This research aims to examine the effect of carbonation on the strength properties and carbonation depth of ordinary Portland cement (OPC) concrete using two different water to cement ratios (w/c) and two different replacement percentages of natural coarse aggregate (NCA) with recycled coarse aggregate (RCA). Two concrete mixes were prepared using w/c of 0.4 and 0.43. The two concrete mixes were subdivided into two subgroups based on the use of NCA and 30% RCA. The first concrete mix having w/c of 0.4 was contained NCA and from this concrete, 42 cylinders of 100 mm dia. and 200 mm height were cast. Six out of 42 cylinders served as control specimens and were not exposed to CO2. A total of 18 out of the remaining 36 cylinders was exposed to the simulated environment and the rest were exposed to the natural environment. The second concrete mix having a w/c of 0.4 contained 30% RCA/70% NCA, and using this concrete, 42 cylinders of similar size were cast. A similar scheme was adopted for w/c of 0.43 and, in total, 84 cylinders using four mix designs were cast. After casting and 28 days of curing, six out of 42 cylinders cast from each concrete mix design were tested for compression and splitting tensile strength, following ASTM C39 and ASTM C496 without any exposure to carbon dioxide (CO2). A total of 18 out of the remaining 36 cylinders was exposed to the simulated environment in a carbonation chamber for an equivalent time duration of 90, 180 and 365 days following CEN test guidelines and the other 18 cylinders were kept in the natural environment for a period of 90, 180 and 365 days. After the completion of simulated and natural exposure periods, these cylinders were distributed equally to test for compressive strength and splitting tensile strength to observe the effect of carbon dioxide (CO2) at each time duration (i.e., 90, 180 and 365 days), and replacement percentage of RCA (i.e., 0 and 30%), which showed that carbonation depth increases incrementally with the w/c ratio and CO2 exposure duration. In both the simulated and the natural environment, the use of RCA in concrete cast using a w/c of 0.4 increased carbonation depth up to 38% and 46%, whereas, in the case of the concrete cast using a w/c ratio of 0.43, the use of RCA increased the carbonation depth up to 16% and 25%. In general, the use of RCA in the concrete exposed to the natural environment significantly affected the compressive strength of concrete, due to multiple interfaces and the porous structure of RCA, and the variation in the temperature, humidity and content of carbon dioxide (CO2) present in the actual environment. The maximum compressive strength variation prepared from the mixes M0-0.4, M30-0.43, M0-0.43 and M30-0.43 differed by 5.88%, 7.69%, 16.67% and 20% for an exposure period up to 365 days. Similarly, the results of splitting tensile strength tests on cylinders prepared from the same mixes exposed to the natural environment differ by 7.4%, 27.6%, 25.41% and 18.2% up to 365 days of exposure, respectively, as compared to the simulated environment.
Mathematical modeling of the equilibrium complete replacement of methane by carbon dioxide in a gas hydrate reservoir at negative temperatures
