Carbon-Cured Concrete: Reducing Embodied Carbon in Structural Elements
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Abstract
About This Research Topic
Cement is one of the most carbon-intensive materials the construction industry relies on, and with concrete demand only rising in fast-urbanising countries like Nigeria, finding a practical way to cut its carbon footprint without redesigning the mix from scratch has real commercial appeal. This article draws on an experimental study that put one such approach — exposing freshly cast concrete to a concentrated carbon dioxide atmosphere, known as carbon curing — through a systematic test using a mix ratio representative of Nigerian construction practice. Readers interested in related structural and materials research may want to browse our civil engineering project topics for further examples. What follows sets out the background to the study, the problem it investigates, its objectives and research questions, and what its findings mean for concrete producers and structural engineers weighing lower-carbon construction materials.
Main Abstract
Cement production remains one of the most carbon-intensive processes in the construction industry, accounting for approximately 7% to 8% of global anthropogenic carbon dioxide emissions, driven principally by the calcination of limestone and the combustion of fossil fuels in clinker manufacture. Carbon curing, a process in which freshly cast concrete is exposed to a concentrated carbon dioxide atmosphere within a curing chamber, offers a promising pathway to both accelerate early-age strength development and permanently sequester carbon dioxide within the concrete matrix through carbonation reactions, offsetting a portion of the embodied carbon associated with cement production. This study investigated the effect of carbon curing duration on the compressive strength, carbonation depth, and net embodied carbon of concrete produced with a 1:2:4 mix ratio and a water-cement ratio of 0.55.
Concrete cube specimens were subjected to carbon dioxide curing at a chamber concentration of 95%, a pressure of 0.1 MPa, and a temperature of 20°C, applied at the pre-set fresh concrete stage, for durations of 2, 4, 6, and 8 hours, and compared against control specimens cured under standard water curing conditions. Carbon dioxide uptake increased with curing duration, from 6.2% by weight of cement at 2 hours to 13.1% at 8 hours, following a logarithmic saturation trend (R² = 0.98). One-day compressive strength rose substantially from 8.5 N/mm² for the control to 21.3 N/mm² for the 6-hour cured specimens, a 150.6% improvement, reflecting the accelerated early strength gain characteristic of carbonation curing.
At 28 days, the 6-hour cured specimens achieved the highest compressive strength of 38.2 N/mm², a 17.5% improvement over the 32.5 N/mm² control, while the 8-hour cured specimens recorded a marginally lower strength of 37.6 N/mm², suggesting a practical optimum curing duration of approximately 6 hours beyond which additional carbonation yields diminishing or slightly reversing strength benefit. Carbonation depth, measured using the phenolphthalein indicator test, increased consistently with curing duration, from 4.5 mm at 2 hours to 10.2 mm at 8 hours, compared to 2.1 mm for the naturally carbonated control, raising a practical trade-off consideration for reinforced concrete elements where excessive carbonation depth could compromise the passivating alkalinity protecting embedded reinforcement. One-way ANOVA confirmed that the differences in 28-day compressive strength across curing durations were statistically significant (p < 0.001).
Embodied carbon accounting indicated that the 6-hour curing regime sequestered 39.68 kgCO2e per cubic metre of concrete, representing a 13.8% reduction in the net embodied carbon associated with cement production for the mix examined. The study concludes that carbon curing offers a technically viable, dual-benefit pathway to simultaneously accelerate early strength development and reduce net embodied carbon in concrete production, with a 6-hour curing duration identified as the practical optimum for the mix and curing conditions examined, subject to appropriate reinforcement cover allowance to manage the carbonation depth trade-off. It is recommended that carbon curing be prioritised for precast, non-reinforced, or lightly reinforced structural elements where the carbonation depth trade-off is of reduced structural significance.
