Performance, Durability and Life Cycle Impacts of Concrete Produced with CO2 as an Admixture

Monkman S., Sargam Y., Naboka O., Lothenbach B.

The early-age impacts of CO 2 activation (using a dose of 0.3% CO 2 by weight of cement) on two binder systems (tricalcium silicate and cement) were studied, principally across the first 3 h of hydration. The investigation included calorimetry, ICP-OES, TGA, and SEM. The in-situ mineralization of CO 2 accelerated the hydration of both the systems, with the effect being more pronounced in cement. During the first 30 min of hydration, the CO 2 addition impacted the solution phase pH and certain elemental concentrations (Ca, Mg, S). At later times of observation, the values were comparable to the level of the reference sample. Thermodynamic modeling revealed that the presence of CO 2 resulted in stronger undersaturation with respect to the binder phases, which implies a strong driving force for their dissolution. The carbonates were observed in SEM micrographs as a multitude of rhombohedral-shaped calcite crystals while TGA confirmed an increased quantity of carbonates in both binder systems. • CO 2 catalyzes hydration of tricalcium silicate and cement paste systems. • A rapid formation of CaCO 3 coincides with a temporary drop in the solution pH. • SEM analysis confirms calcite rhombs in the CO 2 -activated tricalcium silicate. • The total reaction is the same in the hydration and CO 2 activated cases at 3 h. • 15–30% less calcium hydroxide and calcium silicate hydrate gel in activated case.

Monkman S., Lee B.E., Grandfield K., MacDonald M., Raki L.

The industrial injection of carbon dioxide into ready mixed concrete during mixing and batching can produce a measurable increase in hydration and a significant compressive strength increase. Physiochemical aspects of the mechanism were investigated through the analysis of a model tricalcium silicate system activated with a carbon dioxide addition immediately after hydration started. The attendant effects were examined through isothermal calorimetry, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Carbonate reaction products around 70 nm were observed to have formed within 60 s of the CO 2 gas injection. Within 10 min the carbonates have been covered by new hydration products. The initial product formed appears to be an amorphous or poorly crystalline calcium carbonate. The overall reaction over 24 h was the same for the hydrated and CO 2 -activated cases although the latter instance included the formation of carbonate reaction products.

Scrivener K.L., John V.M., Gartner E.M.

The main conclusions of an analysis of low-CO2, eco-efficient cement-based materials, carried out by a multi-stakeholder working group initiated by the United Nations Environment Program Sustainable Building and Climate Initiative (UNEP-SBCI) are presented, based on the white papers published in this special issue. We believe that Portland-based cement approaches will dominate in the near future due to economies of scale, levels of process optimisation, availability of raw materials and market confidence. Two product-based approaches can deliver substantial additional reductions in their global CO2 emissions, reducing the need for costly investment in carbon capture and storage (CCS) over the next 20–30 years: 1. Increased use of low-CO2 supplements (SCMs) as partial replacements for Portland cement clinker. 2. More efficient use of Portland cement clinker in mortars and concretes. However, other emerging technologies could also play an important role in emissions mitigation in the longer term, and thus merit further investigation.

Monkman S., Kenward P.A., Dipple G., MacDonald M., Raudsepp M.

Additions of carbon dioxide to hydrating concrete have been observed to result in performance benefits. In this study, oil-well cement (selected for its very low initial carbonate content) was hydr...

Monkman S., MacDonald M.

Concrete is the world's most important and widely used construction material. Carbon dioxide utilization in the production of ready mixed concrete was investigated through the injection of an optimal amount of CO2 during batching and mixing. The carbon dioxide improved the concrete compressive strength with minimal impact on fresh air content or workability. Three-way comparisons between a reference batch, reduced binder batch and reduced binder batch with CO2 addition, confirmed that the carbon dioxide could allow for a 5–8% reduction in binder loading without compromising strength. A model case shows that integrating a CO2 utilization step into conventional concrete production can, net of process emissions, reduce the carbon footprint of the concrete by 4.6%. The direct utilization is amplified to attain a carbon footprint improvement that is more than 35 times larger than the amount of carbon dioxide required. One year production at a medium sized producer would use about 24 tonnes of carbon dioxide to achieve nearly 897 tonnes of CO2 absorbed and avoided.

Ashraf W.

This article summarizes the existing knowledge regarding the carbonation of cement-based materials and identified the areas which require further investigations. Available studies regarding the carbonation test scenarios, influences of supplementary cementitious materials (SCMs) on carbonation resistance, and effects of carbonation on the properties of cement-based materials are reviewed here. In addition to ordinary portland cement (OPC) based materials, this article has reviewed the performances of sulfoaluminate belite and alkali activated materials (AAM) while subjected to carbonations. Some very recent topics such as the potential of CO 2 storage in concrete and the newly developed carbonate binders are also discussed.

Miller S.A., Horvath A., Monteiro P.J.

