Exploring the viability of various waste materials as aggregates in low-carbon two-part alkali-activated cementitious composites

Abstract

Beginning in October 2020, this research project aimed to answer a fundamental question: Can a variety of waste materials effectively replace natural aggregates in low-carbon cementitious composites? the focus was on alkali-activated materials (AAMs) as an alternative to Portland cement in producing cementitious composites, pavement blocks, and modular prefabricated components. This approach promotes off-site construction, offering advantages such as cost savings, waste reduction, shorter construction timelines, and decreased environmental impact. AAMs are derived from aluminosilicate source materials like fly ash (FA) and ground granulated blast furnace slag (GGBS), aiming to decrease reliance on Portland cement and natural aggregates while promoting circular and durable construction practices. The production of Portland cement, a crucial component in concrete binding, contributes significantly to global CO2 emissions, accounting for about 8% worldwide, with an average emission of 174 kg CO2 per cubic meter. Furthermore, the extensive use of virgin aggregates, such as gravel, sand, and crushed rocks, is concerning, with the UK alone consuming over 155 million metric tonnes annually. To reduce dependence on natural aggregate extraction, this study aimed to promote the recycling and upcycling of waste materials from other industries. In this study, it was found that AAM mix formulations reduced CO2 emissions by approximately 45% compared to OPC-based mixes. Incorporating recycled aggregates further enhanced this reduction while conserving approximately one tonne of river sand per cubic metre of AAM-based sand-free composites. On the other hand, the incorporation of recycled polymer aggregates (PVC, UPVC, and rubber) significantly improved the thermal and acoustic insulation of the composites. For instance, 100% UPVC aggregates led to a 65% reduction in thermal conductivity, while 70% rubber aggregates resulted in a 160% improvement in sound reduction index (SRI) compared to composites with 100% river sand (i.e., control sample). The incorporation of recycled polymer aggregates of low density compared to natural sand resulted in the production of lightweight composites. The incorporation of PVC and UPVC aggregates in AAM-based composites effectively prevented chloride leaching into the environment. While investigating waste glass aggregates, in comparison to samples with 100% sand aggregates, incorporating 100% glass aggregates led to a reduction of approximately 230% in Kg CO2 eq/tonne. Moreover, full replacement of natural sand with waste bricks and geo-cement aggregate (Geo-cement is another term used to refer to geopolymer and Alkali-Activated Materials [1]) in AAM-based composites achieved a compressive strength of about 50 MPa. To some extent, AAM-based composites mixed with used bricks and geo-cement aggregates exhibited compatibility with water absorption, freeze-thaw, acid, and chloride attack tests, and showed good durability traits. However, incorporating recycled inhomogeneous aggregates in AAM-based composites poses challenges due to the lack of alkali-activated materials standards and the varying mix formulations of AAMs with different properties. Recycled aggregates also differ in characteristics from natural aggregates. The findings of this research represent a promising advancement in utilising recycled aggregates for low-carbon cementitious composites, contributing to the decarbonisation of the construction industry and the development of more sustainable, eco-friendly building materials.