Hydration of alite

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. 

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

The added limestone also exhibits an accelerating effect on the hydration process, and mainly on the hydration of alite, as its particles can act as nuclei for the crystallization of poitlandite. This effect may also positively affect the short-term strength of the cement. 

Table 8.1 Hydration of alite and slag in Portland-slag cements with different contents of granulated blast-furnace slag.

Fig. 2.12 Structure of tobermorite- and jennite-phases formed during hydration of alite (schematic)...

In cements, incorporation of calcium carbonate is permitted in some countries. In Canada, the maximum limit is set at 5%. Calcium carbonate is not an inert filler. It is known to react with calcium aluminate. In a study of the hydration of tricalcium silicate in the presence of finely divided calcium carbonate, Ramachandran observed that the carbonate acted as an accelerator. Ushiyama, et al.,t examined the effect of carbonates of Na, K, Li, Cs, and bicarbonates of Na, K, and Li on the hydration of alite. Although small amounts retarded the hydration, larger amounts acted as accelerators. 

Ushiyama, H., Kawano, Y., and Kamegai, N., Effect of Carbonates on Early Stage Hydration of Alite, 8th Int Cong. Chem Cements, Rio de Jeneiro, Brazil, 111(2) 154-166 (1986)... 

The TG technique has been used to follow the hydration of alite in the presence of 2-4% SMF.I Calcium hydroxide and the non-evaporable water contents were determined and not much dilTerence in the hydration rates was observed between the reference alite sample and that eontaining the superplasticizer. 

An extensive work was carried out by Uchikawa l on the conduction calorimetry of superplasticizers, such as SNF (NS in the figure), lignosulfonate (LS), a co-polymer of acrylic acid with acrylic ester (PC), and a three dimensional polycondensate product of aromatic aminosulfonic acid with trimethyl phenol (AS) (Fig. 11). The first peak in the calorimetry corresponds to the heat of dissolution of alite, the heat of formation of the AFt phase, and the calcium hydroxide formation from free lime. The second peak corresponds to the heat of hydration of alite. The admixtures were found to accelerate the formation ofthe ettringite phase. At w/cratios ofO.3 and 0.5 and a later addition of the admixture, the appearance of the second peak was significantly delayed and the peaks were of lower intensity. Most retardation occurred with polycarboxylic acid and amninosulfonic acid-based admixtures (Fig. 11). DSC was used to determine the amount of lime formed at different times. The DSC results show that the addition of admixtures at different w/c ratios generally decreases the amounts of lime in the presence of superplasticizers (Fig. 12). 

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. 

Conduction calorimetric investigations have been earried out on Portland cement in whieh natural pozzolans have been incorporated, It has been found that in the pozzolan-containing pastes, the dormant period is deereased and the normalized height of the seeond main peak is greater than that given by the reference cement. This indieates that the pozzolan aecelerates the hydration of alite. It is possible that the aceeleration effect is due to a eombination of faetors, viz., higher specifie surface area of the pozzolan, its chemical composition, the physical state of the surface, and the rate of release of the alkalis. 

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