Shunsuke Hanehara

Effects of the chemical structure on the properties of polycarboxylate-type superplasticizer

Based on the analysis of the chemical structure of polycarboxylate-type superplasticizers with polyoxyethylene (PEO) side chains, the dispersing properties for cement particles were investigated. Analyzed characteristics were the PEO side chain length, the degree of backbone polymerization, the composition of functional groups such as carboxylic and sulfonic groups, and the purity of polymers. Their relative effectiveness as dispersants was evaluated in cement paste by measuring paste flow, plastic viscosity, and shear yield stress at different water/cement ratios (w/c). Although the effects of chemical structure on the paste fluidity were not significant at high w/c, they became significant at w/c below 25%. Polymers with longer PEO side chains, lower degrees of backbone polymerization, and higher contents of sulfonic groups showed higher dispersing power. Higher concentrations of ionic functional groups in the aqueous phase delayed the setting of cement paste.

The role of steric repulsive force in the dispersion of cement particles in fresh paste prepared with organic admixture

The existence of the steric repulsive force has never yet been determined, though the improved dispersibility of solid particles in concrete by adding the organic admixture is considered to be primarily caused by the dispersion of solid particles by the electrostatic repulsive force based on the formation of an electric double layer made of the admixture adsorbing to the surface of solid particles and the increase of the surface potential, and by the steric repulsive force based on the interaction between the adsorption layers of admixture. The authors proved for the first time the existence of the steric repulsive force acting on the surface of cement particles adsorbing the organic admixture by measuring the interactive force between surface of cement clinker adsorbing the admixture with an atomic force microscope, and measuring the zeta potential of fresh cement paste under actual water-to-cement ratio by electrokinetic sonic amplitude method. It is also clarified that the rate of contribution of electrostatic and steric repulsive force to the dispersion of cement and hydrated cement particles differs by the kind of organic admixture.

Interaction between cement and chemical admixture from the point of cement hydration, absorption behaviour of admixture, and paste rheology

Chemical admixtures can improve the properties of concrete. High performance concrete with high strength, superior fluidity, and self-compactibility can be realized mainly because of chemical admixtures. Rheological properties of fresh concrete can be strongly affected by the combination of cement and chemical admixture, method of admixture addition, or the water-cement ratio. Problems in fluidity, such as stiffening and large slump-loss, occasionally happen under a particular combination of cement and admixture. These phenomena are generally called incompatibilities between cement and chemical admixtures. In this study, the interaction between cement and the chemical admixture types lignin sulfonate, naphthalene sulfonate, melamine sulfonate, amino sulfonate, and polycarboxylate, together with the working factors and mechanisms, are discussed from the viewpoint of cement hydration. Although the polycarboxylate type superplasticizer was considered to have better compatibility in combination with different kinds of cement, the authors show that its compatibility is affected by the amount of alkaline sulfates in cement.

Controlling of the adsorption and dispersing force of polycarboxylate-type superplasticizer by sulfate ion concentration in aqueous phase

Abstract The dispersing force of polycarboxylate-type superplasticizer (PC) with poly(ethyleneoxide) (PEO) graft chain has been known to degrade by the sulfate ion in the aqueous phase of concrete. Alkaline sulfate contents and calcium sulfates in cement, mixing temperature and time elapsed after mixing, and other various material characteristics as well as environmental conditions affect the sulfate ion concentration. Therefore, it is generally very difficult to keep the sulfate ion concentration constant. In this study, the active control of the sulfate ion concentration in aqueous phase by adding soluble salts containing multiple-charged cations, such as CaCl 2 and alkaline sulfate, has been attempted. The adsorption of PC is able to be controlled by adding salts such as CaCl 2 and Na 2 SO 4 . The change of the PC adsorption ratio results directly in the variation of the dispersing force of superplasticizer. It is also indicated that ionic strength in the aqueous phase affects the dispersing force of PC. The dispersing force degrades with the increase in ionic strength and sulfate ion concentration.

Influence of kind and added timing of organic admixture on the composition, structure and property of fresh cement paste

Abstract It is well known that the fluidity and the setting time of fresh concrete are affected by kind and added timing of organic admixture including fluidizing agent, air entraining agent and water reducing agent. The fluidity, setting time and time dependency of fluidity are measured using fresh cement paste prepared with four kinds of organic admixtures, polycar☐ylic acid-based, aminosulfonic acid-based, β—naphthalenesulfonic acid-based and lignin sulfonic acid-based admixture, by two different methods of addition, that is, together with mixing water (simultaneous addition) and specified time after mixing with water (later addition). Hydration of cement, adsorptive behavior of organic admixture to clinker minerals, formation of calcium complex with organic admixture and flocculation structure of cement paste are also estimated with the state-of-the-art method such as Auger electron spectroscopy, X-ray photoelectron spectroscopy and environmental scanning electron microscopy in order to clarify the influence of kind and added timing of admixture on the properties of fresh cement paste and concrete and elucidate the mechanism of the improvement of fluidity of cement paste and concrete in later addition of admixture.

