Gradual transformation of Ca(OH)2 into CaCO3 on cement hydration -: XRD study

The low temperature of decomposition of some calcium carbonates and the bending of the TG curves of hydrated cement between 500 and 800 degrees C suggested the presence of some complex compound(s), which needed complementary investigation (XRD, TG). Stepwise transformation of portlandite (and/or lime) into calcium carbonate, with intermediate steps of calcium carbonate hydroxide hydrates (CCH-1 to CCH-5), was indicated by the previous study of two OPC. This was checked here on four cements ground for t (g)=15, 20, 25 and 30 min and hydrated either in water vapour, successively at RH=1.0, 0.95 and 0.5 for 2 weeks each (WR1, WR2 and WR3, respectively) or as mortars in liquid water (1m), followed by WR as above. The d[001] spacing of portlandite was confirmed to vary: here between the lowest and the highest standard values. The diffractograms of n=32 different samples were analyzed for presence of standard CCH peaks, generally slightly displaced. These were: CCH-1 [Ca-3(CO3)(2)(OH)(2)]: N=11 peaks, of three different d[hkl] spacings, CCH-2 [Ca-6(CO2.65)(2)(OH657)(7)(H2O)(2)]: N=10 for two d[hkl], CCH-3 [Ca-3(CO3)(2)(OH)(2)center dot 1.5H(2)O]: N=14 for five d[hkl], CCH-4, ikaite [CaCO3(H2O)(6)]: N=13 for six d[hkl], CCH-5[CaCO3(H2O)]: N=15 for five d[hkl]. Thus the most probable is the presence of the last three. The stepwise transformation of Ca(OH)(2) into CaCO3 was confirmed: portlandite (varying d[001])-> CCH-1 -> CCH-2 -> CCH-3 -> CCH-4 -> CCH-5 -> CaCO3 The content of CCH was the highest at t (gr)=15 min, decreasing down to t (gr)=25 min and increasing slightly at 30 min, as inferred from the number of the peaks observed. After cement powder hydration at RH=1.0 (WR1) peak number increased gradually from CCH-1 to CCH-5, whereas in the hydrated mortar (1m) the peak number decreased from CCH-1 to CCH-5, indicating the respective progress of the carbonation reaction.