Orthogonal Experimental Study on Mix Proportion Optimization and Mechanical Properties Comparison of Lightweight Aggregate Concrete Made with Recycled Glass Pumice and Ceramsite

To explore the feasibility of using recycled glass pumice (microcellular glass pumice aggregate, MGPA) as a substitute for traditional lightweight aggregates and to compare its mechanical performance with that of expanded clay ceramsite, this study systematically investigated the effects of water–cement ratio (0.40–0.46), cement content (330–360 kg/m3), fine MGPA replacement ratio (0–100%), and coarse MGPA replacement ratio (0–100%) on the dry density and compressive strength of lightweight aggregate concrete through an orthogonal experimental design. The results show that the bulk density of coarse MGPA (312 kg/m3) is only 46% of that of ceramsite (678 kg/m3), while its cylinder compressive strength (3.36 MPa) is slightly lower. The range analysis indicates that the dry density of MGPA concrete is primarily influenced by the replacement ratio of coarse aggregate, followed by fine aggregate replacement, water–cement ratio and cement content; the lowest dry density (1445 kg/m3) was obtained with a water–cement ratio of 0.46, cement content of 330 kg/m3, 100% replacement of coarse MGPA, and partial replacement of fine MGPA (mixture S19). For the 28-day compressive strength, the influencing factors rank as coarse aggregate replacement > water–cement ratio ≈ cement content > fine aggregate replacement. In comparison with the ceramsite concrete reference under the respective mix designs tested in this study, the optimal MGPA concrete exhibited only 4.6% higher dry density but achieved a significantly higher compressive strength of 40.0 MPa, compared with 20.5 MPa for the ceramsite mixture. The specific strength (strength/density ratio) of MGPA concrete is about 1.87 times that of ceramsite concrete. Both types of lightweight aggregate concrete reached 77–80% of their 28-day strength at 7 days. Overall, recycled glass pumice is a promising alternative to ceramsite for lightweight concrete, especially when both high strength and low weight are required for precast components, provided that its long-term durability (particularly ASR resistance) is verified in future studies.