Carbon dioxide utilization in concrete curing or mixing might not produce a net climate benefit

Influence of Optimization Algorithms and Computational Complexity on Concrete Compressive Strength Prediction Machine Learning Models for Concrete Mix Design

The proper design of concrete mixtures is a critical task in concrete technology, where optimal strength, eco-friendliness, and production efficiency are increasingly demanded. While traditional analytical methods, such as the Three Equations Method, offer foundational approaches to mix design, they often fall short in handling the complexity of modern concrete technology. Machine learning-based models have demonstrated notable efficacy in predicting concrete compressive strength, addressing the limitations of conventional methods. This study builds on previous research by investigating not only the impact of computational complexity on the predictive performance of machine learning models but also the influence of different optimization algorithms. The study evaluates the effectiveness of three optimization techniques: the Quasi-Newton Method (QNM), the Adaptive Moment Estimation (ADAM) algorithm, and Stochastic Gradient Descent (SGD). A total of forty-five deep neural network models of varying computational complexity were trained and tested using a comprehensive database of concrete mix designs and their corresponding compressive strength test results. The findings reveal a significant interaction between optimization algorithms and model complexity in enhancing prediction accuracy. Models utilizing the QNM algorithm outperformed those using the ADAM and SGD in terms of error reduction (SSE, MSE, RMSE, NSE, and ME) and increased coefficient of determination (R2). These insights contribute to the development of more accurate and efficient AI-driven methods in concrete mix design, promoting the advancement of concrete technology and the potential for future research in this domain.

Surface frustrated Lewis pairs in titanium nitride enable gas phase heterogeneous CO2 photocatalysis

Gas-phase heterogeneous catalytic CO2 hydrogenation to commodity chemicals and fuels via surface frustrated Lewis pairs is a growing focus of scientific and technological interest. Traditional gas-phase heterogeneous surface frustrated Lewis pair catalysts primarily involve metal oxide-hydroxides (MOH•••M). An avenue to improve the process performance metrics lies in replacing the Lewis base MOH with a stronger alternative; an intriguing example being the amine MNH2 in metal nitrides. This study establishes a proof-of-concept that an amine-type photoactive surface frustrated Lewis pair (MNH2•••M) can be constructed in titanium nitride (TiNxOy) when integrated with a nanoscale platinum spillover co-catalyst. This surface frustrated Lewis pair, comprising Ti-NH2 as the Lewis base and low-valent Ti as the Lewis acid, facilitates the gas-phase light-assisted heterogeneous reverse water-gas shift reaction. The reaction proceeds via a surface-active carbamate intermediate, Ti-(H2N-COO)-Ti, whereby the synergism of Lewis acidic and Lewis basic sites endows it with superior performance indicators compared to TiNxOy alone, as well as conventional platinum supported metal oxides.

Transforming CO2 into advanced 3D printed carbon nanocomposites

The conversion of CO2 emissions into valuable 3D printed carbon-based materials offers a transformative strategy for climate mitigation and resource utilization. Here, we 3D print carbon nanocomposites from CO2 using an integrated system that electrochemically converts CO2 into CO, followed by a thermocatalytic process that synthesizes carbon nanotubes (CNTs) which are then 3D printed into high-density carbon nanocomposites. A 200 cm2 electrolyzer stack is integrated with a thermochemical reactor for more than 45 h of operation, cumulatively synthesizing 37 grams of CNTs from CO2. A techno-economic analysis indicates a 90% cost reduction in CNT production on an industrial scale compared to current benchmarks, underscoring the commercial viability of the system. A 3D printing process is developed that achieves a high nanocomposite CNT concentration (38 wt%) while enhancing composite structural attributes via CNT alignment. With the rapidly rising demand for carbon nanocomposites, this CO2-to-nanocomposite process can make a substantial impact on global carbon emission reduction efforts.

An eco-friendly solution for construction and demolition waste: Recycled coarse aggregate with CO2 utilization

Carbonation of recycled coarse aggregate (RCA) is an eco-friendly solution for the recycling of construction and demolition waste. This paper provides a comprehensive understanding of utilizing CO2 in RCA. The carbonation mechanism associated with CO2 treatment of RCA has been systematically summarized. The methods for CO2 treatment of RCA and the calculation of CO2 sequestration were discussed. Meanwhile, the efficiency of physical properties enhancement of carbonized RCA was analyzed. The microstructure, mechanical properties and durability improvement of recycled concrete containing carbonized RCA were reviewed. Additionally, the environmental benefits of carbonized RCA were provided through carbon footprint, carbon accounting and carbon intensity. Furthermore, the future perspectives of RCA with CO2 utilization were prospected.

