a low water-to-cement (w/c) ratio result in high compressive strength. Typical w/c ratios in concrete range from 0.30 to 0.70. A w/c ratio below 0.30 does not provide enough water for the cement to hydrate completely, and therefore is undesirable. A concise description of cement chemistry and concrete properties is given by Lea (1970), while general aspects of concrete technology are described by Neville (1981) and a valuable review of material science aspects of concrete has been provided in a recent article (Moranville-Regourd, 1997). These articles contain valuable information for those wishing to learn more about cement hydration, structures of hydrates, the colloidal chemistry of cement setting and hardening, the microstructure of concrete, concrete durability, high-performance concrete and organo-cement composites.

4.1.2 Ready-mix Concrete

Today, about 50% of the concrete produced is made in so-called ready-mix concrete plants. These units store cement in huge silos, with different grades of aggregates in covered compartments, and liquid chemical admixtures in dosage tanks (Figure 4). Ready-mix concrete is prepared by dosing the ingredients according to a recipe through automatic dosage units into a large concrete blender. Ready-mix trucks deliver the concrete to the construction site while the mixing drum is slowly rotated to ensure homogeneity upon delivery. The main advantages of ready-mix plants are better quality control of the concrete ingredients and improved quality consistency of the concrete batches. Many of these plants are equipped to recycle waste concrete and concrete wash water rather than attend to their disposal.

4.1.3 Precast Concrete

Large concrete elements such as beams, pillars, floors or walls are often produced at so-called precast concrete plants. There, concrete is poured into molds to produce large numbers of elements of the same shape and size (Figure 5). Steam curing is applied to accelerate the development of early strength of the concrete, thereby improving the economics of the expensive molds. In comparison with ready-mix plants, precast plants typically produce concrete with a higher compressive strength. This is achieved by reducing the amount of mixing water, thus lowering the w/c ratios; consequently, precast plants use a significant volume of water-reducing polymers, referred to as plasticizers or superplasticizers.

Approximately 25% of all concrete is produced in precast plants, 50% as ready-mix, and 25% in small mixers, or by hand.

4.1.4 Self-compacting Concrete

In 1983, Okamura and Ouchi in Japan began to develop a concrete with such exceptional fluidity that it later became known as self-compacting concrete (SCC). This concrete is so fluid that it requires little if any vibration to densify and release air after placement. In fact, it has almost self-leveling properties (Okamura and Ouchi, 1999).

The flowability of concrete is commonly measured by the slump test, as described by ASTM C 143. The test uses a frustum of cone 30 mm (12 inches) high. The test procedure is illustrated in Figure 6. Concrete is filled into the cone, which is lifted slowly. Concrete flowability is determined by measuring the decrease in height of the center of the slumped concrete, with the greater the decrease, the better the flowability. For structural concrete, a slump of 75–100 mm (3–4 inches) is sufficient for placement in forms. Highly workable, so-called flowing concrete shows a slump of about 150–200 mm (6–8 inches).

Other methods are also used to determine concrete flowability, and the German flow table method has recently become accepted as the European norm and is now included in the DIN EN 12 350 standard.

Powerful superplasticizers based on polycarboxylate chemistry are used to obtain SCC with a slump of 270–300 mm. It is easy to imagine that, because of its fluidity, such concrete has a tendency to segregate, but this is only apparent if bleeding water occurs on the concrete surface or if heavy aggregates settle at the bottom.

Two approaches have been taken to stabilize SCC. One method is to add larger amounts of fine aggregates or filler (particle size <0.1 mm), particularly fine sand or limestone. The fine aggregates, by virtue of their large surface area, tie up large amounts of water and prevent bleeding. They also impart some viscosity into the cement paste and reduce the settling of large aggregates. This type of SCC loaded with fine aggregates is referred to as “powder-type” SCC.

An alternate method of stabilizing SCC is to add a polymeric viscosifier that prevents bleed and sag. As will be discussed later, biopolymers have proven extremely effective for this purpose.

4.1.5 Chemicals Used in Concrete

Concrete uses almost exclusively liquid chemical admixtures, the main reason being ease of dosing and mixing. Major chemical admixtures for concrete include: dispersants based on lignosulfonates, β-naphthalenesulfonate resins (BNS), melamine formaldehyde sulfite resins (MFS), or polycarboxylates [PC; e.g., methacrylic acid-poly(ethyleneglycol)methacrylate ester copolymers]; retarders based on sodium gluconate or sugar-rich lignosulfonate; accelerators based on calcium nitrate or calcium formate; air-entraining agents based on root resin extracts, alkylsulfates of phenol ethoxylates; foamers based on protein hydrolysates; anti-segregation admixtures based on welan gum or starch; anti-washout admixtures based on hydroxypropyl cellulose; shotcrete accelerators based on sodium aluminate or fine, amorphous aluminum oxide; and shrinkage-reducing admixtures based on neopentyl glycol. Comprehensive overviews on chemical admixtures used in concrete have been produced by Ramachandran (1995) and Rixom and Mailvaganam (1999).

Clearly, the concrete industry uses a great diversity of admixtures, some important members of which belong to the group of biopolymers.

4.2.1 Floor Screeds

Floor screeds are placed on concrete or wooden panels of floors to provide a firm, planar basis for laying floor carpets or parquet in homes and buildings. The most common are the “non-slump” cement-based floor screeds which are thick and require heavy labor to be placed and smoothed (Figure 7). Anhydrite-based floor screeds using a superplasticizer for high fluidity were introduced during the mid-1970s, and a typical formulation is shown in Table 2. Because of their fluidity, these grouts can be pumped and are easily placed by spreading them on the floor, with minimal labor needed to achieve an even surface (see Figure 7). Recently, calcium aluminate cement-based flowing floor screeds were developed and introduced, and a description of this technology has been produced (Harbron, 2001).

The high fluidity sometimes causes sag of aggregates and bleeding water to appear on the surface, though microbial biopolymers such as xanthan gum may be added to stabilize the grout (see below).

4.2.2 Self-leveling Underlayments

SLUs are placed on floor screeds, concrete or other bases to achieve a completely even surface. They are usually applied in thin layers of up to 15 mm, whereas floor screeds typically have a thickness of 20–50 mm. To achieve a thin layer, only very fine aggregates (particle diameter <0.1 mm) can be used.

SLUs require special dispersants to achieve uniform spread-out and a self-healing effect. Self-healing means that while being placed, the SLU grout does not show any furrowing if it is cut, for example, with a knife (Figure 8). Casein, a natural biopolymer obtained from milk, is the main product on the market to provide self-healing properties. The flowability of an SLU can be measured according to EN 12706 (edition November 1999). SLUs are usually formulated as dry compounds based on either cement or CaSO₄-α-hemihydrate, though on occasion a highly reactive mixture of calcium alumina cement, Portland cement and CaSO₄-anhydrite is used. When mixed with water, this grout gives a workability time for placement of approximately 1 h, and the grout (when laid) can be walked on after only 2 h. SLUs are often mixed by machine and therefore are referred to as machinery grouts.

4.2.3 Tile Adhesives

Tile adhesives are cementitious grouts which are used to fix tiles onto the floor or onto walls. The voids between individual tiles are filled with grouts (see Section 4.2.4).

