MACRO DEFECT FREE CEMENTS: A REVIEW

The usage of polymers in different sectors has been increasing in recent decades, and even our current age may have been defined as polymer age. When concrete is considered polymers are also widely used to modify the properties of both mortar and concrete and the usage of polymers in concrete dates back to 1920’s. On the other hand, Macro-defect free (MDF) cements are one particular type of cement-polymer composites and developed and patented by scientists at Imperial College at the beginnings of 1980’s. MDF cements are produced by mixing cement (commonly calcium aluminate cement) with small amounts of polymer (usually polyvinyl alcohol acetate) and water. High shear, relatively low pressure (about 5 MPa) and moderate temperature (about 80-100 °C) are applied during the production of this material. MDF cements, although consist of more than 80% by weight of cement, show 20-30 times higher flexural strength comparing to ordinary Portland cements. However, MDF cements show a considerable reduction in strength when they are exposed to water even for a short time. Many studies have been conducted to solve the water sensitivity of MDF cements for over 30 years. In this study, production, basic properties and the current state of the art of MDF cements are explained, and the future research works are suggested.

The Influence of Moisture on the Expansion of Macro-Defect-Free Cements

ABSTRACTMDF(Macro-Defect-Free ) cement pastes, which consist of portland cement and polyvinyl alcohol/acetate, were prepared by varying the temperature during pressing and drying operations. We then examined the expansion of MDF cement pastes at various constant humidities. There was a large difference in expansion above 60%R.H. between samples prepared varying temperature at which samples were pressed. Samples pressed at 90 °C showed less expansion than samples pressed at 40 °C.The pore structure of MDF cement pastes before exposure to moisture was measured by nitrogen adsorption, mercury intrusion porosimetry and image analysis. The properties of a matrix containing polyvinyl alcohol/acetate and cemnt hydrates were also investigated by TEM, IR and XPS.There were not large differences in the result of IR and XPS measurement between the MDF cement pastes prepared at various temperatures. Calcium hydroxide crystal, lying perpendicular to cement particles, were often observed only in the MDF cement pastes pressed at 90 °C which occurs by water absorption, seems to be suppressed by calcium hydroxide crystal.