Phase development and mechanical strength of limestone calcined clay cement utilising Australian bentonite and plasterboard waste

The potential of a calcined Australian bentonite (CB) to produce low embodied carbon limestone-calcined-clay-cement with 50 % clinker replacement was compared against metakaolin (Mk). Recycled plasterboard waste (PW) was also investigated as an alternative to virgin gypsum to enhance cement sustainability. CB-based cement blends exhibited greater pozzolanic reactivity, improved phase development and greater 28-day compressive strength compared to Mk-based blends, as confirmed by X-ray diffraction, thermogravimetry, infrared spectroscopy and electron microscopy. Reactive silica from CB promoted formation of calcium-silicate-hydrates with higher Si/Ca and lower Al/Ca ratios, as well as more alumino-silicate phases (strätlingite), whereas Mk favored formation of ettringite and carbo-aluminates. Additional sulphates enhanced ettringite and carbo-aluminate formation, and PW improved early age pozzolanic reactivity in both blends as confirmed by thermal analysis. This study demonstrated that both the CB and PW show great potential for producing low embodied carbon cement, highlighting that other local clay-mineral resources should also be investigated.