Effect of admixture on hydration of cement, adsorptive behavior of admixture and fluidity and setting of fresh cement paste

The adsorptive behavior of admixtures and the hydration of cement in the presence of admixtures were examined and the relationships of them with the physical properties of fresh cement paste including fluidity, variation of fluidity with time and setting time were discussed with the quantitative determination of organic admixture adsorbed on the cement, and with the observation and determination of the surface microstructure and composition of polished clinker dipped in aqueous solution containing a specified quantity of admixture by advanced method of surface analysis. In order to prepare the same fluidity of fresh cement paste, mortar and concrete, the required amount of an easily adsorbed admixture is larger than that of a hard adsorbed one. An admixture having a functional group producing a complex salt with Ca2+ decreases the concentration of Ca2+ in liquid phase at early age and delays the saturation of Ca2+, which influences the morphology of hydrate produced, causes fluidity loss with time and delays the setting time of cement. The microstructural and compositional estimations of the adsorption layer of admixture on the surface of clinker minerals by in-lens FESEM, ESCA-imaging, AES and AFM indicates that the admixture is partially adsorbed to the interstitial phase in a thick layer, forming characteristic three dimensional surface structure.

Effects of water/powder ratio, mixing ratio of fly ash, and curing temperature on pozzolanic reaction of fly ash in cement paste

Abstract The effects of the mix proportion and curing temperature on the pozzolanic reaction of fly ash in cement paste were investigated by examining the productions of calcium hydroxide and the pozzolanic reaction ratio of fly ash. It is possible to accurately determine the reaction ratio of fly ash in cement paste from the insoluble residue and the quantity of dissolved Al2O3. The reaction ratios of fly ash, at the curing temperature of 40°C, the water/powder ratio of 50%, and the substitution rate of fly ash of 40%, are 12% and 32% at the ages of 7 days and 1 year, respectively. The higher the curing temperature or the higher the water/powder ratio is, the higher the reaction ratio of fly ash. The starting time of the pozzolanic reaction at the curing temperature of 20°C is at the age of 28 days or more because the reaction ratio at 28 days is nearly 0%. The pozzolanic reaction of fly ash in cement paste highly depends upon the curing temperature. The reaction ratio of fly ash decreases with an increasing fly ash substitution rate.

Influence of microstructure on the physical properties of concrete prepared by substituting mineral powder for part of fine aggregate

Abstract The hydration reaction of cement, hardened structure and pore structure in concrete prepared by substituting a large quantity of mineral powder including fly ash, slag, limestone and silicious stone for part of fine aggregate in concrete have been studied and the relationships between the substitution of those mineral powders and the physical properties of concrete have been investigated. Increase in viscosity and decrease in fluidity of concrete by the substitution of the mineral powder for part of fine aggregate are mainly caused by the increase of fine particles non-existent in fine aggregate. Higher strength than that of concrete without substitution in the case with the same slump is brought by the densification of hardened concrete structure by filling effect of mineral powder itself and, in some cases, of CSH produced by pozzolanic reaction. The slight increase of creep of the concrete prepared by substituting fly ash for part of fine aggregate may be caused by offsetting the increase of the practical quantity of cement paste in concrete by the improvement of strength of the hardened concrete. The reduction of dynamic Youngs modulus may be caused by the increase of practical quantity of cement paste with the dynamic Youngs modulus almost half that of aggregate.

Influence of CaSO4·2H2O, CaSO4·12H2O and CaSO4 on the initial hydration of clinker having different burning degree

Abstract Relation between early hydration process and properties of fresh cement until setting was studied by using nine kinds of cements prepared from three commercial clinkers with various buring degree and three kinds of calcium sulfates, that is, natural gypsum, β-hemihydrate and anhydrite II. Ca(OH) 2 and CaSO 4 saturation ratio in liquid phase influenced remarkably on the early hydration process of cement paste rather than the hydraulic reactivity of clinker minerals themselves. Setting time well corresponded to the period of maximum Ca(OH) 2 saturation ratio in liquid phase which coincided also with the period of vivid hydration of alite. Soft burning of clinker so as to remain a little amount (0.5% level) of free lime was effective for shortening the setting time but strength at longer age was slightly inferior to that of cement from well burnt linker. Though the setting time was shortened in the order of natural gypsum, β-hemihydrate and anhydrite II in cement from well burnt clinker, no remarkable influence of the kind of calcium sulfate was recognized in cement from poorly burnt clinker.

Influence of microstructural change under stress on the strength-related properties of hardened cement mortar and paste

Abstract Microstructural change of hardened cement paste and mortar under various stresses was studied to obtain the basic data for judging the safety of concrete structure repeatedly receiving the stress. The crack revealed generally at 60% of the fracture stress, and it grew rapidly, exceeding at 80% of the fracture stress. The growth of the cracks in the hardened body was more conspicuous in paste than in mortar, in fly ash cement than in normal portland cement and blastfurnace slag cement, at high W/C than at low W/C, and in repeated loading than in single loading. Results were discussed in connection with the changes of pore structure and backscattered electron images of hardened bodies after loading the stress.