Utilization of CO2-Cured Waste Cement Powder to Enhance the Properties and Microstructure of Cement Mortar and Paste

Abstract

Utilization of waste concrete has been become an urgent requirement for sustainable development due to the huge amounts of construction waste. The most promising strategy is to treat construction wastes by CO2 curing. This study investigates the properties and microstructure of cement mortar with CO2-cured waste cement powder (CWCP). The results showed the CWCP showed a rapid pH value decline from 12.9 to 10.2 within 12 h, the value was maintained at 8.4, and showed no obvious change after carbonation after 72 h. The CO2 uptake of CWCP increased rapidly in the first 2 h and reached 26.4% at 168 h. The C-S-H, Ca(OH)2, and traces of ettringite in waste cement powder (WCP) were transformed to calcium carbonate and silica gel after carbonation treatment. The flowability of cement mortar decreased with addition of WCP and CWCP content. The compressive strength of cement mortar with WCP decreased with replacement of cement beyond 10%, and incorporation of below 20% CWCP increased the compressive strength of mortar. In addition, the replacement of cement with 10%–20% CWCP or 10% WCP showed a lower water absorption compared with control cement mortar. The presence of calcium carbonate in the CWCP resulted in the formation of monocarbonate, which stabilized the ettringite and led to a denser microstructure compared with control cement paste.