Abstract
Macro-defect-free cement (MDF) is a composite material consisting of a tightly bound network of inorganic calcium aluminate (CA) cement grains embedded in an polyvinyl alcohol (PVA) organic matrix. The standard composition starts with a mixed batch made of 83.8 wt% CA, 5.9 wt% partially hydrolyzed PVA, 9.6 wt% water and 0.6 wt% glycerine. The damp powder is further shear-mixed in a Banburry type mixer. During this step, the mix converts into a dough-like, viscoplastic paste which further can be processed by extrusion or molding to a desired shape. Viscoplastic paste formation is a crucial stage in processing and results from mechano-chemical (i.e. mechanically assisted) reactions between the polymer and the hydrating cement phases. The viscoplastic flow is only observed within a certain processing time interval, i.e. window of processability (Regions II and III in Fig. 1). Prolonged shear mixing reverts the system back into a friable, powdery mass. The rheology of the composite paste is monitored in terms of the torque required to flow the mix at a constant rpm.
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