Homogeneity of Particle Dispersion in Slag/Cement Blends and its Effect on Hydration Rate
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HOMOGENEITY OF PARTICLE DISPERSION IN SLAG/CEMENT BLENDS AND ITS EFFECT ON HYDRATION RATE
S. LEE, D.M. ROY, AND R.I.A. MALEK Materials Research Laboratory, The Pennsylvania State University, Park, PA 16802
University
ABSTRACT Effective particle dispersion is believed to play a significant role in hydration reactivity of slag/cement blends and consequently on the development of physical and mechanical properties of the hardened pastes. The effects of various factors (superplasticizer and mixing time) on the particle dispersion were evaluated using a computer controlled scanning electron microscope. Backscattered electron signals and elemental maps were resolved for polished mounts of slag and cement particle mixtures. The adequacy of dispersion of slag and cement particles was determined by analyzing about 1000 particles per analysis with appropriate calculations and statistical treatment. These results were related to the hydration behavior of the slag/cement blends. The results obtained can be used to evaluate some of the physical phenomena and the kinetics of slag-cement hydration.
INTRODUCTION Although both ground granulated blast furnace slag and portland cement possess hydraulic properties, they differ appreciably.in their rates and mechanisms of reactions with water due to the differences in chemical composition and structure. Inhomogeneous distribution of particles in blends of slag and cement may result in differences in chemical reactivity (rate of formation of reaction products) as well as development of physical properties (quantity and rate of heat generation, volume change, etc.) in adjacent clusters of slag and cement particles. This could cause microstructural inhomogeneity that may produce adverse effects on the properties of the hydrated matrix. The objective of the present study was to investigate particle dispersion by addition of a superplasticizer or by the mixing procedure. To relate the results to the hydration behavior, early age heats of hydration are determined and the reaction rate of slag particles are determined by selectively dissolving cement and hydrated cement and slag, leaving residual unhydrated slag.
EXPERIMENTAL ASTM Type I/II portland cement with moderately low C 3 A content and three slags were used in the present study. Table I presents chemical analyses and some physical characteristics of the cement and slags used. The phase composition of the cement as obtained from Bogue calculations is as follows: C3 S - 58.27%, C2 S - 15.95%, C3A - 4.62%, C4 AF - 10.55%, gypsum - 4.77%, free CaO - 0.48%, and MgO - 3.96%. The superplasticizer 2 is a
sodium salt of a naphthalene sulfonic acid formaldehyde condensate (SUP),
ISI, Atlantic NEWCEM; S2, English Purfleet; and S3 , Finnish slag. 2
Mighty 150 (solution).
Mat. Res,. Soc. Symp. Proc. Vol. 85. 11967 Materials Research Society
274
Table I Chemical (Oxide) Composition and Some Physical Properties of Cement and Slags
Portland Cement Type I/II
Si02 A1 2 0 3 Fe 2 03 TiO2 MgO CaO MnO Na 2 0 K2 0 Loss on Ignitiona Index of Hy
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