Thermal, Mechanical, and Microstructural Properties of Novel Light Expanded Clay Aggregate (LECA)-Based Geopolymer Concretes

The construction sector’s reliance on traditional cement significantly contributes to CO2 emissions, underscoring the urgent need for sustainable alternatives. This study investigates fine (0–4 mm), rounded, uncoated, porous-surfaced lightweight expanded clay aggregate (LECA)-based geopolymers, which combine the low-carbon benefits of geopolymers with LECA’s lightweight and insulating properties. Geopolymers were synthesized using lignite-rich fly ash with varying ratios of LECA to aggregate. Mechanical testing revealed that the reference mixture without LECA (REF-GEO) achieved the highest compressive strength of 37.89 ± 0.75 MPa and flexural strength of 7.62 ± 0.11 MPa, while complete substitution of the aggregate with LECA (LECA-100%) reduced the compressive strength to 17.31 ± 0.88 MPa and flexural strength to 3.41 ± 0.11 MPa. The density of the samples decreased from 2.06 g/cm3 for REF-GEO to 1.31 g/cm3 for LECA-100%, and thermal conductivity dropped significantly from 1.15 ± 0.07 W/mK to 0.38 ± 0.01 W/mK. Microstructural analysis using XRD and SEM-EDX highlighted changes in the material’s internal structure and the increase in porosity with higher LECA content. Water vapor permeability increases over time, particularly in samples with higher LECA content. These findings suggest that LECA-based geopolymers are suitable for low-load or non-structural elements. They are ideal for sustainable, energy-efficient construction that requires lightweight, insulating, and breathable materials.