# Preparation of Cementitious Material with Wet Fly Ash by Hydrothermal Reaction and Calcination
###### Abstract  
A large amount of wet-discharged fly ash has caused serious harm to the ecological environment, so the utilization of fly ash has received attention. This paper analyzes the formation of products of fly ash-time system under the autoclave process by X-ray diffraction (XRD) analysis and thermogravimetric-differential thermal (TG-DSC) analysis. The hydrothermal reaction product generation was quantitatively analyzed using the hydrochloric acid selective dissolution method to quantify the degree of reaction of fly ash in the pressure evaporation specimens in combination with the reaction degree of lime in the autoclave specimens. The hydrothermal reaction products were calcined and hydrated, and the mineral composition of the calcined products and the mechanical and microstructure of the hydrated products were analyzed. The results show that hydrothermal reactions occur in the fly ash-time pressure evaporation system to produce C\({}_{2}\)SH, C\({}_{3}\)AH\({}_{6}\), C\({}_{3}\)ASH\({}_{4}\), and other products. The optimum ratio of lime is 22%, and the appropriate autoclave parameter is 140 \({}^{\circ}\)C for 8 h. Under this condition, the reaction degree of fly ash is 15.39%, the reaction degree of CaO is 78.63%, and its f-CaO value is 4.93%. The formation of C\({}_{2}\)SH in the hydrothermal reaction ranged from 14.33% to 18.53%, and the formation of C\({}_{3}\)ASH\({}_{4}\) ranged from 14.06% to 15.26%. The hydrothermal reaction products were calcined at 850 \({}^{\circ}\)C for 1.5 h to produce new gelling materials with gelling phases, such as C\({}_{12}\)Ar, \(\alpha^{\prime}_{\rm L}\)-C\({}_{2}\)S, \(\beta\)-C\({}_{2}\)S, and C\({}_{2}\)AS. The compressive and flexural strengths of the new gelling materials reached 34.4 MPa and 6.4 MPa, respectively, at the age of 180 days.  
fly ash; hydrothermal synthesis; calcination; hydration products +
Footnote †: journal: Applied  
will reduce the strength of the concrete. This kind of fly ash can be used as alkali-activated material [15; 16], or mixed with high limestone content and used in the hydrothermal reaction process at low temperatures, and the fly ash will undergo a pozzolanic reaction and generate hydraulic precursors [17; 18; 19; 20]. Calcination products are related to the amount of water used, reaction temperature, reaction pressure, treatment time in the hydration process, and treatment temperature and time in the calcination process [21; 22; 23]. The high hydration activity of the mineral structure of belite under low-temperature calcination is due to the high number of defects in the mineral structure of belite, which differs significantly from the properties of belite in conventional cement [24]. However, the low early strength of fly ash belite cement is an obstacle to their further development [25; 26]. The compressive strengths of the 1 cm \(\times\) 1 cm \(\times\) 1 cm mortar specimens were only 11-30 MPa [27], and the highest strength of the 4 cm \(\times\) 4 cm \(\times\) 16 cm mortar specimen wet-cured at 40 \({}^{\circ}\)C for 28 days was only 15 MPa [28], as reported by [PERSON] et al. Compared with ordinary Portland cement, its raw material is solid waste, with lower calcination temperature and lower counterfeiting cost.  
As the reaction conditions of hydrothermal treatment determine the type and content of reaction products, they also impact the content of hydraulic minerals such as belite and calcium aluminate in calcined fly ash belite cement. Therefore, studying the reaction degree of reactants in a hydrothermal reaction system and the formation process of hydrothermal reaction products is essential. This paper's hydrothermal reaction degree and products of fly ash are quantitative. The influence of the content of fly ash and lime and the change in autoclave process parameters on the formation of products is discussed to propose the process parameters of the autoclave process for preparing new cementitious materials.