Modern tile adhesives contain a cellulose-based water-retention and thickening agent and a redispersible powder for adhesion. (A typical tile adhesive formulation is shown in Table 3.) Because tiles are often placed on porous surfaces such as brick, water-retention is highly critical, as tiles will not hold firm, or may loosen or even fall off if the tile adhesive dehydrates from loss of water. During the 1970s, highly efficient cellulose ethers achieving water-retention of >97% were introduced; these not only provided a firm glue with the wall, but also allowed a reduction to be made in the overall amount of cementitious grout needed for tile adhesion. So-called “thin-bed” grouts, which are obtained by making furrows in the tile adhesive with a metal claw, have begun to become popular. These grouts are more expensive than conventional tile adhesives because they contain costly admixtures, but the overall material savings is more economic and the technique has become the standard method for tile setting (Figure 9).

Tile adhesives constitute a huge market, especially in countries where ceramic tiles are popular. Germany alone, for example, produces in excess of 1 million tons of tile adhesives per year and uses about 5000 tons of cellulose ethers for this application.

4.2.4 Joint Fillers and Compounds

Joint fillers and compounds comprise a group of materials used to fill voids. Their function is to fill the void entirely without cracks, and to be wipe-resistant even in thin layers. The most important of these compounds include:

Joint fillers would not function well without the water-retention agents and thickeners which provide adhesion, ensure complete hydration of the binder, and prevent solids settling and crack formation. Once filled, the void must not sink in. Cellulose-based ethers (e.g., methylcellulose) are frequently used to provide water-retention and adhesiveness in these fillers; a typical formulation of a joint filler for tile setting is given in Table 4.

4.2.5 Injection Grouts

Cracks in concrete, for example, are repaired by injecting cementitious grouts of very fine particle size. These grouts must be very fluid to achieve optimum penetration into the crack, and therefore high dispersant concentrations are used to provide extreme fluidity. However, this introduces the risk of sedimentation and the formation of bleeding water.

Biopolymers such as xanthan or welan gum, which provide a shear-thinning type of viscosity, are used to prevent the grout from disintegrating.

4.6.1 Drilling Fluids

Drilling fluids, also called “muds”, are circulated (as shown in Figure 15) in order to cool and lubricate the drill bit, to carry drill cuttings to the surface for separation from the fluid, and to counter the formation pressure, thus preventing influxes of oil, gas or reservoir fluids into the borehole. For a comprehensive overview of drilling fluid technology, history, products, processes and test methods, see Darley and Gray (1988).

The most common types of drilling fluids include:

Water-based drilling fluids fall into two categories: “dispersed”, or nondispersed “polymer” mud systems. Dispersed muds contain high amounts of solids and bentonite and therefore require the addition of thinners to reduce viscosity to a sufficiently low level to provide high penetration rate of the drill bit. A basic formulation for a dispersed lignite mud and its properties is provided in Table 6. These fluids represent the classic type of water-based muds and initially used quebracho, tamarind or tannin extracts for thinning. During the 1950s, lignosulfonates and lignite were introduced as a new group of mud thinners, and the resultant chrome lignite/chrome lignosulfonate (CL-CLS) mud subsequently became one of the primary drilling fluid systems used worldwide, thus providing a huge market for the lignite and lignosulfonate biopolymers.

Since the 1980s, “polymer” muds have become increasingly popular, one reason being less formation damage from decreased bentonite concentrations. Viscosity is mainly generated by polymers with thickening effect, and cellulose and starch ethers have been used extensively in this type of fluid.

In addition to providing viscosity for cuttings suspension, these biopolymers also control drilling fluid leak-off into tiny formation pores. This effect, which is also referred to as fluid loss or filtration control, is a key performance characteristic of a drilling fluid. As mentioned previously, drilling muds are weighted to achieve a density which is higher than the formation pressure, and because of this pressure differential the mud is pressed against the porous borehole wall. In the absence of fluid loss control chemicals, quick dehydration of the fluid would occur, resulting in a viscous, unpumpable mud. The filtration process on the borehole wall is shown schematically in Figure 16.

It is important to note that this filtration or fluid loss process is not related to the loss of fluid in larger cracks or voids of the formation. Rather, fluid loss refers to micropores in the formation with, for example, an average pore size in the range of several microns. To seal this off, the drilling fluid must form an impermeable filter-cake after releasing an initial spurt loss. Biopolymers are widely used to reduce filtration rate and to build thin, impermeable filter cakes. This effect is similar to the water-retention effect shown by the same polymers in conventional building systems such as plasters (see Section 4.3).

It is difficult to appreciate fully the benefits that such filtration control agents have provided in modern well construction; indeed, without their introduction the production of hydrocarbons from deeper reservoirs would not have been possible.

4.6.2 Oil Well Cementing

The construction of all oil and gas wells necessitates the cementing into place of a tubular pipe referred to as the casing. The main purpose of this is to create zonal isolation, i.e., adjacent reservoir zones of different pressures which in an uncased borehole may communicate, are sealed off by a cement sheath and casing (Figure 17).

A borehole is cemented in intervals, which means that a certain strata (e.g., 1000 m) is first drilled and then cemented. Formation properties such as pore pressure, rock composition will dictate the casing program for the well. Complete descriptions of oil well cementing technology, equipment, processes and products have been provided by Nelson (1990) and Smith (1976), while details of field practice are described by Smith (1984).

For oil well cementing, a special Portland cement is used. Several types are available, and these are specified according to standards from the American Petroleum Institute (API, 1990). In the manufacture of API grade oil well cements, only pure clinkers can be used, and no secondary raw materials or alternative fuels (e.g., automobile tires) are permitted. The high purity is needed to obtain predictable rheological, hydration and setting properties at elevated borehole temperatures.

Oil well cement commonly is retarded to avoid premature setting while it is pumped downhole. The most common retarders are based on lignosulfonates (see Section 5.1). At increased temperature, they are capable of delaying the set of cement for several hours, and typically a pumping time (i.e., the duration of a fluid state of a cement slurry) of 4–6 h is required. After placement of the slurry, cement setting and strength development should occur immediately.

Like drilling fluids, cement slurries are pumped with overburden pressure against porous rock, and therefore filtration control is as important in well cementing as it is for drilling fluids. The only difference is that cement slurries have somewhat relaxed requirements for water-retention because their filter-cake is laid down on a filter-cake from the drilling fluid. Nevertheless, huge volumes of cellulose-based biopolymers are used for this purpose (see Section 5.5.2).

4.6.3 Well Cement Spacers

Before pumping oil well cement slurry into the hole, a so-called spacer fluid is pumped to displace the drilling fluid and some of the mud filter-cake.

Initially, spacer fluids consisted of abrasive materials (e.g., sharp quartz particles) and a dispersing agent, and were pumped in turbulent flow at high velocity to clean out mud, debris and filter-cake from the hole. Field experience showed, however, that dispersive spacers pumped under turbulent flow often resulted in severe channeling. Thick mud and the filter-cake on the borehole wall were bypassed, thus defeating some of the purpose of spacer application (Figure 18).

Therefore, novel spacers based on shear-thinning rheology and pumped under plug flow conditions were introduced. These were found to be more effective in pushing thick mud upward and in cleaning the hole for the subsequent cement slurry. A schematic illustration of the hole-cleaning effect of a shear-thinning spacer is shown in Figure 18. Spacers rely on microbial biopolymers to achieve shear-thinning rheology, and represent a significant application for these products. Without good spacer fluids, many well bores would not achieve a tight cement sheath, and safety hazards would make costly repairs or even abandonment of the wells necessary. A state of the art report on modern spacer technology has been provided by Crook et al. (2001).

5.2.1 Humic Acid

Humic acid is an important soil constituent and an intermediate in the conversion of vegetation to coal. Because of the complexity of the source materials, details of the structure of humic acid have not yet been agreed upon (Steelink, 1963), though various studies have shown it to contain phenolic and carboxylic compounds. The molecular weights range from 300 to 4000 Da (Steelink, 1966).

