The Recarbonation of Crushed Concrete from a New Zealand Perspective
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The cement industry releases large quantities of CO₂ into the atmosphere during the manufacture of Portland Cement. The intrinsic property of the cement to reabsorb some of this CO₂ over its life time through a process called recarbonation has been investigated. This thesis reports on the development of an accelerated recarbonation apparatus for studying the recarbonation of crushed concrete under controlled conditions. The apparatus involved a series of airtight desiccators into which were placed the crushed concrete samples. The desiccators were then filled to ~50,000 ppm CO₂, which is significantly greater than the ~380 ppm by volume CO₂ available in the earth’s atmosphere. The CO₂ concentration was then monitored with respect to time inside the desiccator using CO₂ specific infrared probes. Two concrete design strengths of 20MPa and 40MPa with various crushed particle sizes were exposed to conditions of 50-60 % relative humidity, a temperature of 20 ± 1.5 °C, an exposure period of 21 days and a maximum CO₂ concentration of ~50,000 ppm by volume. The CO₂ uptake measured by the infrared probes was verified using other detection methods of FTIR, TGA, XRF, phenolphthalein indicator and the weight gain of the crushed concrete samples. The research found that a concrete of 20 MPa design strength and a water to cement ratio of 0.67 could absorb 12-83 % of the original calcination emissions for particle sizes <40, <20 and <10 mm in the 21 day time period. Similar behaviour was also exhibited by the 40 MPa design strength (w/c 0.49) but the extent of CO₂ uptake was not as pronounced. The 40 MPa (w/c 0.49) design mix absorbed 9-70 % of the original calcination emissions for the same particle sizes of <40, <20 and <10 mm. It was found that significant quantities of CO₂ could be absorbed by the smaller crushed sizes of <10 and <20 mm for both design mixes, owing to their much larger surface area. It was also found that about 80 % of the total CO₂ absorbed occurred within the first 10 days of exposure. It is envisaged that the results contained in this thesis will assist in future investigations into crushed concrete recarbonation.