Activation energies of high-volume fly ash ternary blends: Hydration and setting

Because ready-mixed concrete is placed under a wide variety of environmental conditions, the influence of temperature on the hydration reactions and the accompanying setting process is of critical importance. While contractors are generally quite comfortable with the temperature sensitivity of conventional ordinary Portland cement (OPC) concretes, more sustainable mixtures containing high volumes of fly ash (HVFA), for example, often present problems with delayed setting times and increased temperature sensitivity. Based on isothermal calorimetry and Vicat needle penetration measurements, this study demonstrates that the high temperature sensitivity of such HVFA mixtures can be effectively moderated by the replacement of a portion of the fly ash with a fine calcium carbonate powder. In addition to accelerating and amplifying hydration and reducing setting times at a given temperature, the presence of the fine CaCO3 powder also lowers the apparent activation energy for setting for temperatures below 25 °C. The reactivity of the CaCO3 in these mixtures is quantified using thermogravimetric analysis. Comparison of results for CaCO3 powders of nominal sizes of 1 μm and 17 μm, replacing 10% by volume of the cement in an OPC mixture, indicates that the former is highly superior in accelerating/amplifying hydration and reducing setting times.