Cement hydration lignosulfonates

In the absence of knowledge of the surface area of cement hydrates available for adsorption at the time of addition, it is difficult to estimate how many layers of water-reducing admixture molecules are adsorbed, but attempts have been made [40] indicating that over 100 layers may be formed with calcium lignosulfonate and salicylic acid at normal levels of addition. However, these calculations were based on specific surface areas of 0.3-1.0 m g-l, whereas other studies [27, 38, 39] have indicated... 

The addition of a water-reducing admixture to a cement suspension can be shown to disperse the agglomerates of cement particles into smaller particles [33,38, 47] and can be seen clearly in photomicrographs as shown in Fig. 1.21. Maximum dispersion occurs at a level of 0.3-0.5% by weight of calcium lignosulfonate [33, 34] which would indicate the presence at the surface of about 0.2-0.4% calcium lignosulfonate. The separation of particles results in an increase in the surface area of the system by 30-40% [33, 38], which may explain the more rapid rate of cement hydration after the initial retardation period. 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

The effect of carboxylic acids such as gluconic acid, and carboxylic acid and sugars such as glucose or sucrose, on portland cement hydration is very similar to that of lignosulfonate, although different percentages of admixtures are required to obtain similar effects. 

An addition of 0.1% CLS may extend the initial and final setting times of cement mortar by two and three hours, respectively. The influence of 0.3% CLS on the hydration of cement is shown in Fig. 9. Thermograms indicate that the reference cement containing no admixture exhibits a broad endothermal peak below 200°C, representing the formation of both ettringite and C-S-H phase. These peaks increase in intensity as the hydration period is increased. The effect between 450 and 500°C is caused by the dehydration of Ca(OH)2 and its intensity indicates the extent to which the hydration of the C3S component has progressed. The cement hydration, in the presence of lignosulfonate, is retarded as seen by the lower intensity of the Ca(OH)2 decomposition peak. The low temperature effect below 300°C in the presence of CLS is not sharp as that obtained in the reference sample. 

Ramachandran, V. S., Effect of Sugar-Free Lignosulfonates on Cement Hydration, Zement-Kalk-Gips, 31 206-210 (1978)... 

Monocalcium aluminate, a major component of high alumina cement, hydrates to metastable products CAHjo, C4AHj3, and C2AHg which eventually convert to the stable cubic aluminum hydrate, CjAH. In the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA results have shown that the degree of conversion of CAHjo and C2AHg phases to the cubic phase is marginally retarded. 

Water-reducing admixtures are not adsorbed equally by the various anhydrous and hydrated cement constituents and in studies with calcium lignosulfonate, the approximate maximum adsorption figures shown in Table 1.5 have been obtained [38,39], In addition, adsorption isotherms have been studied at various ages of C3A hydration [36] and it has been shown that it is the initial hydration products (less... 

The nature of the bond between the molecules of the water-reducing admixture and the surface of the cement constituent hydrates has been described as ionic group outwards in many references [33, 42,], mainly based on work [33, 43] showing migration of cement particles under the influence of an electric current when lignosulfonate molecules are adsorbed on the surface. Similar results have been reported for hydroxycarboxylic acids [44], Other relevant data are summarized below ... 

The amount of calcium lignosulfonate adsorbed on to hydrating cement is almost independent of initial water-cement ratio within the range 0.4 to 1.5 [34]. 

As far as the final hydration products of ordinary Portland cement are concerned, there is an indication from isothermal calorimetry [57] that there is very little difference in the presence or absence of a calcium lignosulfonate water-reducing admixture. In this work, the heat evolved per unit of water incorporated into the hydrate has been determined for two cements, with the results shown in Fig. 1.25. It can be seen that the relationship between the amount of heat evolved and the amount of water combined with the cement is maintained whether the admixture is present or not. This work also indicated that the retardation in the early stages is compensated for at later times by an acceleration. 

Fig. 1.25 The heat evolved from hydrating cement with and without the addition of calcium lignosulfonate as a function of combined water (Khalil).

Collepardi, M. et al. (1980,1982) Combined effect of lignosulfonate and carbonate on pure Portland clinker compounds hydration. Cement and Concrete Research 10,455-461 12, n -lll, 425-435. 

Extensive work has been carried out on the effect of lignosulfonate on the hydration of individual compounds as well as on cement itself. The effects are similar to what has already been described under the previous section on Retarders. ... 

The lignosulfonate-based admixtures have been used more widely than other water redueers. They are capable of redueing water requirements and retarding the setting times of concrete. They influenee the dispersion and the hydration rate of the individual cement compounds, and, thus, the cement itself. Techniques such as XRD, DTA, DSC, TG, DTG, and conduction calorimetry have been used extensively to follow the hydration of cement and cement compounds containing different t5q)es and amounts of lignosulfonates (LS). 

Thermal investigations have shown that the rate of hydration of C3A and the inter-conversion of ettringite to the low monosulfoaluminate in the C3A-CaS04 2H20-CLS-H20 system, and hydration of individual cement phases C4AF, C3S, and C2S are all retarded by lignosulfonates. Thus, it is expected that the rate of hydration of cement should also be retarded by the addition of lignosulfonates. 

The conduction calorimetric technique has been applied to follow the hydration of Type I and Type V cements containing 0.28% CLS. Table 1 gives the heat of hydration in two cements containing CLS.l l Generally, the amount of cumulative heat is lower in the sample with lignosulfonate. The retarding action of the admixture at later times is compensated for as the time of hydration increases. 

The hydration of cement undergoes a significant modification in the presence of a mixture of lignosulfonate and alkali carbonate. The heat evolution peaks occur separated by two induction peaks. The possibility of a highly anionic complex between lignosulfonate and earbonate ions has been proposed to explain its more effective dispersing effect than lignosulfonate. The first induction period is due to the competitive interaction of one or more of the admixture with C3A.P 1... 

Conduction calorimetric analysis has also been carried out on the effect of sugar-free lignosulfonate on the hydration of cement. It was found that sugar-free lignosulfonate was nearly as efficient as commercial lignosulfonate in retarding rate of reaction and setting of cement. 

Ramachandran and LoweryP l applied conduction calorimetry to the study of the relative effects of various retarders on the hydration of cement. The retarders used were calcium gluconate, glucose, glycolic acid, molasses, sodium borate, sodium citrate, sodium heptonate, sodium hexametaphosphate, sodium pyrophosphate, sugar-free lignosulfonate, and sucrose. The dosage of the chemicals varied between 0.025 and 1.2%. 

Collepardi, M., Manosi, M., and Moriconi, G., Combined Effect of Lignosulfonate and Carbonate on Pure Portland Cement Clinker Compound Hydration, I Tetracalcium Aluminoferrite Hydrate, Cement Concr. Res., 10 455-462 (1980)... 

Ramachandran, V. S., Interactionof Calcium Lignosulfonate with Tricalcium SiUcate, Hydrated Tricalcium Sihcate and Calcium Hydroxide, Cement Concr. Res.,2 119-194 1972)... 

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