Study On The Regulation Of Ettringite Morphology And Its Mechanism

Ettringite is a main product of sulphur aluminate cement hydration,and it has a more significant influence on the performance of cement and concrete.The formation conditions of ettringite affect its morphology.The study of the formation and morphology of ettringite is of great significance to the improvement of cement concrete performance.Pure ettringite samples were synthesized in the solution by using tricalcium aluminate and dihydrate gypsum,calcium sulphoaluminate and dihydrate gypsum,calcium hydroxide and aluminum sulfate according to the molar ratio of 1:3,1:2,6:1,respectively.The effects of synthesis time,raw material,synthesis temperature,p H value,different dosages of boric acid,lithium carbonate,polycarboxylate superplasticizer on the formation and morphologies of ettringite were studied by means of electron microscopy(SEM),X-ray diffraction(XRD),thermal analysis(TG-DSC)and infrared spectroscopy(IR).The results as followed:1.The morphologies of ettringite and the length-width ratio were significantly different due to the use of different synthetic materials.The ettringite synthesized with C3 A and dihydrate gypsum was much smaller than that of other groups of synthetic materials.The length-width ratio was about 1/10 of other sets of length-width ratio.The formation of ettringite could be promoted at elevated temperature;Ettringite was unstable at 100?;The length-width ratio of ettringite decreases with the increase of solution p H value.And the morphologies of ettringite changed from long,thin fiber-like needles to small needles.The formation mechanism of ettringite in solution was: when ettringite was synthesized by solution method,it could form calcium sulphoaluminate hydrates(AFm),and then calcium sulphoaluminate hydrate reacted with SO42-and Ca2+ to form ettringite.2.Boric acid could inhibit the growth of ettringite in the length.With the increase of boric acid dosages,the length-width ratio of ettringite decreased gradually and the shape became shorter,and the formation of ettringite obviously slowed down.When the dosages of boric acid increased to 10%,20%,the flat column-like ettringite formed.Lithium carbonate could inhibit the radial growth of ettringite crystals by adsorbing on the side of ettringite crystals.Lithium carbonate could accelerate the formation of ettringite.Superplasticizers could inhibit ettringite crystal radial growth and make ettringite longer and thinner.Superplasticizers couldenhance ettringite nucleation.3.This paper presents a scheme for the regulation of ettringite morphology.Long rod-like ettringite can be obtained by adding lithium carbonate and polycarboxylate water reducing agent;when the p H value is adjusted to 12.5 or higher,fine needle-like ettringite can be obtained;short columnar ettringite can be obtained by adding boric acid;short and fine ettringite can be obtained by compound doping boric acid and polycarboxylate superplasticizer.