Investigation of the medium calcium based non-burnt brick made by red mud and fly ash: durability and hydration characteristics

Red mud is a type of highly alkaline waste residue produced in the process of alumina smelting by the Bayer process.Based on the idea of medium calcium content,solid wastes such as red mud and fly ash were used to prepare non-burnt bricks;and the mass ratio of CaO/SiO2 was selected in the range of 0.88–1.42.Mechanical properties and durability were investigated with a compressive strength test.X-ray diffractometry(XRD),scanning electron microscope(SEM),and Fourier transform infrared spectroscopy(FTIR)techniques were used to characterize the hydration characteristic.The environmental performance was analyzed by Inductively Coupled Plasma Mass Spectrometry(ICP).The results indicated that the mechanical properties and the durability were optimal when the mass ratio of CaO/SiO2 was 1.23.The hydration products were mostly C–S–H gel,ettringite,Na4Ca(Si10All6)O32·12H2O and Ca3Al2(SiO4)(OH)8.They were responsible for the strength development,and the CaO/SiO2 mass ratio of 1.23 had the best polymerized structure.The results of an environmental performance test showed that the heavy metals in the raw materials were well-solidified in the brick.Therefore,this paper provides an effective solution for use of solid wastes in building material.

Microstructure and cementitious properties of calcined clay-containing gangue

To investigate the optimum calcination temperature and cementitious properties of gangue, the microstructure of clay-containing gangue calcined at different temperatures was analyzed by X-ray diffraction （XRD）, infrared spectroscopy （IR）, and magnetic angle spinning nuclear magnetic resonance （MAS NMR）. The results show that the structure of kaolinite in the gangue sample calcined at 500℃ is destroyed. The XRD spectra show the disappearance of illite at about 800℃ and the formation ofmullite at about 1000℃. With the increase in calcination temperature, octahedral （6-coordinated） aluminum is transformed to tetrahedral （4-coordinated） aluminum gradually. For the gangue sample calcined at 700℃, the 29Si MAS NMR sharp peak of Q4 （framework silicate-quartz） is left. Compared with kaolinite in gangue, the thermal transformed temperature of pure kaolinite is lagged. On the basis of the microstructure and cementitious properties of calcined gangue, the results can be concluded, in order to obtain metakaolinite, the optimum calcination temperature of this gangue is about 500℃, and the optimum temperature is about 700℃ for activated SiO2 and Al2O3.