Humic acid is the starting material for graft polymers used in oil well cementing. Giddings and Williamson (1987) have patented a process of grafting 2-acrylamido-2-methylpropane-sulfonic acid, N,N-dimethylacrylamide and acrylonitrile monomers on a humic acid backbone, and the resultant graft polymer was seen to exhibit excellent filtration control properties in well cement slurries. The effectiveness of this graft polymer in fresh water cement slurries is shown in Table 13.

5.2.2 Lignite

Lignite, variously also called leonardite, mined lignin, brown coal or slack, represents a further degraded humic acid. A natural deposit in North Dakota is the principal source of lignite used in construction applications.

Mined lignite is made water-soluble by addition of alkali such as caustic soda. The resultant sodium lignite, commonly known as “causticized lignite”, is an inexpensive, multi-functional oil well drilling fluid additive. Initially considered only as thinner for water-based muds, it now serves also for filtrate reduction, oil emulsification, and stabilization of mud properties against the effects of high temperature.

The excellent properties of lignite, particularly in combination with lignosulfonate, resulted in the development of a famous water-based drilling fluid system, the CL-CLS mud (chrome lignite-chrome lignosulfonate; see Section 4.6.1). After 1960, this fluid became very common in US land wells, and represented a highly dispersed mud system with a relatively high bentonite content (3–10%) in which both lignite and lignosulfonate dispersed the solids. The chromium salts provided optimum stability at temperatures up to 225°C (Darley and Gray, 1988).

Since the 1980s, cellulose- and synthetic-based polymers have gradually replaced the dispersed CL-CLS muds. Another problem arose due to environmental concerns over the toxic effects of chromium which leached from mud spills or disposed drill cuttings into the ground water. The industry has responded to these concerns by introducing less toxic zirconium salts or synthetic polymers for high-temperature drilling, though at low to medium temperatures sodium or potassium lignite still is used.

A vinyl grafted lignite fluid loss additive has been developed for water-based drilling fluids and oil well cement slurries (Huddleston and Williamson, 1990). For example, a graft polymer containing 20 wt.% lignite, 60 wt.% 2-acrylamido-2-methylpropane-sulfonic acid and 20 wt.% methacrylamido propyltrimethyl ammonium chloride considerably reduces the fluid loss of a drilling fluid aged at 350°F.

Oil-based drilling fluids typically are an emulsion of ∼70% of an oil phase (e.g., diesel) and 30% of an aqueous phase saturated with calcium chloride. Emulsifiers are used to achieve homogeneity and stability of the fluid. A reaction product of sodium lignite with quaternary amines having 12 to 22 carbon atoms in one of the alkyl chains, functions as an effective primary emulsifier and filtration control agent (Jordan et al., 1966), and is often combined with secondary emulsifiers such as organophilic clays.

Although the use of lignite in oil well drilling has declined in recent years, approximately 50,000 tons are still consumed annually by this industry.

5.3.1 Natural Oils

These are divided into subgroups of drying and semi- or nondrying oils. The first group includes linseed oil and fish oil, both of which represent important constituents in paints and lacquers. The second group contains soybean, sunflower, rape, palm, coconut and castor oil, and these have found limited use in the paint industry. The substitution of nonbiodegradable mineral oils used in oil well drilling fluids has become a major application for vegetable oils consisting of C₁₆–C₂₄ unsaturated monocarbonic acids, and esters of C₂–C₁₂ alcohols are produced specifically for this purpose (Müller et al., 1989). In offshore drilling, palm oil- and coconut oil-based esters have successfully replaced toxic diesel oil fluids and have greatly reduced the load of nonbiodegradable hydrocarbon which is disposed of together with drill cuttings into the sea (Degouy and Demoulin, 1993). Recently, biodegradable oils such as rape or beet oil which have replaced the mineral oils are used as surface coating on concrete molds. Their purpose is to prevent concrete adhesion to the wooden mold and to provide an impeccable concrete surface.

Some vegetable oils (particularly soybean oil) contain lecithin, which is a phosphatide and is recovered from raw soybean oil by steam processing. Lecithin is an effective emulsifier which has found some use in solvent-born paints and coatings. Another application is in emulsifier packages to make water-in-oil emulsions for use as a drilling fluid.

5.3.2 Waxes

Many plants form an oil or wax layer on the surface of their leaves to provide protection against desiccation. Waxes from Euphorbia or Copernica cerifera are recovered and used as an important polish for wooden floors and furniture. Like exudate gums, they are complex mixtures, with linear aliphatic C₁₆–C₃₄ mono carbonic acids being the backbone to which C₂₂–C₃₆ alcohols are bound by ester bonds.

Insects can also produce waxes, the most well-known being bees wax, shellac and the wool wax of sheep. These waxes are sometimes used in wood polishes or as anti-graffiti coatings.

5.3.3 Bitumen and Paraffin

Some brown coal deposits contain fossil waxes known as bitumen or pitch, and which were formed from resins or waxes of plants that have degraded over the ages. Paraffins are particularly pure waxes which may occur in such natural deposits. Today, bitumen and pitch are produced by distillation of petroleum and coal, respectively. Bitumen is considered to be harmless to humans, whereas pitch has been proven to be relatively toxic.

Bitumen is a colloidal system in which polar asphaltenes and naphthenic petroleum resins are dispersed in a hydrocarbon phase called maltenes. Bitumen consists of a mixture of alkanes, cycloaliphates or aliphatic substituted aromates, and for industrial use is produced exclusively from the residues of petroleum distillation. Different viscosity grades are manufactured and sometimes oxidized by blowing air at 250–350°C through molten bitumen.

Asphalt is a mixture of bitumen and aggregates or fillers (e.g., sand, limestone etc.), and was used as early as 3000 bc in Mesopotamia as an organic binder in adobe buildings. Today, road construction represents the largest use of asphalt. Tarmac roads use three types of bitumen: distillation bitumen; flux bitumen (distilled bitumen dissolved in low-volatile mineral oils); and cold bitumen (distilled bitumen in highly volatile mineral oils). Occasionally, emulsions of bitumen in water containing emulsifiers and stabilizers (see Section 5.8.2) are also used. Polymer-modified bitumen is also used as a water-repellent coating on basements.

Gilsonite is a naturally occurring asphaltide which has found some use in oil well drilling fluids as a lubricant and filter-cake builder (Davis and Tooman, 1988). As the mud is pumped downhole and the temperature increases, gilsonite melts and forms a thin layer on the borehole wall; this helps to control fluid loss to the porous formation. Sulfonated gilsonite is used for improved dispersion in the aqueous drilling fluid. Likewise, a process for the manufacture of sulfonated asphalt and its use in drilling fluids has been developed (Stratton et al., 1962). However, a lack of biodegradability has reduced the usage volumes of both gilsonite and sulfonated asphalt in recent years.

Pitch (also called “carbolineum”) is produced from the distillation of coal and tar. Previously, huge volumes of pitch were used as a wood preservative, for example on railroad girders or fences. Pitch contains polyaromatic compounds which have been shown to be carcinogenic, and consequently pitch coatings have been used much less in recent years. Today, other less toxic coatings based on synthetic organic compounds such as carbamates are used.

Molten paraffin is sometimes injected into building walls as a moisture barrier, perhaps when a damp wall is continually drawing water from the soil to create an ideal substrate for molds and provide an unhealthy living environment.

5.4.1 Casein

Casein is a biopolymer which is found in milk and is recovered by acid precipitation. Studies have shown that milk casein consists of a mixture of hydrophilic κ-casein and hydrophobic β-casein. Together, these form micelles with specific diameters of 90–140 nm (Figure 22). Further information on casein chemistry can be obtained from a web site (http://class.fst.ohio-state.edu/FST822/lectures/Cafunc.htm).

