LAUSR

Abstract Every year, about a thousand million tires reach the end of their service life, more than half of which are disposed of in landfills. The waste tire rubber has a great potential for application as aggregate phase in production of lightweight concrete/mortar. This study is aimed at evaluating the effects of using crumb rubber (CR) as fine aggregate at replacement ratios of 0–60% (by volume) in alkali-activated slag mortars. Furthermore, polypropylene fibre (PPF) was used at 0.5% and 1% of volume of the mix to enhance the properties of mortar mixes such as flexural strength and shrinkage behaviour. The compressive strength, flexural strength, water absorption, thermal conductivity, drying shrinkage, and microstructural characteristics, as well as the compressive stress–strain relationship of the mortars, were evaluated. Based on the test results, the use of CR significantly reduced the mechanical strength (up to 90%) and increased the water absorption and drying shrinkage by up to 70% and 57% respectively over that of the control mix. However, mix containing 60% CR met the thermal conductivity criterion for thermal insulating concrete. The use of PPF led to significant enhancements in the mechanical strength, especially at an early age, where up to 59% increase in the 1-day flexural strength was achieved. Furthermore, PPF effectively enhanced the shrinkage behaviour of mixes and led to up to 65% reduction in drying shrinkage of the control mortar. Also, the drying shrinkage of mix containing 60% CR and 1% PPF was lower than the control mix, which suggests that such combination can yield a thermal insulating material with superior shrinkage performance. The observations were proved by SEM examination where enclosed pores were detected in the vicinity area of CR and PPF, and fibre agglomeration was more pronounced when CR and PPF were combined. Finally, the ecological and economic analysis of the mortars showed that rubberized mixes were eco-friendlier and more cost-efficient. The AAS mortar incorporating 40–60% CR and 1% PPF has potential application for low-to-medium strength lightweight thermal insulating materials.