The Influence of Moisture on the Expansion of Macro-Defect-Free Cements
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THE INFLUENCE OF MOISTURE ON THE EXPANSION OF MACRO-DEFECT-FREE CEMENTS
H. IGARASHI AND T. TAKAHASHI R&D Center of Cement Division Ube Industries, Ltd.,
Ube,
Yamaguchi, Japan.
ABSTRACT MDF(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. The expansion, which
occurs by water absorption, crystal.
seems to be suppressed by calcium hydroxide
INTRODUCTION High-strength cement pastes, which are prepared from cement with a low water/cement ratio and the addition of water-soluble polymers, have been recently developed [I]. This cement paste does not include defects, such as large pores, which lower flexural strength. Therefore, this type of cement is sometimes called macro-defect-free(MDF) cement pastes. MDF cement pastes have good mechanical properties compared to those of conventional cement pastes. However, MDF cement pastes are known to decrease in strength [2] and expand by water absorption. Many factors controlling water stability of MDF cement pastes, such as hydration of the unhydrated cement in MDF cement pastes[3], microstructure of polymer gel[4l and crosslinking between metal ion and polymer, have been investigated. In this study, various factors which affect the expansion of MDF cement pastes based on portland cement and polyvinyl alcohol/acetate(PVA) are investigated in MDF cement pastes prepared by varying pressing and drying
conditions. The chemical interaction between calcium ion and PVA was also investigated by using X-ray photoelectron spectroscopy(XPS) in the system of C3 S and PVA. EXPERIMENTAL MDF cement pastes were prepared by the method described by Birchall et al. [11. The chemical composition of portland cement used is given in Table I. The MDF cement pastes were prepared as follows. Portland cement(100 parts), PVA(Gohsenol KH-17s, 4 parts) and water(10 parts) were mixed in a
planetary mixer. The resulting mixture was kneaded using a twin roll mill to obtain a sheet. The sheet was pressed and dried under the conditions given in Table II. MDF cement pastes with a thickness of about 1 mm were finally obtained.
Mat. Res. Soc. Symp.
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