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Geopolymer concrete (GPC) represents an innovative green and low-carbon construction material, offering a viable alternative to ordinary Portland cement concrete (OPC) in building applications. However, existing studies tend to overlook the recyclability aspect of GPC for future use. Various structural applications necessitate the use of concrete with distinct strength characteristics. The recyclability of the parent concrete is influenced by these varying strengths. This study examined the recycling potential of GPC across a spectrum of strength grades (40, 60, 80, and 100 MPa, marked as C40, C60, C80, and C100) when subjected to freeze–thaw conditions. Recycling 5–16 mm recycled geopolymer coarse aggregate (RGAs) from GPC prepared from 5 to 16 mm natural coarse aggregates (NAs). The cementitious material comprised 60% metakaolin and 40% slag, with natural gravel serving as the NAs, and the alkali activator consisting of sodium hydroxide solution and sodium silicate solution. The strength of the GPC was modulated by altering the Na/Al ratio. After 350 freeze–thaw cycles, the GPC specimens underwent crushing, washing, and sieving to produce RGAs. Subsequently, their physical properties (apparent density, water absorption, crushing index, and attached mortar content and microstructure (microhardness, SEM, and XRD) were thoroughly examined. The findings indicated that GPC with strength grades of C100, C80, and C60 were capable of enduring 350 freeze–thaw cycles, in contrast to C40, which did not withstand these conditions. RGAs derived from GPC of strength grades C100 and C80 complied with the criteria for Class II recycled aggregates, whereas RGAs produced from GPC of strength grade C60 aligned with the Class III level. A higher-strength grade in the parent concrete correlated with enhanced performance characteristics in the resulting recycled aggregates. [ABSTRACT FROM AUTHOR]

Multiple recycling of waste concrete has attracted widespread attention. This study presented the carbonation behavior of repeatedly recycled aggregate concrete (RRAC) used in a micro-frozen region. The effects of freeze–thaw cycles on the carbonation depth of three generations of RRAC with 25%, 75%, and 100% of replacement rate were evaluated. All RRAC specimens after different numbers of freeze–thaw cycles were rapidly carbonated for 28 d indoors to test the carbonation resistance of concrete. The results suggested that the carbonation depth of RRAC subjected to freeze–thaw cycles is higher than that in the non-freeze–thaw condition. This is because the freeze–thaw damages cause the internal structure of RRAC to become porous and and prone to cracking, thus providing convenient channels for CO2 to react with the alkali in the cementitious materials. With the growth of replacement rate or recycling number, RRAC reveals serious freeze–thaw damage and inferior carbonation resistance, which is due to the continuous deterioration repeatedly recycled concrete aggregate (RRCA) quality. However, when the replacement rate was 25%, the carbonation depth for the third generation of RAC was comparable to the second generation of RAC at a 75% replacement rate, and even the first generation of 100% RAC. To ensure better carbonation resistance durability of multiple recycling RAC, the low replacement rate of RRCA should be considered. For the third generation of RAC with the 100% replacement rate, its highest carbonation depth after freeze–thaw cycles was 9.16 mm, which still met the design requirements for structural use in a micro-frozen region. This indicates that it is feasible for three generations of RRAC to be used in the micro-frozen environment and that RRAC has great engineering application potential and promotional value. [ABSTRACT FROM AUTHOR]

