AbstractsEngineering

Macro-defect-free (MDF) cements were developed following work by the research group led by Birchall at ICI during the late 1970s and early 1980s. MDF cement is a cement-polymer composite seen to provide the high stiffness of a hydrated cementitious material with the high tensile strength of a polymer composite. It is usually made using polyvinyl alcohol (PVA) that undergoes some crosslinking with Al(OH)6 released as the cement hydrates. It is processed using high shear, typically in a twin roller mill. The most remarkable property of MDF is its high flexural strength, 200-300 MPa as compared to 5???10 MPa for hardened cement pastes. The inventors attributed the high tensile strength to the processing, which they claimed removed large voids otherwise seen in cementitious materials such as Portland cement concrete, hence the basis for the name. Later studies demonstrated that crosslinking occurs between polymer and hydrating cement and that this crosslinking plays an important role in the achievement of high strength. However, there are few reports on the study of the fracture toughness of MDF. A common test, the two-parameter fracture method, based on the simple premise that a change in specimen compliance can be correlated to the length of the effective crack at the time the critical (i.e., peak) load is reached, is used to measure fracture toughness of MDF. With the theory of linear elastic fracture mechanics (LEFM), the loads and displacement associated with crack propagation in notched beams can be analyzed easily. A typical fracture toughness value of 1.6-1.8 MPa ??? m1/2 was obtained. Also, a microhardness test is used to measure fracture toughness because it suits the form of the specimens and is a simple and expedient experiment, however, it didn???t generate well-identified cracks.