Mathematical determination of the maximum heat release for fly ash cement paste: Effect of heat flow monitoring time, calculation approach and fly ash content

Abstract

The maximum hydration heat release Qmax is one of the most concerned kinetic parameters for hydration of cementitious materials. In this study, we specially investigated the effect of the mathematical calculation approach, heat flow monitoring time, and fly ash (FA) content on the obtained Qmax values of FA cement paste. The isothermal hydration heat flows of cement paste with 0%, 20%, 40% and 60% FA contents have been monitored for 250 hours at 20 degrees C. Two conventional Qmax calculation approaches, including Knudsen's equation and the three-parameter exponential function, have been utilized to obtain Qmax values for different cementitious systems. The results indicate that irrespective of the calculation approach or monitoring time, mathematically derived Qmax linearly decreases along with the increase of FA content, which is consistent with the determined enthalpy of the binary cement system. Furthermore, the calculated Qmax increases with the extension of heat flow monitoring time, and different calculation approaches can yield varied Qmax values. However, the discrepancies between the two approaches gradually diminish as the monitoring time extends. It is recommended that for cement paste without FA addition, a minimum heat flow monitoring time of 7 days is necessary to obtain a reliable Qmax value using Knudsen's equation, while for the exponential function, a monitoring time exceeding 7 days is advised. For cement paste with FA addition, neither Knudsen's equation nor three-parameter exponential function can derive a relatively accurate Qmax value for FA within the limited heat flow monitoring duration in this study.