Properties and durability of concrete produced using CO2 as an accelerating admixture
Introduction
Carbon dioxide emissions are recognized as a significant issue relating to cement production and the use of concrete as a building material. It is estimated that 5% of the world's annual CO2 emissions are attributable to cement production [1]. The industry has previously recognized a number of approaches to reduce the emissions intensity of the cement produced and used in concrete with the industry goal to reduce emissions 50% below 2006 levels by 2050 [2]. It is clear, however, that practical limits on the impacts of these measures mean that meeting the goal will be difficult [3]. Innovative approaches are sought and are likely to be a part of a portfolio strategy. The most significant improvements in production efficiency and cement substitution with supplementary cementitious materials are already known and available. Future emissions improvements will likely be incremental. Therefore, innovative approaches are sought that can be a part of a portfolio strategy.
One approach that many be relevant is the beneficial use of carbon dioxide to make concrete products. The mechanism of carbonation of freshly hydrating cement was systematically studied in the 1970s at the University of Illinois [4]. The main calcium silicate phases in cement were shown to react with carbon dioxide, in the presence of water, to form calcium carbonate and calcium silicate hydrate gel as shown in Equations (1), (2)):3CaO·SiO2 + (3−x)CO2 + yH2O → xCaO·SiO3·yH2O + (3−x)CaCO32CaO·SiO2 + (2−x)CO2 + yH2O → xCaO·SiO3·yH2O + (2−x)CaCO3
Further any calcium hydroxide present in the cement paste will react, in the presence of water, with carbon dioxide, as shown in Equation (3):Ca(OH)2 + CO2 + → CaCO3 + H2O
The carbonation reactions are exothermic. The reaction proceeds in the aqueous state when Ca2+ ions from the cementitious phases interact with CO32− ions from the applied gas. The carbonation heats of reaction for the main calcium silicate phases are 347 kJ/mol for C3S, 184 kJ/mol for β-C2S [4] and 74 kJ/mol for Ca(OH)2 [5].
When the calcium silicates carbonate, the CaCO3 that forms is understood to be mixed with calcium silicate hydrate (C-S-H) gel [6]. C-S-H gel formation occurs even in an ideal case of β-C2S and C3S exposed to a 100% CO2 at 1 atm according to the observation that the amount of carbonate that forms does not exactly correspond to the amount of calcium silicate involved in the reaction [4].
The reaction of carbon dioxide with a mature concrete microstructure is conventionally acknowledged to be a durability issue due to such effects such as reduced pore solution pH, and carbonation induced corrosion. In contrast, a carbonation reaction integrated into concrete production reacts CO2 with freshly hydrating cement, rather than the hydration phases present in mature concrete, and does not have the same effects. Rather, by virtue of adding gaseous CO2 to freshly mixing concrete the carbonate reaction products are anticipated to form in situ, are of nano-scale and homogenously distributed.
Earlier work had pursued reacting carbon dioxide with ready-mixed concrete to maximize the carbon dioxide absorption [7]. A limited reaction time and effects on workability were identified as challenges to overcome. Subsequent lab work using isothermal calorimetry identified the potential performance benefit of using an optimized low dose of carbon dioxide to promote the development of finely distributed carbonate reaction products. It was concluded that a small dose of carbon dioxide could feasibly be used to provide performance benefits in ready-mixed concrete.
Section snippets
Experimental
Industrial experiments were conducted whereby carbon dioxide was delivered to ready-mixed concrete immediately after batching. A tank of liquid CO2 was connected to a gas control system and injector. The liquid was metered for injection into the truck whereupon it converted into a mixture of CO2 gas and solid carbon dioxide “snow”. The carbon dioxide was delivered into the fresh concrete, at a specified flow rate over a fixed injection interval, whereupon it reacted with the hydrating cement
Plastic properties
An overview of the fresh properties of each of the fives batches can be found in Table 2.
The slumps, air contents, temperatures and unit weights were deemed to be acceptable, with the observed differences consistent with normal production variation. The reference batch had the highest slump as anticipated given that it had the highest dosage of high range water reducer. In all cases the scale of the changes in fresh properties was small enough that the carbon dioxide treated samples of concrete
Discussion
The injection of carbon dioxide into concrete while mixing was associated with an increase in the heat of hydration observed through isothermal calorimetry, a reduction in the concrete set time, a neutral effect on compressive strength, and no negative effect on the durability properties.
The observed acceleration of time-of-set and early strength development with all doses of CO2 may result from one or a combination of two causes. The formation of nanoscale carbonation reaction products may
Conclusions
A series of 4 m3 concrete mixtures were produced in concrete trucks using injection of carbon dioxide during their mixing. The injection of waste CO2 into the concrete mixtures accelerated the hydration and strength development without affecting the fresh properties. The time to initial set was accelerated by 95–118 min (an average 25% time reduction) and the final set was accelerated by 103–126 min (an average 23% time reduction). The mixture batched with the conventional non-chloride
Acknowledgements
The authors thank Phil Zacarias and Stephen Parkes of CBM (Canada Building Materials) for supporting the work through hosting the trial and collecting the data. Further assistance provided by University of Toronto students Gita Charmchi and Soley Einarsdottir was greatly appreciated. Research funding was received from Sustainable Development Technology Canada (SDTC) (project number SDTC-2010-B-1782R) and the National Research Council's Industrial Research Assistance Program (IRAP Project 837459
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