The most effective approach to mitigating the environmental impact of cement production is through the uti-
lisation 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 refer-
ences. 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 mont-
morillonite, 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.
