Influence of mineralogy and calcination temperature on the behavior of palygorskite clay as a pozzolanic supplementary cementitious material

Aiming to limit carbon dioxide (CO2) emissions by cement production and widen the supplementary cementitious materials' (SCM) resource options, recent research has been focused on developing the pozzolanic properties of low-cost calcined clays containing a blend of clay minerals and impurities as an alternative to metakaolin. This study investigated the effects of mineralogical composition and calcination temperature on the use of calcined palygorskite clays as SCM. Rietveld quantitative X-ray diffraction (RQXRD), thermogravimetric analysis and Fourier-transform infrared proved useful in shedding light on the relation of mineralogical composition and thermal treatment temperature of clays, with the properties of pastes and mortars, in which the calcined palygorskite clays substitute cement, at 20 wt%. The results indicated that with increasing palygorskite content in raw clays, the pozzolanic reactivity of calcined clays also increases. Moreover, the calcination of Mg – rich clays between 600°C and 750°C, presented higher calcium hydroxide (CH) consumption and strengths after 28 and 90 days of hydration compared to Al – rich ones. The former displayed the highest pozzolanic reactivity when calcined at 750°C, whereas the latter at 700°C. The substitution of cement by clays calcined at 750°C, and by metakaolin, resulted in a similar reduction of CH content in pastes; up to 60%, relative to neat cement, at all hydration ages. Calcined palygorskite clay could be introduced as a reactive pozzolan in mortar with comparable or higher strengths than neat cement counterparts. •Palygorskite content is positively correlated with the pozzolanic reactivity of SCM.•Pozzolanic reactivity is higher for Mg – rich than Al – rich palygorskite clays.•The optimum calcination temperature is 750 °C and 700 °C for Mg and Al – rich clays.•Calcined palygorskite and metakaolin present similar CH consumption.•Metakaolin pozzolanic reaction produces larger amounts of hydrates than palygorskite.