# Properties and Mechanism of Hydration of Fly Ash Belite Cement Prepared from Low-Quality Fly Ash
## 1 Introduction  
Nowadays, the annual output of fly ash is approximately 600 million tons in China. A part of it obtained by the dry process has been basically applied in cement and concrete. However, still a lot of fly ash obtained by the wet process has not been employed in cement and concrete production due to its coarse particles, high carbon content, and low pozzolanic activity [1; 2; 3; 4]. Accordingly, more than 1 billion of low-quality fly ash has been stockpiled, which not only occupies land but also pollutes the environment. Some recent studies applied ultrafine grinding to improve the activity of stockpiled high-carbon fly ash so as to meet the requirement of fineness and activity index of admixture in cement and concrete. However, this method could not decrease the content of carbon. Moreover, it had significant adverse impacts on the strength and frost resistance of concrete [5; 6; 7; 8; 9]. Therefore, the resource utilization of stockpiled high-carbon fly ash has been an urgent need. Fly ash belite cement (FABC) is a low-carbon cement prepared from fly ashes by hydrothermal synthesis and low-temperature calcination methods. The basic composition of FABC is active dicalciumsilicate (2 CaO-SiO\({}_{2}\); \(\beta\)-C\({}_{2}\)S), calcium aluminate (12 CaO-7 Al\({}_{2}\)O\({}_{3}\); C\({}_{12}\)A\({}_{7}\)), and some inert materials depending on the raw materials such as mullite, quartz, and iron oxide [10]. It has gained increasing attention from scholars because of much less CO\({}_{2}\) emission, low calcium oxide (CaO) content, and low heat of hydration. The hydrothermal synthesis and low-temperature calcination between 700 and 950 \({}^{\circ}\)C also improve the grindability of FABC and reduce energy consumption during preparation. Furthermore, this kind of cement can be synthesized from industrial wastes such as coal fly ash, coal gangue, and straw ash instead of natural quarried clay [11; 12; 13; 14]. The high carbon content of stockpiled fly ash can provide energy for burning in the preparation of FABC. Hence, it is an effective way to comprehensively use the stockpiled high-carbon fly ash by preparing FABC. The preparation methods of FABC are divided into two types based on different hydrothermal conditions of precursors [10; 12]: the slurry-hydrothermal synthesis method and granule-hydrothermal synthesis method. In the slurry-hydrothermal synthesis method, precursors are prepared mainly by hydrothermal synthesis at more than 200 \({}^{\circ}\)C with a water-to-solid ratio of 5-10. However, higher water requirement and lower early-stage strength have been identified as problems in the FABC application. [PERSON] et al. [5] and [PERSON] and [PERSON] [8] made a lot of effort to increase the content of hydraulic minerals by adding alkali activators in the preparation of precursors and reducing the water-to-cement ratio by adding a water-reducing agent in the preparation of cement pastes. Modified by 2% naphthalene water reducer, the 7-day compressive strength and the water requirement of FABC pastes increased by more than 40% and reduced by 15%, respectively. However, the 28-day compressive strength of mortars was lower than 20 MPa according to EN-196-2005, when the water-to-cement ratio dropped only from 1 to 0.85. FABC obtained by the slurry-hydrothermal synthesis method did not have better performance. On the contrary, in the granule-hydrothermal synthesis method, the hydrothermal synthesis temperature was lower than 100 \({}^{\circ}\)C. Some studies [15; 16; 17] suggested a hydrothermal synthesis method to synthesize belite cement, but the 28-day compressive strength of mortars prepared with this cement was lower than 20 MPa because the formation process of precursors was not activated by chemical activators. Previous studies [10; 18] revealed that FABC was prepared from high-carbon, low-activity stockpiled fly ash by the granule-hydrothermal synthesis method. The 28-day compressive strength of mortars modified with 1% water reducer reached 30.2 MPa.