Global decarbonization potential of CO2 mineralization in concrete materials

CO2 mineralization products are often heralded as having outstanding potentials to reduce CO2-eq. emissions. However, these claims are generally undermined by incomplete consideration of the life cycle climate change impacts, material properties, supply and demand constraints, and economic viability of CO2 mineralization products. We investigate these factors in detail for ten concrete-related CO2 mineralization products to quantify their individual and global CO2-eq. emissions reduction potentials. Our results show that in 2020, 3.9 Gt of carbonatable solid materials were generated globally, with the dominant material being end-of-life cement paste in concrete and mortar (1.4 Gt y-1). All ten of the CO2 mineralization technologies investigated here reduce life cycle CO2-eq. emissions when used to substitute comparable conventional products. In 2020, the global CO2-eq. emissions reduction potential of economically competitive CO2 mineralization technologies was 0.39 Gt CO2-eq., i.e., 15% of that from cement production. This level of CO2-eq. emissions reduction is limited by the supply of end-of-life cement paste. The results also show that it is 2 to 5 times cheaper to reduce CO2-eq. emissions by producing cement from carbonated end-of-life cement paste than carbon capture and storage (CCS), demonstrating its superior decarbonization potential. On the other hand, it is currently much more expensive to reduce CO2-eq. emissions using some CO2 mineralization technologies, like carbonated normal weight aggregate production, than CCS. Technologies and policies that increase recovery of end-of-life cement paste from aged infrastructure are key to unlocking the potential of CO2 mineralization in reducing the CO2-eq. footprint of concrete materials.

Industrial by-products-derived binders for in-situ remediation of high Pb content pyrite ash: Synergistic use of ground granulated blast furnace slag and steel slag to achieve efficient Pb retention and CO2 mitigation

Ordinary Portland cement (OPC) is a cost-effective and conventional binder that is widely adopted in brownfield site remediation and redevelopment. However, the substantial carbon dioxide emission during OPC production and the concerns about its undesirable retention capacity for potentially toxic elements strain this strategy. To tackle this objective, we herein tailored four alternative binders (calcium aluminate cement, OPC-activated ground-granulated blast-furnace slag (GGBFS), white-steel-slag activated GGBFS, and alkaline-activated GGBFS) for facilitating immobilization of high Pb content pyrite ash, with the perspectives of enhancing Pb retention and mitigating anthropogenic carbon dioxide emissions. The characterizations revealed that the incorporation of white steel slag efficiently benefits the activity of GGBFS, herein facilitating the hydration products (mainly ettringite and calcium silicate hydrates) precipitation and Pb immobilization. Further, we quantified the cradle-to-gate carbon footprint and cost analysis attributed to each binder-Pb contaminants system, finding that the application of these alternative binders could be pivotal in the envisaged carbon-neutral world if the growth of the OPC-free roadmap continues. The findings suggest that the synergistic use of recycled white steel slag and GGBFS can be proposed as a profitable and sustainable OPC-free candidate to facilitate the management of lead-contaminated brownfield sites. The overall results underscore the potential immobilization mechanisms of Pb in multiple OPC-free/substitution binder systems and highlight the urgent need to bridge the zero-emission insights to sustainable in-situ solidification/stabilization technologies.

Managing carbon waste in a decarbonized industry: Assessing the potential of concrete mixing storage

The effort towards a greener future will entail a shift to more environmentally friendly alternatives of many human activities. Within this context, the path towards a decarbonized society in general, and industrial decarbonization in particular, will require using low carbon solutions and/or capturing carbon emissions at the source. This flux of captured carbon will then require management and one option is to store it in concrete. The incorporation of the captured CO2 can be done during the mixing and/or curing. While the latter is more efficient and effective in terms of the amount of CO2 incorporated, it is limited to concrete in elements that are compatible with chamber curing. In practice, this would be restricted to the concrete pre-fabrication industry and, most probably, only to small size elements. Despite the lower performance, incorporation of CO2 into concrete during the mixing stage is a relatively universal alternative. The present research effort reveals that the latter solution is beneficial from an environmental point of view, with an estimated yearly carbon storage of 23 million tonnes worldwide against emissions of 2.5 million tonnes to do it.

