Determination of the amount of Ca(OH)2 bound by additive nano-SiO2 in cement matrices

Comparison of the content of Ca(OH)2 in hardened cement matrices, which contains the additive SiO2 nanoparticles and in matrice without additives was performed by the method of thermogravimetric analysis. Alite portland cement «SsangYong» and «Denki» were used. Hydrothermal sol of «Geosil» was introduced as an additive of nanoparticles of SiO2. The amount of introduced nano-SiO2 was from 1.15 to 1.74 wt.% in respect to cement consumption. Water-cement ratio was provided at the level of W/C = 0.39–0.26. The compensating amount of polycarboxylate SVC-5Neu was 0.2–0.77 wt. %. The Ca(OH)2 content was calculated from the mass loss of the cement matrice sample in the temperature range 460–510оC. It was determined that the sol «Geosil» additive reacts quickly with Ca(OH)2 (CH) in cement matrice, significantly (up to 40%) reducing its content. By the period of 24 h sol «Geosil» binds 750 [mg CaO /g SiO2] and continues to bind CH up to 100–700 days, to the value δCaO = 1300 [mg CaO /g SiO2]. The pozzolan binding reaction Ca(OH)2 with the formation of calcium hydrosilicates may be one of the mechanisms for increasing the strength of concrete when introducing SiO2 nanoparticles.