Applications of biopolymers and other biotechnological products in building materials

Lignosulfonates

By volume, the largest admixture used in the construction industry is lignosulfonate. It is a very common plasticizer which improves the flowability and workability of concrete. The source is lignin, a natural biopolymer contained in wood at 20–30 wt%. Native lignin is water-insoluble. It is recovered from wood by treatment with alkaline calcium sulfite, producing a “spent sulfite liquor” containing water-soluble lignosulfonate with a low degree of sulfonation, sugars and other by-products (Lin and Lebo 1995). To optimize its plasticizing effect, raw lignosulfonate is further derivatized (Darley and Gray 1988). Sulfomethylation with sodium sulfite and formaldehyde is the most common process. It yields a sodium or calcium lignosulfonate with a degree of sulfonation between 0.5 and 0.6.

Lignosulfonate can be used in concrete for two different purposes: plastification of stiff concrete to enhance its flowability, pourability and workability, or water-reduction, which results in a concrete with good workability but a lower water/cement ratio and corresponding higher compressive strength. By far, ready-mix concrete represents the largest application for lignosulfonates. Their plasticizing effect is used to produce concrete with an improved pourability at the job-site. In pre-cast concrete, lignosulfonates are used to obtain high-strength concrete. Typical dosages of lignosulfonates in concrete are 0.1–0.3% by weight of cement (Plank and Winter 2003).

The plasticizing and water-reducing effects of lignosulfonate are limited. For concrete with superior fluidity or more dramatic water-reduction, so-called superplasticizers based on polycondensate or polycarboxylate chemistry are used. Examples of polycondensates are polymelamine formaldehyde sulfite (PMS) or β-naphthalene sulfonic acid formaldehyde (BNS) condensates with molecular weights below 10,000 Da (Spiratos et al. 2003). The most common type of polycarboxylate-based superplasticizer is a methacrylic acid–methacrylate ω-methoxypolyethylenglycol ester copolymer (Shonaka et al. 1997). These synthetic superplasticizers allow a water-reduction of up to 40%, whereas common lignosulfonate reduces the water content only up to 15%. Lignosulfonate is mainly used because of its cost-competitiveness. For stronger effects, blends of lignosulfonates with synthetic superplasticizers or pure superplasticizers are used.

Sodium gluconate

α-Hydroxycarboxylic acids are very strong set-retarders for cement (Singh 1976). Sodium gluconate and δ-gluco-heptonate are the most widely used cement set-retarders based on this chemistry. Their starting material is d-gluconic acid which is produced in 95% yield by submerged microbial oxidation from glucose syrup using Aspergillus niger (Singh et al. 2003).

The main application of sodium gluconate is ready-mix concrete. It is used to delay early hydration of cement which causes stiffening and loss of workability approximately 30 min after mixing the concrete. A sodium gluconate dosage of 0.05–0.2% by weight of cement prevents workability loss in the first 1–2 h. The effect depends on cement content and concrete temperature. As a result, the gluconate-treated concrete still is very fluid when poured at the job-site. The advantages of gluconate over its main competitor, tetra-potassium pyrophosphate (K₄P₂O₇), are a secondary plasticizing effect which economizes plasticizer dosage and its compatibility with calcium lignosulfonate. A very common, widely used retarding and water-reducing liquid admixture for concrete consists of 30% of calcium lignosulfonate, 5% β-naphthalene sulfonic acid formaldehyde condensate, 5% sodium gluconate, 2% caustic soda and 1% tributyl phosphate defoamer (Reul 1991).

The disadvantage of sodium gluconate is its effect on the early strength of concrete. Like most common retarders, it reduces strength development up to 7 days. Improved superplasticizers based on polycarboxylate chemistry provide 2 h of workability without significant retardation of early strength. Nevertheless, there are still many applications where early strength development is not so critical and sodium gluconate, because of its superior economics, is used.

Pine root extracts

Root resin extracts, particularly from the roots of conifer trees, have found a significant application as air-entraining agents. They allow production of a freeze–thaw resistant concrete (Plank and Winter 2003). Studies have shown that freeze–thaw resistant concrete is obtained by disruption of the capillary pore system in concrete. The introduction of micro air bubbles with diameters of 10–300 μm distributed homogeneously in concrete at a distance of less than 0.2 mm were found to work best. Air-entrained concrete of this quality can withstand several thousand cycles of freeze–thaw change without destruction, whereas untreated concrete cracks after only a few hundred cycles (Kerkhoff et al. 2002).

