These days, the construction industry is facing sustainability issues, leading to the selection of greener, low-carbon, alkali-activated materials. This study examines a low calcium alkali activated system composed of three constituents (ceramic brick, metakaolin waste, and phosphogypsum). The AAB compositions consist of the primary precursor, waste ceramic brick, which is increasingly (20–100 wt%) replaced with waste metakaolin. The alkaline solution was made of sodium hydroxide and water; dosage depended on the Na₂O/Al₂O₃ ratio (1.00–1.36). The AAB specimens were inspected by using XRD (X-ray diffraction) and FT-IR (Fourier transform infrared spectroscopy) methods for the evaluation of mineral composition, accompanied by SEM–EDS (scanning electron microscopy & energy dispersive X-ray spectroscopy) for the analysis of the microstructure. The compressive strength after 7, 28 and 90 days, along with water absorption and softening coefficient were determined. Also, mixture calorimetry was established. The results have shown that the initial materials are suitable for producing medium-strength alkali-activated binder under ambient temperature. The maximum compressive strength was reached by using the combination of 80% CBW and 20% MKW (13.9 and 21.2 MPa after 28 and 90 days respectively). The compressive strength development was linked with the formation N–A–S–H gel and faujasite type zeolite. A higher level of geopolymerization in composition with metakaolin waste led to lower compressive strength. Consequently, binding materials with low demand of high final and especially early compressive strength could be produced under ambient temperature curing, making them more sustainable.

This paper analyzes the efficiency of shrinkage reducing additives for the shrinkage deformations of ordinary Portland cement (OPC) concrete and its mechanical properties. OPC concrete was modified with an organic compound-based shrinkage reducing additive (SRA), quicklime, polypropylene fiber, and hemp fiber. It was found that a combination of 2.5% quicklime and 1.5% SRA led to the highest reduction in shrinkage deformations in concrete, and the values of shrinkage reached up to 40.0%. On the contrary, compositions with 1.5% SRA were found to have a significant reduction in compressive strength after 100 freeze-thaw cycles. Hemp fiber did not show a significant shrinkage reduction, but it is an environmentally friendly additive, which can improve OPC concrete flexural strength. Polypropylene fiber can be used in conjunction with shrinkage reducing additives to improve other mechanical properties of concrete. It was observed that 3.0 kg/m³ of polypropylene fiber in concrete could increase flexural strength by 11.7%. Moreover, before degradation, concrete with polypropylene fiber shows high fracture energy and decent residual strength of 1.9 MPa when a 3.5 mm crack appears. The tests showed a compressive strength decrease in all compositions with shrinkage reducing additives and its combinations after 28 days of hardening.

The construction industry’s growth is causing a surge in CO₂ emissions, driven by increased demand for concrete and other building materials. There is a growing demand for more sustainable building materials, including alkali-activated materials. This study investigates the impact of varying ratios of Na₂SiO₃ and NaOH on the mechanical properties and microstructure of metakaolin (MKW) and ceramic brick waste (CBW) based geopolymer binder. Geopolymer binder precursors were made of three main CBW/MKW ratios: 100/0%wt. (C100), 50/50%wt. (C50M50), and 0/100%wt. (M100). Alkaline activator solutions had three different Na₂SiO₃/NaOH ratios: 0.5, 1.0, and 2.0. The investigation into the geopolymer binder mechanical properties was conducted using a range of analytical methods, including compressive strength, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The findings of the study indicate that the Na₂SiO₃/NaOH ratio alone is inadequate for evaluating geopolymer mechanical properties when different AS/P ratios are employed, given its influence on other parameters, such as the W/S ratio and the total Na₂O content. CBW-based geopolymer binders demonstrate limited capacity to attain substantial compressive strengths because they contain high amounts of unreacted CBW particles, as shown by XRD analysis. The incorporation of MKW precursor resulted in enhanced reactivity and intensified geopolymerization reaction. After the evaluation of all essential ratios, the most favorable Na₂SiO₃/NaOH ratio is 1.0. This determination was based on the highest strengths observed in designs that contained ≥50% of MKW precursor, attributed to predominance of goosecreekite and N-A-S-H gels, as evidenced by XRD and FT-IR analysis.

