Alite, hydration

Temperature has a large effect, especially in the earlier stages of hydration for example, from QXDA, Copeland and Kantro (C39) found that in a Portland cement paste of w/c = 0.57, the fraction of the alite hydrated at 2 days was 0.28 at 5°C, 0.63 at 25°C and 0.81 at 50°C. Apparent energies of activation calculated from such data were 41 kJ moC at a = 0.6 and 26 kJ mol " at a = 0.7 for belite, a value of 56 kJ mol at a = 0.4 was obtained. The decrease in the apparent energy of activation in the case of alite was attributed to a gradual change in rate control from a chemical process to diffusion. 

The final stage of hydration begins when the free space between the alite inner part and the shell formed of hydrates is filled with C-S-H. Alite hydration is continued and the interface shifts toward the centre of the grain In this stage the reaction does not occur presumably through the liquid phase but becomes a topo-chemical process. The product formed corresponds to the type IV C-S-H according to Diamond. 

The beneficial effect of gypsum consists in the simultaneous acceleration of alite hydration, mainly in the pre-induction period [26, 37]. The transfer of SO " ions to the C-S-H was detected [20]. However, these ions are poorly bound, they are rather adsorbed on the surface of C-S-H gel [38] (seep. 3.2.3). According to Bentur [39] the effect of gypsum on cement hydration should be considered in these two aspects. Gypsum has an impact on the amount and constitution of C-S-H gel. On the other hand, in the presence of gypsum the amount of C-S-H increases but the cohesiveness, being the function of C/S ratio is lowered ... 

In Table 5.6 the time needed to replace a half of water content in hydrated alite pastes of varying porosity, according to Parrott [154] is presented. The porosity decreases and this period of half-change increases as the alite hydration proceeds. 

The chemical reactions of cement paste with zinc (galvanized steel) result in the release of zinc to the solution and the formation of calcium zincate, CaZn2(OH)g 2H2O, covering the surface of metal [49], Simultaneously, there is no portlandite crystals in this transition zone because in the presence of zinc alite hydration is delayed (see Sect. 4.1.3.2). The coating of hydrated calcium zincate crystals on the surface of metal increases the bonding forces between paste and reinforcement by the roughness formation on its surface. 

Alite, hydration mechanism, 31 Amorphous silicon-oxygen network, 26... 

Both ordinary and high calcium fly ashes delay the appearance of the peak effect due to alite hydration. The appearance of the peak, however, is delayed to a greater extent by high calcium fly ash. Fly ash with a low calcium adsorption capacity, higher contents of Ca, dissolved alkalis, and unburned carbon retards the hydration of alite, by hindering the saturation rate of Ca(OH)2 in the liquid phase. 

The rate of hydration of cement-fly ash also depends on the particle size of fly ash. Conduction calorimetric curves have been obtained for cements containing 30% fly ash of surface areas of200,450, and 650 m /kg. These mixtures are designated PFA-0, PFA-1, and PFA-2 respectively in Fig. 4. The fly ash cements exhibited sharper peaks at earlier times at 11-12 hours compared to the reference cement. These peaks become sharper as the surface area of fly ash increases. The peak effects are associated with the hydration of C3A after the depletion of gypsum in the system. The initial exothermal effect due to alite hydration is retarded by the fly ashes. 

A detailed study on the influence of binary and ternary blends of Portland cement mixed with silica fume, fly ash, and slag has been presented by Uchikawa and Okamura.l" Applying conduction calorimetry, it was found that in blends with fly ash and slag, the hydration of the interstitial phase is accelerated, and that of alite at stages I and II is slightly delayed. In blends with slag and silica fume, the hydration of the interstitial phase and that of alite was accelerated with the reduction in the induction period. In fly ash-silica fume blends, the hydration of the interstitial phase decreased, and the induction period for alite hydration was lengthened. 

Figure 8.49 STEM-BF images of hydrated samples, (a) Inner (IP) and outer product (OP) regions of a paste of alite hydrated for 90 days at 20°C (w/c = 0.4) (b) inner and outer product region of a CEM I cement hydrated for 90 days at 20°C (w/c = 0.4).

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

An alternative to silicate-based Portland cement is the calcium aluminate cement, ciment fondu, which originated with the Lafarge company in France in 1908. Ciment fondu is typically made by heating limestone with bauxite, which is mainly AIO(OH) but contains much iron oxide (see Section 17.2). As noted above, calcium aluminate hydrates and hardens much more rapidly than alite, and so ciment fondu, either as such or mixed with Portland cement, can be used whenever a rapidly setting cement is required, for example, for construction at low temperatures. Concretes made from aluminate cements remain serviceable at higher temperatures than Portland cements and so are used to make cast refractories for pyrometal-lurgical applications. 

