Study of the hydration of Portland cement blended with blast-furnace slag by calorimetry and thermogravimetry E. Gruyaert • N. Robeyst • N. De Belie Received: 4 February 2010 / Accepted: 22 April 2010 / Published online: 11 May 2010 Ó Akade ´miai Kiado ´, Budapest, Hungary 2010 Abstract 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, iso- thermal, semi-adiabatic and adiabatic calorimetric measure- ments were performed to determine the heat of hydration. At later ages, thermogravimetric (TG) analyses are more suit- able to follow up the hydration by assessment of the bound water content w b . In addition, the microstructure develop- ment was visualized by backscattered electron (BSE) micros- copy. 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 mea- surements clearly indicate a decreasing temperature rise with increasing BFS content. Based on the cumulative heat pro- duction curves, the OPC and BFS reactions were separated to determine the reaction degree Q(t)/Q ? (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 b? . The reaction degrees w b (t)/w b? , Q(t)/Q ? and the hydration degrees determined by BSE-image analysis showed quite good correspondence. Keywords Blast-furnace slag  Hydration  Calorimetry  Bound water Introduction The hydration of the cement minerals C 3 S (3CaOSiO 2 ), C 2 S (2CaOSiO 2 ), C 3 A (3CaOAl 2 O 3 ) and C 4 AF (4CaOAl 2 O 3 Fe 2 O 3 ) is an exothermal chemical process. At early ages, the generated heat can be monitored by isothermal (conduction) calorimetry and five stages can be distinguished, as mentioned by several researchers [1, 2]: (i) the initial period; (ii) the induction period; (iii) the acceleration period; (iv) the deceleration period and (v) the period of slow continued reaction. The possibility to detect the first hydration peak, associ- ated with the wetting of cement and the hydration of free lime, C 3 A and hemi-hydrate (CaSO 4 0.5H 2 O), depends on the calorimeter design [1, 3]. Different methods which allow mixing inside the calorimeter and consequently favour the detection of the first hydration peak, can be found in the literature [4, 5]. Contrarily, the second peak, due to the hydration of C 3 S can always be registered completely. Additionally, a third peak sometimes appears due to the hydration of C 3 A (the reaction of ettringite to monosulphate [6]). According to Bensted [7] only a C 3 A amount of more than 12% results in a visible third peak at 20 °C. However, in presence of fly ash, Baert [8] already recorded this peak for cement containing merely 7.5% C 3 A. When ordinary Portland cement (OPC) is partially replaced by blast-furnace slag (BFS), the reaction of BFS is activated by the release of hydroxyl ions. Although NaOH and KOH are the major components after about 1 day of hydration, rather than Ca(OH) 2 (CH), the pres- ence of solid CH ensures that the supply of OH - ions is maintained [9, 10]. According to some researchers [11, 12], this activation is effective when the pH of the aqueous phase is higher than 11.5 due to cement hydra- tion or alkali-activation, whilst others claim a value of 12 E. Gruyaert  N. Robeyst  N. De Belie (&) Magnel Laboratory for Concrete Research, Ghent University, Technologiepark Zwijnaarde 904, 9052 Ghent, Belgium e-mail: Nele.Debelie@UGent.be 123 J Therm Anal Calorim (2010) 102:941–951 DOI 10.1007/s10973-010-0841-6