Synthesis Of Polycarboxylic Acid Superplasticizers With Three Kinds Of Initiating System

The polycarboxylic acid superplasticizer(PCA)is characterized of high water reducing and environment friendly,which is the most important concrete admixture today.Initiator system can significantly affect the structure and the performance of PCA.In this paper,three kinds of oxidation-reduction initiator system were used to initiate a binary copolymerization composed of acrylic acid and polyether and to synthesize PCA.Furthermore,the third monomer containing cyano or ester group was introduced to the binary copolymerization to modify the PCA.Three kinds of initiator system including hydrogen peroxide(H2O2)/ascorbic acid(ASA),tert-butyl hydroperoxide(TBHP)/ Sodium formaldehyde sulfoxylate(SFS)and cerium salt(OCe)/amino acid(RAm)were studied in a binary copolymerization of acrylic acid(AA)and isoamyl alcohol polyoxyethylene ether(TPEG2400).The preparation conditions of PCA with three kinds of initiator system were optimized through the orthogonal experiment.The optimized preparation conditions for H2O2/ASA are as follows: the dosage of hydrogen peroxide is 0.1 wt% of total monomers,n(H2O2): n(ASA)= 4 : 1,and the reaction temperature is 45?.When OCe/RAm was chosen as an initiator,the optimized PCA can not be prepared until the reaction temp erature up to 60?,the mass amount of cerium salt and amino acid is 1.2 wt% and 0.5 wt%,separately.Instead,PCA that synthesized under room temperature(25?)was obtained using TBHP/SFS with the following optimized reaction conditions: the amount of tert-butyl hydroperoxide is 0.18 wt%,n(TBHP): n(SFS)= 1 : 1.As far as H2O2/ASA initiating system is concerned,PCA(?-TPEG2400)synthesized with AA and TPEG2400 can be obtained at 45?.After 0.12%(mass ratio of PCA to cement)of PCA(?-TPEG2400)was added,the cement paste showing good slump loss resistant property,whose initial fluidity is 302 mm,fluidity loss is 25.8%.GPC test results show that the weight-average molecular weight(Mw)of PCA is 5.7×104 with concentrated molecular weight distribution(Mw/Mn)of 1.2.The cement block added with PCA(?-TPEG2400)and cured for one day possesses a pore rate of 24.39% tested by Mercury injection apparatus.By scanning electron microscope test of the 1d-age cement block,it was observed that the compactness of the hydration products is good,which means th at PCA(?-TPEG2400)is beneficial for concrete strength developing.H2O2/ASA has excellent adaptability in polymerization of large monomers and AA,the obtained PCA has good dispersion stability for cement.1d and 3d of mortar compressive strength is 15.6 MPa and 42.1MPa respectively,and the early strength of mortar grows fast.The fluidities of cement paste adding PCA(?-HPEG2400),PCA(?-TPEG4000)and PCA(?-HPEG4000)are all above 210 mm,indicating that H2O2/ASA has excellent adaptability to TPEG2400,TPEG4000,HPEG4000 and HPEG4000.For TBHP/SFS initiating system,the optimized reaction temperature is 25 ?,demonstrating its ability to initiate the reaction and synthesize the PCA(?-TPEG2400)at room temperature.After 0.12% of PCA(?-TPEG2400)synthesized with AA and TPEG2400 was added,the initial fluidity of cement paste is 315 mm which decreased to 110 mm after 2h,yet.GPC test results demostrate that the Mw of PCA(?-TPEG2400)is 4.8×104,Mw/Mn is 1.1,and the molecular weight distribution is relatively concentrated.The cement block added with PCA(?-TPEG2400)and cured for one day possesses a pore rate of 26.09% tested by Mercury injection apparatus.SEM images of the 1d-age cement block demonstrate that the compactness of the hydration products is less th an H2O2/ASA.1d and 3d of mortar compressive strength is 14.9 MPa and 40.6 MPa respectively,and the early strength of mortar grows slower than H2O2/ASA.A kind of hyperbranched PCA named PCA(?-TPEG2400)was synthesized by OCe/RAm with monomers of AA and TPEG2400 at 60?.Unfortunately,the PCA(?-TPEG2400)exhibits low fluidity and retainability,which render fresh cement paste an initial fluidity of 185 mm and the fluidity after 0.5h of 100mm(the mass amount of PCA is 0.12%).GPC test results indicate that the Mw of PCA(?-TPEG2400)is 2.1×104 with a wide Mw/Mn of 2.0.The cement block added with PCA(?-TPEG2400)and cured for one day possesses a pore rate of 28.73% tested by Mercury injection apparatus.The SEM images of the 1d-age cement block indicate that the hydration products are loose.PCA(?-TPEG2400)show little sacrifice to the early strength of hardened concrete,which give 1d and 3d of mortar compressive strength of 12.2 MPa and 37.5 MPa,respectively.In the above three kinds of initiator system,H2O2/ASA gives PCAs the best comprehensive performance for cement paste.In view of this,H2O2/ASA was chosen to be the initiator system to study the modifying synthesis of PCA,in which the third monomer was added.Here,the third monomer such as acrylonitrile,hydroxyl ethyl acrylate and dimethyl maleate was used which contains cyano or ester groups.When the addition of PCA modified by dimethyl maleate to cement is 0.12%,the water reducing rate of fresh concrete can reach 34.80%,and 1h concrete slump increases by 2.22%.