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The global consumption of Portland cement has risen to over 4 billion tonnes per annum. Its manufacture is energy and carbon intensive and approximately 900 kg of CO2 is emitted into the atmosphere for each tonne of Portland cement produced. The International Energy Agency (IEA) roadmap sets out a goal to reduce emissions due to cement production to 18 % below 2006 levels by 2050. Concrete has the potential to re-absorb CO2 by the process of carbonation, where it reacts with CaO in the concrete to form calcium carbonate. Accelerated carbonation curing (ACC) is a technique for curing fresh concrete that can sequester CO2. ACC of concrete masonry units (CMU’s) can reduce the embodied carbon footprint and play a major role in sustainability by reducing global CO2. ACC also offers potential improvements in the mechanical and durability properties of concrete. Experimental work was carried out which involved the ACC of CMU’s at a CO2 concentration of 50% over various time intervals and exposure conditions. It was calculated that the maximum possible CO2 uptake potential of the cement was approximately 49.5%. A CO2 uptake of 23% per mass of cement was achieved after 7 days of ACC along with compressive strength increases of 15.4% and 28% for ACC samples at 7 and 28 days respectively. The study found that the greatest compressive strength increase occurred between 4 and 24 hours. After 24 hours the ACC process showed a similar proportional rate of strength gain over time when compared to the control. The study shows that ACC is different from weathering carbonation as it accelerates the hydration reaction of the unhydrated cement phases C3S and C2S producing rapid strength gains. Weathering carbonation occurs in concrete after the hydration process has been predominately completed and results in the decalcification of C-S-H and the formation of silica gel which is detrimental to the cement paste.
Gilroy, B., Black, L., Thompson, D., Hogan, R., & Holmes, N. (2020). Effects of accelerated carbonation curing on CO2 sequestration and on the compressive strength of concrete masonry units. CERI: Civil Engineering Research in Ireland (CERI) 2020, 27th. August, online.