Effect of content, gradation, and saturation of expanded waste glass aggregates on physical and mechanical properties of a fly-ash geopolymeric mortar

With low density, high mechanical strength, and high frost resistance, expanded waste glass aggregates (EWGA) can be a promising ecological solution as lightweight aggregates in concrete and mortars. Using a geopolimeric matrix, instead of OPC, could enhance material durability and environmental sustainability by employing waste materials in both the aggregates and the matrix. In the present paper, EWGAs were added to fly ash geopolymeric mortars. The effects of aggregate parameters, namely saturation degree, amount, and gradation, were analyzed. All these factors resulted in varying fluid absorption by aggregates during mortar mixing, leading to different final properties of the materials. The impact of liquid absorption by LWA has been extensively studied in OPC concrete and mortars, but limited research exists regarding geopolymers. This work aims to expand the knowledge of EWGA behavior in geopolymers, proposing strategies for mix design optimization with the ultimate scope of promoting the employment of waste materials in building technology. UPV, weight monitoring, and morphological analysis by SEM were performed to investigate the effects of EWGA parameters on the progress of geopolymerization reactions. Key physical and mechanical properties—including bulk density, total porosity, UPV, compressive/flexural strengths, and thermal conductivity—were evaluated to establish the relationship between EWGA characteristics and the overall performance of the geopolymeric composites. Experimental results highlighted that prewetting aggregates allowed for reducing the alkaline solution (AAS) content in the mortars and, at the same time, for obtaining the same mechanical performances as dry aggregate mortars, reducing density (13 %), thermal conductivity (20 %), and polymerization time. Reducing the content of the alkaline solution in the mix is noteworthy, as it is costly and environmentally impactful. The experimental results also evidenced the critical role of the fine fraction of EWGA in both the geopolymerization process and final mortar properties (lowering of flexural and compressive strength of 40 % and 10 %, respectively, in absence of fines), and the positive effects of increasing the content of EWGA on the mechanical properties (strength increase of 25 % in bending and 10 % in compression) and thermal conductivity (6 % reduction).