Chapter One Preview
Background to the Study
The global construction industry faces mounting pressure to reduce its carbon footprint. According to the International Energy Agency's cement sector analysis, cement production alone accounts for a substantial share of global anthropogenic carbon dioxide emissions, arising principally from the calcination of limestone — a chemical process that releases carbon dioxide independent of fuel combustion — and the substantial fossil fuel energy required to reach the high kiln temperatures needed for clinker formation. As concrete remains the most widely used construction material globally, and given continued growth in construction activity, particularly in rapidly urbanising developing countries such as Nigeria, finding practical, scalable means of reducing concrete's embodied carbon has become an increasingly urgent research and industry priority.
Carbon curing, also known as CO2 curing or carbonation curing, is one such emerging technology. Freshly cast concrete, typically at the pre-set fresh stage before full hydration has commenced, is exposed to a concentrated carbon dioxide atmosphere within a sealed curing chamber. Under these conditions, carbon dioxide reacts with calcium-bearing phases within the fresh cement paste — principally calcium hydroxide and calcium silicate hydrate precursors — to form calcium carbonate. This reaction both permanently sequesters carbon dioxide within the solid concrete matrix and accelerates early-age strength development through the densification of the microstructure that results from calcium carbonate precipitation within the pore structure. This dual benefit — simultaneous carbon sequestration and accelerated strength gain — has attracted considerable commercial and research interest internationally, with several proprietary carbon curing technologies now commercially deployed, principally within the precast concrete industry, where a controlled factory environment readily accommodates the specialised curing chamber infrastructure required.
Despite this growing international momentum, the application of carbon curing technology remains largely unexplored within the Nigerian construction research context, with limited locally generated empirical data characterising its effect on strength development, carbonation depth, and net embodied carbon reduction using materials and mix proportions representative of Nigerian construction practice — a gap that bodies such as the Nigerian Building and Road Research Institute have a direct interest in closing through locally grounded materials research. This study investigates the effect of carbon curing duration on the compressive strength, carbonation depth, and net embodied carbon of concrete produced with a locally representative 1:2:4 mix ratio, with a view to establishing a practical optimum curing duration that balances strength enhancement, embodied carbon reduction, and the carbonation-related durability trade-off relevant to reinforced concrete applications.
Statement of the Problem
The Nigerian construction industry, in common with the global construction sector, faces growing pressure to reduce the embodied carbon associated with concrete production, given the centrality of cement manufacture to national infrastructure and housing delivery and the correspondingly significant contribution of cement production to national and global greenhouse gas emissions. While carbon curing technology offers a promising, dual-benefit pathway to both accelerate strength development and reduce net embodied carbon, its adoption in Nigeria is presently constrained by the absence of locally generated empirical data characterising its performance using materials and mix designs representative of Nigerian construction practice, and by limited understanding of the practical trade-off between the carbon sequestration and strength benefits of the technology and the associated increase in carbonation depth — a durability consideration of particular importance for reinforced concrete elements, where excessive carbonation can compromise the passivating alkalinity that protects embedded steel reinforcement from corrosion. This study addresses this gap through a systematic experimental investigation of carbon curing duration effects on concrete produced with a representative Nigerian mix design, generating locally grounded evidence to inform the practical application of this technology within the Nigerian construction context.
Aim and Objectives
The aim of this study is to investigate the effect of carbon curing on the strength development, carbonation depth, and embodied carbon of concrete, with a view to reducing the embodied carbon associated with structural concrete elements. The specific objectives are to:
1. Determine the physical properties of the fine and coarse aggregates used in the study.
2. Produce concrete specimens subjected to carbon dioxide curing at durations of 2, 4, 6, and 8 hours, and compare these against control specimens cured under standard water curing conditions.
3. Determine the carbon dioxide uptake of the concrete specimens at each curing duration.
4. Determine the compressive strength of the concrete specimens at 1, 3, 7, and 28 days of age.
5. Determine the carbonation depth of the concrete specimens at each curing duration using the phenolphthalein indicator test.
6. Compute the net embodied carbon reduction achieved at each curing duration, accounting for both cement production emissions and sequestered carbon dioxide.
