Influence of the use of fly ash on the adiabatic heat evolution and compressive strength of concretes

Abstract

The construction of high buildings has required the execution of pile caps that reach thousands of m³ of concrete. In general, these structures are built using self-compacting and/or high compressive strength concretes, resulting in high binder contents. In these cases, the use of fly ash replacing Portland cement is a well-known alternative, both for the reduction of hydration heat and for the mitigation of delayed ettringite formation. However, the high binder contents generally used in such concretes results in high temperatures, creating an uncertainty about the efficacy of this mineral admixture for the reduction of the hydration heat. Thus, the current work aims to evaluate the effect of the replacement of Portland cement by fly ash in the adiabatic elevation of temperature and in the compressive strength of concretes. In this regard, concretes were produced with three classes of compressive strength (35, 45 and 55 MPa) and four cement replacement ratios by fly ash (0, 15, 30 and 45%). The temperature elevation of the concretes was monitored by an adiabatic calorimeter for 72 hours. At 28 days, the compressive strength of specimens cured in the adiabatic condition and at room temperature were determined. Finally, the pozzolanic activity of fly ash was evaluated by thermogravimetric analysis (TGA). It was found that, regardless of the resistance class or the replacement content of fly ash, the adiabatic elevation coefficients (corresponding to the temperature variation divided by the consumption of binder / m³ of concrete) were quite similar. The adiabatic condition promoted a reduction in compressive strength at 28 days, on the order of 10% compared to curing at room temperature. Also, the pozzolanic activity of fly ash was confirmed by TGA. Considering the results, it can be concluded that the use of fly ash in large masses of concrete with high binder contents becomes an inefficient alternative for the reduction of the heat release. However, its use is indicated because of the benefits in compressive strength and prevention of delayed ettringite formation.