The effect of temperature on the early hydration of Portland cement and blended cements

The effect of temperature, in the range 10-60 degrees C, on the early hydration of two Portland cements from Mexico and three blended cements has been investigated by means of isothermal conduction calorimetry. The blended cements were based on the two Portland cements with partial replacement by ground granulated blast furnace slag, GGBFS (60%), pulverized fuel ash, PFA (30%) and blast and a Mexican volcanic ash (22%). An increase in the temperature of hydration brought about initial acceleration of all five cement systems, as indicated by the increased magnitude of the maximum of peak II and shifts to earlier times. There was a clear contribution from the GGBFS to the heat evolution curves of the blended cement incorporating GGBFS, which increased with increasing temperature. By way of contrast, there was no enhancement of the cement hydration in the early stages of hydration in the case of the PFA blended cement, and the appearance of peak II was slightly retarded. The incorporation of volcanic ash had little effect on the initial hydration, but gave an additional peak after peak II, similar to that observed in the GGBFS system. At this stage, the total heat evolved was greater than that in the corresponding neat cement, especially at 20 degrees C. If has also been demonstrated that alkali activation of GGBFS and volcanic ash was readily brought about by NaOH at 40 degrees C. The enhanced activity observed in the Portland cement-volcanic ash system is attributed to activation of the ash by the portlandite liberated during the early hydration of alite.