Investigation of different paper mill ashes as potential supplementary cementitious materials

Integrating supplementary cementitious materials (SCMs) in cement-based construction materials is a key strategy for reducing environmental burdens and the greenhouse gas emissions related to cement production. Supplies of commonly applied SCMs such as fly ash and blast furnace slag are becoming limited, and therefore the search for alternative SCMs is of high significance. The present research investigates the viability of four paper mill ashes collected at different stages from the same incineration process as SCMs at different cement substitution rates (5-30 wt%). The ashes were first classified depending on their mineralogical and chemical composition, and their contribution to strength development of the prepared mortars is examined. Furthermore, the effects of the ashes on the hydration kinetics and the fresh and hardened properties of the prepared mortars were studied. The obtained results showed that a high level of SCM replacement accelerates the early hydration reactions and decreases the setting time. In addition, the incorporation of paper mill ashes increased the bound water depending on the ash type and resulted in strength development of the mortars at low replacement percentages (5-10 wt%). This study shows that the ashes belonging to type C medium acid based on bio ash classification could be incorporated as SCMs at a maximum level of 10 wt%, while those corresponding to type C low acid could be used up to 20-30 wt%.