The phase conversion of metastable products in calcium aluminate cement (CAC) at elevated temperatures can significantly affect its durability. This study systematically investigates the degradation and leaching behaviour of CAC-based materials in water environments at 20°C and 60°C. The compressive strength evolution of CAC mortars was tracked over 56 days. The underlying mechanisms were elucidated by analysing the phase composition, microstructure, and pore structure of CAC paste specimens, complemented by monitoring the chemistry of the exposure solution. Crucially, a depth-resolved, layer-by-layer analysis was conducted on larger specimens to reveal the leaching gradient. The results show that at 20°C, the leaching behaviour is not pronounced; instead, ongoing hydration densifies the microstructure, leading to a continuous increase in compressive strength. Conversely, at 60°C, the material undergoes rapid degradation. This is driven by a synergistic process involving the conversion of metastable hydrates to stable C3AH6 and gibbsite, and the accelerated dissolution of these products by the water environment, resulting in a porous microstructure and a severe loss of mechanical properties. The depth-resolved analysis provided direct evidence of these mechanisms, confirming that at 60°C, degradation is a progressive process that attacks the material from the exterior.

The aim of this work was to produce a new type of limestone-based Portland cement (LPC) by minimising the amount of clinker (in the range 50–64%) in accordance with the European standard EN 197-5. Such cements are called CEM II/C-M. The new LPC produced, which meets the specific requirements of EN 197-1, was prepared by partially replacing clinker with 5–20% granite waste powder or brick waste powder (BWP) and adding a constant percentage of 20% limestone filler and 6% gypsum. The new LPC powder was evaluated for chemical characterisation, specific surface area and density. The consistency, setting times and soundness properties of the LPC paste were investigated. In addition, the compressive and flexural strengths, drying shrinkage and chemical resistance (hydrochloric acid and sulfuric acid) of mortars prepared with the new LPC were studied. The LPC materials produced with 5% or 10% BWP showed improved mechanical properties, drying shrinkage and durability compared with the reference systems. This study contributes to the development of new cements by minimising the clinker content. The new LPC prepared by substituting clinker with up to 10% BWP can be classified as 32.5R cement.

This study pioneers a novel methodology to assess the feasibility of utilising industrial byproducts in calcium sulfoaluminate (CSA) cement systems through simulated alkaline environments, specifically employing phosphogypsum (PG) and electrolytic manganese residue (EMR) stabilisation by way of struvite precipitation. The stabilisation protocol comprised two phases. In the first phase, struvite synthesis was optimised using an EMR-to-PG mass ratio of 2:1, a solid-to-liquid ratio of 1:0.7, and pH 9.0 adjusted by magnesium oxide/magnesium sulfate supplementation, followed by 20 days of curing to achieve optimal precipitation. Subsequently, a calcium hydroxide solution-based simulation system replicating CSA hydration conditions (pH <10.5) was established to evaluate the long-term stability of stabilised/solidified (S/S) composites over equivalent 20-day periods. Fourier-transform infrared spectroscopy indicated that the characteristic absorbances of phosphate and nitrogen–hydrogen were markedly intensified after the 20-day S/S treatment and remained stable following alkali exposure. Compressive strength at 14 days (44.4 MPa) was comparable to the control, confirming that 10% cement replacement by the S/S composite does not reduce the later mechanical performance of CSA. These findings significantly advance the application of PG and EMR in CSA.

This study investigates blending belitic calcium sulfoaluminate (BCSA) cement with Portland limestone cement (PLC) and calcined clay (CC). Two blends, PLC-BCSA and PLC-BCSA-CC, were tested using different proportions of BCSA cement, PLC and calcined clay (CC). Workability was assessed using a flow test, and mechanical properties were evaluated through compressive strength and shrinkage tests. The hydration process was analysed using thermogravimetric analysis, scanning electron microscopy and X-ray diffraction. The global warming potential (GWP) of the blends was calculated, and the ‘carbon intensity’ was estimated as carbon dioxide (CO2) equivalent per unit of compressive strength, a first for BCSA and BCSA blends. Although the PLC-BCSA and PLC-BCSA-CC blends showed slower strength development compared to BCSA, they achieved similar compressive strengths at 28 days while improving the early-age compressive strength of PLC and limestone-calcined clay cement. BCSA blends can significantly reduce carbon dioxide intensity, highlighting their potential as more sustainable concrete alternatives.

