Reaserch on the Properties and Mechanism of High Performance Cement–based Materials
Supervisor:wang ru min
High performance cement-based materials are widely used in infrastructure due to their excellent comprehensive properties.However,the mechanism research of polymer latex modified cement lags far behind its application.It is necessary to use chemical analysis method to research the chemical reaction in the polymer latex modified cement system.Otherwise,graphene oxide(GO)is one of the research focuses material in recent years.GO as an additive to cement,can give some new properties to cement.Also,the presence of chemical groups is the starting point for binding the different polymer on the GO nanosheets.The design and synthesis of the new chemical functional GO can obtain a new additive for cement.And chemical functional GO can use to further study the influence of structure on the properties of cement.The main contents and results are summarized as follows:(1)Polyacrylate(PA)latex improved the flexural strength,shrinkage,water retention and viscosity,reduced the compressive strength of the cement.And the optimum content of PA latex in cement is 7.5%,when the content exceeded 7.5%,it will have negative influence on the workability of cement.The chemical mechanism in the polyacrylate latex modified cement system was investigated by Fourier transform infrared spectra(FT–IR),X–ray photoelectron spectroscopy(XPS),gel permeation chromatography(GPC)and compact p H meter.The hydration reaction can create a strong alkalinity,exothermic circumstance in the whole system.In this environment,the ester groups of the polyacrylate chains hydrolyzed to produce carboxyl groups.There is a large generation of carboxyl groups in the second stage,and the dominate reaction in this stage is the hydrolysis of polyacrylate latex.At the third stage;these carboxyl groups reacted with the hydration products of cement [Ca(OH)2] to generate a new product [Ca(HCOO)2].The chemical reaction between polyacrylate latex and cement generated a crosslinked network structure,in which Ca2+ served as the crosslinking points and linked different polyacrylate chains together.(2)With the addition of styrene butadiene latex and carboxylic styrene butadiene latex;the tenacity,shrinkage,water retention and viscosity modified cement increase at different degrees.The mechanism of polymer latex modified cement was investigated by differential scanning calorimetry(DSC),FT–IR,XPS and scanning electron microscope(SEM).Owing to the difference in polymer chains,most of the polymer latexes used in modified cement field can be classified into two kinds.Styrene butadiene latex was used to represent the first kind polymer latex with no active groups in their polymer chains,the mechanism of these polymer latexes modified cement just contained physical modification mechanism.The latex film covered the surfaces of hydration crystals and filled into the cracks and pores of cement thus could be able to give better performance to the modified cement.About the other,these polymer latexes with active groups which can react with hydration products to produce a 3D network structure in the modified cement system,such as carboxylic styrene butadiene latex.The mechanism of these polymer latexes included physical and chemical modification mechanisms.Meanwhile,the modification models of these mechanisms used in most polymer latexes have been put forward.(3)The graphene oxide(GO)nanosheets were compounded by the improved Hummer′s method.The introduction of GO nanosheets improved the mechanical properties of cement composites.But GO has the negative influence on the fluidity of cement,it is due to the strong adsorption ability of GO on the surface of cement.The adsorption layer thickness of GO nanosheets on cement was about 10.16 nm.It means that there are around 12 layers of GO nanosheets adsorbed on the cement surface.And the adsorption data was best fitted with the Freundlich adsorption isotherm model and described by the pseudo–second order kinetics model;the adsorption process contained the chemical reaction.The –COOH in the GO nanosheets reacted with the hydration product of cement(Ca(OH)2).The chemical reactions connected the GO nanosheets each other,producing a 3D network structure in the GO nanosheets modified cement.Moreover,the hydration products were also inserted into this 3D network structure.(4)The chemically functionalized nano–graphene oxide(GOM)was synthesized by the chemical reaction of polyether amine with graphene oxide(GO).The mechanical properties,rheological properties,dispersion effectiveness,slump retention behavior and the mechanism of GOM modified cement were also researched.The results show that GOM have the same toughing function for cement with GO.And GOM can increase the slump flow of cement.The slump flow,the dispersion effectiveness and slump retention of GOM modified cement increased with the increase of molecular weight of polyether amine.The experimental results were fitted better with the Freundlich model than the Langmuir model.The result implied that the adsorption of GOM on cement is mainly a multilayer type.And the adsorption kinetics of GOM on cement is fitted well with the pseudo–second–order.The main chain of GOM adsorbed on the surface of cement,and the branched chains stretched into the water.The electrostatic repulsive forces of adsorption layers and steric hindrance of branched chains endowed the good fluidity of cement.