Water-reducing agents
1.2 The chemistry of the water-reducing admixtures
Although the variety of admixtures that are commercially available are marketed under a multitude of benefit- orientated classifications, namely waterproofers, densifiers, workability aids, etc., it is possible to categorize the basic chemicals used as shown in Table 1.1.
It can be seen, therefore, that only three chemical materials form the basis of all the water-reducing admixtures, i.
e. lignosulfonate, hydroxycarboxylic acid, and hydroxylated polymers.
1.2.1 Lignosulfonates
Lignin is a complex material which makes up approximately 20% of the composition of wood. During the
process for the production of paper-making pulp from wood, a waste liquor is formed as a by-product containing a complex mixture of substances, including decomposition products of lignin and cellulose, sulfonation products of lignin, various carbohydrates (sugars) and free sulfurous acid or sulfates. Subsequent neutralization,
precipitation and fermentation processes [10] produce a range of lignosulfonates of varying purity and composition depending on a number of factors, such as the neutralizing alkali, the pulping process used, the degree of fermentation and even the type and age of the wood used as pulp feedstock [11].
Table 1.1 Formulations of water-reducing admixtures
Type of water-reducing admixture
Normal Accelerating Retarding Air-entraining
Purified lignosulfonate Lignosulfonate + CaCl2 High sugar
lignosulfonate Impure lignosulfonate Lignosulfonate + air Lignosulfonate +
triethanolamine
Hydroxycarboxylic acid Lignosulfonate + surfactant
detraining agent
Hydroxycarboxylic acid at
low dosage Lignosulfonate + Ca
formate Hydroxylated polymer Hydroxycarboxylic acid + surfactant
Hydroxylated polymer at low dosage
Hydroxycarboxylic acid + CaCl2
Lignosulfonate + Sod.
thiocyanate
Page 6 Commercial lignosulfonates used in admixture formulations are predominately calcium or sodium based with sugar contents of 1–30%. Typical analyses of two commercially available lignosulfonate water-reducing admixtures are shown in Table 1.2 [12].
The lignosulfonate molecule is a substituted phenyl propane unit containing hydroxyl, carboxyl, methoxy and sulfonic acid groups [13–15]. A possible representation is a polymer containing the repeating unit shown in Fig.
1.4. The polymer could typically have an average molecular weight of about 20 to 30 000 with a range varying from a few hundred to 100 000 [12, 16]. The range of molecular weight present in the lignosulfonate is
dependent on the manner and conditions under which refinement is done. Three such methods are used, namely, ultra-filtration, heat treatment at a specified pH and fermentation.
Figure 1.5 is a typical molecular weight distribution curve for a lignosulfonate obtained by means of gel permeation chromatography (a sophisticated analytical method where molecules are sieved according to their molecular size).
It has been found [17] that the lignosulfonate polymer is not a simple linear flexible coiled ‘thread’, as found in many high molecular weight materials, but forms spherical microgels of the type shown in Fig. 1.6. Thus the charges are predominately on the outside of the spheroid with the internal carboxyl groups and sulfonate group being non-ionized. Conductivity studies have confirmed that lignosulfonates are only 20–30% ionized [16].
Table 1.2 Typical analyses of lignosulfonate-based water-reducing admixtures (after Edmeades)
Type Spruce wood sulfite lye calcium lignosulfonate
(%) [12] Sodium lignosulfonate
(%)
Solid content 54 30
Ash 6.6 —
Sulfated ash 10.9 —
CaO (in ash) 44 0.1
Reducing sugars (as
glucose) 5.8 0.9
Total sulfur 3.2 2.6
Fig. 1.4 Repeating unit of a lignosulfonate molecule.
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Fig. 1.5 Molecular weight distribution of a typical lignosulfonate.
Fig. 1.6 Schematic representation of a lignosulfonate polyelectrolyte microgel unit.
The sugars contained in lignosulfonates vary both in type and concentration, depending on the source, type and degree of refining that has taken place. In the fermentation process the micro-organisms used preferentially consume the hexoses rather than the pentoses so that the residual sugars present in the refined lignosulfonates are mainly pentoses. The types of sugars found are shown in Fig. 1.7, and Table 1.3 gives a breakdown of sugars found in untreated sulfite lye [15] and two commercial water-reducing admixtures [11].
Page 8
Table 1.3 Analysis of sugars in lignosulfonate materials (after Mouton and Joisel)
Fermented admixtures
Material Sulfite lye A B
Sugar content (%) 30 typical 10.2 5.4
Composition of sugars (%)
Pentoses xylose + acid 15
21
55
70
60
arabinose 6 16 14 74
Hexoses mannose 48
75
11
26
gluose 15
fructose 2
rhamnose — 16
30 15
galactose 10 14
Others 4
Fig. 1.7 Formulae of sugars found in untreated and purified lignosulfonate materials.
low temperatures, thus avoiding sedimentation in winter conditions. In addition, the sodium salt has a higher degree of ionization in solution [16] than the calcium salt. This is reflected in the observation that solutions of higher concentrations of the calcium salt are required to obtain the same reduction in water–cement ratio obtained by using the same dosage of a sodium-salt-based water-reducing admixture. However, calcium lignosulfonate raw materials are invariably cheaper than sodium lignosulfonates so that the higher concentrations can be offered on an approximately equal cost-effectiveness basis.
