Research On The Influence Of Hindrance Effect On Ettringite Growth

Ettringite, one of the important Portland cement hydration products, accounts for7%of Portland cement system. Ettringite has a significant influence on setting time and early strength of cement materials system. Because of positive surface charge, ettringite will adsorb polymer additives containing anionic groups. The macroscopic performance of Portland cement hydration can be reflected by the adsorption effect. The interaction between ettringite and polymer additives was studied in three systems in this research, and the surface charge and growth properties of ettringite was analyzed to understand the influence mechanism of steric hindrance and electrostatic force on ettringite. The results can provide a theoretical basis for practical application of additives.The pure ettringite system, tri calcium aluminate-gypsum system and supersulphated slag cement system are research objects. The comprehensive performance research includes zeta potential, adsorption amount, conductivity, rheological properties, ion concentration analysis, XRD and SEM.The results show:(1) By studying on impact of naphthalene sulfonate formaldehyde, polycarboxylate and melamines water reducer on ettringite zeta potential, it can be found that the zeta potential decreased over time when the water reducer dosage was constant. However, in ettringite system containing welan gum, the zeta potential of ettringite kept constant over time. With increasing water reducer dosage, the ettringite zeta potentials showed an increasing trend, and the ettringite system reached a maximum at1%welan gum dosage. The maximum adsorption amount of naphthalene sulfonate formaldehyde water reducer on ettringite is118mg/g (water reducer/ettringite). Meanwhile, when the content of welan gum reached up to2%, its adsorption amount on ettringite did not achieve a largest value.(2) In tricalcium aluminate-gypsum system, polycarboxylate water reducer accelerated the hydration rate in initial hydration stage, but the ettringite morphology had no significant change. Accordingly, Welan gum induced a retarding behavior and restrained the rate of ettringite nucleation. Subsequently, the morphology of ettringite crystals was changed by welan gum in later hydraion stage.(3) In supersulphated slag cement system, the use of welan gum increased the yield value and plastic viscosity of cement pastes. When mixed with polycarboxylate water reducer in cement pastes, the water retention effect of welan gum was reduced. All pastes can be identified as thixotropic pastes, except for the sample containing0.02%polycarboxylate. In supersulphated slag cement system including general slag, the hydration rate was faster than the sample containing superfine slag. Additionally, since the nucleation sites on general slag was less than that on superfine slag, the generated ettringite crystals of general slag were bigger than that of superfine slag. In compound addition sample including superfine and general slag, the ettringite crystals dimension fell in between two samples above. The paste in the presence of superfine slag was dispersed by polycarboxylate water reducer thoroughly, but few ettringite crystals can be observed in initial hydration stage.