Hydration of calcium silicates

Natural and artificial hydraulic limes offered faster setting and hardening rates than lime. Their hydraulic properties enabled them to set, even under water, by the formation/hydration of calcium silicates and aluminates. The disadvantages of such limes were that the raw materials were not widely available and were often variable in quality (modern hydraulic limes are generally much more consistent and are widely used in some countries). 

The main product of the pozzolanic reaction is an amorphous or nearly amorphous calciiun sihcate/aluminate hydrate phase similar to that formed in the hydration of calcium silicates constituting Portland cement. 

Calcium phosphate-based systems have wide applications in biomedical areas. Brown has outlined the similarities between the hydration of calcium silicates and calcium phosphates. The hydration products in both systems have high surface areas, variable composition, and poor crystallinity. Pozzolanic reactions and Hadley-like grains form in both systems. The primary cement-water reactions for C3S and tetracalcium phosphate are as follows ... 

Bemst, A. van, 1955. The hydrates of calcium silicate. Bull. Soc. Chim, Beiges 64 333. 

Studies of the kinetics of the C3S hydration in the absence and presence of accelerators show that the extent or degree of hydration of the silicate phase in the presence of calcium chloride is considerably increased, right up to at least 28 days, whether measured by the quantity of lime produced [6] (Fig. 5.8), X-ray analysis [15] (Fig. 5.9), or the amount of non-evaporable water [16] (Fig. 5.10). Figure 5.8 also shows that a small amount of TEA retards the hydration of the C S phase for a considerable time, and the trend... 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

No attempt is made here to explain the intricate temperature dependence of the hydrations of the three calcium silicates. Work on the kinetics of these hydration processes is not complete, so the authors could give at present only highly tentative and speculative explanations. The conclusion from Figures 2 to 7 for the purposes of this paper is that the surface area development in pastes of all three silicates is determined primarily by the degree of hydration of the silicate. For a simple reaction, in which the reactants have negligible surface areas compared with the reaction products, such a conclusion would be trivial. However, for as complex... 

In [27-29], hydrothermal, mechanochemical and solid-phase syntheses of calcium silicate from anhydrous and hydrated oxides were compared. Initial components were taken at Ca/Si ratio equal to 0.8 1.0 1.2. According to X-ray analysis, the interaction in the mixtures of anhydrous oxides under mechanical activation is not completed. The product being formed is X-ray amorphous. When heated at 600-800 C, it is crystallized in the form of a -Ca2Si04 (Fig. 3.6a). At higher temperatures, Ca3Si207 is formed P-CaSiOj is crystallized at 860 C. The observed sequence of stages is similar to those observed in solid-phase synthesis of wollastonite. The amount of p-CaSiOj at 900°C does not exhibit any substantial dependence on the initial fractions of the reagents. 

Kosova N.V., Avvakumov E.G. Mechanochemical synthesis of calcium silicates based on hydrated oxides. Sib. Khim. Zhum. 1992 2 135-43. 

Sasaki K., Masuda T., IshidaH., MitsudaT. Synthesis of calcium silicate hydrate with Ca/Si=2 by mechanochemical treatment. J. Am. Ceram. Soc. 1996 80 472-76. KosovaN.V., Avvakumov E.G., Malakhov V.V. et al. About nature ofphases formed in soft mechanochemical synthesis of calcium titanate. Doklady Akad. Nauk, 1997 356 350-53. 

Portland cement, a complex mixture of calcium silicates, aluminates, and ferrates, is one of the world s most important construction materials, with annual worldwide production in excess of lO kg. When mixed with water and sand, it changes by slow hydration to concrete. Water and hydroxide link the other components into larger crystals with great strength. 

Comments many different forms of calcium silicate are known such as CaSiOa, Ca2Si04, and CaaSiOj. Usually these occur in the hydrated form and contain varying amounts of water of crystallization. Calcium silicate is used in pharmaceutical formulations as a glidant and anticaking agent. Also used in food products (GRAS listed). The EINECS number for calcium silicate is 215-710-8. 

