Reducing the emission of climate-altering substances in cementitious materials. A comparison between alkali-activated materials and Portland cement-based composites incorporating recycled tire rubber

Low carbon or near-zero carbon concrete technology is in line with the pillars of sustainable development, where industrial waste or low-carbon binders can reduce or eliminate consumption of Portland cement and natural resources, leading to less environmental pollution. This work presents an experimental study on the comparison between alkali-activated materials (also recognized as geopolymers) and a traditional cementitious matrix (Portland cement) incorporated with rubber particles, deriving from end-of-life tires, as replacement of raw mineral aggregates. To explore the potential of rubber-geopolymer compounds, an experimental comparative analysis with rubber-Portland mortars was performed. Initial investigations (microstructural/compositional analysis, porosity and water absorption measurements, and mechanical tests) were conducted on rubberized samples obtained by varying the binder, the sand-rubber replacement ratio (0 vol%, 50 vol%, and 100 vol%) and the rubber particle size (0–1 mm rubber fine aggregate and 1–3 mm rubber granules). The results revealed a greater compatibility of the alkali-activated matrix with tire rubber aggregates, resulting in better performance in terms of interfacial adhesion, reduced porosity rate, flexural strength, and toughness. However, compressive strength results showed a weaker mechanical performance of rubber-geopolymer mixes compared to Portland counterparts. As also verified by Si/Al elemental analysis, the structural quality and mechanical development of the geopolymer matrix was strongly influenced by the removal of sand as a Si-source. The potential embodied carbon emission performance and cost analysis were also estimated to evaluate the economic and environmental impact related to the use of recycled rubber as complete aggregate in Portland and geopolymer mixes. Sustainability analysis revealed the greater environmental friendliness of geopolymer formulations compared to those in ordinary cement, but higher production costs. The total addition of rubber aggregates induced an increase in emissions and costs (variable according to the type of matrix) which, however, does not directly correlate with the processing/price of the polymer fraction. Deepening the research on cleaner matrices and promoting the use of recycled materials in concrete applications could lead to a gap levelling between Portland and geopolymer rubber-based composites. Building on these findings, future study will focus on the optimization of the mix design as a function of rubber aggregates.