Synthesis And Characteristics Of Polyurethane-modified Pectiniform Polycarboxylate At Room Temperature And Their Interaction With Cementitious Materials

Polycarboxylate superplasticizer(PCE)is a kind of polymeric compound which consists of carboxylic acid type unsaturated monomer,long chain alkane macromolecular monomer and other monomers copolymerization.After being incorporated into Portland cement,PCE adsorbs on the surface of cement particles.The steric hindrance and the electrostatic repulsion between PCE molecules can effectively improve the dispersibility of Portland ceme nt particles in water,thereby improving the performance of cement-based materials.Structural changes in molecular scale of PCE would lead to changes of its properties on dispersion,water retention,enhancement,compatibility with portland cement and so on.Therefore,how to control the structure of polycarboxylates in process of synthesizing and achieve different target production we wanted is still a puzzle.The prerequisite to synthesising polycarboxylate superplasticizer was to prepare new macromonomer with controlled molecular weight,adjustable hydrophilic-lipophilic groups,long-chain alkyl groups and large terminal hydroxyl groups as well;compared with the synthetic products by heating,polycarboxylate synthesised at room temperature had same performance on conversion rate and initial dispersion in cement and broader molecular weight distribution at the same time,but the later had worse performance on collapse resistance.Longer main and side chains meaned better dispersion.Dispersion of polycarboxylate in cement should be explained by a series of theories but not a single one,we should have an overall consideration of various factors.A new kind of polycarboxylates(M-PCE)was synthesized by segmented synthetic method in this research.Isophorone diisocyanate,end group siloxane,bis-hydroxymethyl-propionic acid were used to synthesize polyurethane prepolymer.Polyether macromonomer(TPEG),acrylic acid(AA),polyethylene glycol(PEG)were used as raw material to polymerize at room temperature.Thus,polycarboxylates with different comb structures(i.e.,the same degree of polymerization in side chains but different degree of polymerization in main chains)were synthesized via radical polymerization at room temperature.Test results showed that synthesising polycarboxylate at room temperature needed to use redox initiator system,increasing the initiator and monomer concentration,reaction time and other measures.Single factor and orthogonal experiments and analysis of test results showed that the chain reaction was initiated successfully,acrylic polymerization double bond was opened,the polyoxyethylene segments successfully accessed the macromolecular segments polycarboxylate synthetic molecule containing hydroxyl,a carboxyl group,a methyl group,an ester group and like groups.The optimal synthesis process was: n(AA): n(TPEG)= 2: 1,n(H2O2): n(APS): n(Vc)= 15: 1: 1,n(H2O2)= 0.05 mol,the reaction temperature was 40 ?,reaction time 4h;the highest rate of monomer conversion : 78.5%,synthetic superplasticizer content was 0.15%,the initial flowability of cement paste maximum reached 340 mm within 2 h compared with loss small.To produce M-PCE and other three carboxylate(O-PCE,C-PCE-1 and C-PCE-2)as the foundation,the determination of polycarboxylate in portland cement mixed with supplementary cementitious materials(paste and mortar)adsorption,hydration,Zeta potential,surface tension,flowability,strength,drying shrinkage,autogenous shrinkage,etc.were analyzed.The results showed that relative to O-PCE and the C-PCEs,the adsorption value of M-PCE on the surface of cement particles increased by 14.1%,and the adsorption rate increased by 24%.In the hydration process,the other three polycarboxylates(O-PCE,C-PCE-1 and C-PCE-2)exhibited a delayed effect due to the shielding of their chain alkyl groups;the hydration of the cementitious materials by the M-PCE was not delayed but instead promoted because of its incomplete hydrolysis of the siloxane.Due to the higher adsorption amount,M-PCE with siloxane groups exhibited favorable comprehensive performance in terms of zeta potential,liquid surface tension and flowability in cementitious materials.The average performance of the M-PCE wa the best when the molar ratio of acrylic acid to TPEG is 2: 1 in the same situation.When the water / cement ratio was 0.3,the strength of cement mortar increases by 15.1% Shrinkage,self-shrinkage performance decreased significantly,the lowest were reduced by 21.9% and 60.1%.After the M-PCE was compounded,the liquid surface tension,flowability,and Zeta potential of the cement paste with different supplementary cementitious materials were studied.The results showed that the liquid surface tension of cement pastes decreases and the paste flowability increases with increasing the dosage of supplementary cementitious materials and polycarboxylate.As the water-cement ratio decreases from 0.5 to 0.3,the Zeta potential of cement pastes increases.When the water-cement ratio is 0.3,the Zeta potential decreases with increasing the dosage of supplementary cementitious materials and polycarboxylate.The modified polycarboxylate(M-PCE)was compared with the commercially available Sika high performance polycarboxyliate(mother liquor)to study their application in low water-cement ratio cementitious materials.Based on the different factors of Portland cement,silica fume,slag and fly ash,the quaternary orthogonal design of the low water-cement ratio cementitious material was was carried out.Also,the properties of this kind of cementitious material,including flowability,strength,dry shrinkage and autogenous shrinkage,etc.The results showed that when M-PCE and Sika polycarboxylate were used,the influence of each cementitious material on the flowability was about the same.The dosage of Portland cement and silica fume,Portland cement and fly ash ha d a positive effect on the 28-day strength,and the dosage of fly ash could effectively reduce dry shrinkage and autogenous shrinkage in this kind of low water-cement ratio cementitious materials.