The cement industry is a major source of global CO₂ emissions, driving the development of low-carbon alternatives such as geopolymers. This study examines geopolymer binders produced from ceramic brick waste (CBW) and metakaolin waste (MKW), evaluating their fresh and hardened properties as well as their performance under elevated temperatures. Five binder compositions were formulated by progressively replacing CBW with MKW (25–100 wt.%). The alkaline activator ratio (Na₂SiO₃/NaOH = 1.5) and NaOH molality (8 M) were kept constant. Fresh-state behavior was evaluated using Suttard viscometry and Vicat testing, while hardened-state performance was assessed through compressive and flexural strength, softening coefficient, and drying shrinkage. Thermal resistance was examined at 200, 400, 600, and 800 °C, supported by XRD, FT IR, and SEM analyses. The present study investigates how waste-derived aluminosilicate precursors with differing crystallinity and reactivity affect geopolymerization mechanisms and high-temperature phase evolution. MKW-rich binders were found to form highly reactive amorphous gels, resulting in superior early mechanical strength, whereas CBW-rich binders retained thermally stable crystalline phases that enhanced resistance to structural degradation at elevated temperatures. The MKW-rich formulation (F5) demonstrated the highest ambient mechanical performance, reaching 82.8 MPa after curing at 200 °C, due to intensified secondary geopolymerization. In contrast, the CBW-only binder (F1) exhibited superior thermal stability, maintaining a compressive strength of 46.4 MPa even after exposure to 800 °C. These findings establish a clear structure–property relationship between precursor mineralogy, gel chemistry, and high-temperature performance, offering valuable insights for the tailored design of waste-derived geopolymers with optimized thermal and mechanical properties.

The valorization of construction and demolition waste (CDW) is a significant precondition of establishing sustainable and circular economy (CE). This study investigates the use of ceramic brick waste (CBW) and metakaolin waste (MKW) as precursors in geopolymer mortar systems as sustainable alternatives to conventional Portland cement-based materials. Experimental investigations evaluated the mechanical performance, microstructural development, and thermal resistance of CBW–MKW geopolymer mixtures. The developed geopolymer mortars demonstrated strong early-age performance (compressive strength) and formed a dense N–A–S–H gel matrix during curing. Exposure to 900 °C resulted in microstructural degradation in all mixtures; however, the mortar composition with the CBW–MKW ratio 1/1 (S2) retained the highest residual compressive strength and preserved the most coherent matrix structure. The results highlight that combining crystalline CBW with amorphous MKW improves thermal resistance and mechanical stability. The findings support the potential of CBW–MKW geopolymers as sustainable construction materials that can significantly contribute to CE and construction in the Baltic region.

This paper focuses on developing a sustainable, high-temperature-resistant geopolymer mortar by selecting the most optimal fine aggregate type for high-temperature conditions. The developed building material has superior properties to conventional concrete and could possibly be significant in the context of fire-safety building materials. Ceramic brick waste (CBW) from open landfills and metakaolin waste (MKW) from an expanded glass production company (Lithuania) were used as precursor materials in a geopolymer system. The precursors were systematically substituted from 0 to 100% weight in five distinct compositions (F1-F5). The alkaline activator was a blend of 8 M sodium hydroxide and sodium silicate (Na₂SiO₃/NaOH = 1.5). Five types of fine aggregates were examined: ordinary river sand (OSA), granite (GRA), basalt (BAS), ceramic waste (CBW), and corundum (COR), with a fraction size from 0 to 3 mm and the constant fine aggregate to precursor (FA/P) ratio of 1.5. Geopolymer mortar specimens were prepared to assess their compressive strength and density before and after exposure to high temperatures ranging from 200 to 800 °C. A two-way analysis of variance (ANOVA) was conducted to evaluate the reliability of the residual compressive strength outcomes. The study showed that properly selected fine aggregates in an appropriate binder system improves high-temperature resistance. After 800 °C exposure, mortars with 75% CBW and 25% MKW precursors containing CBW and COR aggregates exhibited minimal microcracking and residual strengths of 34.7 MPa (78.7%) and 53.0 MPa (108.6%), respectively. This performance is attributed to partial reactivity, low thermal expansion, and inherent thermal stability.