To solve the common problems of concrete preparation in low-temperature environments, calcium formate (C2H2O4Ca), anhydrous sodium sulfate (Na2SO4), triethanolamine (C6H15O3N), calcium bromide (CaBr2), and triisopropanolamine (C9H21NO3) are selected as early strength agents and mixed with C40 concrete in different dosages under low-temperature environments of 5 oC and 10 oC to develop a high-efficiency low-temperature compound early strength agent based on the effect of single-doped early strength agents. The effects of the compound early strength agent on the early strength of the concrete, the cement paste setting time, and cement fluidity at 5 oC and 10 oC are investigated, and the corresponding reaction mechanism is discussed from the perspective of micro-products. The best compound early strength agent ratio is found to be 2% of calcium formate + 0.08 % of TEA (C6H15O3N). The compound early strength agent effectively promotes the formation of hydration products, such as Ca(OH)2 and C-S-H gel. In comparison with the control group, the strength of the concrete cured for 18 h, 1 d, 3 d, and 7 d under simulated natural conditions at 5 oC increases by 700%, 540%, 11.4 % and 10 %, respectively, whereas at 10 oC, the corresponding values are 991%, 400%, 19.6 % and 11 %, respectively. The strength of the concrete at each age is close to the normal temperature standard of the curing strength. The addition of the compound early strength agent causes a reduction in cement fluidity and initial and final setting times, and also yields a good effect on the porosity of the early concrete.

Abstract
1. Introduction
2. Materials and Experiments
    2.1 Raw Materials
    2.2 Experimental Methods
3. Experimental Results and Discussion
    3.1 Results and Analysis of the Early StrengthAgent Single-doped Group
    3.2 Results and Analysis of the Early StrengthAgent Double-doped Group
4. Microscopic Analysis of Hydration Products
    4.1 Analysis of Hydration Products
    4.2 Microstructural Analysis
5. Conclusion
References

• Junwei Bao(Anhui Province Engineering Laboratory of Advanced Building Materials, Anhui Jianzhu University/Anhui Province Key Laboratory of Advanced Building Materials, Anhui Jianzhu University)
• Qifang Ren(Anhui Province Engineering Laboratory of Advanced Building Materials, Anhui Jianzhu University/Anhui Province Key Laboratory of Advanced Building Materials, Anhui Jianzhu University)
• Lei Sun(Anhui Road and Bridge Engineering Group Co., Ltd.)
• Yi Ding(Anhui Province Engineering Laboratory of Advanced Building Materials, Anhui Jianzhu University/Anhui Province Key Laboratory of Advanced Building Materials, Anhui Jianzhu University) Corresponding author
• Won-Chun Oh(Department of Advanced Materials Science & Engineering, Hanseo University) Corresponding author