Effect of Different Initial CaO/SiO2 Molar Ratios and Curing Times on the Preparation and Formation Mechanism of Calcium Silicate Hydrate

To better understand the pozzolanic activity in fly ash used as a supplementary cementitious material in cement or concrete, calcium silicate hydrate (C-S-H) has been synthesized by adding silica fume to a supersaturated calcium hydroxide solution prepared by mixing calcium oxide and ultrapure water. Thermogravimetric analysis results have revealed the variation in the weight loss due to C-S-H in the samples and the conversion ratio of calcium oxide (the mu(CaO) value), which represents the proportion of calcium oxide in the initial reaction mixture used to produce C-S-H, with curing time. The weight loss due to C-S-H and the mu(CaO) value were both maximized (13.5% and 90.4%, respectively) when the initial C/S molar ratio was 1.0 and the curing time was 90 d. X-ray diffraction (XRD) analysis has indicated that C-S-H in the samples after curing for 7 d had the composition Ca1.5SiO3.5 center dot xH(2)O. Si-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis has revealed that the degree of polymerization of C-S-H increased with an increase in curing time for samples with an initial C/S molar ratio of 1.0. The ratio of internal to terminal tetrahedra (Q(2)/Q(1)) increased from 2.29 to 4.28 with the increase in curing time from 7 d to 90 d. At curing times >= 28 d, a leaf-like C-S-H structure was observed by scanning electron microscopy (SEM). An ectopic nucleation-polymerization reaction process is proposed for the formation mechanism of C-S-H.

Automated summary

To study the pozzolanic activity of fly ash in cement or concrete, calcium silicate hydrate (C-S-H) was synthesized by adding silica fume to a supersaturated calcium hydroxide solution. Thermogravimetric analysis showed the weight loss due to C-S-H and the conversion ratio of calcium oxide varied with curing time. X-ray diffraction analysis revealed the composition of C-S-H after 7 days of curing. Si-29 magic angle spinning nuclear magnetic resonance analysis indicated an increase in the degree of polymerization of C-S-H with curing time. Scanning electron microscopy showed a leaf-like C-S-H structure at curing times >= 28 days. An ectopic nucleation-polymerization reaction process was proposed for the formation mechanism of C-S-H.