Dicalcium silicate reactive forms

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. 

Portland cement is typically composed of about 25% P-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)... 

The products of dicalcium silicate hydration are identical or almost identical to those formed in the hydration of tricalcium silicate however, the amount of calcium hydroxide formed is distinctly lower. The hydration rate of dicalcium silicate is sigrrificantly lower than that of tricalcium silicate, even though it may be influenced to a certain degree by the selection of the dopant ion and the cooling rate. 8ome highly reactive forms of dicalcium silicate have been synthesized in recent years, but are of theoretical importance only. 

Reactive forms of dicalcium silicate and belite cements 51... 

In conclusion it may be stated that doping with appropriate foreign ions may help to increase the reactivity of dicalcium silicate however, such effects are rather limited, and generally the rate at which the doped material will react with water will remain below that of tricalcium silicate. The resultant reactivity will also depend on the amount of dopant used, and on the conditions of sample preparation. The rate of C2S hydration may be accelerated, to a limited extent, by adding suitable accelerators, such as CaCl2, Ca (N03)2, K2CO3 or calcium acetate (El-Didamony et al 1996). These act by forming insoluble compounds with Ca(OH)2 or catalytically. 

It has been found that very effective activation may be achieved by combining an increased alkali content of the raw meal and a veiy rapid cooling of the clinker formed, in particular in the temperature range 1300-900°C (Stark et al, 1979, 1986 Gies and KnOfel, 1986 Lampe, 1986 Milke, 1992). In this way the a -CjS phase, which is more reactive than P-C2S and exists in the resultant clinker at high temperatures, is at least partially preserved in the cooled product. Moreover, defects are introduced into the crystalline lattice, which increase the reactivity of the dicalcium silicate present even further. To effectively prevent the a - 28 conversion, cooling rates of up to 1000 K/min were found to be necessary. About 5% of alkali oxides and other foreign ions are typically present in the stabilized a -C2S phase. 

Another dopant that may be considered for increasing the reactivity of belite cements is S04 ions (Gies and Knofel, 1987 Stark et al, 1987). Under these conditions the final clinker contains the rather than the a -CjS phase, even if high cooling rates have been employed however, the SO -doped form of yff-dicalcium silicate is much more reactive than its SO -free counterpart. The doped C S typically contains about 3% each of SO3 and AI2O3 in its crystalline lattice. If an SO3-doped belitic clinker that also contains some alite is to be produced, the amoimt of SO3 in the raw meal must not exceed... 

Kuznetsova et al. (1992) produced activated belite cements by introducing into the raw meal industrial waste products containing phosphoras, chromium, and titaniiun, and by employing cooling rates of up to 5000 K/min, Such clinkers contained dicalcium silicate mainly in its form, but a and a modifications were also present. By adding carbon to the raw mix, regions with local temperatures above 2100 °C were obtained before the bulk of limestone present was decarbonated. This resulted in a non-stoi-chiometry of the produced clinker minerals and along with it an increase of reactivity. 

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. 

Owing to the lower reactivity of belite, the overall rate of hydration, and along with it the strength development up to about 90-180 days, is slowed down with increasing belite and decreasing alite contents in the cement (Bei and Ludwig, 1990). At the same time the final strength of a Portland cement with an elevated C2S content may exceed that of an ordinary Portland cement, because more C-S-H and less portlandite is formed in the hydration of dicalcium than of tricalcium silicate. 

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