Effect of Fly Ash Incorporation on Rheology of Cement Pastes
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EFFECT OF FLY ASH INCORPORATION ON RHEOLOGY OF CEMENT PASTES M. RATTANUSSORN*, D.M. ROY** and R.I.A. MALEK Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 *Current address: The Siam Cement Co., Ltd., Bangkok, Thailand **Also affiliated with the Department of Materials Science and Engineering Received 12 November,
1986; refereed
ABSTRACT The predominant spherical shape of fly ash particles combined with mainly glassy composition and texture of its surfaces have a special effect on rheology of cement pastes containing fly ash. The early ages rheological behavior of cement pastes (ASTM Type I) incorporating 30% low-calcium fly ash was monitored by measuring viscosity of the fresh pastes prior to initial hardening and stiffening (up to -2 hours) as a function of time. The viscosities were determined using a co-axial rotoviscometer (HAAKE). The effects of fly ash content, water to cement ratio, and presence and concentration of superplasticizer, were evaluated. In addition, the dispersivity of fly ash spheres was evaluated by determining the zeta-potential of fly ash suspensions in water using a microelectrophoresis technique and the results were correlated to the chemical composition of fly ash as well as the viscosities of fresh pastes. INTRODUCTION The rheological properties of fresh concrete and cement paste have been intensively investigated by many researchers [1-8]. From these studies, it has been established that the rheology of fresh Portland cement pastes and concrete is comparable, both theoretically and practically. Furthermore, it has been recognized that incorporation of finely divided particles such as fly ash generally improves workability by reducing the size and volume of voids in fresh concrete mixtures. Ivanov and Zacharieva [9] studied the effect of two fly ashes having different specific surface areas on the rheology of cement pastes having 0.45 to 0.7 water to cement (w/c) ratios. The fly ash contents ranged from 0 to 100% of the solids by weight. They found that both yield stress and viscosity increased with increasing fly ash content in the paste. The higher specific surface area fly ash (Blaine air permeability = 515 m2 /kg) had much more effect in increasing both yield stress and viscosity at the same proportion (by mass) of fly ash replacement and w/c ratio. However, in terms of water to solid ratio (w/s) both yield stress and viscosity decreased with increasing fly ash content. The lower specific surface area fly ash (Blaine = 465 m2 /kg) was more effective in reducing both parameters. The coarser fly ash, in this case, was considered to be more effective because the specific surface area is closer to that of cement (340 m2 /kg). Nagataki et al. [10] found that addition of fly ash was advantageous for improving fluidity of cement paste, especially in the case of low w/s ratio. The purpose of the present paper is to investigate the effect of fly ash on the rheological properties of fresh cement pastes, and the effects of the amount of fly ash, w/c
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