In construction, casein has found two distinct applications, namely paints and self-leveling underlayments.

Paints require additives which not only viscosify, but also impart self-leveling properties to achieve a leveled layer of paint. Caseinates, obtained by neutralizing casein with alkali hydroxides, provide both properties. At 0.5–4% concentration, they are a major ingredient in water-born casein paints. Their disadvantage is rapid biodegradability, such that the paint must be used quickly before it degrades and develops an evil smell. Synthetic, nondegradable latex compounds such as styrene-butylacrylate dispersions have been developed to replace casein in paints, but casein remains the product of choice for customers who require building paints based on natural, biodegradable products and who believe these products to provide a healthier living environment.

Self-leveling underlayments are another building application in which casein still plays a significant role on the market. SLUs are poured onto uneven floor screeds to provide an even surface before floor carpets are laid and glued with adhesives. SLUs are either based on Portland cement, or mixtures of Portland cement, calcium aluminate cement and anhydrite. Casein provides excellent self-healing properties to SLUs. After being cut with a knife, the scar will immediately close and re-form a totally even surface (see Figure 8). Even today, the mechanism of this self-healing property is not well understood, but over many decades this unique property of casein has made it the product of choice for this application. The typical composition and properties of a casein-based SLU are shown in Table 14. Only very recently have new polycarboxylate-based superplasticizers been presented which provide similar dispersing and self-healing effects in SLUs, and these now appear to have gained a limited foothold (Strauß, 2001).

5.4.2 Protein Hydrolysates

Protein hydrolysates are extracts obtained by cooking animal blood, hair, hides or hoofs, particularly from cattle, with sulfuric acid or, more recently, with enzymes. The resultant extract contains degraded proteins, mainly oligo and polypeptides, of various compositions. Refinement is by filtration and precipitation of inorganic salts. Protein hydrolysates occur typically as 30–35% active, brown aqueous solutions; their nitrogen content (as determined by the Kjeldahl method) is used to specify their quality.

Protein hydrolysates reduce the surface tension of water significantly and are used to prepare foam concrete. This is made by feeding a homogeneous foam, generated by a foam pistol or a large foam aggregate, into a premixed mortar or concrete. By mixing the foam and mortar or concrete together, foam concrete with specific density as low as 0.5 kg dm⁻³ can be obtained. Foam concrete is used to fill ditches, to provide a basement for roads on unconsolidated soil in marshes and moors, and for prefabricated dividing walls in homes.

Compared with synthetic foamers and surfactants such as alkyl sulfonates, alkyl sulfates and betains which are also used to prepare foam concrete, protein hydrolysates are more difficult to apply. They require excellent mixing in a high-quality foam pistol or aggregate (Figure 23) to achieve a foam. The reason why the industry still uses these materials is their superior foam quality. When used correctly, protein hydrolysates produce spherical foam bubbles whereas synthetic surfactants typically generate hexagonal bubbles in concrete. Concrete of the same specific density containing spherical bubbles has been shown to provide 20–30% higher compressive strength, compared with concrete containing hexagonal bubbles. As a consequence, when foam concrete with a low specific weight and a compressive strength as high as possible is required, protein hydrolysates are still preferred over synthetic surfactants. This is an excellent example of how, on occasion, it is difficult to mimic the properties of biopolymers with synthetic materials.

5.5.1 Starch and Derivatives

Starches from whatever source are composed of one or more glucans. The two principal molecular components of starch are the polysaccharides amylose and amylopectin. Amylose is composed of long, straight chains of α-glucose (Figure 24), with molecular weights ranging from 10,000 to 100,000 Da.

Amylopectin, the main component, consists of a mixture of branched molecules (Figures 25 and 26), and has a molecular weight of 40,000 to 100,000 Da.

Commercial starch mostly is produced from corn (maize), though potatoes are another major source, particularly in Europe. The origin of a starch can be identified using microscopy because the sizes and shapes of starch granules are specific for the plant (Whistler et al., 1984).

In construction, very little native starch is used, but pregelatinized and derivatized starches are common. Pregelatinization is a modification process for starch which greatly enhances its cold water solubility and its viscosity yield – two properties which are clearly important in construction. The process consists of heating aqueous slurries of native starch, which causes the granules to swell and the viscosity to increase; the resultant viscous gel of pregelatinized starch is dehydrated to a powder on a drum drier.

Pregelatinized starch is used as an inexpensive, low-temperature fluid loss agent for water-based drilling fluids. It is mainly applied in top-hole drilling or drilling of shallow wells where the temperature stays well below 100°C. Typically, 0.5–2% of this starch is used to obtain an API fluid loss of 2–5 mL at 25°C and 7 bar differential pressure. As seen below, conversion from pregelatinized starch to carboxymethyl starch occurs as the temperature increases.

Pregelatinized starch is also used for plasterboard manufacture (see Section 4.4), where starch is added to the β-hemihydrate slurry. During the drying process, starch migrates to the surface of the gypsum body and glues the cardboard to the gypsum surface.

Among starch ethers, carboxymethyl and hydroxypropyl starch are the most important derivatives used in construction.

Carboxymethyl starch is also prepared by reacting alkali-treated starch with monochloroacetate. This causes some of the hydroxyl groups in the anhydroglucose units to be transformed into ethers with carboxymethyl groups. The degree of substitution (DS) may vary, but cannot exceed 3; commercial products used in construction typically have a DS between about 0.2 and 1.2.

Carboxymethyl starch is an important drilling fluid additive, its main advantage over pregelatinized starch being a higher temperature stability (up to 120°C).

In conventional construction, hydroxypropyl (HP) starch, when prepared by the reaction of propylene oxide with starch, is used to thicken wall plasters. For this purpose, HP starch with a DS of about 0.5 is dry-blended with other ingredients into a dry-mix mortar that is then mixed with water and machine-sprayed onto the wall (see Section 4.3). The primary function of the HP starch is to provide instant high viscosity so that the plaster adheres to the wall – a property known as green-strength. This effect also prevents the plaster from sagging after it has been sprayed onto the wall. Secondary effects of HP starch include: (i) augmentation of the water-retention ability of other additives present in the formulation; (ii) reduction of plaster stickiness on the trowel; and (iii) provision of surface viscosity when the plaster is first leveled and smoothed. Anybody who has ever attempted this leveling process (Figure 27) could not imagine the advantage that an appropriate surface viscosity of the plaster provides in the time needed for application. Hence, the industry has gone to great lengths to develop optimized products.

A typical formulation for a machine-sprayed gypsum plaster is shown in Table 15 (Aqualon Technical brochure, 1996). Clearly, as little as 0.01% of HP starch is required to obtain the desired properties.

The use of starch ethers in tile adhesives has been patented (Schinski and Wunderlich, 1990), but appears to be limited. Crosslinked starch has also found some use in cementitious grouts (Axmann, 1996).

5.5.2 Cellulose Derivatives

Cellulose products represent a very significant portion of the total construction chemicals market. It is estimated that in excess of 100,000 tons of derivatives are consumed annually by the construction industry, representing a value of approximately $500 mio at the manufacturer's level.

Among construction applications, cellulose is mostly used in the form of ether derivatives, with carboxymethyl cellulose and methyl cellulose representing the lion share.

The ethers most commonly used in construction are listed in Table 16. The nonionic cellulose ethers play a dominant role in this market, and a review of nonionic cellulose derivatives, their chemistry, manufacture, performance and market data has been produced by Dönges (1990). Very small quantities of cellulose products other than ethers are used, though one such material is micronized cellulose fibers that are occasionally used as a thickener in dry-mix mortar formulations such as tile adhesives (see Table 3).

