Influence Of Microfines From Manufactured Sand On The Rheological Properties Of Cement Paste And Its Mechanism

Due to the critical shortage of natural fine aggregate,manufactured sand has been widely used to prepare concrete.The manufactured sand contains a large amount of microfine materials.Furthermore,the microfine fraction of manufactured sand can possess a significant difference in mineral composition,particles size distribution,and content.Thus,the rheology of concrete prepared with manufactured sand can vary significantly,which can hinder the use of the manufactured sand as the fine aggregate.This paper studies the effect of different physiochemical properties and contents of microfines on the yield stress,viscosity,and growth of static yield stress at rest of cement paste.Mechanisms affecting the rheology of cement pastes caused by the addition of microfines are determined by investigating the physical interactions between particles,adsorption of superplasticizer onto particle surfaces,interstitial viscosity,and cement hydration.The study also seeks to propose some numerical models to predict the corresponding rheological variations.The main results are as follows:(1)The incorporation of microfines increases the yield stress of cement pastes.The yield stress increases by increasing the addition of finer microfines.Colloidal interactions between particles can be reduced given the additional polycarboxylate ether-based superplasticizer(PCE)that is required to obtain a fixed mini-slump flow.This can reduce the effect of microfines on increasing the yield stress of the cement paste.A simplified YODEL(Yield Stress Model)model for the prediction of yield stress is presented,by which the contributions of colloidal interactions and particle contacts on the strength of internal particle network can be estimated.It is revealed that the addition of microfines reduces the average separation distance between powder particles,leading to an enhancement to the colloidal interactions between the particles.Particle contact interactions are increased by the use of the microfines.The presence of more fine particles can cause stronger contact interactions.Particle flocculation of cement pastes is enhanced by the incorporation of microfines given the stronger colloidal interactions and particle contacts,leading to an increase of the yield stress of cement pastes.(2)The addition of microfines increases the apparent viscosity of cement pastes and the residual viscosity in/near the pseudo-Newtonian regimes(ηres pse-New).The apparent viscosity and ηres pse-New can be increased by increasing the addition of finer microfines.Contact interactions between lubricated powder particles by the interstitial fluid are estimated by introducing a function of particle volume fraction and using the Krieger-Dougherty and Chateau-Ovarlez models.A numerical model of ηres pse-New is then proposed by taking the viscous friction inside interstitial fluid layers and particle contact interactions into account.Particle contacts that are lubricated by the interstitial fluid are enhanced by the incorporation of microfines,while the viscous friction of interstitial fluid is increased given the decrease of average separation distance between particles.Thus,the addition of microfines improves the shear resistance of internal particle network,leading to an increase of ηres pse-New.(3)The use of microfines reduces the shear thickening property of cement paste made with a water-to-cement mass ratio(W/C)of 0.40,resulting in a lower increase of apparent viscosity with shear rate(Δηthic(γ)).Meanwhile,the addition of microfines shows an opposite effect on the shear thickening performance and Δηthic(γ)of the pastes prepared with 0.275 W/C.The effect of microfine content and type on Δηthic(γ)depends on the combined variations of particle inertia,particle volume fraction,and flow index of the Herschel-Bulkley model.Contact interactions between boundarylubricated particles are estimated by introducing the flow index and using the Krieger-Dougherty model.A numerical model of Δηthic(γ)is then proposed by taking particle inertial interactions and contact interactions of boundary-lubricated particles into account.For cement paste made with 0.40 W/C,the use of microfines shows a limited effect on the particle contact interactions due to the decrease of flow index,regardless of the increase of particle volume fraction.Meanwhile,the addition of microfines hinders a fast flow of the paste,which causes a low enhancement of particle inertial interactions.The shear thickening performance of paste mixtures with 0.40 W/C reduces,leading to a lower value of Δηthic(γ).For cement paste prepared with 0.275 W/C,the contact interactions between boundary-lubricated particles are enhanced due to the increase of particle volume fraction caused by the use of microfines,since the microfines exhibit a limited influence on decreasing the flow index.The shear thickening performance increases for the paste mixtures with 0.275 W/C,leading to a higher value of Δηthic(γ).(4)The incorporation of microfines increases the growth of static yield stress at rest for cement pastes made with a fixed mini-slump flow.The limestone fine materials with the smallest specific surface area exhibit the greatest effect on increasing the growth of static yield stress.Models of the growth of static yield stress are presented by evaluating the generation and formation rate of early-age hydration products.Mechanisms affecting the structural build-up at rest of cement pastes prepared with microfines are revealed given the flocculation of powder particles and bridging of early-age hydration products.It is revealed that the use of microfines enhances particle flocculation and early hydration of cement,leading to faster build-up of flocculation micro structure.Besides,the addition of microfines accelerates the build-up of bridging micro structure by promoting the generation of early-age hydration products and its bridging between neighboring particles,given the additional effect of flocculation of hydrated particles.(5)The effect of the content and physiochemical properties of microfines from manufactured sand on flowability of cement mortars are investigated with multi-scale analysis.The high content of microfines can lead to low frictional contact interactions between aggregate particles larger than 0.075 mm.But the particle contact network can be stronger in the presence of more microfines.The finer the microfines,the stronger the particle contact network.Furthermore,the microfines that can absorb more PCE onto unit surface area of the microfine particles lead to stronger colloidal interactions between powder particles.The mini-slump flow of cement mortar made with manufactured sand decreases when the yield stress is increased as a combined results of the abovementioned interactions.In addition,for mortar mixtures prepared with manufactured sand that differs in microfines content and physiochemical properties,the spread loss of the mortar mixtures increases with the increasing generation of early age hydration products.