This study investigates the fresh state and hardened state mechanical and durability properties of 3D-printed concrete. The mechanical tests focused on its anisotropic behavior in response to different load orientations. Compressive, flexural, and splitting tensile strengths were evaluated relative to the print layers orientation. Results showed that compressive strength varied significantly, achieving 85% of cast sample strength when the load was applied parallel to the print layers ([u] direction), 71% when the load was applied perpendicular to the print object’s side plane ([v] direction), while only reaching 59% when applied perpendicular to the top plane ([w] direction). Similar trends were observed for flexural strength, with average values reaching 75% of cast sample strength when the load was applied perpendicular to the print layers ([v.u] and [w.u] directions), but decreasing to 53% when the load was applied parallel to print layers ([u.w] direction), underscoring the weaknesses at interlayer interfaces. The splitting tensile strength remained relatively consistent across print orientations, reaching 90% of the cast sample strength. Durability assessment tests revealed that 3D-printed concrete exhibits reduced resistance to environmental factors, particularly at the layer interfaces where the cold joint was formed, which are prone to moisture penetration and crack formation. These findings contribute valuable insights into the mechanical and durability properties of 3D-printed concrete, emphasizing the importance of print orientation and interlayer bonding in its performance. This understanding helps guide the optimal use of 3D-printed elements in real-life applications by aligning load or exposure to environmental factors with the material’s strength and durability characteristics.

This study investigated the influence of different cooling regimes on the microstructure and consequent reactivity of belite-sulfoaluminate clinkers. The cement clinkers were synthesized by incorporating secondary raw materials, such as titanogypsum and bottom ash, to the natural raw materials. Clinker phases were determined by Rietveld quantitative phase analysis, while the distribution morphology and the incorporation of substitute ions in the phases were characterized by scanning electron microscopy using energy-dispersive X-ray spectroscopy (SEM/EDS). Clinker reactivity was studied using isothermal calorimetry and was additionally investigated through compressive strength, which was determined for the cement prepared from the synthesized clinkers. X-ray diffraction analysis showed that, as well as the three main phases (belite, calcium sulfoaluminate, and ferrite), the clinkers contained additional minor phases (mayenite, gehlenite, arkanite, periclase, and perovskite), the ratios of which varied according to the cooling regime utilized. Microscopic observations indicated that the cooling regime also influenced the crystal size and morphology of the main phases, which consequently affected clinker reactivity. Furthermore, a smaller amount of substitute elements was incorporated in the main phases when cooling was slowed. Results showed that, in comparison to clinkers cooled at slower rates, air quenched clinkers reacted faster and exhibited a higher compressive strength at 7 days.

The potential use of steel slag from treated steel slag in belite-sulfoaluminate cements was investigated in this study. Cement clinkers with two phase compositions were synthesized, allowing the incorporation of different amounts of steel slag. The phase composition and microstructure of cement clinkers at three different sintering temperatures were studied by X-ray powder diffraction and the Rietveld method, as well as scanning electron microscopy with energy dispersive spectrometry. The results showed that the targeted phase composition of clinkers was achieved at a sintering temperature of 1250 °C. However, a higher amount of perovskite instead of ferrite was detected in the clinker with a higher content of Ti-bearing bauxite. Apart from the main phases, such as belite, calcium sulfoaluminate, and ferrite, several minor phases were identified, including mayenite, perovskite, periclase, and alkali sulfates. In both clinker mixtures, a higher content of MgO in the steel slags resulted in the formation of periclase. Furthermore, the hydration kinetics and compressive strength at 7 and 28 days were studied in two cements prepared from clinkers sintered at 1250 °C. As evidenced by the results of isothermal calorimetry, the hydration kinetics were also influenced by the minor clinker phases. Cement with a higher content of calcium sulfoaluminate phase developed a higher compressive strength.

The bottleneck in the process for increasing production of low CO₂ mineral binders, based on BCSA (belite sulfoaluminate) clinkers, is the availability of Al-rich raw materials. For that purpose, a new registry of Al-containing secondary mineral residues (industrial and mine waste) has been developed and is presented in this paper. The methodology of creating the registry consists of three main steps: Gathering ideas, consolidation of ideas, and implementation. In order to achieve this, the following methodology was adopted: Analysis of similar registries by potential end-users and seeking potential solutions and tools to be used, and conducting 3 rounds of stakeholder consultations via workshops in order to determine crucial parameters and features the registry needs to contain. The key discussion points were about which data the registry needs to contain, who shall be the potential users, and what are the stakeholder’s expectations from the registry’s portal. Potential individual registry variables were identified as being relevant/irrelevant or available/unavailable, and potential solutions for the registry’s sustainability were explored. Each Al-rich waste/residue data entry is divided into 10 slots, describing legal status, location, quantities, chemical (REE included), mineralogical, physical and radiological properties, life-cycle assessment, additional data, and data relevancy. The registry will act as a matchmaking tool between producers/holders of Al-rich secondary raw materials and potential producers of cement clinkers.

Concrete is an essential construction material, and infrastructures, such as bridges, tunnels, and power plants, consume large quantities of it. Future infrastructure demands and sustainability issues necessitate the adoption of non-conventional supplementary cementitious materials (SCMs). At the same time, global labor shortages are compelling the conservative construction sector to implement autonomous and digital fabrication methods, such as 3D printing. This paper thus investigates the feasibility of using oil shale ash (OSA) as an SCM in concrete suitable for 3D printing, and collision milling is examined as a possible ash pretreatment. OSA from four different sources was collected and analyzed for its physical, chemical, and mineralogical composition. Concrete formulations containing ash were tested for mechanical performance, and the two best-performing formulations were assessed for printability. It was found that ash extracted from flue gases by the novel integrated desulfurizer has the greatest potential as an SCM due to globular particles that contain β-calcium silicate. The 56-day compression strength of concrete containing this type of ash is ~60 MPa, the same as in the reference composition. Overall, collision milling is effective in reducing the size of particles larger than 10 μm but does not seem beneficial for ash extracted from flue gasses. However, milling bottom ash may unlock its potential as an SCM, with the optimal milling frequency being ~100 Hz.

The cement industry could potentially consume large amounts of solid industrial waste in order to improve its sustainability. The suitability of selected quarry and mine waste as secondary raw materials (SRM) was examined for the sustainable production of cement following the concept of a circular economy. The chemical, mineralogical, and radiological characterization of SRM was conducted in this study. Its potential use in low-carbon and low-energy belite-sulfoaluminate cement was investigated by incorporating the examined SRM into cement clinker. Various characterization methods were used to characterize the cement, including X-ray powder diffraction (XRD), thermal analysis (DTA/TG), and isothermal calorimetry. Depending on the chemical composition of the waste, lower or higher amounts were allowed to be incorporated into the raw clinker mixture for a targeted clinker phase composition. Among the samples, differences were observed in the phase composition of synthesized clinkers, which slightly influenced the reactivity of the cement but did not significantly change the compressive strength of the final product.