Temperature-Dependent Hydration and Mechanical Properties of High-Volume Fly Ash Cement with Chemical Additives.

This research investigates the effects of Na2SO4 and Na2CO3 on the mechanical and hydration properties of high-volume fly ash (FA) cement (HFC) at three curing temperatures: 25°C, 40°C, and 60°C. Setting time, compressive strength, reaction kinetics, phase assembles, and pore structure were explored. The research found that the hydration of HFC was significantly promoted by the chemical activators at all temperatures. At 25°C, the incorporation of Na2CO3 and Na2SO4 decreased the initial setting time by 59% and 50%, respectively, compared to the control sample. This was accompanied by increased heat release from the isothermal calorimetry tests. At the same time, the introduction of Na2CO3 (HFC-C) increased the early compressive strength by 40%, higher than that of samples with Na2SO4 addition (HFC-S). While reduced compressive strength was noticed in HFC-C after curing for a long time, resulting from the increased porosity. Furthermore, temperature rise reduced the contribution index (Ci) to the strength development of both HFC-C and HFC-S. On the other hand, the presence of Na2SO4 contributed to the formation of more ettringite (AFt), while no AFt was detected after the addition of Na2CO3. Increasing the curing temperature did not strongly change the phase assembles of the hydration products, although the amount of AFt content was observed to increase. [ABSTRACT FROM AUTHOR]