Influence of modification of SiO2 on the formation of calcium silicate hydrate

Interactions of Ca(OH)2 with various modifications of SiO2: amorphous (Hi-Sil, TSD) or crystalline (quartz) silica were established. The molar ratio of primary mixtures CaO/SiO2 was 0.5. The samples were cured in a vapour chamber (20 ºC, 100 % humidity) for 7 and 28 days. The products were character-ized by X–ray diffraction, thermogravimetry, differential scanning calorimetry, Fourier-transform infra-red and scanning electron microscopy analysis. SiO2 modification used had a decisive influence on the processes of formation of calcium silicate hydrates. In the mixture with amorphous SiO2, Ca(OH)2 re-acted completely during 7 days of synthesis, while large quantity of Ca(OH)2 remained unreacted even after 28 days when quartz was used. It was found that in the Ca(OH)2–amorphous SiO2–H2O system, after a 28-day exposure, the surface of SiO2 globules was totally covered with well-crystalline C-S-H(I) plates. Moreover, in the samples with quartz, only the gaps between quartz crystals were filled with C-S-H(I). Key words: calcium silicate hydrate; crystalline SiO2; silica fume; Portland cement; X-ray diffraction 1