Generally, cellulose products are applied in building products to provide water-retention and viscosity. An overview of the major fields of application for cellulose ethers in construction is provided in Table 17.

Clearly, the use of cellulose ethers is widespread throughout the industry, and some typical applications are outlined in the following text to illustrate the performance characteristics and special advantages of these materials.

Methyl cellulose (MC) is by far the most important chemical admixture used in dry-mix mortars and grouts as a provider of water retention. Major applications include tile adhesives, cement-, gypsum- or lime-based plaster, and joint fillers for wall boards.

A major use of MC is in machine-applied wall plasters, particularly those based on gypsum which are especially popular in England and Germany. Machine-spray applications cause additional requirements for the MC product in that besides properties of water-retention, anti-sag, and wall adhesiveness, the MC must mix and hydrate within 10–30 s. For this purpose, MC is milled to a particle size of <60 µm, thereby allowing the MC to develop its full effect by the time the plaster hits the wall.

A detailed description of the use of MC, methyl hydroxyethyl cellulose (MHEC) and hydroxyethyl cellulose (HEC) in gypsum machine plaster has been prepared (Schweizer, 1997), in which details of rheology, water-retention and mechanistic aspects are also discussed. The adsorption of cellulose ethers [MHEC, methyl hydroxypropyl cellulose (MHPC), HEC] on CaSO₄ binder has also been shown to be governed by the DS – namely, the higher the DS, the lower the adsorption. For example, at a DS ≤1.2, typically 60–85% of the cellulose ether is adsorbed. It is interesting to note that MC and MHEC with regular DS significantly lose water retention ability at temperatures above 40°C.

Numerous patents on MC and MHEC formulations for plaster applications have been issued, and two are incorporated here for reference (Bietz et al., 1989a, 1989b). In these reports, the MC described had a DS of 1.4–1.95, while the MHEC had a DS of 1.53 and a molar degree of substitution (MS) of 0.3.

Other applications of MC, MHEC and MHPC include among others hand plaster, self-leveling compounds, wall board adhesives, and masonry mortars. A wealth of building product formulations containing cellulose ethers for these diverse applications is provided in a technical brochure (Aqualon, 1996).

Another typical MC application is that of standard and flexible tile adhesives, the compositions of which are detailed in Table 3. Here, the MC provides water retention, adhesiveness of heavy tiles even on vertical walls, and viscosity to allow the formation of a thin-bed on which the tiles may be set.

It has been established that MC viscosity greatly influences the degree of water retention achieved in a binder system (Clariant Technical brochure). Data provided in Table 18 suggest that MC of viscosity grade >60,000 provides maximum water retention, though in practical applications there is always a trade-off between optimum water retention and workability (i.e., viscosity). An MC which imparts too high a viscosity can impede the overall applicability of a system to a point where a slightly lower water retention would be preferable.

Rheological studies on cellulose ethers in mortars using oscillatory rheology technique have been described by Schweizer and Dewald (1996).

Nonionic cellulose ethers are universal thickeners for the paint industry and, by volume, are probably the most important group of thickeners in this field. Among water-borne dispersion paints, MHEC, MHPC, EHEC and HEC are the most common. They provide structural viscosity to the paint, as well as the water retention that is greatly needed on walls or underlayments with capillary suction forces. Other benefits include improved spreadability and enhancement of abrasion resistance. Medium-viscosity MHEC and HEC (2000–6000 mPa⋅s) was found to be best for low-spattering paint formulations. Typical concentrations of cellulose ethers range from 0.15 to 0.60%, though on occasion cellulose products are combined with associative thickeners to reduce pseudoplastic behavior. In solvent-borne paints, or in paints with a low water content, solvent-compatible ethers such as EC and EHEC are preferred. For a review on the use of cellulose derivatives in the paint and building industry, see Mondt (1990).

Considerable volumes of cellulose ethers are also used in oil well drilling. In fact, the water-retention properties of cellulose ethers was first recognized in the oil field, and by 1948 carboxymethyl cellulose had already been added to a water-based, bentonitic drilling fluid to successfully control filtration to the formation (the porous rock on the borehole wall. E.g., sandstone or limestone). This discovery revolutionized the drilling industry by making it possible to drill much longer hole sections without having to case off. Some ten years later, HEC was first mixed into an oil well cement slurry which was pumped into a well in Oklahoma. Again, excellent fluid loss rates were obtained, and this led to the introduction of an entire family of fluid loss polymers. It is an interesting anecdote that in conventional building products, the cellulose ethers did not become established until the mid-1970s.

Carboxymethyl cellulose (CMC), or its purified, desalted form known as polyanionic cellulose (PAC), are the main components of what is referred to as a polymer mud (see Section 4.6.1). This type of drilling fluid has a low content of bentonite as it obtains much of its viscosity from the cellulose product. Bentonite is preferred to provide solids for the filter-cake build up on the borehole wall. The industry provides low-, medium- and high-viscosity CMCs or PACs, with low-viscosity grades being preferred in high-density muds (specific gravity >1.7 g mL⁻¹) and vice versa. Typical concentrations range from 0.2 to 2%. Commercial CMC used in the oilfield typically has a DS of 0.7–0.9, while the DS for PAC is slightly higher and ranges between 1.05 and 1.15.

CMC and PAC are both sensitive to divalent cations such as calcium and, to a lesser extent, magnesium. Low concentrations of divalent cations require increased dosages of cellulose products, and high concentrations of calcium (>3%) render them ineffective. In this case, hydroxypropyl starch or synthetic sulfonated copolymers based on vinylsulfonates are used as these tolerate high calcium/magnesium concentrations. Another limitation for CMC/PAC is temperature stability, as above 150°C they rapidly degrade in the drilling fluid and lose effectiveness. Typically, vinylsulfonate-based copolymers such as 2-acrylamido-2-methylpropane sulfonic acid-N-vinylacetamide-acrylamide terpolymers (Engelhardt et al., 1982) are used at higher temperatures. These are considerably more expensive than CMC/PAC, but are stable up to at least 225°C.

HEC and carboxymethyl hydroxyethyl cellulose (CMHEC) have become important fluid loss additives for oil well cementing. HEC provides good fluid loss under diverse salinity conditions up to 150°C, and even low-viscosity grades cause cement slurries to thicken considerably. However, this effect is generally undesirable as huge pump pressures are then required to transfer the slurry. Consequently, dispersants (also known as friction reducers) are added to the cement slurry containing HEC partly to offset this thickening effect. Test data on the rheology of dispersed and nondispersed HEC cement slurries are listed in Table 19. Typical HEC products used in well cementing have a DS of 1.5 to 2.5.

CMHEC has also gained a foothold in this market. As the carboxylic group introduces a retarding effect on cement, CMHEC can be used as a retarding fluid loss additive for well cementing. Typical commercial products used in well cementing have a DS of 0.3 to 0.4 (CM) and an MS of 0.3 to 2.0 (HE).

In well cementing, cellulose ethers have better maintained their position compared with competing groups of fluid loss chemicals such as polyethyleneimines and polyvinyl alcohols. Reasons for this include their superior stability as well as their overall better environmental acceptability.

5.6.1 Guar Gum

Guar gum is obtained from the seed of the legume Cyamopsis tetragonolobus, an annual plant which grows mainly in India and Pakistan. The seed's endosperm predominantly contains guar galactomannan. Guaran, the polysaccharide in guar gum, consists of a chain of (1→4)-linked β-d-mannopyranosyl units with single α-d-galactopyranosyl units connected by (1→6)-linkages to, on the average, every second main chain unit (Figure 28). The average molecular weight is 1–2×⋅10⁶ Da – one of the highest molecular weights of all naturally occurring, water-soluble biopolymers.