CO2 emission analysis of metakaolin and alccofine replaced cement in M40 grade concrete

Among the largest CO2 emission industries, the cement industry is ranked in 2nd place. A large volume of CO2 is emitted at the clinker production, which is a cement manufacturing intermediate product. Countries around the world were having difficulty reducing atmospheric emissions of greenhouse gases (GHG). Concrete is still being used more and more as the nation's infrastructure advances. The amount of CO2 emitted by concrete can be decreased by using less cement or substituting other materials for cement. In this study, the CO2 emission analysis is made on M40 grade, which is that metakaolin (MK) and alccofine (AL) are replaced to the cement in the manufacturing of concrete and is compared with the conventional concrete. The optimum cement replacement of MK and AL is 10% in the production of M40 grade concrete. MK and AL concrete have advantages and disadvantages. If proper safety precautions are taken during the manufacturing process, the toxicity level can be reduced, as well as the amount of CO2 released by the cement during the production of concrete. The LCA (life cycle analysis) is made for the concrete specimens, and the results were interpreted to know which concrete sample emits less and more carbon dioxide. The LCA study provided insights into the environmental aspects of metakaolin and alccofine concrete, including potential reductions in CO2 emissions, energy consumption and other environmental indicators. It helps identify areas of improvement and informs decision-making processes regarding sustainable material choices and construction practices. In M40 grade concrete, a 10% cement replacement with metakaolin and alccofine was found to be ideal. These results could also help in identifying the major cause of CO2 emission, and they can be used for further research purposes.

Computational Complexity and Its Influence on Predictive Capabilities of Machine Learning Models for Concrete Mix Design

The design of concrete mixtures is crucial in concrete technology, aiming to produce concrete that meets specific quality and performance criteria. Modern standards require not only strength but also eco-friendliness and production efficiency. Based on the Three Equation Method, conventional mix design methods involve analytical and laboratory procedures but are insufficient for contemporary concrete technology, leading to overengineering and difficulty predicting concrete properties. Machine learning-based methods offer a solution, as they have proven effective in predicting concrete compressive strength for concrete mix design. This paper scrutinises the association between the computational complexity of machine learning models and their proficiency in predicting the compressive strength of concrete. This study evaluates five deep neural network models of varying computational complexity in three series. Each model is trained and tested in three series with a vast database of concrete mix recipes and associated destructive tests. The findings suggest a positive correlation between increased computational complexity and the model's predictive ability. This correlation is evidenced by an increment in the coefficient of determination (R2) and a decrease in error metrics (mean squared error, Minkowski error, normalized squared error, root mean squared error, and sum squared error) as the complexity of the model increases. The research findings provide valuable insights for increasing the performance of concrete technical feature prediction models while acknowledging this study's limitations and suggesting potential future research directions. This research paves the way for further refinement of AI-driven methods in concrete mix design, enhancing the efficiency and precision of the concrete mix design process.

Feasibility of utilising porous aggregates for carbon sequestration in concrete

Carbon sequestration in concrete has attracted increasing research attention. CO₂ may be permanently stored in the cement paste of concrete by chemical reaction with the hydration products of cement, but this method leads to a significant reduction of the pH value of the concrete pore solution and may thus put the steel reinforcement at risk of corrosion. This paper proposes a new method for carbon sequestration in concrete using the space in porous coarse aggregates; the method involves presoaking the porous aggregates in an alkaline slurry and then using them for CO₂ sequestration. The potential of utilising the space in the porous aggregates and the cations in the alkaline slurry is first discussed. An experimental study aiming to demonstrate the feasibility of the proposed method is then presented. The results show that CO₂ can be successfully sequestrated and fixed as CaCO₃ in the open pores of coarse coral aggregate presoaked in a Ca(OH)₂ slurry. The amount of CO₂ sequestration by concrete produced using the presoaked coral aggregate was around 20 kg/m³. Importantly, the proposed CO₂ sequestration method did not affect the strength development of the concrete or the pH value of the concrete pore solution.

