Alternative to traditional concrete, sustainable concrete reduces cement content, waste management issues, and CO₂ emissions. To achieve sustainable concrete, waste materials can be used as supplementary cementitious materials (SCMs) to partially replace cement. Fly ash, ground-granulated blast furnace slag, and silica fume have been heavily studied as SCMs. However, due to the retirement of coal-fired power plants and switching to renewable energy, existing SCMs are losing their dominance. With SCMs becoming more widely accepted as partial cement substitutes, there is fear that the current supply will not meet future demand. As a result, researchers have been looking for alternative SCMs. The circular economy can be achieved by reusing non-hazardous construction and demolition materials, timber, and metal/steel production waste as SCMs. This article discusses emerging SCMs, reactivity evaluation methods, their limitations, and treatment methods that may improve reactivity. Emerging SCMs can replace existing SCMs in quantity, but their supply to cement factories and low reactivity due to stable crystallinity hinders their use. Among treatment methods, particle size reduction effectively enhances reactivity; however, very fine SCM may increase the overall water demand due to the large surface area. Decades-old reactivity evaluation methods have relatively weak correlations and thus misreport the reactivity of SCMs. Newer R³ models, such as calorimetry and bound water, give the best correlations (R ≥ 0.85) for 28-day relative strength and better performance. Additionally, more concrete testing with emerging SCMs under different durability and environmental protection conditions is required and life cycle assessments are needed to determine their regional environmental impact.

Fly ash-based geopolymers represent a new material, which can be considered an alternative to ordinary Portland cement. MiniBars™ are basalt fiber composites, and they were used to reinforce the geopolymer matrix for the creation of unidirectional MiniBars™ reinforced geopolymer composites (MiniBars™ FRBCs). New materials were obtained by incorporating variable amount of MiniBars™ (0, 12.5, 25, 50, 75 vol.% MiniBars™) in the geopolymer matrix. Geopolymers were prepared by mixing fly ash powder with Na₂SiO₃ and NaOH as alkaline activators. MiniBars™ FRBCs were cured at 70 °C for 48 h and tested for different mechanical properties. Optical microscopy and SEM were employed to investigate the fillers and MiniBars™ FRBC. MiniBars™ FRBC showed increasing mechanical properties by an increased addition of MiniBars™. The mechanical properties of MiniBars™ FRBC increased more than the geopolymer wtihout MiniBars™: the flexural strength > 11.59–25.97 times, the flexural modulus > 3.33–5.92 times, the tensile strength > 3.50–8.03 times, the tensile modulus > 1.12–1.30 times, and the force load at upper yield tensile strength > 4.18–7.27 times. SEM and optical microscopy analyses were performed on the fractured surface and section of MiniBars™ FRBC and confirmed a good geopolymer network around MiniBars™. Based on our results, MiniBars™ FRBC could be a very promising green material for buildings.

The interfacial transition zone (ITZ) is the weakest phase in concrete, characterised by higher porosity and being prone to microcrack formation. Additionally, the ITZ is created when dispersed fibre reinforcement is present. Although fibres improve flexural strength, they can negatively impact other properties. This research investigates the ITZ of fibre-reinforced concrete where macro-basalt fibres (BFs) and oil shale ash (OSA), as an SCM, were used with the aim of modifying the properties of concrete, enhancing the ITZ, and reducing its carbon footprint. Six different concrete mixes with OSA doses between 10% and 30% and a constant BF dose of 8.0 kg per 1 m³ of concrete were prepared and tested. The ITZ was analysed with SEM images and verified through its mechanical properties. The results showed that the presence of OSA improved bonding and densified the microstructure of the paste, especially in the ITZ, resulting in a nearly constant flexural strength at up to a 20% replacement and only a 6.7% decrease in compressive strength while reducing the global warming potential by 19.24 kg CO₂ equivalent in the mix with 10% OSA replacement. Higher replacement ratios had a negative impact on the mechanical properties, as the OSA had not reacted entirely and served partly as an inert filler.

Geopolymer concrete reinforced with MiniBars™ could be an eco-friendly, innovative, durable, high-strength material substitute for common Portland cement in buildings. AR glass fiber MiniBars™ composites (AR MiniBars™) (ReforceTech AS, Royken, Norway) 60 mm in length were utilized to strengthen the geopolymer matrix for the fabrication of unidirectional geopolymer composites reinforced by AR MiniBars™ (AR MiniBars™ FRBCs). New AR MiniBars™ FRBCs were fabricated by adding different amounts of AR MiniBars™ (0, 12.5, 25, 50, 75 vol.%) into the fly ash geopolymer paste. Geopolymers were obtained by combining fly ash powder with Na₂SiO₃/NaOH in a ratio of 2.5:1, which served as an alkaline activator. AR MiniBars™ FRBCs were cured for 48 h at 70 °C and tested for different mechanical properties. Fly ash, AR MiniBars™, and AR MiniBars™ FRBC were evaluated by optical microscopy and SEM. The addition of AR MiniBars™ increased the mechanical properties of AR MiniBars™ FRBCs. The mechanical properties of AR MiniBars™ FRBCs were heightened compared to the geopolymer without AR MiniBars™; the flexural strength was 18.80–30.71 times greater, the flexural modulus 4.07–5.25 times greater, the tensile strength 3.49–8.27 times greater, the force load at upper yield tensile strength 3.6–7.72 times greater, and the compressive strength for cubic samples 2.75–3.61 times greater. The fractured surfaces and sections of AR MiniBars™ FRBCs were inspected by SEM and optical microscopy analyses, and even though there was no chemical adhesion, we achieved a good micromechanical adhesion of the geopolymer to AR MiniBars™. These results obtained encouraged us to propose AR MiniBars™ FRBCs for application in construction.