Synthesis And Air-entraining & Early-strength Properties Of Polycarboxylate Superplasticizer

Polycarboxylate superplasticizer(PC) is the new generation of high-performance water-reducer after lignosulfonates and naphthalene water-reducers. Because of the advantages of high dispersion, high water-reducing, high slump-loss controlling abilities,the PC can be used in cement system with high-strength, high liquidity and high durability to meet the requirement of high-performance cement in various engineering projects. In China, PC has only been used in some key projects for about 10 years. Nowadays, it becomes a hot topic in the field of water-reducer, and will dominate the market of water-reducer in 10 years as expected.PC can be synthesized by free-radical polymerization of various unsaturated monomers. PC with different structures can be obtained by molecular design and reaction control, e.g. choosing different monomers, changing the ratio of various monomers, and adjusting the reaction process. In this work, a series of PC were synthesized with methoxy polyethylene glycol methacrylate(PMA), methacrylic acid(MAA) and methllyl sulfonic acid(MAS) as monomers, and using ammonium persulfate (APS) as an initiator. To determine the optimum process of PC, various reaction conditions for PC synthesis were intensively investigated, such as the types of monomers, the molecular ratios of monomers, the dosage of initiator, and the feeding technology. Moreover, the influences of contents of sulfonate group, carboxylate group, methoxy polyethylene group, and the molecular weight of PC on the air-entraining and early-strength properties of cement system were also studied.Results show that PC with high dispersion abilities can be obtained by adjusting the dosage of monomers and initiator. PC-45, the optimum product of PC with excellent dispersive performance, is synthesized from PMA45, MAA, and MAS. The optimum reaction conditions are as follow. The ratio of PMA45:MAA:MAS is 1:3.75:0.5, and the dosage of APS is 0.4%. PC-45 has the inherent viscosity of 45.09 ml·g-1. When used in cement system with W/C ratio of 0.25 and dosage of 0.2%, the intial fluidity of cement with PC is 269 mm, and the ageing fluidity is 281 mm in 30 min.Air-entraining performance of PC was studied by means of foaming experiments, motar density testing and mercury intrusion method. Results show that for PC synthesized from PMA45, the air-entraining ability strengthens as the content of MEPG45 increases, and the air-entraining ability weakens as the content of -COO- and -SO3- increases. With the increase of molecular weight of PC, the air-entraining ability shows a trend of first increase and then decrease. For PC synthesized from PMA23, the air-entraining ability first increases and then decreases with the increase of the content of -SO3-. The air-entraining ability decreases with the decreasing molecular weight. The air-entraining ability of PC synthesized from PMA45 is stronger than that of PC synthesized from PMA23. PC synthesized from PMA45 is inclined to introduce bubbles bigger than 20μm, but it is difficult for PC synthesized from PMA23 to introduce bubbles bigger than 20μm.Early-strength property of PC was studied by means of early-strength testing of harden mortar and scanning electron microscope study of cement stone. Results show that for PC synthesized from PMA45, the early-strength ability first strengthens, and then weakens with the increase of content of MPEG45. The early-strength ability strengthens with the increase of the content of -COO- and -SO3-. The lower the molecular weight of PC, the better the early-strength ability. For PC synthesized from PMA23, the early-strength ability shows a trend of first strengthens and then weakens with the increasing content of -SO3- and molecular weight. Scanning electron microscope study of cement stone at the age of 1 d shows that, the coMPaction degree of CSH gel phase increases with the increasing early-strength ability of PC.When PC was used in the actual concrete project, the properties of slump-loss control ability, air-entrain ability and early-strength ability of the corresponding concrete system were measured. Results show that for PC synthesized in this paper, the trend of cement fluidity and ageing fluidity is the same as the trend of slump and slump loss control ability in concrete. The results of mortar density show a great consistency with the air content of concrete. The early strength of harden mortar can reflect the early-strength ability of PC used in concrete.