Nováková Iveta

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.

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.