Guar gum is an excellent viscosifier for aqueous systems, though until recently its use in the construction industry was negligible, with only small amounts of highly substituted hydroxypropyl guar (HPG) being used in dispersion paints to provide viscosity and stabilization. The problem with guar is its susceptibility to bacterial degradation such that preservatives are required to prevent the paint from being spoiled.

More recently, HPG was introduced as a new water-retention and viscosifying agent for cementitious and gypsum-based plasters and grouts (Molteni et al., 1998; Alvarez Berenguer et al., 1999). A high DS and uniform distribution of the hydroxypropyl groups attached to guar in the substitution process are thought to be the key to functionality in building systems. There appears to be a synergistic effect with MC which is used for the same purpose. Details of the chemistry of HPG specifically developed for plaster and grout applications, its solution properties (pseudoplasticity and thixotropy), pH and temperature stability have been provided in a Lamberti Technical brochure. Likewise, formulations illustrating the effects of HPG in plasters and grouts may be found in a brochure produced by the same company.

Native guar is also used to some extent in well construction as a viscosifier in top-hole drilling. Here, the biopolymer provides initial viscosity to a water-based drilling fluid. However, the main advantage of environmental acceptability due to rapid biodegradability occasionally causes problems from a technical point of view. This occurs particularly in top hole sections, where the soil and rock is rich in microorganisms that cause an accelerated biodegradation of guar. In this situation, biocides must be added to the drilling fluid as a preservative for guar.

For the same environmental reason, guar gum is also used as thickener in trench-less drilling operations. Here, large-diameter holes (75–150 cm; 30–60 inch) are drilled using mobile units to lay cables or pipelines under rivers or highways. A common formulation for a guar-based trenchless drilling fluid consists of fresh water, 0.5% guar gum and soda ash, adjusted to a pH of 9–10. A typical rivercross drilling operation is shown in Figure 29.

Guar and guar ethers are used in very large quantities (approximately 20,000 tons) for the stimulation of oil and gas wells to improve production rates.

5.6.2 Locust Bean Gum

Several attempts have been made to use locust bean gum or synergistic combinations of locust bean gum with other hydrocolloids in building materials. Locust bean gum provides a synergistic viscosity-enhancing effect with xanthan, agar and κ-carrageenan. Addition of locust bean gum to xanthan gum (see Section 5.8.1) at a 1:1 ratio, for example, almost triples the viscosity, while higher concentrations of the two hydrocolloids result in a thermoreversible gel. The mechanism behind this is a crosslinking of the rigid helical xanthan gum molecules by the galactomannan strands of locust bean gum. It is interesting to note that guar gum, a galactomannan similar to locust bean gum (see Section 5.6.1), does not exhibit such a synergistic effect.

Although some positive effects of the synergistic blend were observed, its use in construction was ceased due to poor economics. The relatively high price of locust bean gum appears to deter potential users, and the development of synergistic blends with other hydrocolloids that are more acceptable on a cost-effect basis will be required if any volume is to be reached.

5.7.1 Oil of Turpentine and Colophonium

Oil of turpentine, sometimes also called resin oil or pine gum, is the volatile constituent of resin balsam and is isolated by steam distillation of the natural resin. Oil of turpentine essentially contains monoterpenes with 10 carbon atoms and terpenoid compounds derived from the monoterpenes (Figure 30).

Oil of turpentine is used as a solvent and binder for natural resin-based lacquers applied as wood varnish. Its toxic effects must not be ignored; it is an irritant and was shown to cause neurological paralysis when inhaled over a longer period of time. A potential substitute is shellac, a resin produced in the metabolism of female coccids (Tachardia lacca).

Pure colophonium is produced by steam distillation of resin balsam, and contains a mixture of several resin acids, particularly laevo pimaric acid and abietic acid. Colophonium is the film-forming component in many wood glues, varnishes and polishes. It has been shown that these film-forming properties are caused not only by the formation of a dry resin layer, but also by the reaction of its carbon-carbon double bonds with oxygen, thereby forming hydroperoxides and polymers of higher molecular weight.

5.7.2 Root Resins

Root resin extracts, particularly from the roots of conifer trees, have found a significant application as air-entraining agents. Their main use is in the production of freeze–thaw-resistant concrete.

Air-entrainment is regarded as essential for the durability of concrete that will become wetted and then exposed to freeze–thaw conditions. Studies have shown that this protection is best afforded by micro air bubbles with diameters of 10–300 µm. Homogeneous distribution of the bubbles in the concrete is also important, and for optimum performance the inter-bubble distance should not exceed 0.2 mm. The mechanism behind freeze–thaw resistance is disruption of the capillary pore system in the concrete so as to greatly reduce the penetration of water into the concrete. As a result, air-entrained concrete can withstand several thousand cycles of freeze–thaw changes without destruction, whereas untreated concrete cracks and disintegrates after only a few hundred cycles. The effect of freeze–thaw cycles on air-entrained and plain concrete is shown in Figure 31.

Extracts produced from naturally occurring root resins represent one of the most effective and widely used air-entraining agents in concrete. The distillation and extraction of pine stumps leaves a residue which consists of a mixture of phenolics, carboxylics, and other substances. Neutralization with sodium hydroxide renders this soluble; the resultant brown solution, the main active ingredient of which is abietic acid (Figure 32) retains a pine smell and is sold under the name Vinsol^TM resin within the construction industry.

Root resin-based air-entraining agents are used in very small dosages. The superiority of the root resin extract over an alkyl sulfate-based synthetic surfactant is illustrated in Figure 33.

5.8.1 Xanthan Gum

Xanthan gum, produced by the bacteria Xanthomonas campestris, was the first microbial biopolymer to be used in construction. The US Department of Agriculture, Northern Regional Center, Agricultural Research Service, Peoria, Illinois, developed a xanthan gum production method during the 1950s in an effort to enhance molasses consumption and provide financial help to those farmers who were growing maize. An aerobic fermentation process is used to produce the gum; this involves a multi-step inoculation preparation followed by fermentation, pasteurization, precipitation, drying and milling.

Xanthan gum is an exocellular heteropolysaccharide consisting of a poly-β(1→4) d-gluco-pyranose backbone linked with a trimer of mannose, glucuronic acid, and mannose every second glucose unit (Figure 34). The mannose units contain anionic acetate and pyruvate groups which render this biopolymer more water-soluble than other microbial products. As will be seen later, this relatively higher anionic charge in the side chain is less desirable in certain construction applications where other biopolymers excel. The molecular weight of the gum is reported to be ∼2×10⁶ Da, and the molecular conformation is described as helical with the side chains aligned with the backbone (Yanaki et al., 1981).

When dissolved in water, xanthan gum develops significant viscosity. Its main characteristic and advantage is the type of viscosity: While starch- and cellulose-based biopolymers usually increase the plastic viscosity (PV) of their aqueous solutions, xanthan gum and most other microbial biopolymers increase the yield point (YP) and show a low PV. The resultant viscosity profile is referred to as shear-thinning. Figure 35 illustrates a typical viscosity curve as measured with a FANN® rotational viscometer, obtained by addition of various doses of xanthan gum to fresh water. As can be seen, xanthan gum solutions are distinguished by a high YP/PV ratio.

