Research On The Effects Of Water-reducing Agents On The Hydration And Setting Of α-calcium Sulfate Hemihydrates

It is predicted that the up to date developments in bonding materials technology rely on enhanced admixture efficiency rather than on improvement in materials manufacturing.Former researches on admixtures mainly focused on concrete and cement system,while studies on gypsum modification largely lag behind and are limited toβ- calcium sulfate hemihydrates(β-HH).However,the other form of hemihydrates calledα- calcium sulfate hemihydrates(α-HH) actually enjoy much higher value.More work has to be done forα-HH based plasters,especially in seeking further higher strength.In this work,the hydration and setting process of pureα-HH was investigated firstly. The thermogravimetry(TG),mechanical strength test,scanning electron microscopy (SEM) and Brunauer Emmett Teller(BET) adsorption were used to inspect hydration rate,strength development and microstructure ofα-HH based plaster.As one of the most important factors forα-HH application,the influence of water-hemihydrates weight ratio was put on emphasis.Then,two kinds of water-reducing agents,namely sulfonated melamine formaldehyde(EM) and polycarboxylate(EC),were respectively introduced into the preparation of dihydrate(DH) plaster fromα-HH.After their dosages were optimized by identifying their biggest water-reducing efficiency,the effects on the hydration and setting ofα-HH were discussed.Their mechaniEMs were revealed by counting the adsorption quantity and judging the adsorption form with X-ray photoelectron spectrometer(XPS) testing results.Finally,to improve the performance ofα-HH further,EM and EC were impregnated intoα-HH simultaneously. The optimized mixture ratio was acquired by comparing the enhancement of plasters 2h strength.Same methods were employed to study the combined effects on the hydration and setting ofα-HH.The optimized dosages of EM and EC were determined to be 0.5 wt%and 0.3 wt% when they were impregnated separately to modify theα-HH.They all work through chemical adsorption on crystal surface to yield dispersion function and the water-reducing efficiency reached 15.6%.The hydration process was accelerated by EM but decelerated by EC.The strength development of plaster was influenced by EM and EC at all five stages.Higher 2h strength and dry strength of plaster were remarkably achieved.EM worked best in strength improvement.It could raise the 2h bending strength by 38%and 2h compressive strength 29%,the dry bending strength 20%and dry compressive strength 27%.The DH crystals increased and changed from needle form into column or board forms by modifying with EM or EC.Pore size became EMaller and was homogenized to 3～4 nm.When EM and EC were impregnated intoα-HH simultaneously,the optimized mixture ratio was 0.3wt%for EM and 0.1wt%for EC.The combined effects on the hydration and setting ofα-HH involved both the capability of EM in improving initial strength and the capability of EC in postponing setting time.The porosity of combined modified gypsum matrix was reduced by more than 60%which induced the bending strength to decrease more extensively in stageⅢof the strength development.When anti-bubbling agent was added into this compound modification toα-HH,the dry bending strength could be raised to higher than 20 MPa and dry compressive strength higher than 72 MPa,the initial setting time went longer than 15 min and final setting time shorter than 20 min.The strength ofα-HH based plaster can be obviously improved by modifying with EM or EC through water-reducing effect.And the performance ofα-HH based plaster can be further enhanced by compound using of them.