Effect Of Viscosity-enhancing Admixture And Polycarboxylate Superplasticizer On Rheological Properties Of Cement Paste

High-fluidity and high-stability cement-based materials in civil engineering are becoming increasingly important,such as self-compacting concrete,self-leveling mortar,and grouting,etc.Such materials without segregation and bleeding must possess good fluidity and high stability,and achieve a balance between anti-segregation and stability.That is to say,the cement-based materials must have a certain plastic viscosity and maintian high fluidity.Polycarboxylate superplasticizer?PCE?and Viscosity-enhancing admixture?VEA?usually are applied to solve these problems above.High fluidity can be achieved by addition of PCE,high stability and anti-segregation can be achieved by addition of VEA.However,when both VEA and PCE are used at the same time,a slight excess of PCE can cause bleeding and segretation of fresh cement-based materials,which can be solved by addition of VEA to improve the stability of the material.But if the dosage of VEA is improper,it is easy to cause a sharp loss of fluidity.At this time,it is necessary to control the dosage of VEM to increase the plastic viscosity of the slurry and maintain a low flow loss.This means that VEA can interfere with PCE,which easily would make cement-based materials instability and cause a serious impact on the actual engineering.However,this interference is often ignored,and the mutual mechanism between two admixtures has not been explained profoundly.Therefore,it is of great significance to further discuss the interaction mechanism between VEA and PCE.It is hoped that more valuable references and suggestion will be provided for workers in selecting the type and dosage of VEA and molecular design of VEA.This paper relies on the National Natural Science Foundation of China“Study of Photocatalytic Cement-based Materials Based on Layered Aluminosilicate Interlayer Domain TiO2 Assembly”.As for the interference effect of VEA on PCE,cellulose ether,polyacrylic acid and vegetable gum,as common viscosity-enhancing admixtures,were used in this study.The performance of the cement paste was firstly studied by the fluidity,bleeding rate and rheological parameters.Then,the adsorption behavior of VEA and PCE on the surface of cement particles and the conformation behavior in pore solution were studied by adsorption,Zeta potential,conductivity,DLS?Laser particle size dynamic light scatterometer?,XPS?X-ray Photoelectron Spectrometer?.Finally,the relevant model was established to further explain the mechanism of competitive adsorption between VEA and PCE.The results show that:Higher the molecular weight of HPMC would bring about more obvious influence on fluidity and rheology performance,better effect without bleeding.The main reason:HPMC can interfere with PCE,agglomeration and even precipitation of HPMC in pore solution would occur.On the one hand,these small polymer particles might be precipitated on the surface of S-PCE to invalid the steric hindrance.On the other hand,the polymer particles can also adsorb or wrap the PCE to consume PCE,thereby reducing the amount of PCE adsorbed on the surface of cement particles.Additionally,competitive adsorption between PCE and HPMC can reduce the adsorption of PCE,which is one of the main reasons of reducing dispersion ability of PCE.The greater molecular weight of HPMC would bring out stronger competitive adsorption and larger PCE comsumption.The smaller molecular weight of PAA has stronger effect on fluidity and rheological performance of cement paste with PCE.The main reason:PAA can adsorb onto the surface of cement particles,in the presence of Ca²⁺,PAA can be curled and crosslinked,as a result of the combination between carboxyl groups?COO^-?and Ca²⁺,thereby affecting the adsorption performance and conformation behavior.The smaller molecular weight of PAA has stronger adsorption ability,and stronger competitive adsorption between PAA and PCE,the more obvious interference effect on PCE.Thickening effect and improving bleeding effect of three kinds of vegetable gums are in order:Welan gum>Xanthan gum>Konjac gum,which is related to adsorption ability and molecular crosslink.Vegetable gums have obvious effect on the fluidity and rheological parameters of paste plasticized by PCE,indicating that the interference effect of Vegetable gum on PCE.Welan gum has strongest interference effect,which is attributed to strongest competitive adsorption ability of welan gum and strongest consumption of PCE by cross-linked structure.