In the concrete manufacturing industry, a large amount of waste is generated. Such waste can be utilised in the production of more sustainable products with a low carbon footprint. In this study, concrete sludge, a difficult-to-utilise waste that is obtained from residual concrete by washing a concrete truck, was investigated. During washing, aggregates from the concrete mixture are separated, and the remaining insoluble fine particles combine with water to form concrete sludge. Dried and wet concrete sludge were used in the tests. Samples with different compositions were produced with dried and wet concrete sludge, cement, superplasticiser, and tap water. Seven cement pastes with different compositions were made by partially replacing cement with dried concrete sludge (0%, 5%, 10%, 15%, 20%, 25%, and 30%). In compositions with wet concrete sludge, cement was replaced by the same amounts as in the case of dried concrete sludge. The slump, setting time, and their changes with different amounts of concrete sludge were determined for fresh cement pastes. It was found that with different forms of concrete sludge, the technological properties of the mixtures change, and the setting time decreases. The density and compressive and flexural strength results were confirmed by SEM and XRD tests. The research results show that dry concrete sludge causes the deterioration of the mechanical properties of cement stone, while wet concrete sludge improves the mechanical properties of cement stone. However, it was found that replacing 5% cement with dry concrete sludge does not significantly affect the properties of hardened cement stone. In mixes with wet concrete sludge, the recommended amount of replaced cement is 10%, because the technological properties of the mixture are strongly influenced by larger amounts.

The results of this research show that high-alkali biofuel fly ash (BFA) had a significant influence on the mechanical characteristics, microstructure, porosity, freezing–thawing cycle resistance, and ASR resistance of cementitious materials. Different amounts of BFA (varying from 0 to 30%) were used as a substitute for cement in concrete mixes. The impact of substituting cement with BFA on the cement hydration products was analysed. Slump behaviour, mechanical properties, water absorption, porosity, freeze–thaw cycles, and ASR resistance were studied. The analysis of the mechanical and physical characteristics of the developed sustainable concrete revealed that a better structure, higher compressive and flexural strength and density values, and better freeze–thaw and ASR resistance as well as lower water absorption values were achieved when as much as fifteen percent of cement was substituted with high-alkali BFA. The calculations indicate that the substitution of cement with different quantities of high-alkalinity BFA (from 0% to 30% BFA) increased the SiO₂/CaO ratio from 0.32 to 0.51 and the Na₂O + K₂O/CaO ratio from 0.02 to 0.067 in the composition. An evident higher quantity of the hydration products, reflected in the reduction of porosity by up to 27%, the improvement in compressive strength by up to 19.3%, and the calculated freeze–thaw resistance value of up to 51.50%, was observed when the Na₂O + K₂O/CaO ratio did not exceed 0.044. The ASR resistance of the concrete improved with the increase in the Na₂O + K₂O/CaO ratio. This study shows that BFA with high alkalinity is beneficial in the development of sustainable building materials.

Every year, millions of tons of waste glass are generated all over the world and disposed in landfills. Utilization of this waste by substituting a certain share of cement in cement mortars can contribute to the reduction of environmental pollution in two aspects: the utilization of waste and the reduction of the cement content in cement-based mortars. The cement industry is responsible for approximately 6% of global CO₂ emissions. Seven different mortar mixes, containing between 0% and 30% of waste glass powder added by weight of cement, were analyzed. The following physical and mechanical properties of the mortar mixes were measured: compressive strength, flexural strength, and density. The test results revealed that waste glass powder can be used in small amounts in cement-based mortars to reduce the amount of cement and to utilize waste glass. A higher performance, modified cement-based mortar can be produced for civil engineering applications by replacing 5% with waste glass powder. The linear regression equations obtained illustrate the relationships between the density and compressive strength, and between density and flexural strength at 28 days.

The growing demand for low-carbon cementitious materials has increased interest in natural zeolite as a supplementary cementitious material capable of reducing clinker consumption while modifying cement system performance. This study presents an integrated experimental evaluation of natural zeolite-modified cementitious materials by combining rheological behavior, hydration, compressive strength, density, scanning electron microscopy (SEM), and X-ray diffraction (XRD) within a single experimental framework. Natural zeolite was used as a partial replacement for cement at dosages of 5–12.5 wt.%. The results showed that zeolite significantly affected both fresh-state and hardened-state properties. Zeolite increased the rheological resistance of fresh mixtures, shifted the exothermic hydration peak from 12 h to 8–10 h, and reduced the maximum hydration temperature by approximately 8–12%. Among the investigated compositions, the mixture containing 7.5% zeolite exhibited the highest compressive strength (44.9 MPa at 28 days) together with increased hardened density, suggesting more efficient particle packing and matrix development than the reference mixture. SEM observations indicated a more uniform distribution of hydration products in mixtures containing moderate zeolite dosages, while XRD analysis confirmed changes in the crystalline phase assemblage associated with zeolite incorporation. The results demonstrate that moderate natural zeolite replacement, particularly at 7.5%, provides an effective balance between rheological behavior, hydration characteristics, mechanical performance, and microstructural development, highlighting its potential as a sustainable supplementary cementitious material for low-carbon cement-based composites.

This study investigates the combined effect of waste-based metakaolin, cellulose fibres and functional waterproofing additive on the physical, mechanical, and durability-related properties of sustainable concrete. A total of 12 concrete mixtures were produced, varying in cellulose fibre content (0–2%), metakaolin waste replacement levels (up to 10% of binder), and functional waterproofing additive content (1%). The experimental program assessed workability, density, compressive and flexural strength, ultrasonic pulse velocity (UPV) and alkali–silica reaction (ASR) resistance. The presence of metakaolin due to high pozzolanic activity (1451 mg/g) and fine particle size enhance the formation of additional C–S–H phases. The incorporation of cellulose fibres (1–2 mm in length) improved crack-bridging ability and structural integrity, while functional waterproofing additive enhanced water tightness. Results demonstrated that the synergistic use of these materials led to improved mechanical performance (flexural strength varies from 4.87 MPa to 6.81 MPa; compressive strength varies from 24.01 MPa to 32.97 MPa) and enhanced notable ASR resistance (decrease in expansion varies from 0.209% to 0.029%). The findings highlight the potential of combining waste-derived pozzolanic and fibrous materials with functional admixtures to develop environmentally friendly and performance-optimized concrete composites.