Study On Interface Properties And Cracking Resistance Of Thin-layer EVA-modified Cement Mortar

Ethylene-vinyl acetate copolymer (EVA),Hydroxypropyl methylcellulose (HPMC) and lignocellulose is used to modified cement mortars. The W/C is designed by viscosity, the effects of HPMC on the workability is studied by water retention. The dynamic cracking resistance instrument and evaluation methodology are improved, the effects of EVA, HPMC, lignocellulose, white cement and different fillers on interface properties and dynamic cracking resistance is studied specially. The influence of EVA on the microstructure, such as pore structure and interface of the mortar, is studied by Scanning Electron Microscope(SEM). The formed polymer film is analysed simply. X-ray Diffraction (XRD), thermal analysis(DSC,DTG)and Fourier Transform Infrared Spectroscopy(FTIR) are used to research on the effect of EVA on cement hydration in the modified mortars. The influence of EVA on cement hydrates and the degree of cement hydration in pastes is also studied by means of these methods. The modified mortar which has better adhesion strength, dynamic cracking resistance and compatibility with the decorative facing and protective layer are used in external thermal insulation. The researches in this paper show that:1. The viscosity is influenced much by HPMC, litter by cement and quartz sand, no by EVA and lignocellulose. HPMC can rise the water retention and workability modified of mortars markedly.2. EVA can improve the adhesion strength and freeze-thaw resistance of mortar effectively, but the interface bonding strength between mortar and substrate deceases at 5%. the adhesion strength and freeze-thaw resistance of mortar reaches a maximum at HPMC of 5%, the interface bonding strength is low at HPMC below 5%, while the cohesive strength deceases quickly. The adhesion strength of modified mortars is improved with the increase of cement, and come up to inflexion with 30% of cement. The adhesion strength increases with quartz sand and reaches a maximum, and then drops. The interface bonding strength is deceased with quartz sand rising, that more obvious after freeze-thaw. The decline of adhesion strength is 40%～50%. 3. Reference to the dynamic cracking resistance test method on JG/T 157-2004, the instrument and evaluation methods (crack width) are improved in national standard for the performance characteristics of EVA-modified mortar. In the new method (load-warp amount), crack time is ascertained more easy, and evaluating flexibility by crack load and warp amount is various. EVA can improve the dynamic cracking resistance of mortar effectively. The modified mortar can resist crack growth at 2%.The dynamic cracking resistance and crack load is enhanced with lignocellulose to a certain extent.Adding cement can drop the dynamic cracking resistance but rise crack load. The adhesion strength increases with quartz sand ratio at 0～2/3. The crack load is few influenced by quartz sand.4. According to SEM analysis, EVA is formed first by accumulation of polymer particles at less addition and then becomes more and more compact. The formed polymer film is penetrated through by the growing cement hydrates gradually with hydration. Finally, the organic-inorganic interpenetrating structure is formed by two successive phases of mesh network polymer film and cement hydrates. According to XRD, FTIR and thermal analysis, the content of the formed Ca(OH)2 deceases with EVA addition rising, meaning the degree of cement hydration drops with EVA rising. Perfect mesh network polymer film, is formed in the modified mortars, can obstruct the contaction between cement and water. There is physical effect between EVA and cement.5. The decorative facing is provided with the better interface bonding strength performance which is made up of modified mortarⅠandⅡ, acrylic acid dope, water-based fluorocarbon resin paint and metallic paint collection, which is good in freeze-thaw resistance, deformation coordination and air permeability.