Supplementary cementitious material based on calcined montmorillonite standards
Diego Vallina, María Dolores Rodríguez-Ruiz, Isabel Santacruz, Ana Cuesta, Miguel Á. G. Aranda, Ángeles G. De la Torre
Abstract
The most effective approach to mitigating the environmental impact of cement production is through the utilisation of pozzolanic materials to prepare blended cement. Calcined kaolinitic clays are one of the most promising materials to be used as supplementary cementitious materials (SCM) due to their final performances. However, there are locations where kaolinitic rocks are not available. This work presents the study of two dioctahedral smectite standards from the Clay Minerals Society (CMS), SWy-3 and SAu-1, to be used as references. After thermal activation optimisation, their pozzolanic performances are compared with those of low-grade kaolinitic clay. On the one hand, raw SWy-3 has an amorphous content of ∼80 wt%, considered montmorillonite, with quartz and feldspars. The optimum calcination temperature was 800ºC, at which the smectite is fully dehydroxylated (as determined by thermal analysis) but not completely amorphized (as shown by powder diffraction). After calcination (800ºC), with a median particle size by volume (Dv,50) of ∼11 μm, ∼480 J/g were released at 7 days in the R3 test. On the other hand, SAu-1 has a higher amount of amorphous content, ∼96 wt%, which includes interstratified illite/smectite. The optimum calcination temperature was found to be 750ºC. This sample (Dv,50 ∼10 μm) gave 452 J/g at 7 days in the R3 test. The optimised active smectites gave a pozzolanic activity comparable to clay with 30–35 % kaolinite, as determined, for the first time, by the R3 test. This work has established descriptors for the activation of natural smectites/bentonites and reference values for the resulting smectite-based SCMs.