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In the formulation of admixtures from lignosulfonate (Table 1.1), the following comments are relevant:
1. Many lignosulfonates and, in particular, the less pure types and those produced from hardwood lignins entrain a small proportion of air into the concrete. This can be desirable where an air-entraining material is required to enhance durability or cohesion, but is often an unwanted side effect. Thus in the production of normal water-reducing admixtures, a small quantity of an air-detraining agent can be added. The usual material is tributylphosphate at a level of less than 1% of the lignosulfonate, although dibutyl phthalate, water-insoluble alcohols, borate esters and silicone derivatives find some application [18].
2. The lignosulfonate molecule itself and, of course, the sugars present in the lignosulfonate materials do have a retarding influence on the hydration of the cement. In the case of the higher-sugar-content materials, this is utilized to produce the retarding water-reducing admixtures which allow longer
transport or placing times. However, for normal water-reducing admixtures, this is an undesirable effect and, therefore, additions of triethanolamine are occasionally made at a level of about 15% of the
lignosulfonate content of the admixture [19]. At this level of addition the triethanolamine acts as an accelerator and compensates for the retarding influence of the lignosulfonate and its impurities. This has been shown to have certain deleterious effects on some properties of the resultant concrete.
3. The accelerating water-reducing admixtures are simple blends of either calcium chloride, nitrate, thiocyanate or formate with a lignosulfonate or a hydroxycarboxylic acid salt. In some cases it may not possible to obtain a completely sediment-free solution and agitation of store tanks may be necessary.
Typically, a mixture of approximately 33% calcium chloride and 4% calcium lignosulfonate by weight in water would be used.
4. Air-entraining water-reducing admixtures containing lignosulfonates can be based on impure
lignosulfonate raw materials, as stated earlier, where only 2–3% additional air is required. However, this air may not be of the amount, type, and stability required, therefore additions of surfactants are made.
Several different types can be used but in the majority of cases they are based on alkyl-aryl sulfonates (e.
g. sodium dodecyl benzene sulfonate) or fatty-acid soaps (e.g. the sodium salt of tall-oil fatty acids).
Additions of these types will allow incorporation of sufficient stable air of the correct bubble size to meet durability requirements under freeze–thaw conditions.
1.2.2 Hydroxycarboxylic acids
As the name implies, these are organic chemicals which have both hydroxyl and carboxyl groups in their molecules. Generally, the sodium salt is used,
Page 10 although occasionally the materials are found as salts of ammonia or triethanolamine. They are produced from pure raw material feedstocks, by either chemical or biochemical means and, therefore, are of high and consistent purity. Indeed, the primary use of the materials is often in foodstuffs or pharmaceuticals.
In the form of sodium salts all are very soluble and have low freezing points, so that solidification in winter conditions is unlikely. Figure 1.8 shows the types and formulae of materials which have been reported to find application in the formulation of this type of water-reducing admixture. However, the only materials finding widescale application in formulations are the salts of gluconic and heptonic acids.
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Fig. 1.9
Normally, approximately 30% solutions of the salts would be used with additions of other chemical types, depending on the proposed function in concrete. Thus the salts may be present alone to produce normal water- reducing admixtures at low dosages and retarding water-reducing admixtures at higher dosages. Small amounts can be blended with calcium chloride to produce accelerating water-reducing admixtures which are almost colorless, sediment-free solutions and, in a similar manner to lignosulfonates (see earlier), air-entraining agents can be added to form the air-entraining water-reducing admixtures which may or may not be retarding,
depending on the amount of hydroxycarboxylic acid salt present in the formulation.
1.2.3 Hydroxylated polymers
The hydroxylated polymers are derived from naturally occurring polysaccharides, such as corn starch, by partial hydrolysis to form lower-molecular-weight polymers containing from 3 to 25 glycoside units (Fig. 1.9) [27].
Unlike the monosaccharide glucose, these materials are stable under the alkaline conditions of a cement- containing composition and behave as efficient water-reducing agents. They do impart a retardation to the concrete in which they are incorporated, which can be overcome by the addition of small quantities of calcium chloride or triethanolamine [27].
The three categories of major ingredients discussed above for the formulation of water-reducing admixtures account for the majority of commercially available products, but there may be limited use of insitol [28], polyacrylamide [29], polyacrylic acids [30] and polyglycerol [31].