The hydration of tricalcium silicate C3S and dicalcium silicate C2S (for abbreviations see below Table 5.3-6) are responsible for the further. solidification of Portland cement. This reaction only begins in earnest after ca. 4 hours. Initially long needles of calcium silicate hydrate are formed, which bond the cement particles together. Later, smaller needles of calcium silicate hydrate fill the gaps left. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate. 

Figure 2 Hardened paste of hydrated Portland cement observed with MEB a) tabular crystals ofCa(OH)2 (portlandite) on fissured calcium silicate hydrate substratum b) zoom on the intrinsic porosity of calcium silicate hydrate.

Concrete is a mixture of sand and gravel held together by a binding agent which is a mixtnre of calcium silicate hydrates and calcinm hydroxide. The calcium hydrox-... 

The free lime [Ca(OH)2] and carbonate (CaC03) contents of calcium silicate hydrates, ranging from 1.0-20%, were determined by a thermogravimetric method by Biffen (33). The mass-loss curves for a series of calcium silicate hydrates, calcium silicate hydrates plus varying amounts of calcium hydroxide, and calcium silicate hydrates plus varying amounts of calcium carbonate are given in Figure 4.15. 

When synthetic mixtures of calcium silicate hydrate and calcium hydroxide were used, the series of curves obtained all indicated curve breaks, at about 500°C. These were caused by calcium hydroxide decomposition, as was shown by authentic mass-loss curves for the pure compounds. By taking the vertical distance from the point at which the straight-line curve starts to change due to evolution of the combined water from the calcium silicate hydrate to the point where it resumes the calcium silicate decomposition drop, and calculating the calcium hydroxide from the loss in mass of water equivalent to this vertical distance, one obtained a good estimate of the amount of calcium hydroxide in all cases. 

The decomposition of calcium silicate hydrate samples containing added amounts of calcium carbonate, and in some cases calcium hydroxide, is given in Figure 4.15. The presence of calcium carbonate is indicated by the curve break, due to the evolution of carbon dioxide, in the temperature range 700-900°C. If a vertical distance is measured between the points where this... 

Figure 4.15. Mass-loss curve of ( ) calcium silicate hydrale preparation (/ ) calcium silicate hydrate preparation plus added Ca(OH) (c calcium silicate hydrate preparations plus added amounts of CaC03 (33).

Figure 19. Cement hydration process. Calcium silicate hydrates to form C-S-H, a quasi-amorphous gel of composition close to C3S2H3. The excessive rate of hydration of the aluminate phase is controlled by gypsum through the formation of a calcium trisidfoaluminate hydrate, ettringite. (Reproduced with permission from reference 5. Copyright 1991 Elsevier.)...

Stabilisation is a much slower process, which occurs progressively over several months, and involves the reaction of lime with the siliceous and aluminous components of the soil. The lime raises the pH to above 12, which results in the formation of calcium silicates and aluminates. These are believed to form initially as a gel, which coats the soil particles, and which subsequently crystallises as calcium silicate/aluminate hydrates. Those hydrates are cementitious products, similar in composition to those found in cement paste. The rate of crystallisation is temperature dependant and may take many months to reach completion. The resulting gain in strength (measured by the California Bearing Ratio Test [26.11]) is progressive, as illustrated in Fig. 26.2. 

Grutzeck, M., Benesi, A., and Fanning, B., Silicon-29 magic angle spinning nuclear magnetic resonance study of calcium silicate hydrates, J. Am. Ceram. Soc., 72 (4), 665 (1989). 

Set Biodentine material consists substantially of hydrated calcium silicate [79], some of which forms more rapidly in Biodentine than in MTA because of the presence of calcium chloride and calcium carbonate. This latter component, which accounts for 15% of the powder by mass, acts as nucleation sites for the deposition of calcium silicate hydrate. Consequently, the precipitation of calcium silicate hydrate occurs more rapidly within this material, which leads to a shorter setting time [79]. 

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