Concrete: A carbon sink
Recent tests prove that significant carbonating of concrete structures occurs rapidly with its demolition and crushing, making this material a valuable carbon sink in world gone mad over reducing CO2 emissions.
About half of the CO2 emissions from cement production arise from the calcination of limestone, the chemical reaction process that occurs when limestone is burnt, releasing embodied CO2. However, it is becoming increasingly understood that concrete structures and demolition concrete have the potential to reabsorb much of this CO2 in a process known as re-carbonation.
With approximately half of the carbon dioxide emissions in the manufacture of cement arising from calcination of limestone, the ability of concrete to reabsorb some, or all, of this carbon dioxide obviously has important implications for the cement industry. Currently, there is a dearth of knowledge surrounding the re-carbonating capacity of demolished and crushed concrete in New Zealand, although we are using more and more recycled concrete, which means that the cement industry’s contribution to global CO2 emissions is likely to be considerably overestimated.
Much of the current research has been carried out in Europe, among the Nordic countries, but a recent study by set up between the University of Canterbury, Holicm NZ, the Cement and Concrete Association (CCANZ) and Golden Bay Cement suggest as much as 57 percent of the CO2 emitted during calcination in the production of Portland cement will be reabsorbed over a 100-year period.
This preliminary research in New Zealand confirms that re-carbonation is occurring here at a similar rate to that measured in Europe. Tests on samples of historic crushed concrete from Auckland and Christchurch locations show that re-carbonation increases with the age of concrete and C02 uptake is at its highest when fresh concrete surfaces are exposed to the atmosphere and reduces with time. While the study concludes that most of the carbon dioxide emissions from the calcination of limestone during cement manufacturing can be reabsorbed (particularly when aged concrete is crushed for recycling), further research has been planned to determine optimal conditions for carbon dioxide sequestration, and the timeframes over which this can be expected to occur.
Carbonate titration and phenolphthalein indicator tests were used to test 20 samples consisting of crushed and in-situ concrete from Auckland and Christchurch localities dated between zero and 84 years.
The results indicate that carbonation increases over time, with the rate the carbon dioxide uptake rapid at earlier ages and progressively slowing with time.
Significantly, demolition and crushing of post-service concrete appears to have a very important effect on carbonation during the life cycle of concrete, says the study’s authors, as they increase the surface areas exposed to the atmosphere and significantly increase the rate of carbonation reactions.
The study was initiated by the NZ Portland Cement Association sustainability committee, which engaged a senior student, Kiran Dayaram, from the University of Canterbury to undertake this research. The 20 samples of demolition and crushed concrete were collected from the Christchurch and Auckland areas late in 2007 and early in 2008.
Samples taken from some crushed concrete stockpiles were initially problematic as the source and duration of stockpiling was not always obvious. Another challenge in the project was with obtaining demolition and crushed concrete with ages less than 15 years. Therefore several examples of newly produced concrete were collected for comparison from core and masonry block.
Once collected, all samples where sealed in plastic bags until they were relocated to the laboratory for testing. X-Ray Fluorescence Spectroscopy was initially considered as a test method, but later rejected after giving inconsistent results. As an alternative a traditional carbonate titration method was adopted as the main test method for the study.
The titration method involved crushing samples in a primary crusher, before pulverizing into a fine powder. The powder was digested in hydrochloric acid and heated for three minutes. Following heating, the solution was titrated with sodium hydroxide and phenolphthalein indicator.
The end point was observed when the colour changed from colourless to a light pink. The amount of carbonate present was determined by measuring the volume of sodium hydroxide used for this reaction to occur. It was noted that this titration method gave the result of total carbonates present not just calcium carbonate.
Residual whole concrete samples were also sprayed with a phenolphthalein solution (1g phenolphthalein powder dissolved in 100ml methanol) as a secondary indicator of carbonation. This phenolphthalein test was used to verify the carbonate titration test results and as a method of visually demonstrating the extent of carbonation. After 30 minutes, each sample was photographed as a record and the extent of coloration compared with the titration results. Where a purple colour was observed, little carbonation was assumed to have occurred. Where no purple coloration was observed, a large amount of carbonation was assumed.
The phenolphthalein test method is the basis of the vast majority of carbonation rate data cited in literature. While this method for the determination of the carbonation front has its limitations (it only indicates the pH change in the concrete rather than carbonation), the indicator test was used albeit with an assumed underestimate of the carbonation.
The study observed no obvious difference between samples from Auckland and Christchurch or the different environments in which samples were originally located. It is possible that the two oldest samples (from the Wallace Block and Canterbury Saleyards), which displayed high degrees of carbonation, have absorbed most, if not all, of the original CO2 produced from calcination during the cement manufacturing process.
While the extent of carbonation is affected by environmental and concrete composition parameters, the post-service demolition and crushing of concrete is considered to be an extremely significant factor in determining CO2 uptake.
The carbonation of concrete in New Zealand represents an important process that should be considered when discussing the embodied CO2 of cement and concrete, says the study’s authors. With the wider use of crushed recycled concrete, the effects of carbonation must be considered in any rigorous life cycle assessment of the sustainability of concrete structures.
Further research by the group is underway regarding the carbonation of crushed modern concrete compositions in a laboratory environment. The additional research is designed to help give a better understanding of the likely uptake of CO2 from current concrete mix designs into the future.
Contractor Vol.33 No.3 April 2009
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