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.

摘要 高层建筑施工需浇筑方量可达数千立方米的承台（pile caps）。此类结构通常采用自密实混凝土（self-compacting concrete）和/或高抗压强度混凝土配制，胶凝材料用量较高。在此类工况下，以粉煤灰（fly ash）替代硅酸盐水泥（Portland cement）是广为人知的技术方案，既可降低水化热（hydration heat），又可缓解延迟钙矾石形成（delayed ettringite formation）问题。然而，此类混凝土普遍采用的高胶凝材料用量会产生较高温升，使得该矿物掺合料在水化热抑制方面的有效性存在不确定性。为此，本研究旨在评估粉煤灰替代硅酸盐水泥对混凝土绝热温升及抗压强度的影响。
本研究制备了3个抗压强度等级（35、45及55 MPa）、4种粉煤灰替代率（0、15、30及45%）的混凝土试件。通过绝热量热仪（adiabatic calorimeter）连续72小时监测混凝土的温升过程。分别测试了绝热养护与室温养护试件在28天龄期的抗压强度。此外，采用热重分析（thermogravimetric analysis, TGA）评估了粉煤灰的火山灰活性。
研究发现，无论混凝土抗压强度等级或粉煤灰替代率如何，绝热温升系数（即温度变化量除以每立方米混凝土胶凝材料消耗量）均较为接近。与室温养护相比，绝热养护会使28天抗压强度降低约10%。同时，热重分析结果证实了粉煤灰的火山灰活性。综合上述结果可得出结论：在高胶凝材料用量的大体积混凝土中使用粉煤灰，在抑制水化热方面并非高效方案，但由于其在抗压强度提升及延迟钙矾石形成防控方面的益处，仍具备应用价值。