Key Takeaways & Executive Findings
- •• • Optimal residual water-to-cement ratio (r/w) ≤ 0.15, curing pressure ~0.3 MPa, and curing time ~5 h yield a balanced combination of compressive strength and carbon sequestration; higher pressures increase CO2 uptake but provide diminishing or negative strength returns. • • Under constant total solid waste content, steel slag content dominates the synergistic improvement of strength and carbon sequestration; formulations with ≥40% steel slag perform well, with 50% steel slag + 10% fly ash identified as the 'synergistic optimal zone'. • • CO2 mineralization promotes the formation of highly crystalline calcite that fills pores, reduces interfacial transition zone (ITZ) continuity and crack connectivity, leading to crack deflection/blunting and enhanced fracture energy, thereby driving strength gains. • • Lower residual water-to-cement ratios, higher steel slag content, and longer curing durations increase both the content and crystallinity of CaCO3, as confirmed by XRD and TG-DTG; the mass loss increase in the 500–950 °C region corresponds to highly crystalline CaCO3, supporting the crystallinity–mechanical response correlation.
Abstract
This study investigates the optimization of CO2 mineralization curing on the performance of a ternary cementitious system comprising steel slag, fly ash, and Portland cement. Specimens were fabricated with varying residual water-to-cement ratios (r/w), steel slag and fly ash contents, curing pressures, and durations, then subjected to standard curing and CO2 mineralization curing. Compressive strength and carbon sequestration rate were measured, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to analyze mineral composition and microstructural evolution. Results indicate that compressive strength first increases then rapidly decreases with increasing residual water-to-cement ratio, with an optimal r/w below 0.15. Both compressive strength and carbon sequestration rate increase with higher steel slag content; the 50% steel slag + 10% fly ash formulation exhibited the highest values. Microstructural analyses revealed that CO2 mineralization primarily consumes hydration products such as Ca(OH)2, C-S(Al)-H, and AFt, generating abundant calcium carbonate that densifies the pore structure, thereby enhancing mechanical properties. Lower residual water-to-cement ratios, higher steel slag content, or extended curing durations increase the content and crystallinity of calcium carbonate. SEM observations confirmed the presence of densely packed, well-crystallized rhombohedral calcite in specimens with lower water-to-cement ratios and higher steel slag content. These findings provide a mechanistic basis for the engineering application of CO2 mineralization curing in ternary solid-waste cementitious materials.
1. Introduction
Portland cement (PC) production is a major source of global CO2 emissions, contributing approximately 13.4×10^8 tonnes annually, about 9.8% of anthropogenic emissions. The industry faces mounting pressure to reduce its carbon footprint. Utilization of supplementary cementitious materials (SCMs) such as steel slag and fly ash has emerged as a common strategy to lower the carbon intensity of cement-based materials. However, conventional curing methods do not fully exploit the carbonation potential of these SCMs, leaving their inherent CO2 sequestration capacity underutilized. CO2 mineralization curing offers a promising alternative, where CO2 reacts with alkaline phases in the cementitious matrix to form stable carbonates, potentially enhancing early-age strength and durability while permanently storing CO2.
Despite prior research on CO2 curing of binary systems (e.g., steel slag-cement or fly ash-cement), systematic studies on ternary systems and the critical role of residual water-to-cement ratio are lacking. The residual water content significantly influences CO2 diffusion and reaction kinetics, yet its optimization remains underexplored. This study addresses this gap by systematically investigating the coupled effects of residual water-to-cement ratio, steel slag content, curing pressure, and time on the mechanical properties and microstructural evolution of a steel slag-fly ash-cement ternary system. By quantitatively linking CaCO3 crystallinity to process parameters and macroscopic strength, this work provides a mechanistic framework for optimizing CO2 mineralization curing in ternary solid-waste cementitious materials, facilitating their engineering application.
Loading authentic research manuscript (Pages 1–5)...
