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In the construction industry, it is imperative to replace natural sand with manufactured sand. The environment in northwest China is very complex and harsh. The application of manufactured sand concrete in such a harsh environment needs scientific and comprehensive research to meet the requirements. However, there is a gap in the study of sulfate freeze-thaw resistance of manufactured sand concrete in the complex environment of Northwest China. Therefore, this study investigated the northwest environment, designed an indoor sulfate freeze-thaw cycle experiment system close to the real environment, and designed a conductive titration method to test the sulfate ion content in hardened concrete. The influence of the freeze-thaw cycle on the diffusion depth of sulfate ions in manufactured sand concrete was studied. Finally, the following conclusions are obtained: In the process of conductometric titration, hydrochloric acid is used as the dissolution solution, Ag 2 O and CO 2 are used as neutralizers, and Ba(NO 3) 2 is used as the titration solution, which can perfectly eliminate the significant influence ions in the test solution, and can accurately test the content of sulfate ions in hardened concrete. The sulfate ion content of concrete at different depths was tested, and it was found that the two sides of sulfate erosion caused by freeze-thaw cycles were mainly reflected in about 100 freeze-thaw cycles. The limit number of freeze-thaw cycles N 0 and the limit diffusion depth X 0 are proposed, and the diffusion equation of manufactured sand concrete under the action of sulfate freeze-thaw cycles is derived. The proposed diffusion parameter v is unique to the concrete, which can well judge the sulfate freeze-thaw resistance of the concrete. When v is closer to 0, it shows that the resistance of concrete to sulfate freeze-thaw erosion is better. For the northwest environment of China, the manufactured sand concrete should ensure v < 6; the environment in Northwest China has a very complex multi-factor erosion environment. It is worth noting that this study is only applicable to laboratory conditions. To improve the accuracy and reference value of experimental research, future research needs to add more comprehensive environmental impact factors and collect actual data in the northwest environment of China for verification. • A detailed conductivity test method for sulfate ion content in hardened concrete was designed. • The diffusion equation of manufactured sand concrete under sulfate freeze-thaw cycles is derived by proposing the limit number of freeze-thaw cycles N 0 and the limit diffusion depth X 0. • The diffusion parameter v proposed by is a new concept, which can be used to judge the key index of sulfate freeze-thaw damage, and is of great significance for evaluating the frost resistance of manufactured sand concrete in the northwest environment of China. [ABSTRACT FROM AUTHOR]

• The carbon reduction rate can reach 23.01% and 28.52% when the fly ash replacement rate is 24.5% and the stone powder is 8.33%. • The calculation method of carbon emission factor of manufactured aggregate is proposed. • The carbon emission factor of manufactured sand is 7.651 kgCO 2 /t, and the carbon emission factor of manufactured crushed stone is 8.483 kgCO 2 /t. • The change rate of compressive strength and carbon reduction rate were fitted by Gaussian distribution function. The fitting accuracy can reach more than 93 %. In recent years, it has become an inevitable trend to replace natural sand with manufactured sand. Manufactured sand is accompanied by carbon emissions from production to use. This study regarded this process as the process of manufactured sand from "cradle" to "adult". Based on the case, the energy consumption and carbon emissions of the process were tracked, the carbon emission factors of the manufactured sand and the manufactured gravel were determined, and the carbon emissions of the manufactured sand concrete (MSC) were calculated. On the other hand, the Gaussian distribution function is used to fit the compressive strength change rate and carbon reduction rate of manufactured sand concrete under different stone powder replacement rate and fly ash replacement rate. The fitting accuracy can reach more than 93%. The fitting results show that the fly ash replacement rate is 24.45%, and the stone powder replacement rate is 8.33%. The mix ratio is the optimal mix ratio, and the carbon reduction rate can reach 23.01% and 28.52%, respectively. [ABSTRACT FROM AUTHOR]