Sustainable Cements with up to 80 wt% Calcined Common Clay: Challenges in Microstructure and Compressive Strength of Concretes

In this article, binders containing 0, 40 and 80 wt% calcined common clay are investigated in pastes, mortars and concretes. The investigation covers heat flow, microstructural aspects, compressive strengths, carbonation and sustainability calculations. The initially slow pozzolanic reactivity of the calcined common clay causes a decline of the 2-day strength exceeding its replacement level. As the pozzolanic reaction develops with time, concretes with 40 wt% binder replacement achieve up to 10% higher strengths than their reference at day 90. A further increase in the replacement to 80 wt% calcined clay yields concrete strengths that are at least 30% lower than the reference, because portlandite lacks for a complete pozzolanic reaction of the calcined common clay. This is counteracted by adding hydrated lime, which improves strength by up to 10 MPa. Microstructural investigations substantiate the strength findings. As the amount of hydrated phases declines and porosity rises, strength decreases, and vice versa. Replacing 40 or 80 wt% of Portland cement by calcined clay reduces the Global Warming Potential of concretes by nearly a quarter or half. The concretes with 40 wt% replacement exhibit the best strength eco-efficiency from day 28 onwards, while the concretes with 80 wt% replacements achieve a strength eco-efficiency comparable to that of the reference concretes.