Enhancement of the wet carbonation of artificial recycled concrete aggregates in seawater

This study aimed to improve the carbonation efficiency and reduce processing time of recycled concrete aggregates (RCAs). Different liquid mediums were used for wet carbonation and compared with dry carbonation. Reaction kinetics, phase assemblage, microstructure and performance of RCAs and recycled aggregate concrete (RAC) prepared with carbonated RCAs were evaluated. RCAs carbonated under seawater for 10 min achieved >9 % reduction in water absorption and 3 % increase in density, which were more efficient than dry carbonated samples. Compressive strength of RAC prepared with 1-h seawater carbonated RCAs was significantly improved. Formation of ettringite and calcite under wet carbonation increased density and strength by contributing to solid volume, also leading to improved bond strength in interfacial transition zone in RAC. Seawater presents several advantages as a medium for wet carbonation due to its abundant availability, CO2 capture capacity and accelerated hydration and carbonation, thereby enabling rapid improvement of RCA's and resulting concrete formulations' performance

Investigation of the properties of reactive MgO-based cements and their effect on performance

Reactive MgO cement (RMC) is a promising alternative cementitious material benefiting from a relatively low calcination temperature during its production and strength development in concrete formulations linked with its CO2 sequestering capacity. One of the main challenges with RMC is the variations in its performance in line with the significant differences observed in the properties of the main phase, MgO. To identify and analyze the effects of these properties on the performance of RMC, this study presents a detailed characterization of 9 commercial RMC powders from different sources and precursors and an investigation of their performance in terms of reaction mechanisms and strength development. The results showed that the progress of hydration was highly dependent on the reactivity of RMC, whilst the early stages of the reaction were influenced by the purity. Additionally, agglomeration ratio revealed a strong correlation with the strength after 7 days of carbonation curing and 28 days of hydration. Finally, a regression analysis was employed to propose a model for the prediction of strength based on the initial properties of the RMC powder. The results emerging from this study can serve as a guideline for the selection of most suitable RMC-based binders for various building applications.</p

Effect of preconditioning on carbonated reactive MgO-based concrete samples

This study investigated the influence of preconditioning on the strength and microstructural development of carbonated reactive MgO cement (RMC)-based concrete mixes. The hydration mechanisms of the prepared formulations were studied via isothermal calorimetry. Compressive strength and porosity measurements were conducted to assess sample performance. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used for the microstructural analysis and quantifications of phases within each sample. Subjecting RMC samples to moderately elevated temperatures (50°C–60°C) for 1–2 days before the start of the curing process enhanced the hydration process. This increase in the degree and rate of the hydration reaction increased the amount of phases available for the subsequent carbonation reaction. The increase in the content and size of hydromagnesite [4MgCO3⋅Mg(OH)2⋅4  H2O] crystals led to denser microstructures, thereby facilitating higher strengths in samples subjected to preconditioning. The obtained results showed that the application of this practical approach to the preparation of RMC-based samples can not only present a more efficient use of the binder component, but also enable the increased sequestration of CO2 within these samples

Passivation of reinforcing steel in reactive MgO cement blended with Portland cement

Reactive MgO cement (RMC) has an ability to gain strength by carbonation. One of the main concerns in RMC systems is the potential corrosion of reinforcing steel in structural applications. This study evaluated the feasibility of replacing small proportions of RMC with Portland cement (PC) to promote the passivation of reinforcing steel. Reinforcing steels embedded in RMC with different proportions of PC were investigated by electrochemical measurements and microstructural analysis. Pore solutions extracted from these pastes were evaluated for their chemical compositions. Inclusion of ≥2% PC enabled the passivation of steel. Passive film forming in RMC-PC blends consisted of Fe3O4 and Mg(OH)2, which was thicker than those in pure PC mixes. The formation of passive film was mainly attributed to the increased pH of the pore solution, which reached over 12 in RMC mixes containing only 2% PC. These findings highlighted the potential of using RMC mixes in reinforced concrete applications.</p

Modeling the mechanical properties of recycled aggregate concrete using hybrid machine learning algorithms

To explore the complicated functional relationship between key parameters such as the recycled aggregate properties, mix proportion and compressive strength of recycled aggregate concrete (RAC), a complete database involving 607 records from relevant published literature was built. Two standard algorithms (artificial neural network (ANN) and support vector regression (SVR)) and two optimized hybrid models (Particle Swarm Optimization based SVR (PSO-SVR) and grey Wolf optimizer based SVR (GWO-SVR)) were adopted. Furthermore, two interpretable algorithms (Partial Dependence Plot (PDP) and SHapley Additive exPlanations (SHAP)) were utilized to assess the global and local approaches independent of machine learning models, contributing towards decision-making rationales. Results indicated that the coefficient of determination (R2) of ANN, SVR, PSO-SVR and GWO-SVR were 0.7569, 0.5914, 0.8995 and 0.9056 respectively, showing that hybrid models outperformed the conventional models. However, GWO-SVR was the most problematic with overfitting when analyzing its three subsets. The two feature importance analyses revealed cement content, water content, natural fine aggregates, and water absorption as significant characteristics that affect mechanical performance

