South Africa is endowed with a wealth of coal-fired power stations that can produce extremely high volumes of fly ash per year exceeding 34 million tonnes. The use of high-volume fly ash (HVFA) binders in the construction sector has the capacity to significantly reduce greenhouse gas emissions associated with traditional cement production and offset the carbon footprint of Eskom. The excessive production of fly ash by Eskom warrants the need for developing ultra-high-volume fly ash binders (UHVFA, fly ash/binder > 60 wt%). Nonetheless, fly ash (FA) replacement of cement is still largely limited to 35% regardless of more ambitious research indicating the potential to surpass 60%. In view of the urgent need for South Africa to offset and reduce its carbon footprint, this work reviews and summarises the literature on the performance of HVFA binders with a focus on two specific areas: (i) strength and (ii) durability. On HVFA binder strength, the focus is drawn on work that analysed the compressive strength, flexural strength, and split tensile strength. This review focuses on the extant literature analysing the durability of HVFA binders using various tests, including sorptivity, resistivity, permeability, tortuosity, rapid chloride penetration tests, resistance to sulphate attack, and microstructural analysis. As the FA content increases towards optima, i.e., 50–80%, the most indicative composite characteristics of the strength and durability properties are UCS (30–90 MPa) and permeability (low). This review reveals the leading methodologies, instrumentation, findings, challenges, and contradictions.

The objective of this research was to investigate the effect of Fly ash as a source of Si and Na on the mechanical performance of the Basic Oxygen Furnace Slag based (BOFS) geopolymer. BOFS has the lowest SiO₂, Al₂ O₃, Na₂ O content compared to other aluminosilicate sources (fly ash, metakaolin, granulated blast furnace slag and mine tailings); which have been used to effectively in geopolymer synthesis. SiO₂ and Al₂ O₃, contents of BOFS are respectively 5-7 times and 6-8 times lower than those of fly ash, metakaolin and granulated furnace slag (GBFS). This study evaluated the potential use of fly ash as a source of Si and Al to improve the mechanical performance of a BOFS based geopolymer. The influence of varying amount of Coal Fly Ash (FA) (10–50%) on UCS was studied. The effects of several factors on the UCS of BOFS geopolymer were also investigated. The test variables were molarities of sodium hydroxide (NaOH) (5 M, 10 M and 15 M); the solid to liquid ratio (20 %, 25 % and 30 %); and the curing temperature (20°C, 40°C, 80°C and 100°C). It was established that most favorable conditions for the geopolymer synthesis were FA was 10% of the mixture, 5M NaOH and 80°C curing temperature. Attempts has been made to relate the microstructure of BOFS/FA based composite with properties of geopolymer. Aggregation was identified as the main particulate process as established by the evolution of the particle size distribution (PSD) and its derived moments during the geopolymerisation process. The morphology of the particles appeared flaky and fluffy. The developed composite met the minimum requirement of ASTM C34-13 for a structural clay load bearing non exposed masonry and load bearing exposed side construction masonry.

The application of natural fibres in soil reinforcement is cost-effective and environmentally friendly. However, natural fibres exhibit dimensional instability when exposed to moisture. Their hydrophilic characteristics affect mechanical performance and durability of natural fibre-reinforced composites. In this study, the effects of fibre surface coating on the mechanical properties of soil–fibre composite were investigated. Natural-based water repellent was coated onto sisal fibres to prevent water absorption. The single-fibre pull-out tests were conducted with varying soil matrix moisture contents and densities. Unconfined compression tests were performed on the soil composite containing 25 mm coated fibres at 0.75% fibre content. The results showed that interfacial shear strength (IFSS) of coated fibres was lower than uncoated fibres at the matrix optimum moisture content. This was attributed to the presence of thin film of moisture at fibre–matrix interface, which suppressed adhesion and mechanical interlock between fibre and soil matrix. Increasing matrix density led to high IFSS of both fibre types due to the enhanced interfacial frictional resistance. The composite with coated fibres exhibited higher rate of moisture loss due to the hydrophobic characteristics of fibres. Shear lag model was used to predict axial load transfer of fibres. The model prediction accuracy was dependent on the material properties, geometry and friction at fibre–matrix interface. Rosin can effectively be used to prevent water absorption of natural fibres and enhance long-term strength properties of natural fibre-reinforced soil.