Maris Sinka

This study presents the development and comprehensive characterisation of a sustainable 3D-printable cementitious composition in which up to 40 wt.% of Portland cement was replaced by a ternary binder containing oil shale ash (OSA) and metakaolin (MK). Following laboratory optimisation, the developed formulations were successfully transferred to industrial production as pre-blended dry mixes at Sakret Latvia Ltd., demonstrating the feasibility of large-scale manufacturing of printable cementitious materials. Attention was devoted to the characterisation of the raw materials and dry mixtures using particle size distribution (PSD), scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), and X-ray diffraction (XRD). Two compositions—a reference mixture (REF) and the ternary OSA mixture—were evaluated in terms of printability, mechanical performance, durability, and the influence of the type of sample production. The ternary composition (due to the pozzolanic activity of MK and OSA) exhibited strength development resulting in compressive strength (60.6 MPa) exceeding that of the reference mixture after 90 days of curing (55.3 MPa). Mechanical testing of compression and flexural properties indicated direction-dependent differences between printed and cast specimens. Durability assessment, including capillary water absorption and surface freeze–thaw scaling tests performed using two standardised methods, confirmed frost resistance and the suitability of both mixtures for outdoor applications. The results further indicate that the layered manufacturing process governs moisture transport and direction-dependent mechanical behaviour associated with interlayer interfaces. The developed pre-blended OSA–MK composite represents a promising low-carbon material for industrial 3D concrete printing, combining reduced cement consumption with reliable printability, mechanical performance, and durability.

This study presents the development and comprehensive characterisation of a sustainable 3D-printable cementitious composition in which up to 40 wt.% of Portland cement was replaced by a ternary binder containing oil shale ash (OSA) and metakaolin (MK). Following laboratory optimisation, the developed formulations were successfully transferred to industrial production as pre-blended dry mixes at Sakret Latvia Ltd., demonstrating the feasibility of large-scale manufacturing of printable cementitious materials. Attention was devoted to the characterisation of the raw materials and dry mixtures using particle size distribution (PSD), scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), and X-ray diffraction (XRD). Two compositions—a reference mixture (REF) and the ternary OSA mixture—were evaluated in terms of printability, mechanical performance, durability, and the anisotropic behaviour of 3D-printed elements. The ternary composition (due to the pozzolanic activity of MK and OSA) exhibited strength development resulting in compressive strength exceeding that of the reference mixture after 90 days of curing. The anisotropy study revealed a difference in the mechanical properties of printed samples compared with conventionally cast samples. Durability assessment, including capillary water absorption and surface freeze–thaw scaling tests performed using two standardised methods, demonstrated satisfactory frost resistance and confirmed the suitability of both mixtures for outdoor applications. The results further indicate that the layered manufacturing process governs moisture transport and mechanical anisotropy through interlayer interfaces. The developed pre-blended OSA–MK composite represents a promising low-carbon material for industrial 3D concrete printing, combining reduced cement consumption with reliable printability, mechanical performance, and durability.

Freeze–thaw resistance is a key durability concern for cementitious materials intended for use in cold climates, especially when alternative binders are introduced into extrusion-–thaw resistance is a key durability concern for cementitious materials intended for use in cold climates, especially when alternative binders are introduced into extrusion-based 3D concrete printing (3DCP). This study examines the effect of oil shale ash (OSA), a by-product of oil shale combustion, on the fresh and hardened performance of 3D-printable cementitious mixtures, with particular focus on durability under freeze–thaw exposure. Four mixtures were prepared with 0%, 10%, 20%, and 40% replacement of ordinary Portland cement by OSA. Fresh-state properties, including flowability, density, and buildability, were evaluated alongside hardened properties such as density, water absorption, mechanical properties, microstructure (SEM) and crystalline phase composition (XRD), and ultrasonic pulse velocity. Freeze–thaw resistance was assessed using NaCl-solution cycling with mass loss measurements. The results indicate that moderate OSA replacement (10–20%) contributes to a denser microstructure, resulting in higher compressive strength and reduced permeability. At 28 days, the OSA-10 and OSA-20 mixtures achieved compressive strengths of approximately 54 MPa, compared with 50 MPa for the reference mixture. The OSA-20 mixture also exhibited the best freeze–thaw performance, with mass loss after 56 cycles reduced from 35 g/m² to 26 g/m², corresponding to an improvement of approximately 26%. In addition, 3D-printed specimens exhibited 20–30% lower compressive strength than corresponding cast specimens. These mixtures also showed improved or comparable resistance to freeze–thaw action compared to the reference mix. In contrast, higher replacement levels (40%) increase porosity, weaken the microstructure, and significantly reduce durability. At 40% replacement, freeze–thaw mass loss increased to 77 g/m² after 56 cycles. The findings suggest that controlled incorporation of OSA can improve the sustainability and durability of 3D-printed cementitious materials for cold-region applications.