Due to its prevalence in modern infrastructure, concrete is experiencing the most rapid increase in consumption among globally common structural materials; however, the production of concrete results in approximately 8.6% of all anthropogenic CO2 emissions. Many methods have been developed to reduce the greenhouse gas emissions associated with the production of concrete. These methods range from the replacement of inefficient manufacturing equipment to alternative binders and the use of breakthrough technologies; nevertheless, many of these methods have barriers to implementation. In this research, we examine the extent to which the increased use of several currently implemented methods can reduce the greenhouse gas emissions in concrete material production without requiring new technologies, changes in production, or novel material use. This research shows that, through increased use of common supplementary cementitious materials, appropriate selection of proportions for cement replacement, and increased concrete design age, 24% of greenhouse gas emissions from global concrete production or 650 million tonnes (Mt) CO2-eq can be eliminated annually.

Xie H., Yue H., Zhu J., Liang B., Li C., Wang Y., Xie L., Zhou X.

The issues of reducing CO 2 levels in the atmosphere, sustainably utilizing natural mineral resources, and dealing with industrial waste offer challenging opportunities for sustainable development in energy and the environment. The latest advances in CO 2 mineralization technology involving natural minerals and industrial waste are summarized in this paper, with great emphasis on the advancement of fundamental science, economic evaluation, and engineering applications. We discuss several leading large-scale CO 2 mineralization methodologies from a technical and engineering-science perspective. For each technology option, we give an overview of the technical parameters, reaction pathway, reactivity, procedural scheme, and laboratorial and pilot devices. Furthermore, we present a discussion of each technology based on experimental results and the literature. Finally, current gaps in knowledge are identified in the conclusion, and an overview of the challenges and opportunities for future research in this field is provided.

Damineli B.L., Kemeid F.M., Aguiar P.S., John V.M.

At present, the cement industry generates approximately 5% of the world’s anthropogenic CO 2 emissions. This share is expected to increase since demand for cement based products is forecast to multiply by a factor of 2.5 within the next 40 years and the traditional strategies to mitigate emissions, focused on the production of cement, will not be capable of compensating such growth. Therefore, additional mitigation strategies are needed, including an increase in the efficiency of cement use. This paper proposes indicators for measuring cement use efficiency, presents a benchmark based on literature data and discusses potential gains in efficiency. The binder intensity ( bi ) index measures the amount of binder (kg m −3 ) necessary to deliver 1 MPa of mechanical strength, and consequently express the efficiency of using binder materials. The CO 2 intensity index ( ci ) allows estimating the global warming potential of concrete formulations. Research benchmarks show that bi ∼5 kg m −3 MPa −1 are feasible and have already been achieved for concretes >50 MPa. However, concretes with lower compressive strengths have binder intensities varying between 10 and 20 kg m −3 MPa −1 . These values can be a result of the minimum cement content established in many standards and reveal a significant potential for performance gains. In addition, combinations of low bi and ci are shown to be feasible.

Schiessl P., Bamforth P., Baroghel-Bouny V., Corley G., Faber M., Forbes J., Gehlen C., Helene P., Helland S., Ishida T., Markeset G., Nilsson L., Rostam S., Siemes A.J., Walraven J.

BERGER R.L., YOUNG J.F., LEUNG K.

LIME mortars were one of the first cementitious materials and were used extensively in Greek and Roman times. Strength development by the reaction of atmospheric CO2 with Ca(OH)2 to form CaCO3 is slow because the carbonation reaction is controlled by the diffusion of CO2 at small partial pressures. If high partial pressures of CO2 are used, however, Ca(OH)2 can carbonate very rapidly and recently compressive strengths of 2,000–4,000 pound inch2 have been achieved in compacted blends of lime with soils1 and coal refuse2. The experimental data2, 3 suggest that the carbonation reaction is taking place primarily in the aqueous film surrounding the particles.

Shaqraa A.A.

Hwang H., Park J., Moon J., Kang S., Hong S.

Meesaraganda L.V., Kazmi M.A.

This paper presents a beneficial use of CO2 as an admixture in concrete made with ordinary Portland cement. In current practice, accelerated carbonation is the primary method of adding CO2 to concrete. This method has several drawbacks, including low CO2 diffusion, which is applicable only to precast construction, CO2 diffusion to the immediately surrounding outer surface of the concrete element, the need for a carbonation chamber, and high handling costs. This study suggests using CO2 as an addition directly mixed with concrete using a two-stage procedure to overcome the drawbacks of earlier methods. Through the in-situ generation of calcium carbonate particles in the nano- to submicron range in fresh concrete, densifying the concrete improves its performance. At 28 days of testing, the compressive strength of M20 and M40 grades with 0.25% and 0.20% of CO2 is 21.98% and 20.59%, higher than that of corresponding conventional M20 and M40 grades of concrete, respectively. The improved performance can be used to develop concrete mixture proportions that better use Portland cement and CO2 to reduce concrete’s carbon footprint. The suggested method can save 6 to 7.5% of Portland cement, resulting in about an 8% reduction in carbon emissions. In brief, the present method is cost-effective for CO2 mineralization in concrete, offering double benefits, improving concrete properties and making beneficial use of the recovered CO2, hence resolving the problem of greenhouse gas emissions.

Monkman S., Kenward P., Dipple G.