AbstractThis study aims to compare the microstructural and mechanical properties of lightweight polymer concrete (LPC) and lightweight ordinary cement concrete (OCC) before and after exposure to a corrosive acidic environment. In this regard, LPC mixes were prepared with five different unsaturated polyester resin contents (15%–27%) and combinations of lightweight expanded clay aggregate (LECA) and natural river sand. Thereafter, specimens were exposed to sulfuric acid solution for up to 90 days. In addition to visual inspections, physical and mechanical properties [mass and volume changes, compression strength, and modulus of elasticity (MOE)], petrographic inspections, and scanning electron microscope investigations were carried out. Mechanical results showed that LPCs had by far better mechanical features compared with OCC, where at the optimum mix comprised of 24% polymer, reduction of 6% for compression, and 7% for MOE was observed, whereas these values were respectively 70% and 55% for OCC at the same conditions. Outcomes of microscopic observations indicated that increasing the polymer content resulted in strengthening the bond between binder and aggregates where pores were filled by resin. In addition, the impact of the acidic solution on the LPC mixes was less pronounced than for OCC mixes, and lamination of the matrix and affected polymer areas were reduced in LPC mixes with a resin content of 24% and 27%. After 90 days, a slight reduction in properties of LPC specimens and degradation was observed due to the reaction of the acid with the surface of LPC specimens.