Cement compounds hydration

The hydration of the ferrite phase (C AF) is of greatest interest in mixtures containing lime and other cement compounds because of the strong tendency to form soHd solutions. When the sulfate in solution is very low, soHd solutions are formed between the cubic C AH and analogous iron hydrate C FHg. In the presence of water and siUca, soHd solutions such as C3 ASH4-C3FSH4 may be formed (33). Table 7 Hsts some of the important phases formed in the hydration of mixtures of pure compounds. 

Dicalcium silicate (2CaO SiO ) is very important in the final strength of the cement. This compound hydrates very slowly. The average dicalcium silicate content is 25% to 35%. 

Figure 2.9 SMF adsorption on cement compounds and cement during hydration.

The hydration reactions may also be influenced by changes in the pH value of the solution in contact with hydrated cement, which may alter the solubility or stability of some hydrated cement compounds or inhibit the formation of protective coatings. 

In general, acid solutions attack concrete in any combination of four ways (a) by dissolving both hydrated and unhydrated cement compounds, (b) by dissolving calcareous aggregates present in the mix, (c) through physical stresses induced by the deposition of... 

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. 

The chemical reaction of cement compounds with water is very complex because the hydration of each compound (C3S, C2S, C3A, and C4AF) is closely related with the reactions among the rest of the compounds. The OPC is a mixture of four major compounds, as mentioned in Section 13.1. Therefore, at first, it is important to understand the hydration of pure cement compounds in order to have complete understanding of the hydration mechanism of OPC. 

On addition of water, Portland cement reacts immediately to form new compound, hydration products. These compounds function by... 

Knowledge of the hydration behavior of individual cement compounds and their mixtures forms a basis for interpreting the complex reactions that occur when portland cement is hydrated under various conditions. For a given particle size distribution and water solid ratio, tricalcium silicate and alite harden in a manner similar to that of a typical Portland cement. 

Figure 4. Compressive strength of hydrated cement compounds. (Reprintedfrom Beaudoin, J. J., and Ramachandran, V. S., Cement and Concrete Res., 22 1992, with kind...

Quantitative x-ray diffraction has been used to determine the degree of reaction of individual cement compounds present in cement. Some errors in these estimations are recognized. Figure 5 shows the fractional amounts of alite, belite, aluminate, and ferrite phases that hydrate in cement when hydrated for different times.These rates are not the same when the individual compounds are hydrated. 

The mechanisms that have already been described for pure cement compounds form abasis for a study ofthe hydration mechanism of portland cement. The conduction calorimetric curves of C3S and portland cement are similar except portland cement may yield a third peak for the formation of monosulfate hydrate (Fig. 1). The detailed influence of C3A and C4AF on the hydration of C3S and CjS in cement is yetto be worked out. The delayed nucleation models and the protective layer models, taking into account the possible interactions, have been reviewed. 1 1 Although the initial process is not clear for C3S (in cements), it appears that C3A hydration products form through solution and topochemical processes. 

Conduction calorimetry is another technique that is extensively used for following the hydration reactions of cement and cement compounds. In this method, heat evolved during the hydration reactions is followed as a function of time from the moment water comes into contact with the cement. The curves are obtained under isothermal conditions. This technique can also be used to study the rate of hydration at different temperatures. Conduction calorimetry has been used to determine kinetics of hydration and for studying the role of admixtures, relative setting times of cement, and for identification purposes. 

The potentiality of the emanation thermal analysis for the investigation of the hydration of cement and cement compounds has been discussed by Balek.t This technique is based on the measurement of radioactive gases released from the hydrating phase. The amount of gas released depends on the physico-chemical processes taking place in the solid. A quantitative estimation of the rate of hydration of cement and C3S at early stages has been obtained. 

The rate of hydration of cement and cement compounds is influenced by superplasticizers. The reported data on the effect of superplasticizers on the hydration of C3A and C3A + gypsum systems are contradietory because of the variations in the materials and conditions of hydration. There is a general consensus that SNF and SMF retard the hydration of... 

In this chapter, typical examples of the application of various thermal techniques to the study of the effect of admixtures on the hydration of cement and cement compounds is emphasized. Where relevant, the results obtained with these techniques are compared with those derived from other tools. 

Many inorganic and organic salts have been examined for their action on the hydration of cement and cement compounds utilizing thermal techniques. They include sodium and calcium salts of chloride, bromide, nitrite, thiosulfate, thiocyanate, iodide, nitrate, hydroxide, carbonate, hydroxide, etc. A few typical examples are given illustrating the application of thermal techniques in the investigation of these compounds on cements and cement components. 

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). 

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., and Lowery, M. S., Effect of Phosphonate-Based Compound on the Hydration of Cement and Cement Compounds, 4th CANMET/AClInt Conf. on Superplasticizers and Other Chem. Admixtures in Conor., Montreal, Canada, SP-148 131-151 (1994)... 

A study of the hydration of cement and cement compounds in the presence of superplasticizers is useful for theoretical and practical considerations. Many t5 es of thermal techniques including DTA, DSC, TG, DTG, Conduction Calorimetry, and EGA have been used for such studies. They have yielded important results that could be correlated with physical and mechanical characteristics of cement systems. Investigations have included the measurementofheat of hydration, the mechanism of reactions, strength development, microstmcture, permeability, durability aspects, compatibility problems between cement and superplasticizers, the prediction of some properties, material characterization and selection, mathematical modeling of hydration, development of test methods, and cement-superplasticizer interactions. 

Figure 3. SMF adsorption on cement and cement compounds during hydration.

Hydration of fly ash cement differs from pure cement in terms of the hydration rates of the clinker phases, amount of calcium hydroxide formed, composition of the clinker hydration products, and additional hydration products from the reaction of the fly ash.I l Lower amounts of lime are formed in the presence of fly ash because ofthe pozzolanic reaction between the fly ash and lime formed in cement hydration. Fly ash generally retards the reaction of alite in the early stages and accelerates the middle stage reaction. The accelerated reaction is attributed to the existence of nucleation sites on fly ash particles. The aluminate and ferrite phases hydrate more rapidly in the presence of fly ash, and also there is a significant difference in the hydration rate of the belite phase up to 28 days. Table 1 gives the relative hydration rates of cement compounds in the presence of fly ash as derived from conduction calorimetry. [" 1 It can be seen that the earlier rates of hydration are generally retarded, and the later stage hydration rates are accelerated. 

Table 1. Relative Hydration Rates of Cement Compounds in the Presence of Fly Ash...

Table 6. Hydration Rate of Cement Compounds Containing Slag...

The physicochemical characteristics of concrete depend on the behavior of the individual components of portland cement as well as on the cement itself. The second chapter provides essential information on cement and cement components so that the informationpresented in subsequent chapters can easily be followed. In this chapter, the formation of cement, the hydration of individual cement compounds and cement itself, physicochemical processes during the formation ofthe pastes, the properties ofthe cement paste, and the durability aspects of concrete are discussed. 

The information presented in Ch. 3 clearly demonstrates the extensive applicability ofthermal techniques for investigations of raw materials forthe manufacture of cement, clinkerformation, hydration of cement compounds and cement, the oxide systems of relevance to cement chemistry, and durability processes. Some examples of the usefulness of associated techniques forthese investigations are also given... 

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