Investigations on carbon-sequestration optimization of recycled coarse-aggregate and its effects on concrete performances

Recycled coarse-aggregate (RCA) derived from waste concrete can be re-used for concrete preparation, which is now limited due to its drawbacks such as micro-cracks, high-porosity, and reduced concrete strengths. To remedy these deficiencies, carbon-sequestration was employed to enhance RCA and the conditions were optimized, including the temperature (20 degrees C-40 degrees C), pressure (0.1 MPa-0.3 MPa), time (5 h-24 h) and initial water content (25%-75 %). There parameters were optimized based on the orthogonal test with scheme L-9(3(4)), which was evaluated based on RCA carbon-sequestration amount as well as its properties. With the optimized parameters, RCA was enhanced as CRCA. Both RCA and CRCA were utilized to fully replace natural coarse-aggregate (NCA) in concrete and concrete basic performances were investigated systematically, including strengths, shrinkage and medium transport properties. The results show that the effects of carbon-sequestration time and initial water-content of aggregate on the carbon-sequestration efficiency of RCA are the most significant. The optimized parameters were 30 degrees C temperature, 0.3 MPa pressure, 24 h time and 25 % initial water content with the maximum amount of carbon-sequestration at 8.42 %. The carbon-sequestration reduced the width of the recycled aggregate interfacial transition zone (ITZ) in concrete, increased the microhardness of ITZ and mortar, and decreased the porosity of CRCA. The CRCA elevated the compressive strength, splitting tensile strength and flexural strength of the concrete (28d) by 13.5 %, 8.5 % and 7.4 %, respectively. The drying shrinkage at 28d of RCA-concrete was decreased by 12.0 % when RCA was replaced with CRCA, however with the CRCA value still 16.2 % higher than that of NCA-concrete. Moreover, CRCA can decrease the medium transport coefficients of water, chloride, and gas in concrete when compared with the RCA-concrete while they are still greater than those of NCA-concrete.