Effect of carbonated recycled concrete fines on early-age engineering properties, hydration and microstructure of cement paste under high substitution level and low water-to-binder ratio

The application of carbonated recycled concrete fines (CRCF) as supplementary cementitious material has emerged as a solution that offers efficient use of construction waste and excellent CO2 capture potential. This study explores the effect of CRCF (up to 40 % replacement of cement) on early-age engineering properties, hydration and microstructure of cement paste under the low water-to-binder (w/b) ratio of 0.20, in comparison with recycled concrete fines (RCF) blended cement paste. The results suggest that although the incorporation of both RCF and CRCF reduces the mechanical properties, the blended cement paste with 20-40 % CRCF shows excellent compressive strengths reaching over 85 % that of plain cement paste. CRCF has a significant promotion effect on the acceleration period and the peak value of heat evolution rate as compared with RCF, due to the C-S-H nucleation sites provided by calcite. Moreover, the gel pore of blended cement paste with 20 % CRCF at 7 d increases by about 15 % due to the dense structure resulting from the vertical growth of hydrates on the CRCF surface. Overall, CRCF can be utilised as an excellent supplementary cementitious material even under a high substitution level of 40 % and low w/b ratio of 0.20 with acceptable mechanical and microstructural properties of blended cement paste.