The shear-thinning type of fluid viscosity is extremely useful in certain oil well construction applications. As pointed out earlier (see Section 4.6.1), drilling fluids need to be low in viscosity when being pumped through the hole, but highly viscous when circulation is halted. Xanthan gum exactly provides this property; by way of its shear-thinning rheology, xanthan gum-treated drilling fluids require low pump pressures to circulate the fluid and to clean the hole from drill cuttings. However, when circulation stops and the fluid is at rest, it suspends the cuttings quite well because of its high YP. Consequently, settling of drill solids and weighting materials (e.g., barite) that are suspended in the drilling fluid is greatly reduced. This effect is extremely important, because solids settling at the bottom of the hole may bury the lower part of the drill string. Likewise, when restarting rotation of the drill string, a large quantity of settled material may cause rupture of the drill pipe, the latter effect being considered a major accident. If this happens, it is necessary to side-track the hole from the point of disruption, which clearly adds extra cost to the construction of the well bore and is highly undesirable.

During the mid-1960s, the KELCO Company introduced xanthan gum as a novel drilling fluid component under the name “XC polymer”. Since then, most water-based drilling fluids are treated with 0.01 to 0.2% of xanthan gum, though for viscous sweeps to clean out large debris in the hole concentrations of up to 0.8% may be used.

Xanthan gum is also effective in high-salt brines based on sodium chloride. For example, NaCl brines of 27%, weighted with ground salt or calcium carbonate, are used in workover operations on well bores. Xanthan viscosifies these brines, though somewhat more gum is required to achieve the same viscosity increase in salt solution as compared with fresh water.

The application of xanthan gum in well construction is limited by its temperature stability and sensitivity to divalent cations. Rheology data produced on an HT/HP rotational viscometer (e.g., FANN®50 of Halliburton Co.) indicates that xanthan gum viscosity starts to decline at 60°C and is completely lost at 120°C. Interestingly, the viscosity is regained when the fluid is cooled back to moderate temperatures (Figure 36).

These data suggest that the gum should not be used at well bore temperatures significantly exceeding 100°C unless continuous replenishment is possible. Fortunately, there are other, more temperature-stable microbial biopolymers to take over at elevated temperatures. From a market point of view, the temperature limitation does not pose a major problem for xanthan gum, because by far the majority of oil and gas wells drilled never reach a bottom-hole temperature of 100°C. For example, the average well depth of all wells drilled in 2000 in the US was ∼1700 m, which corresponds to an average bottom-hole temperature of 60°C. This explains why, in comparison with other microbial biopolymers, so much more xanthan gum is used in drilling.

The shear-thinning type of rheology imparted by xanthan gum is also appreciated in building materials. Highly fluid floor screeds based on anhydrite, for example, sometimes have a tendency to drop out the binder or aggregates. Bleeding water may appear on the surface, and coarse particles may settle at the bottom, and such inhomogeneity of the screed results in reduced compressive strength and poor surface. Xanthan gum (0.03 to 0.1% weight of binder) may be added to the dry formulation to prevent disintegration of the floor screed.

Other minor uses of xanthan in building materials include gypsum-based wall plasters to enhance plaster adhesion to the wall; and water-borne paints and coatings. In the latter application, xanthan gum addition supports the formation of a film when applying the paint by brushing or spraying. Dripping of the paint, for example from ceilings, is also reduced.

As approximately 9000 tons of xanthan gum is used annually in well construction, this application by far exceeds the use of the gum in conventional building products. Although expensive, xanthan gum provides benefits to the overall drilling process which makes its use worthwhile from performance and safety points of view and, as indicated below, the oil drilling industry has accepted the use of even more expensive microbial biopolymers for the sake of their overall cost benefits. In conventional building products, however, xanthan gum is more often than not a “nice-to-have” additive, allowing the technical perfection of certain products, albeit at great expense. In fact, this added cost is often significant and prevents a more widespread use of xanthan gum in the highly competitive market for building products. Hope persists, however, that this situation may ease in the future as biopolymer manufacturers increase their capacity such that unit costs for xanthan gum reach a downward trend.

5.8.2 Scleroglucan

Scleroglucan is a branched homopolysaccharide which consists of a main chain of (1→3)-linked β-d-glucopyranosyl units. Every third unit bears a single β-d-glucopyranosyl unit linked (1→6) (Figure 37).

Scleroglucan is produced by an aerobic submerged culture of a selected strain of the fungus Sclerotium rolfsii. The fermentation medium contains ∼3% d-glucose as a carbon source, corn-steep liquor, nitrate, and mineral salts. Fermentation takes place at 28–30°C and requires ∼60 h for completion. Mycelial growth and polysaccharide production occur simultaneously, the rates of mycelium to polysaccharide being about 1:3. In construction applications, native scleroglucan is used still containing the mycelium. Purified scleroglucan, from which the mycelium has been removed by filtering the dilute broth, is used as thickener in cosmetic applications.

Dissolved scleroglucan chains assume a rod-like triple helical structure in which the d-glucosidic side groups are on the outside and prevent the helices from coming close to each other and aggregating (Yanaki et al., 1981).

Scleroglucan has found a major application in oil well drilling. Today, horizontal wells are drilled to extract oil and gas from extended, yet thin, reservoirs. These wells are often drilled over distances of several thousand feet (500–3000 m) and typically are not cemented and cased off. Rather, sand screens are placed in the hole to prevent plugging from sand influx. Because of this technique, these wells are referred to as open-hole completions. Figure 38 shows the drilling of a horizontal well and illustrates the importance of suitable drilling fluid properties.

Drilling fluids used in open-hole completions must provide shear-thinning rheology, and this property is provided equally by xanthan gum and scleroglucan. A fluid for open-hole completions also must not plug the micro pores of the oil- or gas-bearing reservoir, as this will severely reduce the flow of hydrocarbons to the borehole and reduce the production rate that determines the overall profitability of a well bore. Oil producers therefore are prepared to undergo significant expense to ensure high productivity of their well.

In 1996, Johnson discovered and patented a novel nondamaging reservoir drilling fluid essentially consisting of 2.6 parts scleroglucan viscosifier, 10.4 parts carboxymethyl starch fluid loss control additive and 87 parts graded calcium carbonate weighting agent and filter cake builder (Johnson, 1996). Tests conducted with a so-called return permeability apparatus indicate this fluid to show significantly less pore plugging than a xanthan-based system. The improved performance is attributed to the nonionic character of scleroglucan. Xanthan gum, with its anionic charge in the side groups, is believed to adhere more to the rock and hence plug reservoir pores. Because of its field-proven advantages, scleroglucan-based so-called drill-in fluids have found a significant use in horizontal well drilling as they allow the extraction of more oil from a given reservoir. The introduction of such “nondamaging” fluid systems represent a major technological advance in oil and gas production.

Another advantage of scleroglucan over xanthan gum is its higher temperature stability. The viscosity of a fresh water scleroglucan solution in comparison with xanthan at temperatures ranging from 20°C to 150°C is illustrated in Figure 36. The gain in temperature stability by using scleroglucan is clear, and as a result when bore hole temperatures exceed 100°C, a conversion from xanthan gum to scleroglucan is recommended to control solids settling. It must be noted however that once heated above a threshold temperature of 140°C, scleroglucan viscosity abruptly breaks down and is not retrieved on cooling (unlike xanthan gum). It is believed that the excessive temperature destroys the helical structure of scleroglucan, thereby destroying its viscosifying effect.

In conventional building products, scleroglucan has found a niche application in asphalt emulsions. Generally, road tarring is carried out either by laying down molten asphalt or by spraying an aqueous asphalt emulsion onto the road. The latter method is popular in warmer climates. When the water evaporates from the emulsion an asphalt layer is formed. Scleroglucan is used to thicken these asphalt emulsions, thus preventing them from seeping away on steep roads or inclined curves.

