The coupling effect of calcium concentration and pH on early hydration of cement

Pore solution provides the physical and chemical environment for cement hydration, in which calcium, silicon, and hydroxyl ions released by dissolution of cement clinker are transferred into the hydration products such as calcium silicate hydrates (C-S-H) and calcium hydroxide (CH). Therefore, both calcium ions and pH not only have significant influence on the dissolution of cement clinker but also on the precipitation of CH and C-S-H. However, how they influence the kinetics of cement hydration, especially at the very early stage, remains unclear. With the attempt to explore this influence potassium oxalate and oxalic acid were used to regulate the calcium concentration and pH within pore solution independently. Cement pastes with different formulations were investigated by the combination of calorimetric measurements and pore solution analysis. The results suggest that simultaneous removal of calcium and hydroxyl ions can significantly increase the length of induction period and reduce the height of the silicate reaction peak in the calorimetric curves, whereas the removal of hydroxyl or calcium ions alone cannot. The calculation of saturation index of alite, CH, and C-S-H indicates that undersaturation degree of alite during induction period is well related to the length of induction period and simultaneous removal of calcium and hydroxyl ions is likely to cause lower undersaturation degree of alite, longer induction period, and postponed time of silicate peak. The increase of pH could result in a higher sulfate concentration and subsequently a higher aluminate peak, and a shorter time interval between silicate and aluminate peak. (C) 2018 Elsevier Ltd. All rights reserved.