Effect of a calcined Westerwald bentonite as supplementary cementitious material on the long-term performance of concrete

Abstract The objective of using supplementary cementitious materials (SCMs) in cement is to reduce costs and the overall CO2 output of the cement production. Against the background of an increasing demand for cement and a decreasing availability of common SCMs, calcined clays serve as alternative material for the industrial-scale manufacture of composite cements. In the presented study, a raw bentonite clay of high volume capacity and small exploitation options was selected to check its pozzolanic activity after suitable thermal treatment and the consequent effects on the concrete performance. The results show that, at high cement substitution rates of 30%, the properties of the concrete were not changed to a significant extent. In certain durability issues namely sulphate resistance, inhibition of alkali silica reaction (ASR) and chloride migration, the mixture with calcined clay even performed better than the reference. Other properties like final strength and frost and de-icing salt resistance were changed in an uncritical way. Negative effects were observed on the early strengths and on the carbonation velocity. However, giving careful consideration to these aspects during concrete projecting, the calcined clay would be a suitable material for the majority of concrete applications.