The traditional interlayer bonding testing methods used for cement-based materials is not suitable for three-dimensional (3D)-printed geopolymers and they have some drawbacks that prevent efficient and accurate testing; accordingly there is an urgent need for a new testing method. In this briefing article, a newly designed fixture method (N-FM) was proposed to test the interlayer bonding strength of a 3D-printed geopolymer made from TJC-1 lunar regolith simulant as a case to show the improvement from the previously used traditional adherence method (T-AM). The results show that these two methods exhibit different failure modes. Interlayer failure, intralayer failure and epoxy resin detaching can be observed from the T-AM, while interlayer failure and diagonal failure can be observed from the N-FM. Compared to 50% by the T-AM, the success rate can be improved up to 90% by the N-FM and the distribution of data is more uniform. The 28-day interlayer bonding strength result from the N-FM was 2.16 MPa, which is considered to be more accurate, and is 30.91% higher than that of the T-AM.

Thallium (Tl) poses a significant pollution risk due to its high volatility during clinker calcination. Thermodynamic calculations and multi-stage monitoring have revealed that, despite sulfur suppression, over 90% of Tl volatilised as thallium chloride at 900°C due to chlorine. Volatilised Tl migrated with gas, condensing in cooler zones, enriching Tl in mill-outlet raw meal. The recycling of Tl-enriched dust from the suspension preheater boiler and bag filter further intensified the Tl enrichment in the kiln-feed raw meal. As this enriched material reached the two-stage cyclone, it re-adsorbed and condensed Tl from the gas and reintroduced Tl back into the high-temperature zone again, forming an internal ‘volatilisation-condensation’ cycle. After calcination, only 0–27.85% of input Tl resided in the clinker, while the bypass system released merely 1.31–6.66%. Crucially, the bag filter efficiently intercepted volatile Tl, preventing detectable Tl emissions from kiln gas. Therefore, despite high volatility, the internal cycle, effective dust capture and dust recycling led to closed-loop circulation and enrichment of most Tl within the system, preventing environmental release. In addition, co-processing Tl-containing solid waste amplified the imbalance between Tl input and output, leading to greater internal circulation and enrichment within the kiln system, without significantly altering clinker Tl content or emissions.

To promote the application of seawater–sea sand concrete (SSC) in marine engineering and investigate the influence of ions in the marine environment on the alkali–silica reaction (ASR) in concrete, the impact of calcium ions on the formation and evolution of ASR products was explored. By incorporating varying amounts of calcium oxide, the characteristics of ASR products (composition and microstructure) in SSC under different calcium/silicon molar ratios (CSRs) were systematically examined, elucidating the influence of the CSR on ASR pathways and product stability. The results showed that, with an increase in the CSR from 0.30 to 0.55, excess Ca2+ ions progressively facilitated the staged transformation of sodium shlykovite into the intermediate phase ASR-P1, followed by further conversion to calcium silicate hydrate gel. Concurrently, the pore solution exhibited significant dilution of Na+ and K+ ions (concentrations reduced to 26.13 mg/l and 7.41 mg/l, respectively), effectively suppressing the formation of deleterious alkali silicate gels. Specimen expansion rates at 14 and 28 days decreased from 0.212% and 0.345% to 0.085% and 0.109% (reductions exceeding 60%), respectively, with 14-day expansion rates remaining below 0.1% at CSR ≥ 0.50. This study proposes a CSR optimisation strategy for ASR mitigation, providing theoretical foundations for the engineering application of SSC in marine environments.