The increasing generation of wood ash (WA) from biomass combustion presents both an environmental challenge and an opportunity for sustainable resource utilization. This review provides a comprehensive assessment of recent advances in the valorization of WA for the development of sustainable engineering materials within a circular economy framework. Unlike previous studies that primarily focus on isolated applications of WA, this work integrates multiple technical dimensions, including material characterization, advanced manufacturing technologies, mechanical performance evaluation, computational modelling, and industrial commercialization pathways. Wood ash typically exhibits alkaline characteristics (pH 9–13.5) and particle sizes ranging from 1 to 1000 µm, enabling its application in a wide range of material systems. In cementitious materials, partial replacement of cement with WA (0.10–20%) generally improves mechanical performance, whereas excessive incorporation may reduce structural integrity. The high silica content (>62%) in certain WA types also enables its utilization in lightweight glass systems and radiation-shielding materials. Furthermore, WA has emerged as a promising functional filler in polymeric and ceramic composites, where additions above 0.5% can enhance dynamic mechanical properties and thermal stability. The review also examines standardized inspection and testing procedures, including quality control (QC) and quality assurance (QA) frameworks based on American Society for Testing and Materials (ASTM), Canadian Standards Association (CSA), and European standards, to ensure the reliability of WA-derived materials. Recent developments in artificial intelligence, machine learning, and computational modelling are highlighted for predicting mechanical behavior, optimizing processing parameters, and enabling digitalized manufacturing systems. In addition, circular manufacturing strategies and economic evaluation models, including break-even analysis, are discussed to assess the industrial feasibility of WA-based products. By integrating circular economy principles with materials engineering, digital technologies, and economic assessment, this review establishes a holistic framework for transforming wood ash from an industrial residue into value-added sustainable materials for construction, energy, and advanced composite applications.

Steel slag aggregates (SSA) offer a potential route for reducing reliance on natural aggregates (NA) and diverting steelmaking by-products from landfill. However, facility-specific life cycle data for SSA production in Abu Dhabi, United Arab Emirates, remain limited. This study quantifies the cradle-to-gate greenhouse gas footprint of SSA produced at two recycling facilities in Abu Dhabi. The assessment follows ISO 14040/14044 and uses a functional unit of 1 tonne of SSA. Primary 2024 operational data were combined with emission factors from the UK Government 2024 dataset, Intergovernmental Panel on Climate Change default values, and published literature. The combined emissions for 500,000 tonnes of SSA were 3,270.22 t CO2eq, corresponding to an average intensity of 6.54 kg CO2eq/tonne SSA. Al Fayah emitted 2,007.88 t CO2eq for 250,000 tonnes of output, equivalent to 8.03 kg CO2eq/tonne SSA, while KEZAD B emitted 1,262.33 t CO2eq for 250,000 tonnes of output, equivalent to 5.05 kg CO2eq/tonne SSA. Inbound material transportation dominated the footprint at both plants, particularly last-mile road transport. Compared with the NA benchmark of 7.75 kg CO2eq/tonne, the average SSA intensity was approximately 16% lower. Under the stated boundary and assumptions, SSA showed lower cradle-to-gate emissions than NA. The way forward should prioritize primary metering, carrier data, and the development of a third-party-verified Environmental Product Declaration to strengthen comparability and market uptake.

