Reactivity of alternative supplementary cementitious materials assessed by the R³ method

The necessity to reduce carbon emissions in concrete production has accelerated the partial replacement of Portland cement by Supplementary Cementitious Materials (SCMs). As the range of SCMs studied is constantly expanding, screening tests to evaluate their reactivity have gained considerable attention. One example is the R³ method, which consists in quantifying the chemical reactivity of an SCM in a simplified mixture simulating the environment of a cement paste. Different parameters measured by various techniques have been proposed in previous work: heat release, bound water, portlandite consumption and chemical shrinkage.

In the present study, the R³ method was applied to both traditional and alternative SCMs, to further investigate the validation range of the method. The experimental matrix included limestone, fly ash, calcined clays, biomass ashes, crushed brick, glass beads and sewage sludge ash. Three parameters were measured: heat release by isothermal calorimetry, bound water by oven-drying and portlandite consumption by thermogravimetry. The results indicated that both heat release and bound water correlated well with the relative compressive strength at 28 days, even for alternative SCMs. Thus, the R³ method appears as an efficient screening test to identify promising SCMs. In addition, the study confirmed the potential of bound water, which can be measured with basic equipment available in many laboratories.

As most SCMs were first tested as received, the particle size varied significantly between the materials. Crushing the SCMs did not always improve the reactivity, presumably only when the amorphous part was affected. Finally, the effect of sulphates and carbonates on the reactivity was tested. Both compounds influenced the measured parameters, but no conclusion could be drawn regarding the correlation with the compressive strength. Thus, it is suggested to keep both sulphates and carbonates in the R³ mix design, following the initial idea of mimicking a cement paste environment.