The Adsorption Behavior Of Polycarboxylate Superplasticizer On The Surface Of Cement And Supplementary Cementitious Particles

Due to its excellent properties, e.g. high water reducing ratio, goog control of slump loss,especially environmentally friendly and modulated molecular structure, polycarboxylatesuperplasticizer (PC) is a very important development direction of superplasticizers used inconcrete in the future and becomes more prevalent in the application of modern concreteengineering. Further, with the development of concrete technology nowadays, supplementarycementitious materials are used with an increased dosage to partially replace Portland cementin concrete. Therefore, the workability problems for early-age concrete, e.g. poor early-agefluidity, abnormal setting and hardening behavior, over dosage of superplasticizer has beengradually emerging due to the compatibility among raw materials, especially the compatibilitybetween cementitious components and admixtures. This paper mainly investigated theadsorption behavior of PC on the surface of cementitious component particles and the mainparameters influencing the adsorption behaviors, including the molecular structure of PC,particle surface characteristics of different cementitious materials. The results can provide atheoretical guidance for exploring the compatibility between PC and cementitious systemwith multi-components.The adsorption behaviors and dynamic process of PC with different molecular structureon the surface of cement and supplementary cementitious materials (including slag and flyash) were investigated. The adsorption of PC on the surface of cementitious particlesexhibited a typical Langmuir monolayer adsorption feature, driven by the charge interactionbetween PC molecules and cement particles; the adsorption dynamics process basicallyfollowed the Langmuir adsorption rate equation and the adsorption rate showed the similartrend as the adsorption amount. The carboxyl content of the main-chain and branched-chainlength of PC molecular had an important influence on the adsorption behavior and adsorptionrate of the superplasticizers: a higher carboxyl content of the main chain and more suitableratio between the main-chain and branched-chain length were more beneficial to reduce thecritical dosage; a longer main-chain and branched-chain length and higher molecular weightwere related to a higher adsorption amount; a higher carboxyl content of the main chain led toa greater adsorption rate. the adsorption behaviors and adsorption rate of the superplasticizers were also affected by the types and particle size of cementitious materials: a smaller particlesize and greater specific surface area led to a greater adsorption amount; with the samespecific surface area, the adsorption amount of PC on the surface of cement was larger thanslag and fly ash; the hydration degree of slag was higher than fly ash when particles sizebecame smaller; there is a coverage limit with a value about70%for PC on the surface ofcementitious particles; a higher activity and smaller particle size of cementitious materialswere related to a more easy adsorption of PC molecules on the surface of cementitiousparticles, a higher pre-adsorption-rate and a lower post-adsorption-rate, the adsorption ratebecame zero when the saturated adsorption was achieved.Based on the analysis on the adsorption behavior of PC on the surface ofsingle-component cementitious particles, the adsorption behavior and dynamic process of PCon the surface of blended cement particles were studied. The influence of PC molecularstructure on the adsorption behavior and dynamic process of the superplasticizers on thesurface of blended cement particles showed the same trend as on the surface of thesingle-component cementitious materials particles. The influence of types and particle size ofthe cementitious materials on the adsorption behaviors of PC were basically independent andthe surface coverage was in the range from40%to50%.Further, according to the interaction between PC and the various cementitious materials,the rheological properties of different “PC-cementitious materials” systems were brieflydiscussed. The carboxyl content of the main chain in PC molecule had an important effect onimproving the initial fluidity of the cementitious paste, resulted from the strong adsorptionability of PC on the surface of cementitious particles. The initial rheological properties ofcementitious paste basically agreed with Bingham rheological model. Generally,supplementary cementitious materials, including slag and fly ash can improve the rheologicalproperties of cementitious paste, and a larger particle size and better morphology ofcementitious materials were related to a smaller water requirement, better compatibility between PC and cementitious materials, smaller paste viscosity and better fluidity. Generally,a greater adsorption amount of PC on the surface of cementitious particles, smaller pasteviscosity and better fluidity.