Calcium and magnesium silicate hydrates formed in the presence of sodium hydroxide: Insight from experiments and DFT simulation

In this paper, calcium and magnesium silicate hydrates were synthesized in the presence of sodium hydroxide to explore the effect of alkaline environment on the structure of synthetic phases via XRD, DTG, and 29Si NMR. Further, the competition relations between Ca(2+ )and Mg2+ in bonding with [SiO4]4-were investigated by DFT method. The results indicate that both C-S-H and M-S-H were formed, when Ca2+, Mg2+, and [SiO4]4-coexisted in the sodium hydroxide solution. Moreover, the presence of sodium hydroxide as well as the low polymerization degree of silica tetrahedron in sodium silicate solution could reduce the silicate chain length or Q(3)/Q(2) ratio of synthetic products. The Q(3) content of M-S-H was low no matter sodium hydroxide existed or not in this study. Even so, the presence of sodium hydroxide could still reduce the Q(3) proportion of M-S-H by 6%. The calculated binding energies of [SiO4]-Ca/Mg in NaOH environment were both negative. This implies that the [SiO4]-Ca/Mg could be formed when the [SiO4] coexisted with Ca and Mg in NaOH environment, which supports the exper-imental results. Moreover, the DFT results suggest that the presence of calcium could break the Mg-O bond and weaken the charge transfer between Mg and O in [SiO4]-Mg system, while the presence of magnesium has slight effect on the bonding of [SiO4]-Ca system.

Automated summary

The effect of an alkaline environment on the structure of synthetic calcium and magnesium silicate hydrates was explored in this study. It was found that both C-S-H and M-S-H were formed when Ca2+, Mg2+, and [SiO4]4- coexisted in a sodium hydroxide solution. The presence of sodium hydroxide and low polymerization degree of silica tetrahedron in sodium silicate solution reduced the silicate chain length or Q(3)/Q(2) ratio of the synthetic products. The DFT results showed that the presence of calcium broke the Mg-O bond and weakened the charge transfer between Mg and O in the [SiO4]-Mg system.