Development of sustainability systems for assessment of environmental impacts remains a paramount challenge for green and circular manufacturing of polymers. In this study, a comprehensive life cycle assessment (LCA) framework is developed for European polymeric waste by integrating OpenLCA, Ecoinvent v3.11, and Python-based machine learning (ML) algorithms. Cradle-to-gate, service-life, and cradle-to-grave assessments are performed for representative thermoplastic composite systems, including PP–PET–cotton, HDPE–glass fiber, and PEEK–carbon fiber composites, covering domestic, engineering, and high-performance polymer categories. The results demonstrate that raw material extraction and manufacturing stages dominate environmental impacts, contributing the highest shares to climate change, ecotoxicity, and non-renewable energy consumption. PP-based composite systems exhibit the lowest overall environmental burdens due to lower processing energy and simpler molecular structures, while HDPE-based systems show moderate impacts. PEEK-based composites present the highest impacts per unit mass, driven by energy-intensive synthesis and high processing temperature. Environmental impacts are evaluated using EF v3.1 and ReCiPe methodologies, supported by Monte Carlo simulations and ML-assisted uncertainty quantification. Monte Carlo simulations and ML-assisted LCA provide probabilistic ranges, uncertainty quantification, and predictive insights into impact indicators, enabling the development of a quantitative sustainability system based on probability–impact relationships. A Europe-wide assessment of 57 Mt of polymeric waste highlights that environmental burdens are concentrated in countries with high polymer production and consumption, emphasizing the importance of energy mix, recycling efficiency, and waste management strategies. Overall, this work demonstrates that digitalized LCA coupled with ML offers a powerful decision-support framework for sustainable polymer design, recycling optimization, and circular economy policy development, supporting the transition toward low-carbon and resource-efficient polymer systems in Europe.

This study investigates the feasibility of incorporating wood-based waste in cementitious composites for extrusion-based three-dimensional (3D) printing through the production of artificial aggregates. Because lignocellulosic residues can retard cement hydration, wood dust was chemically modified with a calcium nitrate-based accelerator and granulated into aggregates using disc granulation. The resulting aggregates were characterized for mechanical robustness, and their influence on cement hydration and microstructural development was evaluated using X-ray diffraction (XRD) and thermogravimetric/differential scanning calorimetry (TG/DSC). The modified aggregates were then incorporated into 3D printable cementitious mixtures to assess fresh-state properties, printability, and mechanical performance. The accelerator affected hydration by increasing bound water content and altering the development of hydration products. The produced aggregates exhibited sufficient crushing resistance for practical handling. The incorporation of artificial aggregates resulted in reduced compressive and flexural strengths compared to the reference mixture. However, the differences between mechanical properties measured in different loading directions were reduced, indicating a more uniform structural response in printed elements. The findings demonstrate that chemically treated wood-based aggregates can be successfully integrated into 3D printable cementitious systems, offering a promising pathway toward more sustainable construction materials.

Ternary gypsum–cement–pozzolan (GCP) binders represent a promising low-carbon alternative to traditional Portland cement-based systems for additive 3D printing (3DP). This study presents a systematic three-stage experimental framework for the development of printable and durable GCP mixtures: (i) optimisation of gypsum–cement–metakaolin binder proportions based on a ternary diagram for 25 formulations, (ii) comparative evaluation of different pozzolanic additives and secondary gypsum sources alongside comprehensive durability testing, and (iii) adaptation of the optimised mixtures for 3DP, focusing on rheological properties. The optimal composition was determined with 55 wt% gypsum, 22.5 wt% Portland cement, and 22.5 wt% metakaolin, achieving a 28-day wet compressive strength of 36.2 MPa and a softening coefficient of 0.85. Successful integration of secondary gypsum sources was demonstrated. The GCP 3DP mixtures were developed with water/binder ratios of 0.38–0.45 and sand/binder ratios of 0.5–1.4, with an open time of 20–40 min. The mixtures exhibit pronounced thixotropic behaviour, characterised by increasing yield stress over time and relatively stable plastic viscosity. Printability tests confirmed the stable application of 29–39 layers before structural buckling. 3DP under laboratory conditions successfully demonstrated the feasibility of producing architectural and structural elements from sustainable GCP compositions.