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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]

The use of recycled aggregate obtained from the crushing of construction solid waste can not only save natural sand and gravel resources, but also promote the recycling of construction solid waste. The quality of recycled aggregate determines the performance of recycled aggregate concrete. There are many factors affecting the quality of recycled aggregate, and the strength of parent concrete is one of the important influencing factors. The article takes strength of parent concrete as the research object, and summarizes its influence law on the properties of recycled aggregate and recycled aggregate concrete. Different strengths of parent concrete affect the quality and content of recycled aggregate adhesive mortar, which in turn affects the properties of recycled aggregate and recycled aggregate concrete. When the strength of parent concrete increases, the quality and content of adhesive mortar are improved. While the quality of adhesive mortar is improved, the properties of recycled aggregate and recycled aggregate concrete are strengthened. The increase in the content of adhesive mortar deteriorates the physical properties of recycled aggregate, increases the porosity and water absorption of recycled aggregate, but strengthens the internal curing effect of concrete, which contributes to the mechanical properties of recycled aggregate concrete. Using microscopic analysis, it is found that different strength of parent concrete make a difference in the content of hydration products in the interfacial transition zone of recycled aggregate concrete, which has an impact on the mechanical properties of concrete. The article provides a theoretical basis for further grading and utilization of recycled aggregate. [ABSTRACT FROM AUTHOR]

As a material for road deicing and snow removal, electrically conductive concrete (ECON) can convert electrical energy into thermal energy to automatically melt ice and snow, avoiding manual and mechanical operations. However, to achieve ideal electrical conductivity, the content of single-doped conductive fillers in ECON needs to be high, which leads to poor workability of ECON and agglomeration of conductive fillers. Combing conductive fillers with different scales helps reduce the content of single-doped conductive fillers in ECON. This study prepared composite conductive concrete using multilayer reduced graphene oxide (RGO), carbon fiber, and steel fiber as conductive fillers. The effect of multilayer RGO on composite conductive concrete's compressive strength, flexural strength, and electrical conductivity was investigated. Besides, the mechanism of RGO on composite conductive concrete's conductive network was analyzed by scanning electron microscope (SEM). Results showed that the 28-day mechanical properties and electrical conductivity of composite conductive concrete first increased and then decreased. Composite conductive concrete containing RGO with 0.4% cement weight (RGO-0.4) exhibited better mechanical and electrical properties. The improvement of the performance of composite conductive concrete was attributed to the filling effect of RGO on the gaps of concrete matrix and the wrapping effect of graphene flocs on fibrous conductive fillers because these effects increased the compactness of the matrix and conductive contact area, respectively. Also, a snow melting test was carried out using an RGO-0.4 slab at an ambient temperature of −15°C. The snow melting efficiency was improved, showing that 20-cm-thick snow melted in 1.2 h. In conclusion, the hybrid addition of RGO and fibrous conductive fillers has positive effects on ECON, providing a new way to develop ECON. [ABSTRACT FROM AUTHOR]

This work assessed the effect of compressive fatigue on sulfate ion diffusion in standard-cured and steam-cured concrete containing slag. After compressive fatigue, the slag concretes were immersed in 5% sodium sulfate solution for 18 months. The sulfate ion content in concrete was measured and the sulfate ion effective diffusion coefficient was calculated. The results showed that compressive fatigue accelerated sulfate ion diffusion in concrete, and the acceleration was more prominent in steam-cured concrete than standard-cured concrete. During sulfate attack, the sulfate ion content in the same layer of standard-cured fatigued concrete grew with an increase in stress level. Massive ettringite with large size was generated in steam-cured fatigued concrete during sulfate attack. In addition, a sulfate ion diffusion model of fatigued concrete was established. Based on this model, the measured values agreed well with the simulated distribution curve of sulfate ions in fatigued concrete. [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]

In this study, the shrinkage performance of recycled aggregate thermal insulation concrete (RATIC) with added glazed hollow beads (GHB) was investigated and a time-dependent shrinkage model was proposed. Two types of recycled fine aggregate (RFA) were used to replace natural fine aggregate in RATIC: RFA from waste concrete (RFA1) and waste clay brick (RFA2). Besides, the mechanical properties and thermal insulation performance of RATIC were also studied. Results showed that the pozzolanic reaction caused by RFA2 effectively improved the mechanical properties of RATIC; 75% was the optimal replacement ratio of RATIC prepared by RFA2. Added RFA decreased the thermal conductivity of thermal insulation concrete (TIC). The total shrinkage strain of RATIC increased with the increase of the replacement ratio of RFA. The 150d total shrinkage of RATIC prepared by RFA1 was 1.46 times that of TIC and the 150d total shrinkage of RATIC prepared by RFA2 was 1.23 times. The addition of GHBs led to the increase of early total shrinkage strain of concrete. Under the combined action of the higher elastic modulus of RFA2 and the pozzolanic components contained in RFA2, the total shrinkage strain of RATIC prepared by RFA2 with the same replacement ratio was smaller than that of RATIC prepared by RFA1. For example, the final total shrinkage strain of RATIC prepared by RFA2 at 100% replacement ratio was about 18.6% less than that of RATIC prepared by RFA1. A time-dependent shrinkage model considering the influence of the elastic modulus of RFA and the addition of GHB on the total shrinkage of RATIC was proposed and validated by the experimental results. [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]