The influence of cement extenders on the mechanical and durability properties of concrete incorporating waste tyre rubber aggregates

Abstract

South Africa faces critical ecological issues stemming from the mismanagement of waste tyres, leading to the atmosphere, hydrosphere and lithosphere pollution due to improper disposal and incineration practices. This environmental concern extends to the mining and manufacturing of aggregates and cement. To mitigate these ecological effects, researchers have used waste tyres as fine-crumb-rubber aggregates (FCR) and coarse-crumb-rubber aggregates (CCR) in concrete technology to create rubberised concrete. However, incorporating these tyre derived aggregates in concrete has compromised its durability and mechanical characteristics, thus limiting their use in construction projects. To address the limitations posed by rubber aggregates, a comprehensive review of existing literature, coupled with in-depth experimental analysis, was performed. As a result, this study explored the influence of cement extenders, such as Fly Ash (FA) and Ground Granulated Blast Furnace Slag (GGBFS), when combined with either Portland Cement (PC) or FA, on enhancing the concrete mechanical properties, namely compressive and splitting tensile strength and improving the durability characteristics, such oxygen permeability index, water absorption, porosity, and chloride conductivity, of concrete containing FCR aggregates. This improvement was achieved by modifying and mixing these extenders with alkaline solutions composed of sodium silicate and sodium hydroxide, a process known as geopolymerisation or alkali activation. The cement extenders in this context are commonly known as alkali-activated binders or geopolymer binders. The experimental methodology was categorised into three stages (A, B, and C). Stages A and B involved utilising a water-to-cement (w/c) ratio of 0.6 and sand replaced with 10% and 20% of FCR aggregates, as well as Portland cement-CEM I 52R (PC) substituted with 10% and 20% FA. Stage C involved using geopolymer binders of FA, a blend of 40%PC+60%GGBFS, and 40% FA+60% GGBFS blend and sand replaced with 10% and 20% FCR aggregates. All concrete specimens in these stages were tested for workability, durability index and mechanical tests. The study thus concludes that while FCR aggregates can enhance certain durability characteristics, including water sorptivity and chloride ion conductivity, they adversely affect mechanical properties. Furthermore, FA negatively impacts both the mechanical and durability characteristics of conventional concrete as well as concrete that incorporates FCR aggregates. However, geopolymer binders improve the mechanical characteristics of concrete incorporating FCR aggregates. These binders also exhibit varying effects on durability properties, with some specimens showing improvements in oxygen permeability index (OPI) while others demonstrate increased water sorptivity (WS) and porosity. The choice of binder and the presence of FCR aggregates significantly influence chloride conductivity (CC), with some specimens displaying poor resistance to chloride ion diffusion. Lastly, the geopolymer concrete mix design made of 40% PC and 60% GGBFS demonstrated superior mechanical and durability characteristics compared to other mix designs.