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Title:Effect of phase transformations, chemical doping, and matrix constraint on the microstructural development of dicalcium-silicate
Author(s):Chan, Chin-Jong
Doctoral Committee Chair(s):Young, J.F.
Department / Program:Materials Science and Engineering
Discipline:Materials Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Materials Science
Abstract:The factors controlling the monoclinic ($\beta$) to orthorhombic ($\gamma$) transformation of dicalcium silicate (Ca$\sb2$SiO$\sb4$) at about 490$\sp\circ$C have been studied. This transformation is of interest in portland cement technology because the low temperature $\gamma$ phase is unreactive with water, while the high temperature $\beta$ phase is. The transformation, which is accompanied by a 12% volume increase and a 4.6$\sp\circ$ unit cell shape change, is analogous to the tetragonal-to-monoclinic transformation of ZrO$\sb2$. Thus Ca$\sb2$SiO$\sb4$ is a potential transformation toughener alternative to ZrO$\sb2$.
Pure Ca$\sb2$SiO$\sb4$ was studied extensively from different aspects. A particle size effect in controlling the transformation was confirmed by low temperature annealing on Ca$\sb2$SiO$\sb4$ particles of different sizes. The high temperature $\beta$ phase was also observed to be stabilized by a low temperature synthesis method or by a laser-melting/roller-quenching technique. Fast quenching through the $\alpha$ $\to$ $\alpha\sp\prime\sb{H}$ transformation was also observed to have a strong effect on the stabilization of $\beta$ phase at room temperature.
The effects of chemical stabilizers and departures from stoichiometry on the $\beta$-to-$\gamma$ transformation were also investigated. Changes in the CaO/SiO$\sb2$ molar ratio from 1.8 to 2.2 were not able to stabilize the high temperature $\beta$ phase without concomitant additions of K$\sb2$O or Al$\sb2$O$\sb3$. EPMA and TEM/EDS studies revealed that the dopants tended to concentrate at amorphous grain boundaries. However, higher levels of Al$\sb2$O$\sb3$ were also observed to enter the $\beta$-Ca$\sb2$SiO$\sb4$ grains under lime-rich conditions (CaO/SiO$\sb2$ = 2.2) up to 3.0 wt%. Some additional crystalline phases were observed.
Ca$\sb2$SiO$\sb4$ was successfully dispersed in a calcium zirconate matrix (CaZrO$\sb3$; CZ). Dense pellets of CZ-30 vol% Ca$\sb2$SiO$\sb4$ were hot-pressed and yielded microstructures of intergranular, irregularly shaped Ca$\sb2$SiO$\sb4$ particles. Results suggested that the critical particle size may be determined by overall Ca$\sb2$SiO$\sb4$ grain size or twin width and twin length. A characteristic "herring-bone" type or parallel-banded twin structure resulting from fast quenching through the $\alpha$ $\to$ $\alpha\sp\prime\sb{H}$ transformation is believed to modify the $\beta$ twin length and width. Transformation toughening was proved to be possible. An approximately 5-fold increase in fracture toughness in a CZ-30 vol% Ca$\sb2$SiO$\sb4$ composite was observed in a parallel study.
Issue Date:1989
Rights Information:Copyright 1989 Chan, Chin-Jong
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI8924786
OCLC Identifier:(UMI)AAI8924786

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