Study of the Interaction Between Cement Phases and Polycarboxylates Possessing Silyl Functionalities

【Author】

Plank J;Yang F;Storcheva O;

【Abstract】

<正>In 1981,pofycarboxylate comb polymers were introduced as a new class of superplasticizers.Their structural characteristic is an anionic polymer backbone which holds lateral graft chains.These side chains instigate a steric hindrance effect between the cement particles suspended in water.In spite of the great arsenal of currently available superplasticizers,the industry tries to further improve this technology,with a focus on PCEs.The main goal is to present novel PCE structures which are more robust with diverse cement compositions,which are not subject to desorption by free dissolved sulphate and which can tolerate contaminants such as e.g.clay. In this study,PCEs possessing silyl functionalities were synthesized and their dispersion effectiveness,working mechanism and interaction with cement surface was investigated. For this purpose,PEG-type PCEs containing silyl functionalities were prepared by free radical copofymerisation from methacrylic acid,MPEG-methacrylate ester and N-maleic acid-y-amidopropyl triethoxy silane(MAPS) at different molar ratios.The polymers were characterised with respect to their elemental composition,molar masses (Mw,Mri),polydispersity index(PDI) and anionic charge amount.To probe whether MAPS instigates a stiffening of the PCE main chain,the hydrodynamic radii of the modified and non-modified PCEs were measured by light scattering.Dispersion effectiveness of the silylated PCEs was probed via 'mini slump' test on cement pastes. Adsorption of these polymers on cement and pure clinker phases(C_3S/C_2S) was Quantified to prove the working mechanism.The positive effect of highly silylated PCEs is explained by their increased adsorption.The interaction between these PCEs and silicate phases of cement were investigated by a desorption experiment with sulphate at different concentrations.The evidence for a chemical bond between the polymer and the silicate hydrate surface was investigated by 29Si MAS-NMR and X-ray photoelectron Spectroscopy. The study demonstrates very effective dispersing perfomances of silyl modified PCE superplasticizers at low dosages,which can be explained by the formation of chemical bonds between PCEs with silyl functionalities and silicate phases in cement. Originality The purpose of the study of PCEs with silyl functionalities was to clarify whether PCEs can interact with the surfaces of the silicates present in cement.Common anionic superplasticizers adsorb almost exclusively on aluminate hydrates such as ettringite or monosulfoaluminate.The silanol groups formed after alkaline hydrolysis of the siloxane functionality can undergo a condensation reaction with≡Si-OH groups present on the surfaces of hydrating cement phases(C_3S/C_2S).The aim was to attain a PCE which is not only physically sorbed,but chemically bound to cement through a covalent bond.Silyl grafted PCEs would be robust against desorption from sulphate,and thus can exhibit a more stable performance.

【Keywords】

polycarboxylates;;cement;;silane;;sulphate resistance

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