Study of the hydration of Portland cement blended with blast-furnace slag by calorimetry and thermogravimetry

The hydration of ordinary Portland cement (OPC) blended with blast-furnace slag (BFS) is a complex process since both materials have their own reactions which are, however, influenced by each other. Moreover, the effect of the slag on the hydration process is still not entirely known and little research concerning the separation of both reactions can be found in the literature. Therefore, this article presents an investigation of the hydration process of mixes in which 0-85% of the OPC is replaced by BFS. At early ages, isothermal, semi-adiabatic and adiabatic calorimetric measurements were performed to determine the heat of hydration. At later ages, thermogravimetric (TG) analyses are more suitable to follow up the hydration by assessment of the bound water content w (b). In addition, the microstructure development was visualized by backscattered electron (BSE) microscopy. Isothermal calorimetric test results show an enhancement of the cement hydration and an additional hydration peak in the presence of BFS, whilst (semi-)adiabatic calorimetric measurements clearly indicate a decreasing temperature rise with increasing BFS content. Based on the cumulative heat production curves, the OPC and BFS reactions were separated to determine the reaction degree Q(t)/Q (a) (Q = cumulative heat production) of the cement, slag and total binder. Moreover, thermogravimetry also allowed to calculate the reaction degree by w (b)(t)/w (ba). The reaction degrees w (b)(t)/w (ba), Q(t)/Q (a) and the hydration degrees determined by BSE-image analysis showed quite good correspondence.