Preparation and Hydration of Low-Carbon UHPC with High Fraction of Activated Tuff and Fine Recycled Powders

Due to the high energy consumption and carbon emission of cement, using active supplementary materials in concrete is widely studied. In this paper, waste tuff micropowder and recycled fine powder (RFP) with maximum replaced ratio of 70% were investigated by alkali activation in ultra-high performance concrete (UHPC), the mechanical properties and hydration process were studied with different dosages and activators. The results show that the strength activity index of tuff and RFP both enhanced after calcium hydroxide activation. The alkali activator accelerated the hydration speed of tuff and RFP, also leaded to a low heat generated. The compressive strength was reduced with the replaced ratio of tuff and RFP, but the flexural strength was improved once tuff added and cured in appropriate temperature. In the cement-tuff/RFP system, calcium hydroxide accelerates cement hydration and activates part of the tuff and RFP to generate C-(A)-S-H to improve the strength activity of the mortar. Due to the low degree of tuff activation, less hydration products were formed, leaded to an increase in porosity. The RFP was effectively activated by calcium hydroxide, and the generated C-(A)-S-H and CaCO3 effectively refined the pore structure. The activation of RFP consumed limited mixing water, while the excessive CaCO3 generated by activation will cause the loss of UHPC strength.