REN Bin, JU, WANG Wei, FENG Xihao, SHI Junjie, ZHANG Changqing (2026). Mechanical Properties and Microstructural Evolution of CO2-Mineralized Cured Steel Slag-Fly Ash Composite Cementitious Materials. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202507095
Research & Educational Purpose Only: The translations, structured abstracts, analytical annotations, and data reports provided by SinoGreenTechare intended exclusively for academic research, internal corporate R&D, and educational benchmarking. They do not constitute formal engineering, chemical safety, legal, or professional advice.
Copyright & Intellectual Property Notice: Original copyright of the underlying source articles and experimental data remains with the respective authors, institutions, and original publishing journals. SinoGreenTech claims intellectual property only over its proprietary translations, analytical syntheses, and AEO structured enhancements in accordance with international fair use and academic citation principles.
Frequently Asked Questions
What is the optimal residual water-to-cement ratio (r/w) for maximizing compressive strength and carbon sequestration in this ternary system?
The optimal r/w is ≤0.15. At this level, compressive strength and carbon sequestration are balanced; higher r/w values lead to a rapid decline in strength due to reduced CO2 diffusion and lower carbonate crystallinity.
How does steel slag content influence the performance of the ternary cementitious material under CO2 curing?
Steel slag content is the dominant factor for synergistic improvement of strength and carbon sequestration. Formulations with ≥40% steel slag perform well, with 50% steel slag + 10% fly ash achieving the highest compressive strength and carbon sequestration rate, attributed to the formation of highly crystalline calcite that densifies the microstructure.
What is the effect of curing pressure and time on the mechanical properties and CO2 uptake?
Curing pressure around 0.3 MPa and time around 5 hours provide optimal strength and carbon sequestration. Increasing pressure further enhances CO2 uptake but yields diminishing or negative strength returns. Extending curing time increases CaCO3 content and crystallinity, but strength gains become limited beyond a certain point.
How does CO2 mineralization affect the microstructure and what is the role of CaCO3 crystallinity?
CO2 mineralization consumes hydration products like Ca(OH)2, C-S(Al)-H, and AFt, generating CaCO3 that fills pores and reduces ITZ continuity. Highly crystalline calcite (rhombohedral) forms a dense, interconnected skeleton, enhancing strength. Lower r/w, higher steel slag content, and longer curing times increase CaCO3 crystallinity, which correlates with improved mechanical performance.
What are the practical implications for scaling up CO2 mineralization curing in precast concrete production?
The findings suggest that optimizing r/w (≤0.15) and steel slag content (≥40%) can achieve high strength and significant CO2 sequestration, making the process viable for precast elements. However, careful control of curing pressure and time is necessary to balance strength and carbon uptake, and further studies on long-term durability and cost-effectiveness are required.
Related Chinese Research & Cross-Citations
Analysis of National and Local Policies for Medical Waste Treatment and Disposal in China
The escalating generation of medical waste, driven by healthcare expansion and frequent medical activities, poses significant environmental and public health risks. Under the framework of ecological civilization, China is developing a comprehensive policy system for medical waste treatment and disposal, yet the current framework remains nascent and exhibits inconsistencies between national and local policies. This study systematically analyzes the status of national and local policies from 2003 to 2024, collecting 413 policy documents (166 from national ministries and 247 from provincial governments). The analysis examines temporal evolution, regional distribution, and policy focus, alongside the influence of medical waste output, treatment technologies, facility infrastructure, and major epidemic responses. Findings reveal distinct policy phases: initial self-disposal, exploratory management, foundational system building, and rapid development. Regional disparities are pronounced, with eastern coastal areas showing more advanced policies due to greater technical and financial resources. The surge in medical waste, particularly during the COVID-19 pandemic, underscores the need for enhanced regulatory guidance. Non-incineration technologies are gaining traction for their environmental and cost benefits, and facility coverage has improved but remains uneven. The study proposes five policy principles to foster technological innovation and industrial upgrading, ensuring safe medical waste management and environmental protection.
