Micro-forces And Rheological Property Of Cement-based Mineral Dispersion Systems

Mineral and chemical admixtures have become important components in cement-based materials.The use of these admixtures not only affects the mechanical property and durability of concrete,but also the flowability and workability of fresh mixtures.At present,more and more kinds of admixtures have been used in cementitious materials.However,the mechanism how these admixtures,especially mineral admixtures,affect the rheological behavior of cement-based materials is still unclear,and most theories focus on the water film theory or filling and packing theory.With respect to chemical admixtures,there is also a lack of research on their adsorption and dispersion mechanism,especially the variation of micro forces between particles with different water reducing agents,which restricts the development and application of new chemical admixtures.The rheological property of a mineral suspension is related to the micro forces in the system,while ionic adsorption and chemical admixtures will change the interparticle force thus affecting the suspension rheology.In the case of cement paste,the main forces affecting the rheological behavior include van der Waals attraction,electric double layer（EDL）repulsion and steric repulsion.This thesis aims to reveal the effect of different admixtures on the rheology of cement paste by studying the micro forces in different suspension systems including unitary mineral systems and binary cement-admixture systems.For the condition of mineral admixtures,two simple minerals i.e.quartz and calcite were selected,and a series of model solutions were prepared based on the ionic composition of cement pore solution.Through the model solutions,we investigated the effect of different ions on the zeta potential and interparticle force in unitary mineral dispersion system.Based on the study of unitary mineral system,the mechanism how different mineral admixtures affect the rheological property of cement paste was revealed based on DLVO theory.For chemical admixtures,we selected five different water reducing agents,their functional groups were investigated by Fourier transform infrared spectroscopy,and their influence on the electric property of cement particles was investigated by zeta potential test.Through out this study,the following conclusions can be drawn:Different mineral admixtures exhibit distinct adsorption characteristics for various hydrated ions.On the other side,ionic adsorption can change the interparticle forces thus affecting the rheological property of unitary and binary mixtures.As for quartz,the surface charge and interparticle force are related to the p H of the solution as well as the valence and concentration of counterions.The high p H in cement pore solution leads to the deprotonation of silanol groups and enhances the negative charge on quartz’s surface,this promotes the adsorption of Ca²⁺which in turn reduces the EDL repulsion between particles.As a result,quartz flocculates under the effect of van der Waals attraction and finally decreases the fluidity of cement-quartz paste.Different from solid oxides,the surface charge of calcite and the interparticle force are mainly dominated by potential determining ions（PDI）.Ca²⁺adsorption can enhance the EDL repulsion between calcite particles,while OH^-and SO₄^2-will reduce the EDL repulsion.In the condition of cement pore solution,calcite,with Hamaker constant much smaller than cement,exhibits much smaller flocculation degree,and finally improves the fluidity of cement-calcite system.The influence of specific surface area on cement paste rheology is related to the van der Waals attraction between particles,rather than the conventional water film theory or filling and packing theory.In a suspension with constant solid volume fraction,the van der Waals attraction between particles is inversely proportional to the particle size.Therefore,the larger the specific surface area of mineral admixtures,the lower the fluidity of cement slurry.Similarly,the influence of water to cement ratio（w/c）is also related to van der Waals attraction.When the w/c is below 0.6,the increasing w/c can significantly reduce the van der Waals attraction between cement particles,which helps to improve the fluidity of cement paste.When w/c is above 0.6,the fluidity is mainly dominated by the hydrodynamic effect of the fluid.Polycarboxylate superplasticizer,which takes effect by steric repulsion,provides the best efficiency in improving the flowability of cement paste.On the contrary,the lignosulfonate mainly depends on EDL repulsion and exhibits the worst effect since the cement pore solution is saturated with ions and the EDL repulsion is significantly screened.The effect of different water reducers follows the following relationship with the same dosage:Calcium lignosulfonate（CL）<Polynaphthalene sulphonate（PNS）<Sulfonated melamine formaldehyde（SMF）<Sulfonated acetone-formaldehyde（SAF）<Polycarboxylate ether（PCE）.Rheological models were established in this paper based on the micro forces between particles,which can effectively predict the rheology of unitary mineral dispersion system and cement-water reducer systems.