Reports on CO2 Uptake from the Carbonation of Concrete - Crushed concrete

Carbon dioxide uptake in demolished and crushed concrete
by C.J. Engelsen, J. Mehus, C. Pade, & D.H. Sæther, Byggforsk, Norway (www.byggforsk.no)

Abstract
Recycled Concrete Aggregates (RCA) produced in the Nordic countries was summarized and its scenario applications with regard to the grain size were described. The CO₂ uptake to different crushed concrete types was then measured by conducting extensive accelerated laboratory tests, in order to document any differences in the uptake rate between the different crushed concrete samples. Furthermore, the maximum uptake of CO₂ within reasonable testing time in laboratory was also determined.

The annually volumes of concrete rubble generated in the Nordic countries, except for Island, was estimated to be in the range of 0.6 to 1.2 million ton. From these concrete rubble volumes, the production of RCA were calculated to be in the range of 0.2 to 1.0 million ton corresponding to a recycling level of 30-90 %. In Norway, Finland and Sweden the current recycling level is at 30, 50 and 60 % respectively. However, these countries reported that the target recycling level is 70 % by the end of 2010. The current recycling level in Denmark was reported to be 90 & and is expected to be the same by the end of 2010. In Findland, however, a major increase in the concrete rubble generation (from 45 % to 60%) as well as the expected increase in the recycling level results in a major increase approximately 50 000 ton which is landfilled.

In the laboratory different concrete mixes were tested for CO₂ uptake. After hardening the mixes were crushed into different grain sizes. It was found that 60-80 % of the CO₂ released during calcination is reabsorbed to the concrete mixtures with w/c of 0.6 or higher for the grain size of 1-8 mm within 20-35 days of exposure. Furthermore, the calculation showed that 60-90 % of the total CaO in the same samples was carbonated. Determination of the total carbon in the carbonated samples by total combustion and CO₂ detection showed reasonable agreement with the measured CO₂ uptake.

The w/c ratio was found to be crucial as expected. Comparison of the mixes with the w/c ratios from 0.4 up to 0.75 showed large differences as the highest w/c ratio gave the highest carbonation rate. Thus, it was found that more than 90 % and less than 10 % of the CO₂ was absorbed within the first 50 hours of exposure for the mixes with w/c of 0.75 and 0.4 respectively testing samples with grain size of 1-8 mm. Coarser aggregate samples carbonated significantly slower.

Although the reaction kinetics varied due to the changing CO₂ partial pressure in the exposure desiccators, this test setup provided a fairly rapid quantification of the carbonation rate in between different concrete mixes. The tests also gave a realistic measurement of the total CO₂ uptake for the different samples which can support the documentation of concrete carbonation during service life and secondary use.