Preparation And Characterization Of Polyacrylamide Flocculant For Non-dispersible Underwater Concrete

With the development of China’s economy and national defense strategy,the construction of cross river,sea bridge,tunnel,protection and other buildings(structures)puts forward higher requirements for the performance of non-dispersible underwater concrete and environmental protection.Non-dispersible underwater concrete is to add flocculant based underwater anti-wash admixtures into ordinary concrete,so that the concrete can maintain good workability and anti-dispersity,At present,the commonly used flocculant-polyacrylamide has strong water absorption.When it is directly applied to concrete for underwater construction,the large amount of water absorption leads to problems such as poor workability of concrete,shrinkage of strength after hardening,poor durability and others.Therefore,it is of great theoretical and engineering significance to develop a kind of polyacrylamide and composite anti-dispersant with low water absorption and suitable for the construction of underwater concrete under complex working conditions.Aiming at the problem of water absorption of polyacrylamide,an anionic polyacrylamide(APAM)grafted with water reducing functional group(-COOH)was synthesized at room temperature.The effects of monomer concentration,monomer ratio(n(AM):n(AA)),initiator dosage,reaction temperature and pH value on the turbidity of cement paste were discussed.The crucial process parameters were optimized by partial factor design and response surface design and the structure and morphology were characterized by UV,FTIR,TG-DSC and SEM.Then the anti-dispersity and fluidity of cement paste was studied.The influence of APAM content,water-binder ratio and sand ratio on the rheological properties of cement-based materials was analyzed by rheological test,and the rheological model of cement-based materials with APAM was established.Finally,the relationship between compressive strength and micro pore structure of mortar was studied with water reducing agent,and macro performance of concrete was further optimized by adjusting APAM content through analyzing pore structure characteristics.The main conclusions are as follows:(1)Through partial factorial design and response surface design methods,and by means of UV,FTIR,TG-DSC and SEM,it was confirmed that an anionic polyacrylamide with molecular weight of 11 million and solid content of 90.19%was synthesized by using AM and AA as main monomers under the action of a redox-azo complex initiator system.The most suitable control process parameters as follows:temperature was 35℃,pH was 8,monomer concentration was 27.30%,initiator dosage was 0.60%,monomer ratio(n(AM):n(AA))was 3.The turbidity value of underwater slurry was only 9.75 NTU when the corresponding dosage was 1.0%of cement mass,and the anti dispersion effect was good.(2)APAM had good flocculability and low water demand when it was used in cement paste.The water demand only increased by 12%when the dosage was 1.0%of cement mass,and it can meet the self leveling and anti dispersion of the underwater slurry at the same time.The results show that the cement paste and mortar mixed with APAM had obvious positive thixotropy.From the analysis of rheological parameters,When the water-binder ratio was 0.45,the sand ratio was 40%,and the APAM content was 1.0%of cement mass,the cement-based material has smaller yield stress and larger plastic viscosity,and has better anti dispersion and fluidity when forming underwater(3)Based on the modified Bingham model,the rheological model of non-dispersible underwater mortar was established.The following factors were fully considered in the model:(1)yield stress of slurry;(2)the interaction between polyacrylamide and cement particles;(3)the shear stress caused by the flow of cement slurry;(4)the interaction between cement slurry and fine aggregate;(5)the interaction between fine aggregate particles.Through the rheological curve fitting,the correlation coefficient R~2can reach 0.976 29,which proves that the rheological model was reliable.(4)When the water-cement ratio was 0.45,and the polycarboxylic acid water-reducing agent and APAM were respectively 2.0%and 1.0%of the mass of the cementitious material,the fluidity and anti dispersion of the mortar reach the best.The water-land strength ratio of mortar were 79.18%in 7 d and 89.14%in 28 d,respectively.The microscopic porosity of the 3D X-ray nano-tomography scan 7 d underwater non-dispersible mortar was only 6.10%,harmless pores and less harmful pores account for the majority,and the pore size distribution was relatively reasonable.