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This study used recycled aggregate (RA) and mineral additions to design large-pore sandy recycled concrete (LPSRC) using a design porosity (DP) of 25 %, 20 %, and 15 %. Thus, the related effects of recycled coarse aggregate (RCA), recycled fine aggregate (RFA) and fly ash (FA) on the compressive strength, frost resistance (using slow and fast freezing methods), and sulfate attack properties of LPSRC were systematically studied. Moreover, to reduce the error between the design and theoretical porosity values, the bulk density ratio adjustment method was employed to design the theoretical porosity. The results of binarization analysis demonstrated that the deviation of the actual porosity caused by the sidewall effect was ∼2 %, which should be considered during DP design and performance evaluation. The compressive strength of the LPSRC after 56 curing days increased by 35.9 % on average as the DP was reduced from 25 % to 15 %. In a freeze–thaw cycle test, the slow freezing method was more accurate for evaluating the frost resistance of the LPSRC. Under the same conditions, the mass loss rate of the LPSRC calculated using the slow and fast freezing methods was 3.2 % and 7.62 %, respectively. This distinction is pivotal for establishing reliable frost resistance criteria for LPSRC in cold-weather applications, complementing previous research in the field. In terms of sulfate deterioration resistance, compressive strength corrosion factors decreased with increasing DP. Additionally, damage was mainly concentrated at the contact interface between RCA and the cement matrix, while the aggregate itself showed relatively little damage, suggesting RCA application has a positive effect on improving preparation of the LPSRC. Specifically, as DP increased from 15 % to 25 %, the average reduction in the compressive strength corrosion factors for LPSRC after 30 and 60 times of dry–wet cycle were 9.64 % and 8.37 %, respectively, illustrating a clear correlation between higher DP and reduced sulfate resistance. This study contributed to the LPSRC design and production using RCA, RCF and FA, that positively impacts to the sustainable development of the construction industry and helps to evaluate the effectiveness of RA in macroporous concrete. Moreover, the strategic implementation of design porosity has been shown to optimize the structural integrity and performance of the concrete, achieving a balance between strength and workability, which is crucial for the concrete's durability and its ability to adapt to various environmental conditions.

Although the energy structure has been gradually enriched in recent years, China, as the world’s largest producer and consumer of coal, still accounts for more than 50% of resources. Therefore, in this paper, the high carbon fly ash raw ash (HCFARA) was mechanically activated, and the flotation was treated to obtain decarburized fly ash (DFA) with a loss on ignition of 1.18%, water demand ratio of 101% and strength activity coefficient of 74.5%. With the exception of for water demand ratio, the rest meet the national requirements for the use of Class I fly ash. The foam concrete was prepared by partially replacing cement with decarburized fly ash in different proportions and at different water-to-cement ratios (0.3, 0.4 and 0.5) in order to study its dry density, porosity, compressive strength and thermal conductivity. The results show that the porosity of foam concrete grows with the increase in water-to-cement ratio and fly ash replacement rate. In addition, the dry density and thermal conductivity of the prepared foam concrete can meet the requirements of A05 grade foam concrete (Density less than 500 kg/m3, thermal conductivity less than 0.12 W/(m·k)). The compressive strength can meet the requirements of C0.3 grade foam concrete (Minimum use standard).

The CO2 emissions from the cement industry and the production of waste wood chips are increasing with the rapid growth of the construction industry. In order to develop a green environmental protection building material with low thermal conductivity and up to standard mechanical properties, in this study, pine waste wood chips were mixed into cement-based materials as fine aggregate, and three different kinds of cementitious binders were used, including sulfur aluminate cement (SAC), ordinary Portland cement (OPC), and granulated blast furnace slag (GBFS), to prepare a recycled light cementitious composite material. The mechanical, thermal conductivity, shrinkage, water absorption, and pore structure of a wood chip light cementitious composite material were studied by changing the Ch/B (the mass ratio of wood chip to binder). The results showed that the strength, dry density, and thermal conductivity of the specimens decreased significantly with the increase in the Ch/B, while the shrinkage, water absorption, and pore size increased with the increase in the Ch/B. By comparing three different kinds of cementitious binders, the dry density of the material prepared with OPC was 942 kg/m3, the compressive strength of the material prepared with SAC was 13.5 MPa, and the thermal conductivity of the material prepared with slag was the lowest at 0.15 W/m/K. From the perspective of low-cost and low-carbon emissions, it was determined that the best way to prepare a light cementitious composite with waste wood chips is to use granulated blast furnace slag (GBFS) as the cementitious binder.

Poor mechanical properties and durability of recycled aggregate concrete (RAC) hinder its application in the construction field. In this study, pre-wetted recycled coarse aggregate was used as the internal curing material for prepared RAC with low water-to-binder ratio (W/B), aiming to improve the mechanical properties and durability. The results show that the workability decreases with increasing contents of pre-wetted recycled coarse aggregate. The variation in compressive strength of RAC with different contents of pre-wetted recycled coarse aggregate is obvious within 28 d. After 28 d, the effect of internal curing of pre-wetted recycled coarse aggregate starts to occur, causing a sustained increase in compressive strength. The sealed concrete with 50% and 75% pre-wetted recycled coarse aggregate contents presents the highest compressive strength and better internal curing effect. The pre-wetted recycled coarse aggregate decreases the relative humidity inside the concrete and effectively inhibits the development of shrinkage in the early stages. The RAC with pre-wetted recycled coarse aggregate presents little effect on the drying shrinkage. Additionally, the electric flux of RAC cured for 28 d increases from 561C to 1001C, which presents good resistance to chloride permeation. Microscopic tests indicate that the incorporation of pre-wetted recycled coarse aggregate is beneficial to the improvements of internal structure of RAC.

In this paper, the properties of sulphoaluminate based recycled concrete, with high workability and low and medium strength, prepared by recycled powder (RP), recycled coarse aggregate (RCA), and high belite sulphoaluminate cement (HBSAC), were systematically studied. Under the condition of a water binder ratio of 0.45, sulphoaluminate based recycled concrete, with different mix proportions, was prepared by replacing sulphoaluminate cement with RP and natural coarse aggregate (NCA) with RCA. The workability, mechanical properties, durability, and hydration products of the prepared concrete were analyzed. The results showed that when RP and RCA were used together, the workability of recycled concrete could fully meet the pumping demand in actual construction. When the mass replacement rate of RP was less than 30% and that of RCA was less than 20%, the strength of recycled concrete could completely reach the design strength grade, while those that did not reach the design strength grade could reach the next grade. The durability performance was also good.