###### Abstract  
Based on the promotion and application of green and low-carbon technology, this study aims to develop a high-safety performance cement concrete incorporating a large dosage of fly ash (FA). The safety and effect of FA content on the mechanical properties of FA composited cement were studied through compressive strength, flexural strength, and microscopic tests. The results show that when the FA replaced 20% cement, the properties of concrete were the best in this study. The flexural strengths and compressive strengths of the standard cured concrete for 28 days with 20% FA content are 0.82 MPa and 4.32 MPa larger than that of the pure cement concrete. The XRD and SEM analysis suggested that the mechanical properties of the composite cement FA system are improved significantly since the replacement of cement by FA promotes secondary hydration of calcium hydroxide in the concrete, leading to a more compact and safe interface between cement and FA. low-carbon concrete; fly ash; mechanical property; microstructure; high-safety performance +
Footnote †: journal: Applied Sciences  
**Citation**: [PERSON]; [PERSON]; [PERSON] Safety and Effect of Fly Ash Content on Mechanical Properties and Microstructure of Green Low-Carbon Concrete. _Appl. Sci._**2020**, **14**, 2796. As an important link in sustainable development, the preparation methods of green and low-carbon concrete have been developed and promoted successively. The mechanical properties of concrete are affected by many factors, such as temperature and loading ratio [11; 12]. Previous studies demonstrated that the reasonable composition ratio of mineral admixtures and recycled aggregates can greatly improve the performance of composite admixtures [13; 14; 15]. On the one hand, the compaction method and microstructure of concrete are also important factors affecting its performance [16]. [PERSON], et al. [17] investigated the effect of four compaction techniques on the performance of concrete after 28 days through uniaxial compression and splitting tests. Results showed that the mechanical properties and mixing parameters of concrete are closely related to compacted density. On the other hand, an appropriate amount of mineral admixtures can achieve an effective supply of cement materials and improve the technical performance at a low cost [18; 19]. [PERSON] and [PERSON] [20] found that the waterproofing admixture presents a strong inhibiting effect on the micro-cracking of the concrete surface by producing calcium silicate hydrates. [PERSON], et al. [21] performed laboratory tests to evaluate the hydration properties of concrete by adding magnesium slag, and the results suggest that a magnesium slag content of 30% can improve the working performance of concrete in frost resistance and crack resistance. Therefore, a series of mineral admixtures provide a convenient solution to enhance the mechanical properties of concrete considering the low-carbon and sustainability requirements. A large amount of fly ash (FA) is produced in the coal industry every year as a by-product with a low utilized value [22]. At present, given that the utilization rate of FA is about 50% in the world, a lot of FA waste is piled up in landfills, occupying much arable land and space [23]. [PERSON], et al. [24] calculated the CO\({}_{2}\) emissions and cost of the concrete with FA addition and suggested that the effect of FA content on CO\({}_{2}\) emissions is more obvious for higher-strength concrete. [PERSON] and [PERSON] [25] reported the relative contribution of FA to reduce CO\({}_{2}\) emissions is higher than 15% in the production process. Based on the above consideration, many studies declared that FA can be applied suitably as a substitute for cement to meet the needs of hydration products in concrete preparation, which is also an effective measure to produce low-carbon concrete [26]. Therefore, the exploration of FA admixture has become a research topic of green low-carbon concrete [27].