Investigation of the viscoelastic evolution of reactive magnesia cement pastes with accelerated hydration mechanisms

Viscoelasticity of reactive magnesia cement (RMC) pastes containing 3 different hydration agents (HCl, Mg(CH3COO)2 and MgCl2) were investigated. Amplitude sweep, frequency sweep and time sweep of RMC pastes were examined within 3 h of hydration. Time-dependent evolution of storage modulus, loss modulus, phase angle, and shear stress were recorded. Measurements of pH, isothermal calorimetry, XRD, TG-DTG and FTIR were used to analyze hydration reaction and products. Addition of hydration agents (HAs) accelerated the growth rate of storage modulus/loss modulus over time. MgCl2 demonstrated the greatest acceleration influence, also reflected in non-destructive structural build-up and buildability related to 3D printing applications. Addition of MgCl2 and HCl advanced the initial setting time of RMC pastes to 100–110 min, during which yield stress reached maximum, and decreased afterwards. Within 3 h of hydration, pastes containing MgCl2 revealed lowest pH, highest heat release and brucite concentration. HAs inclusion precipitated brucite away from MgO particles in the bulk solution, creating a bridge between MgO particles and enabling denser microscopic network structure

Quantification of carbonated Mg-based cement pastes by Raman spectroscopy

This study presented a detailed investigation of the carbonation of reactive magnesia cement (RMC) and brucite cement by using Raman spectroscopy and other characterization techniques. Quantification of carbonation at different depths and formation of various reaction products in each system were demonstrated. Correlations between Raman peak intensities and carbonation degree were established by Raman mapping. Established correlations were employed to evaluate the carbonation behaviour of RMC and brucite cement. Results indicated the higher sensitivity of Raman spectroscopy than XRD in detecting different carbonation products at early stages. This information can be used to assess the strength and microstructural development of RMC and brucite cement since different HMCs have distinct properties and performance contributions, and their transformation plays a key role in microstructural and strength development. This study provided further understanding of the carbonation mechanisms of Mg-based systems, which could enable carbonation degree and performance prediction via the established correlations.</p

Emerging CO2 utilization technologies for construction materials: a review

The construction industry is a major contributor of CO2 emissions. Carbonation, involving the reaction of CO2 with alkaline reactants, immobilizes CO2 into thermodynamically stable carbonates used in construction materials such as concrete and aggregates. The utilization of CO2 in construction materials is considered as one of the most promising routes for carbon sequestration, with a $400 billion market opportunity and a potential to reduce annual CO2 emissions by up to 3 Gt by 2030. This paper reviews the current status of the utilization of CO2 in construction materials from the perspective of scientific research and commercial applications. The explanation of the fundamental carbonation reaction mechanisms was extended to cover different binder systems involving Portland cement, non-hydraulic calcium silicate, industrial solid wastes and magnesium-based materials. Factors affecting the kinetics of the carbonation reaction and properties of the final products were reviewed. Furthermore, the current state of research and commercial initiatives involving the utilization of CO2 in the production of various building components were presented. Finally, key issues regarding the challenges faced in the scaling up of CO2 utilization technologies from the perspective of academia, industry and relevant regulatory bodies were highlighted. Recommendations to address the current utilization dilemma and promote large-scale application of CO2 in the production and development of construction materials were provided

Environmental assessment of magnesium oxychloride cement samples: A case study in Europe

This study is the first in the literature to systematically assess the environmental impacts of magnesium oxychloride cement (MOC) samples, which are regarded as a more eco-friendly construction material than Portland cement. The environmental impacts of MOC samples prepared with various molar ratios of MgO/MgCl2∙6H2O and sources of reactive magnesia were obtained via a life cycle assessment (LCA) approach (from cradle to grave), and the obtained outcomes were further compared with the counterparts associated with the preparation of Portland cement (PC) samples. Meanwhile, a sensitivity analysis in terms of shipping reactive magnesia from China to Europe was performed. Results indicated that the preparation of MOC samples with higher molar ratios led to more severe overall environmental impacts and greater CO2 sequestration potentials due to the difference of energies required for the production of MgO and MgCl2∙6H2O as well as their various CO2 binding capacities, whereas in terms of CO2 intensities, the molar ratios in MOC samples should be carefully selected depending on the strength requirements of the applications. Furthermore, various allocation procedures and MgO production processes will greatly influence the final outcomes, and allocation by mass is more recommended. Meanwhile, the environmental impacts associated with the transportation of reactive magnesia from China to Europe can be ignored. Finally, it can be concluded that MOC concrete is no longer a type of ‘low-carbon’ binder in comparison with PC concrete in terms of CO2 emissions, and in view of the single scores and mixing triangles for weighing, MOC concrete can only be identified as a type of more sustainable binder than PC concrete when the main component MgO in MOC samples is obtained through the dry process route rather than the wet process route