Evolution of mechano-chemistry and microstructure of a calcium aluminate-polymer composite: Part I. Mixing time effects
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Paste development and evolution of microstructure, microchemistry, and mechanical properties of macro-defect-free composites were investigated. Mixing torque plots from a Banbury mixer showed a "window of processibility" within which an optimum polymer-particle network structure formed. This processing window can be controlled by mixing rate and temperature. Network development during paste formation governed the flexural strength and microstructure of the cured material. Electron microscopy studies of the composites corresponding to various points along the mixing torque curve revealed a matrix formation-destruction process that was linked to the mixing activity and the mechano-chemistry of the system. Microchemical evolution in the composite was studied by EDS. These studies suggested that a combined convection and diffusion mechanism was responsible for the migration of Al 3+ and Ca 2+ ions into the different regions of the microstructure. Increasing concentrations of these elements in the polymer regions stiffened the matrix, leading to complete degradation on further mixing.
I. INTRODUCTION Macro-defect-free (MDF) cements are polymercement composites consisting of a tightly bound network of cement grains embedded in a polymer matrix. The standard formulation1 starts with a premixed powder made up of a calcium aluminate cement, partially hydrolyzed polyvinyl alcohol, water, and a small amount of glycerine. The damp powder undergoes a shearcompression step during further mixing on either a two-roll mill2"4 or a Banbury mixer,5-6 which transforms the formulation to a dough-like, viscoplastic paste that can be extruded or molded to form a desired shape. Upon curing, the hardened materials have volume porosities of less than 1% and may attain flexural strengths in excess of 250 MPa (compared to porosities of 10-20% and strengths of 5-10 MPa which are characteristic of conventional cement-based materials). Paste formation, regarded synonymously with the development of the polymer-particle network, is governed by the mechano-chemical (i.e., mechanically induced) reactions during the high shear mixing step.6 High resolution electron microscopy studies7-8 of tworoll milled material indicated that an interphase zone on the order of a few hundred nanometers in thickness forms between the bulk polymer and the partially hydrated cement grains. The primary hydration product in this 1746
J. Mater. Res., Vol. 10, No. 7, Jul 1995
region is believed to consist of metastable octahydrate (Ca2Al2O5 • 8H2O) crystallites. Apparently, the presence of PVA inhibits the formation of the stable hexahydrate (Ca 3 Al 2 0 6 • 6H2O) phase.7 Although the two-roll mill can produce good MDF composites, the procedure is labor intensive and often leads to inconsistent mechanical properties of the hardened material. Another disadvantage of the two-roll mill is the relatively poor control over processing conditions, particularly temperature and humidity, as well as a complete lack of quantitative information which might be correlated with the mi
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