Study on Preparation and Performance of CO2 Foamed Concrete for Heat Insulation and Carbon Storage

Environmental problems caused by large amounts of CO2 generated by coal-electricity integration bases have raised concerns. To solve these problems, this study develops a CO2 foam concrete (CFC) material with both heat insulation and carbon fixation characteristics to realize CO2 in situ storage and utilization. In this study, a Portland-cement-based CO2 foam concrete (PC-CFC) with good thermal insulation performance and carbon fixation ability is prepared using carbonation pretreatment cement and a physical foaming method. The effects of CO2 on the compressive strength, thermal insulation, and carbon fixation properties of PC-CFC are studied. The internal relationship between the compressive strength, thermal insulation, and carbon fixation performance of PC-CFC is analyzed, and the feasibility of PC-CFC as a filling material to realize the in situ mineralization and storage of CO2 in the coal-electricity integration base is discussed. The experimental results show that the compressive strength of PC-CFC is significantly improved by CO2 curing. However, CO2 in the PC-CFC pores may weaken the strength of the pore structure, and the compressive strength decreases by 3.62% for each 1% increase in PC-CFC porosity. Using CO2 as a foaming gas and the physical foaming method to prepare CFC can achieve improved thermal insulation performance. The thermal conductivity of PC-CFC is 0.0512-0.0905 W/(m·K). In addition, the compressive strength of PC-CFC increases by 19.08% when the thermal conductivity of PC-CFC increases by 1%. On the premise of meeting the thermal insulation requirements, PC-CFC can achieve improved compressive strength. The carbon sequestration rate of the PC-CFC skeleton is 6.1-8.57%, and the carbon storage capacity of PC-CFC pores is 1.36-2.60 kg/ton, which has obvious carbon sequestration potential; however, the preparation process and parameters of PC-CFC still require further improvement. The research results show that PC-CFC has great potential for engineering applications and is of great significance for realizing carbon reduction at the coal-electricity integration base.

Cooperative Catalysis of Vibrationally Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation

Using nonthermal plasma (NTP) to promote CO₂ hydrogenation is one of the most promising approaches that overcome the limitations of conventional thermal catalysis. However, the catalytic surface reaction dynamics of NTP-activated species are still under debate. The NTP-activated CO₂ hydrogenation was investigated in Pd₂Ga/SiO₂ alloy catalysts and compared to thermal conditions. Although both thermal and NTP conditions showed close to 100% CO selectivity, it is worth emphasizing that when activated by NTP, CO₂ conversion not only improves more than 2-fold under thermal conditions but also breaks the thermodynamic equilibrium limitation. Mechanistic insights into NTP-activated species and alloy catalyst surface were investigated by using in situ transmission infrared spectroscopy, where catalyst surface species were identified during NTP irradiation. Moreover, in in situ X-ray absorption fine-structure analysis under reaction conditions, the catalyst under NTP conditions not only did not undergo restructuring affecting CO₂ hydrogenation but also could clearly rule out catalyst activation by heating. In situ characterizations of the catalysts during CO₂ hydrogenation depict that vibrationally excited CO₂ significantly enhances the catalytic reaction. The agreement of approaches combining experimental studies and density functional theory (DFT) calculations substantiates that vibrationally excited CO₂ reacts directly with hydrogen adsorbed on Pd sites while accelerating formate formation due to neighboring Ga sites. Moreover, DFT analysis deduces the key reaction pathway that the decomposition of monodentate formate is promoted by plasma-activated hydrogen species. This work enables the high designability of CO₂ hydrogenation catalysts toward value-added chemicals based on the electrification of chemical processes via NTP.

CO2 Curing on the Mechanical Properties of Portland Cement Concrete

This study was to evaluate the CO2 curing on mechanical properties of Portland cement concrete. Three different specimen sizes (5 × 10 cm, 10 × 20 cm, and 15 × 30 cm cylinders), three CO2 concentrations (50%, 75%, 100%), three curing pressures (0.2, 0.4, 0.8 MPa), three curing times (1, 3, 6 h), two water cement ratios (0.41, 0.68) for normal and high-strength concretes, and two test ages (3, 28 days) were used for this investigation. Before using the CO2 curing process, the concrete samples reached the initial set at approximately 4 h, and the free water in the samples was gradually removed when dry CO2 gas was injected. The test results show that the 3-day early compressive strength of normal concrete cured by CO2 is higher than that of concrete cured by water, but the difference is not obvious for high-strength concrete cured by CO2. In addition, there is a size effect on the strength of the 5 × 10 cm and 15 × 30 cm cylinders, and the strength conversion factor ks5 value obtained for the 28-day compressive strength is greater than 1.18. Compared to conventional water-cured concrete, the elastic modulus of carbon dioxide-cured one generally increases in proportion to the square root of the 28-day compressive strength. It was observed that there are only minor differences in the four EC empirical equations obtained by CO2 curing from 5 × 10 cm and 10 × 20 cm cylinders, respectively.

