Hydration characteristics of waste sludge ash that is reused in eco-cement clinkers

The study reports on the hydration characteristics of eco-cement clinkers produced with waste sludge ash as raw components. The tested mixtures were composed of different types of waste sludge ash, including sewage sludge ash, water purification sludge ash, limestone, and ferrate, prepared using the optimum proportioning method. The mixtures were burned at 1400 °C for 6 h. The clinkers thus obtained were quantified and the hydration characteristics of the eco-cement pastes prepared from the waste sludge ashes. The setting time, compressive strength, hydrates and porosity distribution were examined at various ages. The 28-day compressive strength of the early high strength developing of eco-cement C paste outperformed that of ordinary Portland cement paste by 3 MPa. It is supposed that the large quantity of limestone used provided CaO, which in turn enhanced the formation of C₃S, leading to the greater compressive strength development in the eco-cement C paste. From the porosity distribution, shown by the Mercury Intrusion Porosimetry results, it was found that, with increasing curing ages, the gel pores (<0.01 μm) increased and the total porosity and capillary pores (>0.01 μm) decreased—a result that shows that hydrates had filled the pores. This resulting densification and enhanced later strength were caused by the shifting of the pore size distribution to a smaller diameter range.