Preparation Of Alkali-activated Cementitious Materials From Thermally Activated Bayer Red Mud And Study Of Their Propertie

Bayer red mud is the residual of bauxite when extracting alumina products by Bayer process,the annual discharge of red mud in China’s aluminum industry factories is as high as hundreds of millions of tons,the p H of red mud lye is high and contains trace radioactive elements,therefore,the large accumulation of red mud has a negative impact on water resources,land resources and ecological environment.At present,red mud is mainly used in the production of building materials,recovery of valuable metals,improvement of soil and environmental protection.Although the utilization range is wide,a method for mass application of red mud has not been found.In addition,the metal content of red mud is low and the activity is poor,which usually requires additional treatment costs in the application.The phase composition of red mud is mainly composed of a large amount of aluminosilicate,so it has the potential to prepare alkali-activated cementitious materials,in this paper,bayer red mud was activated at high temperature,red mud,slag,fly ash and alkali activator were used as research objects,and red mud was used instead of fly ash to prepare alkali-activated slag-fly ash-red mud cementitious materials.The effects of activation temperature and dosage of red mud on the compressive properties,shrinkage properties and freeze-thaw cycle properties of alkali-activated cementitious materials were studied,and the optimum activation temperature and dosage of red mud were obtained.XRD,SEM,hydration heat and pore size distribution were used to further reveal the reaction mechanism of cementitious materials.The specific research contents are as follows:1.The effect of high temperature calcination on the phase composition of red mud was studied,the activity of red mud could not be activated at low temperature,and the activity of red mud increased gradually with the formation of Si O₂ and Al₂O₃ during high temperature calcination.When the activation temperature exceeded the optimum temperature,the formation of a large amount of Fe₂O₃ affected the activity of red mud.When red mud completely replaces fly ash to prepare mortar,the compressive strength of slag-red mud system is 3.9MPa lower than that of slag-fly ash system,if the activation temperature of red mud is less than 300°C,the activity of red mud is not activated,and the compressive strength of mortar under different red mud content is relatively close,the compressive strength of each group of specimens fluctuates around 60MPa for 28 days,and the compressive strength of mortar increases with the increase of age,the growth rate gradually decreases,indicating that the early strength of the alkali-activated system develops rapidly,and the specimens have good mechanical properties at the age of 14 days.When the activation temperature of red mud is between 520°C and 600°C,its activity is the best,and the compressive strength of alkali-activated slag-fly ash-red mud cementitious material mortar is obviously improved,the best ratio of raw materials is:slag 50%,fly ash 40%,red mud 10%,and the peak compressive strength of mortar reaches 83.24MPa.2.The mechanism of red mud content on the compressive strength of mortar was studied.The results showed that the heat release and heat release rate of the cementitious system increased after the addition of activated red mud,the addition of appropriate amount of red mud could promote the hydration of the cementitious system,accompanied by the formation of a large number of hydration gels.The interior of the slurry became denser and the compressive strength increased,at the same time,the addition of red mud changed the hydration products of the alkali-activated material.The crystal phase of sodium ferrite（Na₄Fe O₃）was detected by experiments,and the amorphous phase gel was the same as that of 0%group,the main components are hydrated calcium silicate（C-S-H）,hydrated calcium aluminosilicate（C-A-S-H）and polymer sodium aluminosilicate（N-A-S-H）.The interweaving of hydration products ensures the development of mechanical properties of the slurry.3.The change law of drying shrinkage performance of alkali-activated slag-fly ash-red mud cementitious material mortar under different activation temperature and dosage of red mud was studied.Adding unactivated red mud does not improve the drying shrinkage performance of mortar,especially the early shrinkage performance of mortar,when the activation temperature of red mud exceeds 450°C,the drying shrinkage curve of mortar moves down after adding red mud.It can be seen that the increase of activation temperature is beneficial to improve the drying shrinkage of mortar,and the drying shrinkage performance increases first and then decreases with the increase of red mud content.The test results of drying shrinkage and mass loss of mortar show that the optimum activation temperature is 520°C and 600°C,and the content is 20%.The pore size distribution of the paste with red mud content between 0%and 50%was determined by porosity test.It was concluded that the capillary pore volume was the largest when the red mud content was 0%.Therefore,it was concluded that the incorporation of a certain amount of red mud reduced the capillary pore volume of the specimen,thereby improving the shrinkage of the specimen.4.The frost resistance of mortar under the optimum activation temperature and content（0%～30%）was studied by freeze-thaw cycle test,and its frost resistance grade was evaluated,it was concluded that the mortar specimens with different red mud content did not show obvious appearance damage after 100 freeze-thaw cycles.However,with the increase of freeze-thaw cycles,the mass loss of the specimens increases gradually,and the compressive strength decreases gradually,and the mass loss rate of all specimens is less than 5%.Based on the compressive strength results of the specimens after 100 freeze-thaw cycles,the ability of the specimens to resist freeze-thaw cycles from strong to weak is RM-520-0.2>RM-600-0.1>RM-520-0.1>RM-600-0.2>RM-0>RM-520-0.3>RM-600-0.3（take RM-520-0.2 as an example,where RM is red mud,520 is the activation temperature,and 0.2 is the dosage of red mud）,at the same time,the specimens are more prone to frost heaving damage under 30%red mud content.Except for this group（RM-600-0.3）,the frost resistance grade of the mortar can reach 100 cycles.