Portland cement clinker potential phase composition is presented in Table 4. It could be seen that the C3A content in the clinker was 9.46% which is important for the cement hydration rate and cement sulfate resistance. Common Portland cement is not resistant to the sulfate influence because of the significant C3A content, whose hydrates react with sulfate ions resulting in expansive compounds. Portland cement with the higher resistance to sulfates must have low C3A content. Moderate to high content of mineral alite - C3S (54.72%) is usual for the Serbian cement plants and enables the addition of higher quantities of mineral admixtures without influencing the quality of final cement. 

In the previous investigations in this laboratory, most of the data were obtained on well-hydrated specimens prepared at room temperature. The present investigation includes specimens of this type, as well as specimens hydrated for shorter periods of time. Three temperatures were used—5°, 25°, and 50°. The hydration of alite, the form of Ca3Si05 found in portland cement, was also investigated under the same conditions. In addition to time and temperature, the effect of stoichiometry on the development of surface area was investigated. 

The degrees of hydration of Ca2Si04, Ca3Si05, and alite pastes as functions of time at the three temperatures are given in Figures 2, 4, and 6, respectively. The data, obtained from pastes whose compositions are given in Tables III, IV, and V, are plotted on a logarithmic time scale only as a matter of convenience. 

Figure 7. Surface development in hydration of alite vs. time...

The loss of one molecule of lime is not accompanied by an immediate uptake of one molecule of water. The H20/Si02 ratios for the tobermorites in all 13 Ca3Si05 and alite pastes that are 2 days old or younger are significantly less than 1.0. Subsequently, the H20/Si02 ratio increases to an ultimate value, while the CaO/ Si02 ratio decreases to an ultimate value. In the hydration of Ca2Si04, both ratios increase, but the uptake of water is slower than the uptake of lime, as pointed out above. 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

For pastes of typical ordinary Portland cements cured for 3-12 months, the CH content found by thermal methods or QXDA is typically 15-25%, referred to the ignited weight (P29,R14,M37,H37,T17,D12). Pressler et al. (P29) found that for pastes of various ages of ordinary (US Type I) Portland cements, it was linearly related to the content of non-evaporable water, but that for cements high in belite (US Type IV), it tended to a maximum while the latter continued to increase. This is readily explained, since belite yields only a little CH on hydration. The author has noticed similar behaviour even with modern cements high in alite, and that the CH content can possibly even decrease slightly after 28-91 days (T5). il... 

In an ordinary Portland cement, only some two-thirds of the Fe occurs in the ferrite, the rest being contained largely in the alite and aluminate (Table 4.3). On hydration, the Fe " in these other phases probably does not enter a hydrogarnet, but goes into AFm phases or layers formed in situ. This would account for the observation by analytical electron microscopy that small amounts of Fe are present in the AFm phases. 

The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20°C and 100% RH, Patel el al. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, 1.00 and 0.51. If the RH was lowered to 80%, the corresponding values were 0.77, 0.19, 0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of RH on the hydration rate (da/d/) of each of the phases could be represented by a factor (RH — 0.55)/0.45. ... 

Parrott and co-workers (P30,P32,P35,P33) described a more sophisticated method for modelling the hydration process. The fraction of the total water porosity that was below 4nm was calculated by multiplying the volume fraction of C-S- H by an appropriate factor, which depended on whether the C-S-H was formed from alite or belite, the temperature and the amount of space available. The constants assumed were based on experimental data obtained using a procedure based on methanol sorption (Section 8.3.4). The effect of drying was allowed for (P35) by introducing a factor of 0.7 - -1.2(RH — 0.5) for 0.5 < RH < 1, or of 0.7 for RH 0.5. These refinements allow some deviation from the Powers-Brownyard postulate of a fixed volume ratio of gel porosity to product. Typical results for the volume fractions of pores larger than 4 nm in mature pastes of a cement with an alite content of 56% were approximately 0.26, 0.16 and 0.07 for w/c ratios of 0.65, 0.50 and 0.35, respectively (P32). For the two higher w/c ratios, these results are near the capillary porosities of Powers and Brownyard, but for w/c 0.35 the latter value is zero. 

---