7. Establish the statistical relationship between curing duration and carbon dioxide uptake, and determine whether the observed differences in compressive strength across curing durations are statistically significant.
8. Determine the optimum carbon curing duration that balances strength enhancement, embodied carbon reduction, and carbonation depth considerations.
Research Questions
The study seeks to answer the following research questions:
1. What are the physical properties of the fine and coarse aggregates used in this study?
2. How does carbon dioxide uptake vary with curing duration?
3. What is the effect of carbon curing duration on the compressive strength of concrete at 1, 3, 7, and 28 days?
4. How does carbonation depth vary with carbon curing duration, and what are the implications for reinforced concrete durability?
5. What net embodied carbon reduction is achieved at each carbon curing duration examined?
6. Is there a statistically significant relationship between curing duration and carbon dioxide uptake, and are the observed differences in compressive strength across curing durations statistically significant?
7. What is the optimum carbon curing duration for the mix design and curing conditions examined in this study?
Significance of the Study
This study generates locally grounded empirical evidence on the performance of carbon curing technology using materials and mix proportions representative of Nigerian construction practice, addressing a notable gap given the predominantly international origin of existing carbon curing literature. The findings are of direct practical benefit to precast concrete manufacturers, construction material producers, and structural engineers seeking evidence-based means of reducing the embodied carbon associated with concrete production, while simultaneously benefiting from the accelerated early strength development demonstrated in this study — a practical production efficiency advantage, particularly for precast concrete manufacturing where rapid mould turnover is commercially valuable.
The study also contributes a rigorous, quantified characterisation of the carbonation depth trade-off associated with carbon curing, giving structural engineers a practical basis for deciding how the technology should be applied to reinforced versus non-reinforced concrete elements. More broadly, it supports Nigeria's and the global construction industry's transition toward lower-carbon construction materials, offering a locally validated pathway for embodied carbon reduction that does not require wholesale reformulation of the underlying concrete mix design. Researchers designing a comparable experimental study may find it useful to work through their methodology and statistical analysis plan with our research coaching service.
Scope of the Study
This study is limited to the experimental investigation of carbon curing applied to concrete produced with a nominal 1:2:4 mix ratio and a water-cement ratio of 0.55, subjected to carbon dioxide curing at a chamber concentration of 95%, a pressure of 0.1 MPa, and a temperature of 20°C, applied at the pre-set fresh concrete stage for durations of 2, 4, 6, and 8 hours. It covers carbon dioxide uptake, compressive strength development at 1, 3, 7, and 28 days, carbonation depth assessment, and embodied carbon accounting. The study does not extend to the assessment of other durability parameters such as chloride penetration resistance, freeze-thaw resistance, or long-term carbonation progression beyond the initial curing period, nor does it investigate carbon curing applied to concrete mixes incorporating supplementary cementitious materials, both of which are recommended for further research.
Operational Definition of Terms
Carbon Curing (CO2 Curing): A concrete curing process in which fresh concrete is exposed to a concentrated carbon dioxide atmosphere, promoting carbonation reactions that accelerate early strength development and sequester carbon dioxide within the concrete matrix.
Carbonation: A chemical reaction between carbon dioxide and calcium-bearing compounds within cement paste, forming calcium carbonate and, in the context of hardened concrete exposed to atmospheric carbon dioxide over time, progressively reducing the alkalinity of the affected zone.
Carbon Dioxide (CO2) Uptake: The mass of carbon dioxide absorbed and chemically bound within a concrete specimen during carbon curing, typically expressed as a percentage of the specimen's cement content by weight.
Carbonation Depth: The depth from the exposed surface of a concrete element within which carbonation has occurred, typically measured using the phenolphthalein indicator test, which produces a colour change delineating the carbonated (colourless) from the non-carbonated (pink/purple) zone.
