Low-temperature Synthesis of Belite Cement from Reactive Mixtures Based on Coal Fly Ash

This paper summarizes the selected results of an extensive investigation of application of two methods （hydrothermal and mechanochemical） assisted by calcination for synthesizing belite cement from reactive mixtures （CaO/SiO2 molar ratio of 2） consisting of various waste kinds from fluidized brown coal combustion in Slovakian power plant and CaO addition. Based on XRD diffraction patterns and infrared spectra ofpre-treatment products, the formation of the new profiles corresponding to CSH phases with low degree of ordering as belite precursors after hydrothermal treatment as well as metastables calcium silicates and aluminosilicates in mechanosynthesized products was confirmed. Calcination of hydrothermally treated products led to transformation of CSH phases to wollastonite （CS）, belite and gehlenite phase, whereas creation oft^- and I^-C2S or wollastonite in milled reactive mixture took place. Differences in phase composition of products before and after calcination depend upon waste quality and precursor＇s synthesis conditions. Bottom ash isn＇t suitable as raw material for synthesizing belite phase because of high CaO content fixed in anhydrite form （44.1%）. Coal fly ash with low CaO content in anhydrite form （4.2%） and its mechanochemical or hydrothermal treatment in combination with subsequent heating offer opportunities for the utilization of coal fly ash as raw material for belite production.

Solutionizing of Additional Ions in Belite-rich Clinkers and the Properties of the Resulting Cement

Belite-rich cement （BRC） can be made at lower temperature, but it has unsatisfactory reactivity. The crystal structure of dicalcium silicate （C2S） was modified by solutionizing some additional irons. By adding barium sulfate （BaSO4） in the raw meals, the clinkers were easier to be burnt, and the compressive strength of BaSO4-modified BRC was considerably improved. The distortion of the crystal structure of C2S was confirmed by the interplanar distance change and nuclear magnetic resonance （NMR） of ^29Si in C2S. An effective way was found to activate C2S and to broaden the application field of Belite-rich cement.

INFLUENCE OF ORDINARY ACCELERATORS ON HYDRATION OF BELITE CEMENT

The study revealed that ordinary acceler- ators for portlandcement also improves early strength of Belite cement remarkably, butone serious defect exists Which lead to a high strength reduction atthe age of 28 days. The paper analyzes the action mechanism ofordinary accelerators on Belite cement. It shows that theseaccelerators can not accelerate the hydration of β-C_2S directly.

Influence of Hydrothermal Pre-Treatment of Coal Fly on the Synthesis of Belite Phase

In this paper, the results of an extensive investigation of hydrothermal pre-treatment for synthesizing belite phase from reactive mixtures （CaO/SiO2 molar ratio of 2） consisting of one waste kinds （bottom ash-BA or fly ash-FA） from fluidised brown coal combustion in Slovakian power plant and CaO （analytical grade reagent） addition are summarized. Changes in structure and phase composition of hydrothermally synthesized belite precursors and subsequent calcinated products were compared with those of starting mixtures. Based on XRD diffraction patterns, the formation of the new profiles corresponding to CSH phases with low degree of ordering as belite precursors after hydrothermal treatment was confirmed. Calcination of hydrotermally treated products at 900℃ led to transformation of CSH phases to wollastonite, belite and gehlenite phase. Differences in phase composition of products before and after calcination depend upon waste quality and precursor＇s synthesis conditions. Bottom ash isn＇t suitable as raw material for synthesizing belite phase because of high CaO content fixed in anhydrite form （44.1%）. Coal fly ash with low CaO content in anhydrite form （4.2%） and its hydrothermal treatment in combination with subsequent heating offer opportunities for the utilization of coal fly ash as raw material for belite production.