Kinetic Analysis and Simulation of Pollutant Removal in Sewage Networks
This study investigates pollutant removal characteristics and kinetic behaviors in sewage networks, and analyzes their impact on the carbon-to-nitrogen ratio (C/N, as COD/TN) of influent to wastewater treatment plants. Source water quality sampling at drainage outlets revealed spatial and temporal variations in C/N, with domestic sewage exhibiting higher C/N than industrial sewage, and diurnal peaks reaching 6.92 versus 4.71 during off-peak hours. Using a pilot-scale adjustable sewage network system in Kunshan, experiments were conducted under high (0.491 m·s−1) and low (0.089 m·s−1) flow velocities, monitoring pollutant removal over 144 hours. Pseudo-first-order kinetics were applied to model COD and TN removal. Results showed that COD (including SCOD and PCOD), BOD5, and SS achieved approximately 80% removal within 144 h, with higher removal at low flow velocity. TN, NH3-N, and TP exhibited lower overall removal rates. Kinetic fitting revealed that COD removal rate constants (kCOD) were significantly higher than those for TN (kTN), and both decreased with increasing flow velocity: at low velocity, kCOD=0.0167 h−1 and kTN=0.0029 h−1; at high velocity, kCOD=0.0127 h−1 and kTN=0.0020 h−1. Simulations based on actual source pollutant concentrations indicated that the time for C/N to drop to the denitrification critical value of 4.50 was 12.24 h at high velocity, but shortened to 9.49 h at low velocity. These findings demonstrate that increasing flow velocity effectively retards the decline of C/N. Therefore, regulating network flow velocity to reduce hydraulic retention time is a key strategy for maintaining adequate C/N at the terminal and ensuring denitrification efficiency in wastewater treatment plants.
Preparation of Trimetallic-Carbon Composite Catalysts and Their Application in Catalytic Ozonation of Industrial Wastewater
Advanced oxidation processes (AOPs) are promising for degrading organic pollutants in water treatment. Heterogeneous catalytic ozonation (HCO) has gained attention due to its high oxidation efficiency, strong interference resistance, and low secondary pollution. In this study, a series of trimetallic-carbon composite ozone catalysts were prepared via an organic precursor calcination method using γ-Al2O3 as support. This method enhanced catalytic activity and mechanical strength while overcoming the limitations of carbon materials (low mechanical strength) and metal-based materials (poor mass transfer). The optimized catalyst, CA-FeCoCu, comprising Fe, Co, Cu, carbon, and alumina, exhibited excellent performance in phenol degradation and real industrial wastewater treatment. Characterization revealed that the synergistic effect of trimetals and the introduction of multiple carbon types increased specific surface area and hydroxyl radical (·OH) generation. In a pilot-scale fixed-bed reactor, the CA-FeCoCu/O3 system reduced COD from 120 mg·L−1 to below 40 mg·L−1, with an O3 consumption ratio (O/C) of less than 1, effectively lowering operational costs. This work provides a new strategy for developing efficient and stable heterogeneous O3 catalysts and offers a reference for the practical application of HCO in industrial wastewater treatment.