Embodied Carbon: The total greenhouse gas emissions, expressed as kilograms of carbon dioxide equivalent (kgCO2e), associated with the extraction, manufacture, and transportation of construction materials, computed in this study net of any carbon dioxide sequestered through the carbon curing process.
Pre-Set Fresh Concrete: Concrete that has been cast but has not yet undergone significant hydration or hardening, representing the stage at which carbon curing is typically applied to maximise carbon dioxide diffusion and reaction with the available calcium-bearing phases.
Conclusion
The results make a fairly compelling practical case for carbon curing as a near-term, retrofit-friendly lever for lowering concrete's carbon footprint: a 6-hour CO2 curing regime delivered the highest 28-day compressive strength, sequestered nearly 40 kgCO2e per cubic metre, and cut net embodied carbon by close to 14% — all without changing the underlying mix design. The catch is the carbonation depth trade-off, which grows steadily with curing duration and matters most for reinforced elements, so the clearest near-term opportunity sits with precast, non-reinforced, or lightly reinforced structural components rather than reinforced concrete generally. For Nigerian precast manufacturers and structural engineers exploring lower-carbon materials, this gives a locally grounded starting point rather than an imported assumption. Students and researchers exploring related concrete technology, materials science, or sustainability topics can browse comparable studies in our full project topics library, which spans civil engineering and related departments.
Frequently Asked Questions
1. What is carbon curing (CO2 curing)?
Carbon curing is a concrete curing process in which freshly cast concrete is exposed to a concentrated carbon dioxide atmosphere within a curing chamber, triggering carbonation reactions that accelerate early strength development while permanently sequestering carbon dioxide within the concrete matrix.
2. Does carbon curing actually reduce embodied carbon in concrete?
Yes. This study found that a 6-hour carbon curing regime sequestered 39.68 kgCO2e per cubic metre of concrete, representing a 13.8% reduction in the net embodied carbon associated with cement production for the mix examined.
3. What is the optimum carbon curing duration according to this study?
A 6-hour curing duration was identified as the practical optimum, achieving the highest 28-day compressive strength of 38.2 N/mm², with 8-hour curing yielding a marginally lower strength of 37.6 N/mm².
4. Does carbon curing affect the strength of concrete?
Yes, significantly. One-day compressive strength rose from 8.5 N/mm² for the control to 21.3 N/mm² for the 6-hour cured specimens, a 150.6% improvement, and at 28 days the 6-hour cured specimens showed a 17.5% strength improvement over the control.
5. What is the trade-off associated with longer carbon curing durations?
Carbonation depth increases consistently with curing duration, from 4.5 mm at 2 hours to 10.2 mm at 8 hours, which raises durability concerns for reinforced concrete elements where excessive carbonation could compromise the alkalinity that protects embedded steel reinforcement from corrosion.
6. How was carbonation depth measured in this study?
Carbonation depth was measured using the phenolphthalein indicator test, which produces a colour change that delineates the carbonated, colourless zone from the non-carbonated, pink or purple zone of the concrete.
7. What mix ratio and curing conditions were used in the experiment?
The study used a 1:2:4 mix ratio with a water-cement ratio of 0.55, cured in a chamber at 95% carbon dioxide concentration, 0.1 MPa pressure, and 20°C, applied at the pre-set fresh concrete stage for durations of 2, 4, 6, and 8 hours.
8. Where does the study recommend carbon curing be applied?
The study recommends prioritising carbon curing for precast, non-reinforced, or lightly reinforced structural elements, where the carbonation depth trade-off is of reduced structural significance compared with heavily reinforced concrete.
9. Was reinforced concrete directly tested in this study?
No. The study was limited to plain, unreinforced concrete cube specimens; the actual performance and corrosion risk implications of carbon curing applied to reinforced concrete elements were not directly investigated through reinforced specimen testing.
10. Does this study apply to concrete mixes with supplementary cementitious materials?
No. The study did not investigate carbon curing applied to concrete mixes incorporating supplementary cementitious materials such as fly ash or slag, which the study identifies as a direction for further research.
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