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This study investigates the effect of a novel negative temperature early-strength agent (NEA) in enhancing concrete performance under varying curing temperatures, with a discussion on the underlying mechanism through analysis of hydration products and microstructure.The results indicate NEA markedly accelerates early cement hydration, reduces setting time, and enhances compressive strength. Moreover, the efficacy of NEA is more pronounced at lower temperatures. With an optimal dosage of 3 %, the incorporation of NEA leads to a remarkable 285.9 % increase in compressive strength at 28 days under −5°C curing temperature. Despite accelerating cement hydration can induce increased chemical shrinkage, NEA effectively mitigates early freezing expansion under negative temperature and significantly diminishes chloride ion permeability. Higher temperatures facilitate early cement hydration but also compromise long-term hydration continuity. Consequently, the highest hydration degree and loweast porosity are observed at 28 days under 5°C curing temperature. NEA's air-entraining component introduces numerous superfine pores into the paste, augmenting early pore volume while optimizing pore structure. As the early-strength component fosters hydration, these pores undergo further filling and refinement, ultimately reducing the total pore volume at 28 days. This study provides insights into optimizing concrete performance in cold environments.

Magnesium oxysulfate (MOS) cement, with its low carbon footprint, faces challenges in water resistance and volume stability. This study examines how fly ash (FA) and ground granulated blast-furnace slag (GGBS) improve MOS cement properties. Findings show that adding FA and GGBS significantly enhances the mechanical properties, water resistance, and volume stability of MOS cement. An optimal admixture content of 45 % and an FA to GGBS ratio of 2:1 increase compressive and flexural strengths to 71.8 MPa and 16.5 MPa. The study also reveals that FA and GGBS do not introduce new hydration crystalline phases and limited secondary reactions occur due to the low alkalinity of MOS cement. The improvement of MOS cement is primarily attributed to the micro-aggregate effect, which optimizes the pore structure by reducing harmful pores and total porosity. The findings suggest that FA and GGBS are effective in enhancing the performance of MOS cement.

Numerous clay bricks produced by the demolition of old buildings occupy large amounts of valuable land resources and cause serious environmental pollution in the process of transportation, landfill and storage. The objective of this work is to evaluate the feasibility of using waste clay bricks as fine aggregates (CBFA) to prepare sustainable plastering mortar meeting the requirements of GB/T 25181-2019. The effects of CBFA on fresh and hardened properties of plastering mortar have been studied by water requirement, water retention, consistency loss rate, setting time, compressive strength, tensile bonding strength and drying shrinkage. Furthermore, the microstructure of plastering mortar has also been studied to understand the mechanism of CBFA on plastering mortar. The results demonstrate that CBFA can improve water retention of plastering mortar at all dosages (30%, 40%, 50% and 60%), and enhance compressive strength as well as tensile bond strength at 30% dosage. In addition, CBFA can refine the pore diameter and optimize interfacial transition zone (between cement paste and CBFA) of plastering mortar. Nevertheless, CBFA can increase consistency loss rate, drying shrinkage and porosity, and shorten setting time of plastering mortar. In summary, the maximum amount of CBFA in the plastering mortar that can meet the standard requirements is 50% of the total amount of fine aggregates. The utilization of CBFA to prepare plastering mortar not only expands the application range of waste clay bricks and reduces the consumption of natural river sand, but also produces economical green plastering mortar and benefits the sustainable development of building materials.