CO2 and Cost-Based Optimum Design of Sustainable Metakaolin-Modified Concrete

Metakaolin is a highly reactive pozzolanic material that is widely utilized for enhancing the performance of concrete. This study offers a framework for the mixture design of sustainable metakaolin-modified concrete with low CO2 emissions and low costs. Different design strengths after 28 days are first formulated, with values such as 30, 40, 50, and 60 MPa. A genetic algorithm is then used to determine the optimal mixtures. Minimized CO2 and cost are set as the aims of the genetic algorithm. The strength of the concrete, its workability (slump), and carbonation service life with climate change are set as constraints of the genetic algorithm. Five design cases are considered: 1) low-CO2 concrete with no carbonation, 2) low-CO2 concrete with carbonation, 3) low-material cost concrete with carbonation, 4) low-total cost concrete with carbonation, and 5) low-total cost concrete with climate change. Based on the analysis, the following results are found: 1) When the design’s strength is 30 MPa, to satisfy the requirement of carbonation durability, the concrete real strength should exceed 30 MPa. Moreover, after considering climate change, the concrete real strength should be further improved. 2) When the design strength is 40, 50, or 60 MPa for low-total-cost concrete, climate change has no impact on the optimal design because the concrete has sufficient carbonation resistance. 3) Low-material-cost concrete has the same mixture as low-total-cost concrete because compared with the material cost, the CO2 emission cost is much lower. Moreover, for low-material-cost or low-total-cost concrete, the metakaolin content is at the lower limit because the price of metakaolin is much higher than that of cement. Summarily, the proposed model covered different aspects of sustainable concrete, such as cost and CO2 emissions, clarified various decisive factors of mix design, such as strength and carbon durability, and considered different conditions of climate change, such as no climate change and Representative Concentration Pathways (RCP)8.5. The proposed method is valuable for designing sustainable metakaolin-modified concrete with low CO2 emissions and costs.