Analysis of the Strength Development, CO2 Emissions, and Optimized Low-Carbon Design of Fly-Ash-Enhanced Composite Concrete

Fly ash is commonly used as a partial replacement for cement. Although extensive research has been conducted on mixed design schemes for fly ash concrete, these studies commonly overlook carbonation durability, which may lead to an insufficient service life. This study investigates the strength development and optimal low-carbon design of composite concrete that incorporates fly ash. Initially, a regression analysis was performed to assess the compressive strength of concrete with various fly ash (FA) to FA plus cement (C) mass ratios (FA/(C + FA)), ranging from 0% to 55%, and water-to-binder mass ratios (W/(C + FA)), between 0.30 and 0.50 in the following different stages: early-term (3 and 7 days), mid-term (28 and 56 days), and long-term (90 and 180 days). The correlation coefficient between the predicted and actual strength values was 0.98208. The parameter analysis indicates that, for a given FA/(C + FA) ratio or curing duration, the relative strength of the concrete increases more rapidly with a lower W/(C + FA) ratio. Following this, with the water content held constant at 170 kg/m3, the CO2 emissions of the cementitious materials in the concrete and the CO2 emissions per unit of concrete strength were calculated. As the FA/(C + FA) ratio increased from 0% to 55%, the CO2 emissions per unit of strength decreased. Similarly, reducing the W/(C + FA) ratio from 0.50 to 0.30 also lowers the CO2 emissions per unit of strength. Finally, a genetic algorithm was employed for the optimization of the low-carbon design. As the water content decreases and the concentration of CO2 increases, the critical design strength of the concrete must also increase to meet the requirements for carbonation durability. The fly ash replacement rate in each optimized mix design reached the maximum allowable replacement rate. Compared with considering only the carbon emissions of materials, the optimal mix ratios for each case remain unchanged when accounting for transportation and production processes; only the corresponding carbon emissions differ.

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