Processing-Property Interactions in Macro-Defect-Free Cements
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PROCESSING-PROPERTY INTERACTIONS IN MACRO-DEFECT-FREE CEMENTS A.J. McHugh and L.S. Tan Department of Chemical Engineering and Center for Cement Composite Materials, University of Illinois, Urbana, IL 61801 ABSTRACT Relationships between mixing conditions, viscoelastic paste properties and flexural strength of hardened matrices have been investigated for an MDF cement. The correlation of paste relaxation times and matrix mechanical properties with mixing intensity suggests the build up and eventual breakdown of a network structure during mixing. INTRODUCTION Macro-Defect-Free (MDF) cements are low water content, polymer-cement composites consisting of a tightly bound network of cement grains embedded in an amorphous polymer matrix. The polymer-particle network which forms during high shear mixing is believed to be responsible for the transformation of the premixed powder to a dough-like, visco-plastic paste [1]. MDF Cements have generated considerable interest for their potentially useful mechanical, dielectric and acoustic damping properties, among others [2] and the ability to extrude and shape the paste at room temperature using conventional polymer processing methods. Most published studies have been concerned with the elucidation of MDF microstructure and the polymer-particle reactions which occur during setting, or, the role of moisture in the final properties [2]. However, with the exception of the patent reports [3], a near-void exists in the literature concerning the dynamics of the mixing mechano-chemistry and its relationship to paste rheological properties and the mechanical properties of the hardened matrix. This paper reports preliminary results of a study we are undertaking to address these issues. EXPERIMENTAL Materials and formulation Cement pastes were prepared using the "standard" formulation which is considered to produce optimal properties in the hardened matrix [2]. This consists of 40 parts by weight calcium aluminate cement (Lafarge Secar 71), 2.8 parts 80 mol % hydrolyzed polyvinyl alcohol in powder form (Nippon Gohsel Gohensol KH-17s), 4.6 parts deionized water and 0.3 parts glycerine.
Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
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Equipment High shear mixing was carried out in a Haake-Buchler Rheocord 90 with a water-cooled Rheomix mixing head attachment. Rotors were of the Delta conical type. In some runs a plexiglass front plate was used for visualization and video recording of the mixing dynamics. Stress relaxation of mixed pastes was monitored using a biaxial squeezing flow rheometer of the type described elsewhere [4]. The device consists of two circular 1" steel disks, the upper of which is attached to a 100 lb. load cell. Squeezing motion is controlled by a timer-attached motor which produces rise times between 0.1 and 1 s with equibiaxial strains ranging to 0.25. Mixing and viscometry protocols Cement, PVA powder, and glycerine were mixed in a low shear planetary mixer for 30s to obtain a homogeneous, dry powder, followed by addition of the water and
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