Utilization of Bayer Red Mud Derived from Bauxite for Belite-Ferroaluminate Cement Production

Bayer red mud(BRM)is a kind of industrial solid waste characterized by huge volume and high alkalinity.Its disposal generates serious environmental pollution and occupies a large number of farmland.The utilization and recycling of BRM is currently a crucial issue and needs to be addressed as soon as possible.The chemical composition of BRM is similar to cement clinker.In this study,the feasibility of preparing Belite-ferroaluminate clinker(BFAC)with different BRM was explored.The physical properties,mechanics performance,radioactivity levels and trace harmful metals leaching were measured.XRD,BEI and EDS were used to characterize the mineral formation,and SEM is used to reveal the solidified mechanism of trace harmful metal.The results show that the preparation of BFAC using a certain amount of BRM was feasible.The formed phases in clinkers mainly included C_(4)A_(3)Š,C_(2)S and C_(4)AF.The flexural strength and compressive strength of BFAC at 3 days increased whereas 28 and 90 days decreased with the increase of BRM due to the formation of higher C_(4)AF and lower C_(2)S.The formation of large amounts of Al_(2)O_(3)·3H_(2)O gel and Fe_(2)O_(3)·3H_(2)O gel in hydration products enhanced the adsorption capability to heavy metals and other ions.The trace harmful metal concentration in the leaching solution was much less than the upper limits.The radioactivity level of leaching solution was close to natural radioactive background.BRM is safe as raw material of BFAC.

Preparation and Performance of a Fluorine-Free and Alkali-Free Liquid Accelerator for Shotcrete

Based on aluminum sulfate,a fluorine-free and alkali-free liquid accelerator(FF-AF-A)was prepared in this study.The setting time and compressive strength of three cement types with different FF-AF-A dosages were fully investigated.The compatibility of the FF-AF-A with the superplasticizers were also investigated,and the early hydration behavior and morphology of the hydration products of reference cement paste with the FF-AF-A were explored by hydration heat,X-ray diffractometry(XRD),and scanning electron microscopy(SEM).Test results indicated that adding the FF-AF-A at 8 wt%of the cement weight resulted in 2 min 35 s initial setting time and 6 min 30 s final setting time.The 1-day compressive strength of the cement mortar with 8 wt%of FF-AF-A reached 13.5 MPa,which represents an increase of 35%as compared to the strength of cement mortar without the FF-AF-A,and the 28-day compressive strength ratio was 119%.In addition,the FF-AF-A also showed good compatibility with different superplasticizer dosages.The results show that,when the FF-AF-A was added to the cement paste,it promoted the formation of ettringite crystals due to the aluminum ions(Al^(3+))and sulfate ions(SO_(4)^(2-))reacted with gypsum in the cement,as well as promoted the hydration of tricalcium aluminate(C_(3)A)and tricalcium silicate(C3S)leading to the overall structure becomes more compact.As a consequence,the hydration heat rate of the cement sharply increased,the cement paste setting time is shortened,and the compressive strength of cement mortar is improved.