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 hydra tion of two Portland cements from Mexico and three blended cements has been investigated by means of isothermal conduction calorimetry. The blended ce ments were based on the two Portland cements with partial replacement by gr ound granulated blast furnace slag, GGBFS (60%), pulverized fuel ash, PFA ( 30%) and blast and a Mexican volcanic ash (22%). An increase in the tempera ture of hydration brought about initial acceleration of all five cement sys tems, 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 enh ancement of the cement hydration in the early stages of hydration in the ca se of the PFA blended cement, and the appearance of peak II was slightly re tarded. The incorporation of volcanic ash had little effect on the initial hydration, but gave an additional peak after peak II, similar to that obser ved 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 o bserved in the Portland cement-volcanic ash system is attributed to activat ion of the ash by the portlandite liberated during the early hydration of a lite.