Calcined Illitic Clays: An Effective SCM to Reduce the Alkali–Silica Reaction

In concrete technology, when evaluating ways to mitigate processes that affect durability, such as alkali–silica reaction (ASR), the use of active supplementary cementing materials (SCMs) is the most accepted way to proceed. In the last few years, to achieve decarbonization purposes, attempts have been made to find alternatives to replace fly ash and blast furnace slag, since their availability is finite and they are subproducts with an impact on CO2 emissions. Thus, it has been proposed to reuse calcined clays as SCMs, as was done in the past, and to study them using them in concrete to see how they behave from the point of view of hydration and their resulting durability. The advantage of using calcined illitic clay (CIC) in concrete is that CIC consumes less water than metakaolin and makes it possible to produce concrete that is adequately workable. This study evaluates CIC as a potential solution for regions with large clay deposits, such as Argentina, Brazil, and the United States. Various American standard expansion-based tests and microstructural techniques (petrographic thin section, electron probe microanalysis, and energy-dispersive spectrometry) were performed to objectively evaluate the CIC effectiveness and the minimum level of replacement to control ASR expansion of a very highly reactive aggregate (R3). Results confirm the beneficial effect of CIC replacement in mitigating ASR expansion, despite its significant alkali content (Na2Oeq=4.95%). A minimum level replacement of 25% or 35% by mass of CIC has been proven to guarantee an appropriate protection level for structures for which a minor risk of ASR is accepted. Microstructural analysis results proved that CIC replacement mitigates ASR expansion through the known role of Al introduced into the system, the reduction of calcium hydroxide, and the long-term alkali binding in the gel.