Extracts produced from pine wood hair roots are among the most effective and widely used air-entraining agents. Their main ingredient is abietic acid. Root resin extracts are often superior over synthetic surfactants such as sodium tetradecyl sulfate because they provide air bubbles in exactly the desired size range.

Protein hydrolysates

Protein hydrolysates are extracts obtained by cooking animal blood, hair, hides or hoofs, particularly from cattle, with sulfuric acid or, more recently, treating these animal parts with enzymes. The resultant extract contains degraded proteins, mainly oligo- and polypeptides, of varied composition.

Protein hydrolysates lower the surface tension of water significantly and are used to prepare foam concrete with a specific density as low as 0.5 kg/dm³. This concrete is used to fill ditches, to provide a base for roads on unconsolidated soil and for prefabricated dividing walls in homes.

Compared with synthetic surfactants such as alkyl sulfonates, alkyl sulfates or betains, protein hydrolysates stand out because they generate spherical foam bubbles, whereas synthetic foamers typically generate hexagonal bubbles. This difference is important because concrete containing spherical bubbles has been shown to provide 20–30% higher compressive strength than concrete with hexagonal bubbles. As a consequence, protein hydrolysates are preferred over synthetic foamers when it comes to foam concrete with low specific weight and yet high compressive strength (Plank 2003).

Welan gum

Welan gum is a microbial biopolymer produced from d-glucose by bacteria from the Alcaligenes species in an aerobic, submerged fermentation (Dimitru 1998). The Welan gum 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.

In spite of its high unit cost, Welan gum finds a significant application in concrete as a stabilizer (Hibino 2000). Highly fluid concrete, such as self-compacting concrete, has a tendency to disintegrate, indicated by the formation of a water layer at the surface, or by the settling of large aggregates. Various viscosifiers have been experimented with to prevent this effect. Welan gum, at 0.01–0.1% by weight of cement, has been shown to stabilize concrete and prevent these negative effects. Its main advantage is stabilization without destroying the fluidity of the concrete. This effect is attributed to the shear-thinning type of rheology typically observed for aqueous Welan gum solutions. It provides a high yield point which prevents sag of solid particles, but does not increase plastic viscosity to an extent where concrete flowability is reduced (Khayat and Yahia 1997).

Wall plasters

Wall plasters require a water-retention agent to prevent the loss of water into the wall, which typically consists of brick, light-weight concrete or porous concrete. Capillary forces suck water from the plaster, thus dehydrating it. The plaster cannot develop its full strength and will show cracks or spall off.

Cellulose ethers are the dominant water-retention agent used in plasters. Methyl cellulose or, more often, methyl hydroxypropyl cellulose is added at 0.05–0.3% by weight of the total blend. It is manufactured by alkoxylation of alkaline cellulose linters of certain molecular weight and subsequent derivation with methyl chloride (Dönges 1990). The dry powder is ground to <200 μm particle size to achieve dissolution in water within 10–15 s. Slower solvation is unacceptable because of the fast mixing process in a machine-applied plaster. Typical degrees of substitution (DS) range over 1.4–2.0 for methylation and 0.1–0.7 for propoxylation. It is well established that higher viscosity ethers provide better water retention. Therefore, typical grades show a Brookfield viscosity of 15,000–60,000 mPa s⁻¹ in 2% aqueous solution. Higher viscosity products may cause excessive viscosity and poor workability of the plaster, whereas lower viscosity grades require higher dosages to achieve acceptable water retention (Clariant 1998).

Other benefits from cellulose ethers include anti-sag, adhesiveness to the wall and air-entrainment into the plaster, which increases its yield and reduces cost. Many cellulose products are blended with up to 10% of synthetic polymers, such as polyacrylamide, polyethylene oxide or polyvinylalcohol, to perfect their properties. The industry offers custom-made blends tailored to specific binders for major users.

Hydroxypropyl guar of high DS (1.5–2.0) was recently introduced as a novel water-retention agent for plasters (Molteni et al. 1998). Good field results were achieved in gypsum plasters (Lamberti 2002).

