Properties and CO2-curing enhancement of cement-based materials containing various sources of waste hardened cement paste powder

Recycling hardened cement paste powder (HCPP) as an alternative binder is a high-efficiency approach to reducing the amount of hardened cement paste waste. In this work, the performance of the cement-based ma-terials doped with HCPP from various sources is investigated. The results show that the HCPP particle is irregular, and that a CO2-curing treatment makes HCPP denser through changing unstable CH into stable CaCO3 and silica gel. Substituting HCPP for 50% cement results in a 13.3% increase in the drying shrinkage, and the blended HCPP is unfavorable to the mechanical strength and transport properties. For example, the 28-d compres-sive strength and sorptivity coefficient of recycled mortar including 30% HCPP are approximately 27.6% lower and 55.3% higher than those of plain mortar without HCPP. Original hardened cement paste with low w/c ratio or blended with SCMs (fly ash, silica fume and mineral powder) benefits the properties of preparing HCPP and its mortar, but cement grade in original hardened cement paste has little influence on the properties of preparing HCPP and its mortar. Employing CO2-curing treatment on HCPP improves the properties of its prepared cement -based materials; for instance, the 28-d compressive strength and sorptivity coefficient of recycled mortar with 30% carbonated HCPP are 12.6% higher and 34.8% lower compared with recycled mortar including 30% un-carbonated HCPP. Recycled mortar including 30% carbonated HCPP, as well as incorporating 30% HCPP from SCM-blended paste, shows water transport properties similar to plain mortar.