Dicalcium silicate pastes

The important compounds in Portland cement are dicalcium silicate (CazSi04) 26%, tricalcium silicate (CasSiOj) 51%. tricalcium aluminate (Ca3Al206) 11% and the tetracalcium species Ca4Al2Fe2 Oio (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations ... 

The conclusion is that both the body structure and the surface structure of tobermorite are highly reproducible. Whether we use tricalcium silicate or fi-dicalcium silicate as starting solids, whether we use a water to solid ratio of 0.7 or 9.0, whether we use paste hydration or ball-mill hydration or a third type which I have not discussed (which gave the six other points on the curve), we wind up with a tobermorite having very nearly the same body structure and surface structure. 

The two most important constituents of Portland cement are alite, a form of tricalcium silicate, and belite, a form of dicalcium silicate. In their hydration both calcium silicates yield—in addition to calcium hydroxide—a nearly amorphous calcium silicate hydrate phase (C-S-H phase), and this hydration product is mainly responsible for the strength and other physico-mechanical properties of the hardened cement paste. 

The amount of free calcium hydroxide in Portland cement-microsilica mixes increases initially, as its formation in the hydration of tricalcium silicate is faster than its consumption in the pozzolanic reaction with microsilica. Later on, however, the amount of free calcium hydroxide may start to decline, when the amount of it consumed in the pozzolanic reaction exceeds the rate by which it is formed in the hydration of tiicalcium and dicalcium silicate (Papadakis, 1999). This crossover point— that is, the time at which the rate of Ca(OH)2 consumption exceeds the rate of its formation— will depend on the amount and reactivity of the microsilica present, as well as on the reactivity of the clinker, and can occur after several hours or a few days of hydration, or not at all (especially at low microsilica additions). The amount of residual free calcium hydroxide in mature paste will generally decline with increasing amounts of nucrosilica in the original mix. It will also decline with decreasing watei/solid ratio, as under these eonditions the C/S ratio of the formed C-S-H phase tends to increase. 

C-P-S powders preignited to 1300 °C contain neither hydroxyapatite nor dicalcium silicate, but only tricalcium phosphate as the sole crystalline phase. Both pastes and compacts made from such powders exhibit setting and hardening that may be enhanced significantly by the catalytic action of ammonium, sodium or potassiiun phosphates present in the mixing water or solution in which the compacts are stored (Vanis and Odler, 1996). The product of the hardening reaction is hydroxyapatite in combination with a C-S-H and/or C-S-P-H gel. 

As to the phase transformations, in blast furnace slag cement pastes increasing amounts of melilite [a sohd solution of gehlenite (C2AS) and akermanite (C2MS2)] are formed as the amount of slag in the original cement increases, at the expense of yff-dicalcium silicate, which is formed in Portland cement pastes under similar conditions. In pastes made from cements that contain distinct amoimts of fly ash, yff-dicalcium silicate is also the main final decomposition product. In parallel, quartz, which is a common constituent of fly... 

Thermal analysis data on the hydration of dicalcium silicate are sparse because it is time consuming to follow the reaction of this phase which is very slow. The characteristic products obtained during its hydration are not much different from those formed in C3S hydration. Also, the major strength development that occurs in cement in the first 28 days (a period of practical significance) is mainly due to the tricalcium silicate phase. TG, DTG, and DTA investigations of C2S were carried out by Tamas.t The sensitivity of the instrument had to be increased substantially to detect the peaks due to the decomposition of calcium hydroxide and calcium carbonate, especially at earlier times. In Fig. 21, the DTA, DTG, and TG curves of C2S hydrated for 21 days and 200 days are given. A comparison of these peaks with those obtained from C3S pastes shows substantial differences in the intensity value of the peaks. The 200 day C2S sample shows a weight loss of 4%, whereas C3S hydrated for 21 days indicates a loss of 13%. 

Kantro, D. L., Weise, C. H., and Brunauer, S., Paste Hydration of B-Dicalcium Silicate, Tricalcium Silicate and Alite, Symp. Structure of Portland Cement Paste and Concrete, pp. 309-327, Highway Res. Board (1966)... 

Compared to the extensive thermal investigations of C3S hydration in the presence of CaCl2, only a limited amount of work has been reported on the effect of calcium chloride on the hydration of C2S. Calcium chloride accelerates the hydration of dicalcium silicate. In the thermograms of C2S paste hydrated for 1-3 months, lower amounts of calcium hydroxide are formed in the presence of calcium chloride, compared to that hydrated... 

Richardson, I. G. 2004. Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H applicability to hardened pastes of tricalcium silicate, p-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume . Cement and Concrete Research 34 (9) 1733-1777. 

Note that unlike calcium hydroxide formed in the hydration of calcium oxide, calcium hydroxide hberated in the hydration of dicalcium or tricalcium silicate under no circumstances generates expansion of a cement paste. 

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