• It is feasible to recycle waste RAC in harsh environments by carbonation treatment. • Accelerated carbonation shows excellent repair effect on the damage of RRCA. • Producing C-RRCA per ton can capture 38.3 kg CO 2 with great environmental benefits. • RRAC with C-RRCA (content ≥ 6 %) shows good chloride ion permeability resistance. The recycling of waste recycled concrete (RC) has become a focus of many scholars. But the repeatedly recycling of RC used in harsh environments is still a difficult issue, due to the poorer quality of repeatedly recycled aggregate. This paper studied the repair effect of accelerated carbonation treatment on repeatedly recycled coarse aggregate (RRCA), which was collected from the failed parent recycled concrete via coupling action of high stress and freeze-thaw (F-T) cycles. Then the properties of carbonated repeatedly recycled aggregate concrete (C-RRAC) containing 0 %, 25 %, 50 %, 75 % and 100 % carbonated RRCA (C-RRCA) were evaluated. And the recycling potential of C-RRCA was explored, finally. The results suggested that accelerated carbonation treatment had excellent ability on repairing coupling damage of high stress and F-T in RRCA. Compared with RRCA, the apparent density of C-RRCA increased by 5.5 %, and water absorption, crushing value and soundness (mass loss) decreased by 26.6 %, 23.7 % and 19.9 %, respectively. The quality of C-RRCA was improved from type III to type II. With the growth of C-RRCA replacement rate, the water absorption, porosity, and chloride ion permeability coefficient of C-RRAC decreased by 34.4 %, 48 % and 55.6 %, respectively, and the compressive strength increased by 34.2 %. In addition, the C-RRAC could satisfied requirement of 100 years design service life of concrete structures in the E-type chloride environment, totally. According to the numerical fitting analysis, the minimum content of C-RRCA is 6 % under the condition that the compressive strength and chloride ion permeability of recycled aggregate concrete are met. It is feasible to recycle waste or even failed RC in high stress and F-T environment so as to reuse it in chloride environment, which provides a data support for effective utilization of waste RC under freeze-thaw environment. [ABSTRACT FROM AUTHOR]

The existing researches on the recycled aggregate concrete (RAC) mainly focus on the recycling of waste concrete used in atmospheric environment, while the ones on the recycling of waste concrete subjected to complex environments are seldom reported. Reusing waste concrete in freeze-thaw (F-T) environment relates to the multi-generation recycling system of "high performance parent concrete-waste concrete-recycled aggregates (RAs)-next generation of recycled high-performance concrete (RHPC)-next generation of RAs". The efficient operation and sustainability of such recycling system depends on not only the performances of concrete, but also environmental load and economic benefits. The optimal balance between performance, environmental load and economic benefits for recycling waste concrete in F-T environment is generally affected by the performance of parent concrete and quality of RAs in the recycling system. Based on the theories of the emergy analysis and life cycle cost of structure, a sustainable design theory model of this innovative recycling system is proposed. In this model, high performance concrete (HPC) or RHPC is chosen as parent concrete and the performances of middle and low-quality RAs are improved by carbonation treatment method to gain high-quality RAs. The feasibility of the theoretical model is verified by the emergy analysis of the data of three-generation RAC, which provides a theoretical basis for the frost resistance quantitative design of multi-generation recycled concrete. [ABSTRACT FROM AUTHOR]

• The feasibility of recycling waste RAC in freeze–thaw environment is evaluated. • Freeze-thaw damage of RAC adversely affect the properties of second-generation CRA. • Freeze-thaw damage threshold of waste RAC as CRA for structural concrete is 0.36. • RAC with freeze–thaw failure cannot reuse as CRA for structure concrete. • Adhesive mortar and ITZ are the main carriers of freeze–thaw damage transfer. The reuse of waste recycled aggregate concrete (RAC) from freeze-thaw (FT) environment as coarse recycle aggregate (CRA) for structural concrete can effectively improve the application of second-generation CRA. This paper evaluated the reuse potential of waste RAC suffered different FT cycles as CRA for structural concrete by FT damage. C40 RAC specimens with 100% replacement rate were produced and the FT damage through a coupled action of FT cycles and 35% bending load was assessed. The RAC were crushed into second-generation CRA after every 100 FT cycles until FT failure. Results showed that the evolution process of FT damage on physical properties of CRA can be divided into four stages: reinforcement stage, transition stage, weakening stage and failure stage. In the evolution process, the apparent density of second-generation CRA decreased with FT cycles, while the crushing value, water absorption and soundness (mass loss) increased. The particle size of second-generation CRA gradually decreased with the accumulation of FT damage, and the screening curve of the CRA after 400 FT cycles cannot meet the requirements of continuous particle size. Relative dynamic modulus of elasticity (RDME) was more sensitive than mass loss rate in assessing the frost resistance durability of RAC. The FT damage threshold of RAC based on the properties requirements of type II and type III CRA was 0.085 and 0.359, respectively, indicating that the FT damage threshold of waste RAC crushing as CRA for structural concrete was 0.36, and the corresponding RDME was 64% rather than 60%. The microscopic analysis showed that the weakening of adhesive mortar and ITZ was the fundamental reason for the decrease of physical properties of the second-generation CRA. The CRA obtained from the RAC with FT failure cannot be reuse as aggregates for structure concrete. [ABSTRACT FROM AUTHOR]