High-belite calcium sulfoaluminate cement (HBCSA) is a low carbon dioxide binder but suffers from slow early strength development due to its low hydration rate. Adding calcium oxide (CaO) can increase the hydration rate, but the synthesis of HBCSA clinker containing a designed amount of calcium oxide remains unexplored. In this study, a method was developed to produce calcium oxide–C2S–C4A3S¯ (CBCSA) clinkers for autoclaved aerated concrete (AAC). Experiments were performed to systematically examine the effects of the calcination temperature, retention time and minor oxides on the mineral composition of the clinkers, along with the influence of the mineral composition on the slurry properties and the physical–mechanical performance of the AAC. The results indicated that the optimal calcination temperature was 1230–1290°C and the optimal retention time was 30–60 min. The CBCSA clinker prepared with 26.5% calcium oxide, 8.8% C4A3S¯ and 44.7% C2S produced an AAC slurry with well-matched foaming and thickening rates, shortening the pre-curing time. The resulting AAC blocks achieved a bulk density of 724 kg/m3 and a compressive strength of 7.00 MPa, demonstrating that the CBCSA preserved environmental benefits while enhancing the production efficiency of AAC.

To advance sustainable construction, in this study, red mud-based geopolymer recycled aggregate concrete (GRAC) was synthesised using industrial waste (red mud) and construction waste (recycled coarse aggregate (RCA)). Orthogonal experiments optimised the geopolymer mortar mix design, identifying sodium silicate concentration (30%), ground granulated blast furnace slag (GGBS) content (40%), lime (5%) and gypsum (0%) as the optimal proportion. Sodium silicate concentration was the most critical factor influencing compressive strength, followed by GGBS content, while lime and gypsum exhibited minimal effects. The optimised mortar achieved a 28 days compressive strength of 61.48 MPa – which is 36.90% higher than ordinary Portland cement (OPC) mortar. GRAC specimens with 0–100% RCA replacement were subsequently prepared. Compressive strength decreased with increasing RCA content, peaking at 50% replacement (45.96 MPa at 28 days, 2.64% above OPC concrete). Microstructural analysis by way of scanning electron microscopy/X-ray diffraction analysis revealed: depolymerisation of red mud and GGBS minerals released active Ca, Si and Al components; geopolymerisation formed a hybrid zeolite-like framework and calcium aluminosilicate hydrate (C–A–S–H) gel; the geopolymer matrix filled RCA surface microcracks, enhancing interfacial bonding. GRAC’s superior strength stems from synergistic RCA–geopolymer interactions, reducing cement consumption and natural resource extraction. This work demonstrates high-value recycling of dual waste streams for sustainable infrastructure development.

Low-carbon-dioxide belite–ye’elimite–Q phase clinker (BYQ clinker) was designed and prepared, and its performance was explored in this work to realise the utilisation of gold mining and alumina industrial solid waste and the reduction of carbon dioxide emission. This new system of BYQ clinker was designed based on the investigation involved in the Q phase formation and Q–C4A3$ coexistence. In detail, the effects of temperature and magnesium oxide (MgO) composition on the calcining of the Q phase, the coexistence mechanism of Q phase and C4A3$, as well as the effects of sodium oxide (Na2O) on Q phase and C4A3$ were discussed. The results showed that the Q phase was calcined at 1350°C when the content of magnesium oxide was 3.37–5.37%. The ratio of Q phase/C4A3$ was greater than 1:1 in order to realise the coexistence of Q phase and C4A3$. Also, C4A3$ and Q phase can accommodate some amount of sodium oxide. These findings provide the foundation for the successful preparation of BYQ clinker using 35% gold mining and alumina industrial solid waste, with red mud accounting for 4%. The BYQ clinker was sintered at 1350°C and demonstrated excellent hydration activity, high early strength and a remarkable capability for immobilising heavy metal ions present in industrial solid waste raw materials.