Differentiated Characteristics of Suspended Particulate Matter and Their Effects on Water Quality in the Middle and East Routes of the South-to-North Water Diversion Project
This study investigates the spatiotemporal differentiation of suspended particulate matter (SPM) characteristics, sources, and their impacts on water quality between the Middle Route (closed artificial channel) and East Route (open natural water system) of the South-to-North Water Diversion Project. Thirty sampling sites (13 on the Middle Route, 17 on the East Route) were established, and samples were collected during dry and wet seasons. Water quality parameters and SPM characteristics were analyzed, including particle size distribution, total suspended solids (TSS), chlorophyll a, and stable carbon and nitrogen isotopes. Results show that the Middle Route maintains good and stable water quality, with SPM dominated by coarse particles (>63 μm, 61.43%–94.68%), total phosphorus (TP) <0.01 mg·L−1, and a significant positive correlation between chlorophyll a and coarse particles (r=0.60), indicating algal aggregation dominates particle formation. In contrast, the East Route exhibits high and fluctuating nitrogen and phosphorus concentrations, with SPM dominated by fine particles (<20 μm, 51.26%–88.61%), TP ranging from 0.03 to 1.11 mg·L−1, and a positive correlation with fine particles, suggesting significant external inputs. Carbon and nitrogen isotope analysis reveals that Middle Route SPM primarily originates from autochthonous algae (contribution >46.75%), while East Route SPM is influenced by both terrestrial C3 plants and algae. The distinct engineering and management approaches of the two routes lead to significant differences in SPM characteristics and sources, thereby affecting water quality dynamics. The Middle Route requires an 'algal reduction and hydrodynamic optimization' strategy to control algal-derived coarse particle deposition, whereas the East Route benefits from 'retention-sedimentation and wetland purification' to reduce external fine particles and pollutant inputs. This research provides theoretical support and practical guidance for differentiated SPM management in long-distance water diversion systems.
Combined Ozone Micro-Nano Bubble Oxidation and Powdered Activated Carbon Adsorption for Removal of Taste and Odor Compounds from Drinking Water
Algal-derived taste and odor compounds (2-methylisoborneol, 2-MIB, and geosmin, GSM) in drinking water sources are poorly removed by conventional treatment. This study systematically evaluated the standalone and combined performance of ozone micro-nano bubbles (O3-MNBs) oxidation and powdered activated carbon (PAC) adsorption for removing 2-MIB, GSM, and algal cells from source water. Results showed that O3-MNBs pre-oxidation achieved >97.5% removal of odorants at 400 ng·L−1 and 67.2% algal cell removal within 30 min. When applied as a deep treatment stage, the degradation rate constant (k) was 10.1%–25.6% higher than in pre-oxidation due to lower background matrix interference. Both pre-oxidation and deep treatment reduced effluent concentrations of 2-MIB and GSM to below 10 ng·L−1, with oxidation kinetics fitting pseudo-first-order models (R²>0.95). PAC adsorption of both compounds followed pseudo-second-order kinetics (R²>0.99), with GSM equilibrium adsorption capacity approximately 20.0% higher than that of 2-MIB. In pure water, adsorption capacity increased by >10.0% compared to raw water. Based on kinetic models, a quantitative prediction method was established for O3-MNBs oxidation and PAC adsorption processes, aiming to achieve efficient odorant removal and cost optimization, providing theoretical support for advanced drinking water purification and smart water plant construction.
Pulsed Electric Field Enhancement of Nitrogen Removal Performance and Microbial Community Structure Response in Anammox Granular Sludge
This study investigated the effects of a ring-shaped pulsed electric field (PEF) (1.5 V, 4 h on-time per cycle) on nitrogen removal performance and microbial community structure of anammox granular sludge (AnGS). Two anaerobic sequencing batch reactors (R1 control, R2 with PEF) were operated under stepwise increasing nitrogen loading rates (NLR). At NLR below 1,155 mg·(L·d)−1, R2 exhibited total nitrogen removal efficiency (TRE) 7.5%–17.0% higher than R1, with biomass, specific anammox activity (SAA), and extracellular polymeric substances (EPS) increased by 5%–7%, 21%–71%, and 54%–77%, respectively. However, at NLR above 1,320 mg·(L·d)−1, the toxic effect of nitrite dominated, and PEF enhancement diminished or even reversed to inhibition. Microbial community analysis revealed that at low-to-moderate NLR, PEF increased the relative abundance of Planctomycetes and key anammox bacteria (Candidatus Brocadia and Candidatus Jettenia), along with enhanced community richness (Chao1) and diversity (Shannon/Simpson indices). At high NLR, PEF decreased microbial richness compared to R1. Principal component analysis and redundancy analysis indicated that PEF was the key factor driving community differences at low-to-moderate NLR, whereas nitrite concentration became the dominant factor at high NLR. This study provides theoretical support for enhancing the resilience and engineering application of anammox processes.