5.8.3 Welan Gum

Welan is produced by bacteria from the Alcaligenes species (ATCC-31555) in an aerobic, submerged fermentation. The medium contains d-glucose as the carbon source, phosphate as a buffering agent, ammonium nitrate and soy peptone as sources of nitrogen, and trace elements (iron, magnesium, cobalt, zinc, copper, manganese, and molybdenum) (Dimitru, 1998).

The welan repeat unit shows a single sugar side chain containing either l-rhamnose or l-mannose substituted on C3 of every (1→4)-linked glucose unit (Figure 39). The welan side chain shields the carboxylate group in the double helix, thereby rendering welan unsusceptible to cation-mediated bridging. The welan molecule is rather stiff, whereby it becomes relatively insensitive to temperature and pH variation. This, together with its tolerance of high calcium ion concentrations, makes it ideal for applications in concrete, mortars, cement grouts and high-temperature drilling.

The usefulness of welan gum in construction was first documented in patents describing cement compositions with low-viscosity welan (Allen et al., 1990), but later on further reports were made for the use of welan in grouts (Skaggs et al., 1994; Skaggs, 1997).

There are two major applications of welan in construction, namely its use in concrete as a stabilizer, and in oil well construction for spacer fluids.

As discussed in Section 4.1.4, highly fluid concrete such as SCC with a slump in excess of 270 mm has a tendency to disintegrate. Of all microbial biopolymers, welan gum has been found the best to stabilize this concrete and to prevent surface bleeding. With concrete requiring liquid admixtures rather than dry powders for ease of homogeneous mixing, considerable effort was devoted to develop a liquid, yet highly concentrated colloidal welan suspension (Skaggs et al., 1996, 2001). Dial et al. (2001) discovered and patented an aqueous suspension containing 10% welan gum, 40% of a sulfonated melamine formaldehyde condensate, 0.75% ultra-fine cellulose fibers for suspension stability, with the remainder being water.

Published test data (Hibino, 2000) suggest that in concrete the welan gum suspension effectively prevents the formation of surface bleeding water and the sag of large aggregate particles. It does however impede early strength development to some extent when used at higher concentrations. Nonetheless, welan gum suspensions are the most widespread admixture used to stabilize highly fluid concrete.

Liquid welan-superplasticizer suspension is also used in cementitious injection grouts, generally for crack repair, for sealing of submerged anchorages, and for filling post-tensioned ducts. With these applications, as for SCC, it is essential to control the bleeding of mix water and sedimentation of suspended cementitious materials. Details on grouts treated with the welan suspension, their rheology, set times, etc. have been presented by Khayat and Yahia (1997), while the use of welan powder in a dry formulation of an injection grout is described by Teichert and Umlauf (2000).

Among all applications, probably the largest volume of welan is used in spacer fluids for well construction (Carpenter et al., 1997). In this application, welan has several advantages over any other commercially available biopolymer, including:

This combination of properties has turned the market almost exclusively to welan. As described earlier (see Section 4.6.3), spacer fluids are pumped and circulated through the bore hole to separate the drilling fluid from the cement slurry. While previous spacers typically contained dispersing and abrasive agents and were pumped under turbulent flow at high annular velocity to achieve complete removal of drilling fluid and mud cake, it is generally accepted now that spacer fluids showing a shear-thinning type of viscosity imparted by a microbial biopolymer and pumped under plug flow conditions are more efficient for the job. Typical compositions of dispersed and shear-thinning spacer formulations used in well construction are listed in Table 21.

Typically, 25 kg of the above dry spacer formulation is mixed with 1 m³ of water and circulated through the hole prior to cementing the casing. When surfacing after circulation, the spacer fluid either is blended into the drilling fluid to be used later on, or is disposed of.

5.8.4 Succinoglycan

Succinoglycan, a β-(1→3)-linked d-glucan, was discovered in 1966, the producing organism being of the Alcaligenes faecalis type. In addition to the pyruvate groups, the polysaccharide contains covalently bound succinic acid and therefore was referred to as succinoglycan (Matulova, 1994).

Succinoglycan has found some interesting uses in the construction industry for self-leveling underlayments. These cementitious grouts require an admixture which imparts shear-thinning rheology. Succinoglycan seems to provide similar or even better effects than casein (see Section 5.3). Detailed test data on succinoglycan performance in an SLU based on Portland cement and calcium alumina cement have been reported by Charrin (1997).

The use of succinoglycan as an anti-sag and stabilizing agent for SCC, similar to welan gum, has also been proposed. A liquid admixture consisting of 5% active suspension of succinoglycan in a vegetable oil (e.g., rape oil ester) has been reported to work well in concrete when combined with nanosilica (Sari et al., 1999).

When heated above 55–60 °C at concentrations >1%, succinoglycan forms a firm, resilient gel, with gellation occurring over a wide range of pH values from 3 to 10. On occasion, this property has been used in drilling operations to shut off water influxes into bore holes or to prevent water influx into product reservoirs.

The major draw-back of succinoglycan use in construction is its high cost, and while this persists its uses will be limited to highly specialized niche applications.

5.8.5 Curdlan and Rhamsan

Curdlan is a β-(1→3)-glucan that was first described in 1964 by Harada and Yoshimura (1964). These authors isolated a soil bacterium that grew in a medium with 10% ethylene glycol as the sole carbon source and produced Curdlan in high yield. The bacterium was a member of the Alcaligenes faecalis family which also produces succinoglycan.

In Japan, Curdlan is used to some extent as an anti-sag and stabilizing agent in SCC, similar to welan gum. Performance details and studies on the swelling and solubility behavior of Curdlan in concrete have been described by Matsuoka et al. (1997). It is proposed that Curdlan forms a calcium complex with cement, and this enhances concrete flowability.

Rhamsan has a two glucose side chain substituted on C6 of every 3-linked d-glucose residue. Its disaccharide side chain partially shields the carboxylate group and reduces cation-mediated cross-linking. In comparison with welan and xanthan, rhamsan produced by far the highest viscosity in Portland cement grouts (Skaggs, 1997), and also showed the best effect with respect to reduction of bleeding water. Commercially, rhamsan plays a far smaller role in construction than welan or scleroglucan, with its high cost being the major reason for its limited use.

5.10.1 Polyaspartic Acid

Polyaspartic acid is made by reacting maleic anhydride with ammonia. The linkage of the aspartic acid unit can occur on either the α- or β-position to the amide groups (Figure 40).

Polyaspartic acid has been recognized as a cement dispersant (Vickers et al., 2001) and gypsum retarder (Staffel et al., 2001) that shows significant biodegradation. Published test data indicate a biodegradability of >70% in 28 days, according to OECD 302 B test protocol (Bayer Technical brochure, 2000).

5.10.2 Polyesters

A snag for polyaspartic acid in cement is its strong retardation effect. Esterification of polyaspartic acid with poly(ethylene oxide) has been demonstrated to reduce the retardation exerted by the carboxylic groups (Vijn et al., 2002), though unfortunately the ester side chains reduce the biodegradability to some extent.

Another polyester with significant biodegradability has been proposed for the dispersion of hydraulic binders (Wegener et al., 1996). The polyester can be prepared by reacting a diol (e.g., diethylene glycol) with a dicarboxylic acid (e.g., maleic anhydride), followed by sulfitation with sodium bisulfite or sodium pyrosulfite. The sulpho group-containing polyester shows a biodegradability of >80% in 28 days according to OECD 302 B test protocol. Test data describing the plastification of Portland cement and gypsum are presented by Wegener et al. (1996).