Accelerated Carbonation Curing as A CO2 Sequestration And Water Conservation Technique

Abstract

Global warming has become one of the biggest problems for a sustainable future ever since the start of industrial era. CO2 is a major greenhouse gas which is responsible for global warming, with cement industry contributing towards 5% of worldwide CO2 emissions. CO2 sequestration is described as long-term storage of atmospheric CO2 in stable forms, and is identified as one of the solutions to further mitigate higher levels of CO2.The ability of cement to react with the atmospheric CO2 makes the cement-based materials viable carbon sinks. The process of interaction of cement with atmospheric carbon dioxide is called natural carbonation. Instead of natural carbonation, the process of deliberately supplying CO2 to the cement system during very early ages of strength development is called accelerated carbonation curing (ACC). During ACC, the captured CO2 gas is passed over freshly cast concrete for a certain duration, pressure and relative humidity. In this process, CO2 reacts with unhydrated cement components (calcium silicates) and hydrated cement compounds (calcium hydroxide and C-S-H) to form CaCO3 that is stored as stable precipitate in concrete. The objective of this research work is to evaluate the performance of concrete when subjected to ACC. The testing programme is divided into various subparts. In the first part, the performance of carbonation cured concrete was evaluated vis-à-vis water curing in terms of mechanical properties, permeation properties and microstructural changes of concrete, with special emphasis on surface characteristics.ACC was carried out for a period of 6 hours after initial preconditioning of 2 hours and followed by water spray for three days. The microstructural analysis of the mixes indicated considerable modification of the concrete surface by adopting carbonation curing. It was observed that calcium carbonate formed during the carbonation process gets accumulated in the concrete pores, thereby refining the pore structure and improving the permeation characteristics of concrete. Due to the pore refinement and formation of additional CSH gel formed during the carbonation curing process, an improvement in mechanical properties was also visible. The benefit of ACC was more pronounced on abrasion resistance of concrete as compared to compressive strength. Both the techniques were further analysed by ANOVA and stoichiometric analysis. ANOVA data indicated that ACC has a more prominent effect on surface of concrete than on the bulk properties. Further, the effect of carbonation curing duration on the properties of concrete was investigated. For this, the concrete specimens were subjected to carbonation curing duration of 6, 12 or 24 hours and the results were compared with water cured concrete. The mechanical properties of concrete were found to vary significantly with variation in duration of ACC. During the early age testing, the strength of concrete was observed to increase with the increase in carbonation duration. However, at the later ages, highest strength and abrasion resistance was achieved by C12 mixes. Mixes subjected to 24 hours of ACC exhibited lower strength and abrasion resistance; which was attributed to the higher intensity of carbonation in the C24 specimens that resulted in decalcification of CSH into weak silica gel. Secondly, the higher intensity of carbonation led to extensive pore water loss from concrete, which not only created internal voids in the mix, but also depleted the available water required for further hydration in concrete. The durability of concrete subjected to different duration of carbonation curing was analyzed using drying shrinkage test and resistance to weathering carbonation. The drying shrinkage of concrete was observed to be considerably reduced for carbonation cured specimens and the magnitude of reduction in drying shrinkage was observed to be directly proportional to the duration of carbonation curing. In all, carbonation curing of 12 hours was observed to be optimal as it provided better mechanical and durability characteristics to concrete. In the second part of this work, effect of ACC was studied on performance of mineral admixture based cementitious systems.Three types of mineral admixture based systems were considered: fly ash based portland pozzolana cement (PPC),ground granulated blast furnace slag (GGBS) and cement kiln dust (CKD). Both GGBS and CKD were incorporated as partial replacement of cement in replacement levels varying from 0% to 50%. It was observed that the later age performance of mortars made with fly ash based PPC or GGBS was negatively affected by adopting carbonation curing. It was due to fact that carbonation curing procedure consumed most of the Ca(OH)2 formed at early ages in the carbonation reaction and converted it into calcium carbonate. Due to this, Ca(OH)2 was not available in the matrix to allow the continuation of the pozzolanic reaction. Therefore, the advantage of presence of pozzolanic material could not be achieved in carbonation cured mortars. The post-conditioning of the specimens after carbonation reaction compensated to some extent on the negative effect of carbonation curing on the pozzolanic reaction. On the other hand, the performance of CKD based mortars was observed to be significantly improved with carbonation curing at both the early ages as well as later ages. The cementitious nature and fine particle size of CKD was advantageous in the carbonation curing regime. The CO2 sequestration potential of ACC procedure was investigated for all mixes from the TGA analysis of the mixes. ACC samples registered an effective CO2 uptake of 14%, 20% and 24% respectively for carbonation curing duration of 6 hours, 12 hours and 24 hours. In terms of CO2 sequestration ability of different cementitious based systems, it was observed that CO2 uptake capacity was 18%, 15%, 12% and 20% for OPC, PPC, GGBS and CKD based cementitious systems, respectively.