Role of Siliceous Raw Materials on the Structure of Nano-CSH and Their Effect on the Properties of Cement

Synthetic nano-calcium silicate hydrate (CSH) is widely known for its ability to act as crystal nuclei in ordinary portland cement. Material performance is largely influenced by its microstructure. Here, nano-CSH was prepared by different siliceous raw materials (Na2SiO3 or Na2SiO3·5H2O). It was found that nano-CSH can be obtained from Na2SiO3 or Na2SiO3·5H2O in the presence of Ca(NO3)2·4H2O and NaOH solution and were labeled as CSH-0 and CSH-5, respectively. The average particle size of CSH-0 and CSH-5 was about 80 nm, and the polymerization degrees of silicon and oxygen atoms in CSH-0 and CSH-5 were different. CSH-0 and CSH-5 can increase the rate of hydration and significantly enhance the early strength of portland cement. Further, CSH-0 and CSH-5 can provide nucleation sites for CSH hydration product; however, CSH-0 and CSH-5 can also react with the calcium ions in the cement paste and produce calcium silicate hydrate with a high Ca/Si ratio. Compared with CSH-5, CSH-0 with a lower Ca/Si ratio can consume more calcium ions in the cement paste. CSH-0 promoted cement hydration better, decreased the volume fractions of macropores, and increased compressive strength more significantly due to the synergistic effect of the nanocrystal nucleus and reactivity with calcium ions in solution.