Utilization of Bio-Mineral Carbonation for Enhancing CO2 Sequestration and Mechanical Properties in Cementitious Materials

Microorganisms can perform mineral carbonation in various metabolic pathways, and this process can be utilized in the field of construction materials. The present study investigated the role of bio-mediated mineral carbonation in carbon sequestration performance and mechanical properties of cementitious materials. Bacterial-mediated ureolysis and CO2 hydration metabolism were selected as the main mechanisms for the mineral carbonation, and a microorganism, generating both urease and carbonic anhydrase, was incorporated into cementitious materials in the form of a bacterial culture solution. Four paste specimens were cured in water or carbonation conditions for 28 days, and a compressive strength test and a mercury intrusion porosimetry analysis were performed to investigate the changes in mechanical properties and microstructures. The obtained results showed that the pore size of the specimens incorporating bacteria was reduced by the precipitation of CaCO3 through the mineral carbonation process, thereby improving the mechanical properties of the paste specimens, regardless of the curing conditions. In addition, in the case of the paste specimens cured in carbonation conditions, more amorphous CaCO3 was observed and a larger amount of CaCO3 in the specimens incorporating the bacteria was measured than in the specimens without bacteria. This is attributed to promotion of the inflow and diffusion of CO2 via mineral carbonation through bacterial CO2 hydration metabolism.

Macroscopic and Microscopic Properties of Cement Paste with Carbon Dioxide Curing

Carbon dioxide is the main component of greenhouse gases, which are responsible for an increase in global temperature. The utilization of carbon dioxide in cement-based materials is an effective way to capture this gas. In this paper, the influence of carbon dioxide curing on the setting time, the electrical resistivity, dry shrinkage ratio, water absorption by unit area and mechanical strengths (flexural and compressive strengths) were determined. The scanning electron microscope, X-ray diffraction and thermogravimetric analysis were obtained to investigate the mechanism of carbonation reaction of cement paste. Water-cement ratios of cement paste were selected to be 0.3, 0.4 and 0.5. Results showed that carbon dioxide curing could accelerate the setting of cement paste. The electrical resistivity decreased with the increasing water-cement ratio and increased with the carbon dioxide curing. Moreover, the evaluation function for the curing age and dry shrinkage rate or the mechanical strengths fit well with the positive correlation quadratic function. The water absorption by unit area increased linearly with the testing time. The carbon dioxide curing led to increasing the mechanical strengths and the dry shrinkage ratio. Meanwhile, the carbon dioxide curing demonstrated a decreasing effect on the water absorption by unit area. The mechanical strengths were improved by the carbon dioxide curing and increased in the form of quadratic function with the curing age. As obtained from the microscopic findings, that the carbon dioxide curing could accelerate the reaction of cement and improve the compactness of cement paste.

Assessing the Relative Climate Impact of Carbon Utilization for Concrete, Chemical, and Mineral Production

Estimates show that 6.2 gigatons of carbon dioxide (CO₂) can be captured and utilized across three pathways, concrete, chemical, and minerals, by 2050. However, it is difficult to compare the climate benefit across these three carbon capture and utilization (CCU) pathways to determine the most effective use of captured CO₂. The life cycle assessment methods to evaluate the climate benefit of CCU chemicals should additionally account for the change in material properties of concrete due to CO₂ utilization. Furthermore, with most CO₂ utilization technologies being in the early stages of research and development, the uncertainty and variability in process and inventory data present a significant challenge in evaluating the climate benefit. We present a stochastically determined climate return on investment (ROI) metric to rank and prioritize CO₂ utilization across 20 concrete, chemical and mineral pathways based on the realized climate benefit. We show that two concrete pathways, which use CO₂ during concrete mixing, and two chemical pathways, which produce formic acid through hydrogenation of CO₂ and carbon monoxide through dry reforming of methane, generate the greatest climate ROI and are the only CCU pathways with a higher likelihood of generating a climate benefit than a climate burden.

Spiers Memorial Lecture: CO2 utilization: why, why now, and how?

This introduction to the Faraday Discussion on carbon dioxide utilization (CDU) provides a framework to lay out the need for CDU, the opportunities, boundary conditions, potential pitfalls, and critical needs to advance the required technologies in the time needed. CDU as a mainstream climate-relevant solution is gaining rapid traction as measured by the increase in the number of related publications, the investment activity, and the political action taken in various countries.