Hydroxypropyl starch of low DS (<0.5) is used at 0.05% to provide instant viscosity for adhesion of the plaster to the wall. This effect is achieved through a synergism with cellulose ethers. Hydroxypropyl starch prevents the plaster from sagging once it has been sprayed on and reduces stickiness to the trowel (Aqualon 1996).

Gypsum-based plasters typically consist of a blend of β-hemihydrate gypsum (CaSO₄·0.5H₂O) and several forms of anhydrite as a binder. This blend is called “multiphase gypsum”. Pure β-hemihydrate will hydrate and stiffen within a few minutes and therefore requires a retarder to provide enough workability.

Tartaric acid or its potassium salt, K₂C₄O₆H_4, is by far the most common retarder for gypsum plaster. The best effect is obtained from natural tartaric acid [also termed l(+)- or (2R, 3R)-tartaric acid] obtained in the wine manufacturing process. Meso- or (2S, 3S)-tartaric acid or their salts show much less retardation (Forg 1989).

Tartaric acid develops its full retardation at pH>9, which is why gypsum plasters typically contain up to 5% of lime or cement. There is one effect why tartaric acid stands out far above other α-hydroxy carboxylic acids, such as citric or malic acid: while it retards the initial set of the plaster only slightly and provides sufficient early strength for the plaster to stick, it delays the final set very strongly, thus allowing the worker several hours to smooth the plaster. This specific property, unknown from other retarders, has turned the gypsum plaster market almost exclusively to natural tartaric acid. The improvement in workability is so important that the disadvantage of up to 50% lower compressive strength typically occurring with tartrate is tolerated. The working mechanism of tartaric and citric acid in gypsum retardation was investigated by Hill and Plank (2004) and Hummel et al. (2003).

Tile adhesives

Cement-based grouts, commonly about 5 mm thick, are used to glue ceramic tiles onto walls or place them on the floor (Bayer and Lutz 2003). They must be viscous enough to prevent tile sag. Tile adhesives are a huge market in countries where ceramic tiles are popular. Germany alone, for example, produces in excess of 1×10⁶ t of tile adhesives per year.

Self-levelling underlayers

Self-levelling underlayers (SLUs) are poured on uneven grounds to provide an even surface for laying out carpets, parquet or tiles. Their characteristic is self-flowing and self-healing behaviour, i.e. they spread out evenly by themselves. SLUs are either based on Portland cement or CaSO₄-α-hemihydrate. Sometimes, a rapid-setting, highly reactive mixture of calcium alumina cement, Portland cement and CaSO₄-anhydride is used (Wöhrmeyer 2000). This grout gives a workability time for placement of approximately 1 h and can be walked on after only a few hours. SLUs are often dry-mix blends. They are machine-mixed on-site and therefore referred to as machinery grouts. Special dispersants are required to achieve the perfect fluidity and self-healing properties of a SLU (Plank 2003).

Casein, a biopolymer recovered from milk by acid precipitation, is by far the most widely used dispersant in SLUs (Kumosinski et al. 1996). It is used at concentrations between 0.4% and 2.0% by weight of binder and provides perfect self-levelling properties. The disadvantages of casein as perceived by the industry are volatility in quality and pricing, occasional development of ammonia or evil smells and its fostering of moulds such as Aspergillus niger.

Succinoglycan, a microbial biopolymer produced in a fermentation process by organisms like Alcaligenes faecalis, has made some inroads into the SLU market (Charrin 1997). This β-(1→3)-linked d-glucan provides the shear-thinning type of rheology which is necessary to achieve self-levelling and self-healing. Like casein, it also prevents the sag of fine aggregates in the grout. The major draw-back for succinoglycan is high cost.

Very recently, new polycarboxylate-based superplasticizers were introduced into SLU applications. They provide excellent flowability (Strauβ 2001). A typical polycarboxylate copolymer consists of maleic (polyethylenglycol) ester, maleic anhydride and vinylethers. As they do not impart a high yield-point into the grout, solids-settling has been observed. Xanthan or Welan gum, both microbial biopolymers, were found to prevent particle sag at very low concentrations (0.01–0.05%; Skaggs 1997).