Mechanical and microstructural properties of glass powder-modified recycled brick-concrete aggregate concrete

In order to achieve a better recycling of construction waste and explore the mechanical properties of concrete after incorporating multiple types of construction waste, this paper uses discarded concrete as recycled concrete aggregate (RCA), waste clay bricks as recycled fine brick aggregate (RFBA), and waste glass powder (GP) as an auxiliary cementitious material. Taking fully into account the modification effect of GP on the mortar matrix, a new type of green recycled concrete, namely GP modified Recycled Brick-Concrete Aggregate Concrete (GBCC), is prepared. Through a four-factor, four-level orthogonal experimental design combined with microstructural analyses (XRD, SEM, EDS, MIP), the mechanical properties and synergistic mechanisms of GBCC were systematically investigated. Results demonstrate that under the optimal mix ratio (15 % RCA, 40 % RFBA, 10 % GP, and water-binder ratio of 0.48), the 28-day cube compressive strength of GBCC reaches 39.2 MPa (equivalent to 100 % of C30 concrete), while the axial compressive strength and splitting tensile strength are 29.8 MPa and 2.72 MPa, respectively, meeting the design requirements of C30 concrete. Notably, at 40 % RFBA replacement, GBCC achieves over 90 % of the compressive strength of conventional C30 concrete. Microscopic analysis indicated that C-(A)-S-H gels formed by GP and RFBA reduced the total porosity by approximately 18 % (MIP test) and increased the proportion of harmless pores (<20 nm) to 25∼28 %, effectively refining the pore structure. SEM-EDS observations revealed dense gel filling at the interfacial transition zone, with the Ca/Si ratio of the gel reduced to 0.29, significantly enhancing interfacial bonding. This study pioneers the efficient co-utilization of RCA, RFBA, and GP, and for the first time integrates SEM-EDS microstructural characterization with molecular chemical analysis to elucidate the formation mechanisms of gels. ● A new type of green concrete (GBCC) was proposed and validated. ● The mechanical properties and microscopic mechanism of GBCC were researched. ● Pozzolanic activity of RFBA and GP in recycled concrete was investigated in depth. ● An innovative explanation of the mechanism of the production of slender fibrous gels—C-(A)-S-H was proposed.