Modification of recycled aggregate using calcium carbonate formed by homogeneous precipitation and its application in concrete

The recycled aggregate (RA) from construction waste needs to be modified to promote its high-value recycling and utilization. In this study, an innovative and efficient approach to modify the quality of RA was developed, namely the in-situ homogeneous precipitation method of calcium carbonate. The principle is that the mixed solution composed of urea (as a uniform precipitant) and calcium acetate (as a source of calcium ions) first enters the internal pores or defects of RA through the impregnation process. After water bath heating treatment, urea in the mixed solution will undergo hydrolysis to uniformly produce carbonate ions, which react with the calcium ions in the mixed solution to uniformly form calcium carbonate precipitates, gradually filling the internal pores or defects of RA in situ. The investigation was divided into two parts. Firstly, using crushing index, water absorption and apparent density as evaluation indicators, the modification effect of RA was studied under different mixed solution concentrations, modification temperatures, and reaction times. The treated recycled aggregate (TRA) was analyzed through SEM/EDS, XRD, and TG-DTG. On this basis, the modified recycled aggregate under the optimal modification process was replaced with natural aggregate (30 %, 50 %, 70 %, and 100 % replacement ratio) to prepare recycled concrete, and its workability, mechanical properties, and durability were evaluated. The results indicated that when the reaction temperature was 80 °C, the reaction time was 5d, and the solution concentration was 1.0 mol/L, the RA achieved the best modification effect. Under these conditions, the water absorption and crushing index of RA decreased by 32.0 % and 29.8 %, respectively. Compared with the untreated RA concrete, the TRA concrete with a 100 % replacement ratio had a 50 % increase in slump, a 23.9 % increase in compressive strength and a 31.1 % increase in splitting tensile strength, a 45.1 % decrease in capillary water absorption coefficient and a 40.2 % decrease in electrical flux. The microscopic results indicated that the needle like aragonite calcium carbonate whisker was formed in the urea-calcium acetate mixed solution after water bath heating treatment, which can deposit on the surface and internal pores of RA, thereby improving the quality of RA and enhancing the properties of concrete.