Study On Synthesis Of Ettringite Crystal By Electrodialysis

Ettringite is one of the key hydration products of high-sulfur bearing cementitious materials,such as sulphate aluminate cement and circulating fluidized bed combustion ash.The formation mechanism of ettringite is unclear,so that the effective control of ettringite crystal growth is not possible,which leads to the expansion and cracking of blocks prepared using high sulfur cementitious materials.In this paper,ettringite is synthesized by electrodialysis.Under the help of external electric field,AlO2,Ca2+and SO42-ions mobility to the crystallization chamber fast and directly.The ions exchange membrane blocked impurities.High-purity ettringite crystals formed in the crystal chamber.the results of this investigation shows as follows:(1)Thermodynamic calculation show that reaction of Ca2+,SO42-,OH-and Al(OH)4 ions to synthesize ettringite has the greatest tendency in electrodialysis system.Increasing temperature promotes the formation of ettringite crystals.Meanwhile,side reactions such as Ca(OH)2,CaCO3 and CaSO4 may occur.The mobility of three ions was tested by experiments,of which the mobility rate of Ca2+ions was the largest,followed by SO42-.ions.Because of the large hydraulic radius of AlO2-ions,the transmission efficiency is the lowest.45℃ is the best electrodialysis reaction temperature.(2)At low ion concentration(SO42-0.3165 g/L;AlO2-0.0708 g/L;Ca2+ 0.1701 g/L),the growth rate of ettringite crystal is greater than the nuclear rate,the crystal aspect ratio increases.The(0001)crystal planes are higher than that of(1010)crystal planes;Relatively excessive SO42-ions adsorb on the(1010)crystal planes containing Ca2+ions,inhibiting the growth of ab axis and promoting the crystal elongation along the c axis,with average length of 55.89 μm;At high ion concentration(SO42-0.5564 g/L;AlO2-0.0890 g/L;Ca2+ 0.6346 g/L),the nuclear rate of ettringite crystal is greater than the growth rate.The difference between the growth rate of(1010)and(0001)crystal planes decrease.The cell parameters(a0=1.1108 nm,c0=2.1467 nm)are slightly smaller than low concentration conditions(a0=1.1178 nm,c0=2.2588 nm).Crystals have poor uniformity and short rod shaped,with average length of 33.74 μm.(3)Increasing the concentration of AlO2-ions to approximate the reaction environment in the cement system.On this basis,the formation mechanism of ettringite crystal was studied.The results show that a large number of Ca2+,S04",AlO2-and OH-ions,under the condition of electric field force and stirring action,collide and bond with each other to form line crystals,and when they increase to certain limit,they come together to form crystal surfaces;Due to the strong bond energy of Ca-0 and Al-O,(0001)crystal planes are preferentially formed,grow rapidly,and gradually shrink to disappear.The growth rate of {1010} crystal plane family are slow,which are the main exposed surface of ettringite crystals.New microcrystals grow on the surface of the formed crystals,showing longitudinal fibrils,and multiple fibrils are stacked laterally to attach/surround to form ettringite crystals.(4)In the early stage of electrodialysis and curing of cement system,due to low AlO2and high mobility rate Ca2+ions,the system tends to form calcium hydroxide first.As the concentration of AlO2-ions increase,it is transformed into ettringite with more stable thermodynamics.If the solution is contaminated with CO2,it will react with Ca2+ ions in the system to generate calcium carbonate,forming a radial aggregate morphology.