Re-cementation effects by carbonation and the pozzolanic reaction on LWAs produced by hydrated cement paste powder

This paper aims to evaluate lightweight aggregates produced with hydrated cement paste powder (HCP) using both carbonation and normal curing. The comparative study was conducted to figure out the effects of the portlandite amount in HCP I and HCP III on the aggregates during carbonation curing. The addition of up to 10 wt% silica fume to HCP I was attempted to make full use of the considerable amount of portlandite to prepare artificial aggregates under normal curing. The mechanical properties, reaction products and microstructure were analysed and the results show that the optimal carbonation periods for HCP I- and HCP III-type aggregates are different due to different amount of portlandite. HCP I-type aggregates can gain 3.14 MPa after 7-day carbonation and contain 35.60 wt% calcium carbonates. The remaining 13.95 wt% portlandite shows the enormous potential in elevating the strength and CO2 capture capacity jointly via the optimized carbonation curing method. On the contrary, HCP III-type aggregates gained 2.97 MPa after 1-day carbonation and further carbonation decomposed C–S–H and lead to the formation of calcite and amorphous silica gel with significantly elevated specific surface area (from 10.69 m2/g to 42.96 m2/g). Additionally, the individual strength development of the prepared aggregates containing silica fume benefits from the sufficiently available portlandite due to the formation of secondary C–S–H, obtaining 2.39 MPa after 28-day normal curing.