The role of sulphates in cement clinkering reactions: phase formation in the system CaOAℓ2O3Fe2O3SiO2CaSO4K2SO4

The chemistry of Portland cement clinker

“…However, significant incorporation of sulfur can still occur as solid solution in belite with concentrations of up to 2% SO 3 at high SO 3 /alkali ratios (Herfort, 2002). Glasser clearly demonstrated the immiscibility of the alkali sulfate liquid in a series of simple but elegant experiments (Pliego-Cuervo and Glasser, 1979).…”

“…Higher temperatures than 12008C are needed to stabilise alite, so that at a maximum burning zone temperature of say 14508C the driving force needed to overcome the kinetic barriers for alite formation is reduced. Although sulfates thermodynamically destabilise alite they will kinetically favour alite formation by increasing the amount of low temperature melt, but the thermodynamic effect dominates at total SO 3 contents in the clinker in excess of about 1% (assuming that the alkali content remains constant; of course a partition must exist between sulfate dissolved in the immiscible sulfate liquid phase, the main clinker liquid and the solid phases of C 3 S and C 2 S. The higher the alkali level the lower the substitution level in C 2 S (Herfort, 2002;Pliego-Cuervo and Glasser, 1979). The negative thermodynamic effect of high sulfate contents can be offset by introducing a mineraliser for alite formation (to counteract the mineralising effect of sulfates on belite).…”

Mineralizers and Fluxes in Clinkerization

“…Other phase equilibrium studies of interest to cement chemists and relevant to this article are the ones in presence of sulphates [25,26], In the Ca0-Al 2 03-Si0 2 -S0 3 system, C 3 S was not detected below 1175°C. Pliego-cuervo and Glasser [27] studied the solid phase assemblages compatible at sub-solidus temperatures in several sulphate bearing systems. Pliego-cuervo and Glasser [27] studied the solid phase assemblages compatible at sub-solidus temperatures in several sulphate bearing systems.…”

“…In the system Ca0-Si02~S03, Gutt and Smith [26] delineated the phase assemblages in the sub-system^ Ca0-CS-C2S at 1000 and 1200°C. It is doubtful whether C5S2S has a primary phase field in the system, though with AI2O3 and I^SOi* Glasser [27] showed its compatibility with C4A3S at sub-solidus temperatures. The silico-sulphate was found to be_incompatible with C3S and it decomposed around 1300 C (on the join C2S-CS) into a -and a'-C2S containing 1 mole % CaSO^.…”

Effect of alkalis on the mineral formation and properties of alite-sulfoaluminate cement

“…It has been claimed that minor amounts of alkalis (usually less than 1%) form the fluxing agent in the burning of cement clinkers, which could decrease the forming temperature of the liquid phase and increase the amount formed to facilitate the absorption of free lime and the formation of tricalcium silicate (C 3 S) Skalny, 1977, 1978;Pliego-Cuervo and Glasser, 1979;Taylor, 1999). However, if the amount of alkali is excessive, the obviously increased viscosity of the liquid and the alkali solid solution (such as KC 23 S 12 , NC 8 A 3 ) formed at lower temperature would seriously impede the absorption of free lime and the formation of C 3 S. Excess amount of alkali would thus affect the setting time of cement while the 28-day strength of cement would decrease considerably (Gies and Knöfel, 1986;Jelenić et al, 1977;Wonnemann, 1983a, 1983b).…”