1 Thermo-kinetics of Cement Hydration: Temperature effect and Activation Energy Yanfei Peng * , Will Hansen † , Claus Borgnakke ‡ , Ivindra Pane § , J.C.Roumain ** & Joseph J. Biernacki †† Abstract The purpose of this study is to quantify the temperature effect on cement hydration. In this study, long-term ( ≈ 21 days) isothermal heat of hydration rate data is obtained at three different temperatures (15 o C, 23 o C, 35 o C) for ordinary portland cement (OPC) and for blends of OPC containing supplementary cementitious materials (SCM). Between 1000 and 2000 data points are obtained for rate of hydration between the ages of 1 hour and 21 days. The results show that the absolute rate decreases by three orders of magnitude as the hydration process gradually becomes diffusion controlled. The cumulative heat of hydration is evaluated using the exponential hydration function of Freiesleben-Hansen. A new rate model for absolute hydration rate is proposed. This model predicts that the rate of hydration is a product of a temperature function (absolute rate constant) and a degree of hydration function (diffusion controlled) that is unique for a given system. The absolute rate, instead of the conventional relative rate, is used so that the effect of temperature on long-term hydration can be incorporated. This model is then used to predict the temperature effect on total rate in the temperature range of 5 to 40 o C. Within this range the temperature effect on rate of hydration at 40 o C is rapidly decreasing to less than 1% of total peak rate within 3 days, while at 5 o C temperature effect is still pronounced at the age of 7 days. During the deceleration stage of hydration the relative temperature sensitivity (i.e. activation energy) is constant at about 50 KJ/mole. Keywords: hydration, cement, rate, maturity Introduction Temperature has been shown to significantly influence cement hydration [1, 2, 3, 4, 5], especially during early age. Previous studies have demonstrated that higher curing temperature will result in higher early degree of hydration (strength), but lower long-term hydration (strength) [1, 6, 7, 8], and vice versa. It has also been shown that curing temperatures can change the cement paste microstructure [4]. The understanding of the temperature effect on cement hydration and concrete * Research Assistant, Department of Civil and Environmental Engineering † Professor, Department of Civil and Environmental Engineering ‡ Associate Professor, Department of Mechanical Engineering § Computational Materials Laboratory, Swiss Federal Institute of Technology ** Holcim Co., Colorado †† Professor, Tennesee Technological University