Durability of waste concrete powder-based geopolymer reclaimed concrete under carbonization and freeze-thaw cycles

In order to realize the complete recycling of waste concrete, this paper used waste concrete powder (WCP), fly ash (FA) and granulated ground blast furnace slag (GGBS) as raw materials to prepared geopolymer, mixed with recycled waste concrete aggregates to prepare waste concrete powder-based geopolymer reclaimed concrete (WCPGRC). The WCPGRCs cured for 28 days were carried out carbonization test (group C-28d) and freeze-thaw cycle carbonization test (group DC-28d), and the WCPGRCs cured for 84 days were carried out carbonization test (group C-84d) to study their durability. The carbonation depth, uniaxial compressive strength (UCS) and dynamic elastic modulus (DEM) of three groups of WCPGRC under different carbonation times and different freeze thawed-carbonation times were tested respectively. The mineralogical phases and micromorphology characteristics of WCPGRC before and after carbonization and freeze-thawed-carbonization were characterized through XRD and SEM. The results show that: (1) The carbonation depth of WCPGRC in groups C-28d, DC-28d and C-84d all increased with the carbonation time and freeze-thawed-carbonation time, and the maximum carbonation depth was 4.4 mm, 4.9 mm and 4.1 mm, respectively. (2) The UCS of C-28d increased by 9.7% from 39.11 MP to 42.56 MPa with carbonization time; The C-84d increased by 4.6% from 40.51 MPa to 43.31 MPa; The DC-28d first increased and then decreased with the increase of freeze-thaw-carbonization time, and the maximum UCS was 40.51 MPa on the 28th day. (3) The DEM of C-28d increased by 16.5% from 17.22GPa to 20.06GPa with carbonization time; The C-84d increased by 7.6% from 20.05GPa to 21.57GPa; The DC-28d first increased and then slightly decreased with the increase of freeze-thaw-carbonization time, the maximum DEM was 18.41 GPa on the 14th day. (4) Prolonging curing time increased the UCS and DEM, while freeze-thaw decreased the UCS and DEM of WCPGRC. (5) After the carbonization of WCPGRC, new carbonate crystals and geopolymer gels were generated, continued to fill the voids and cracks, making the overall structure denser. However, after the freeze-thaw-carbonization, the freeze-thaw cycle inhibited the geopolymerization, and the freeze-thaw damage intensified the carbonization, which reduced the structural integrity of WCPGRC.