Interface characterization and modelling of cement composite systems.

Interface characterization and modelling of cement composite systems.

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Title: Interface characterization and modelling of cement composite systems.
Author: Xie, Ping.
Abstract: A detailed review of cement paste-aggregate interface phenomena in concrete is presented as a precursor to identification of the specific problems addressed in the current interface investigation. The principal deficiencies with current interface investigations described in the literature are as follows: (i) a lack of effective and simple investigative methods to quantitatively characterize interfacial microstructure; (ii) no simple and effective method of interfacial modification. A systematic electrical conductivity theory for the interface characterization of cement composites is proposed in this study. Based on the proposed theory, a novel investigative tool, the electrical conductivity method, has been developed. The proposed theory indicates that the characteristics of interfacial microstructure and interfacial zone formation and development can be described uniquely by a parameter, referred to as the "$\theta$-parameter" or "interfacial excess conductance". The developed method was successfully applied to practical cement paste-aggregate systems. Several new findings pertaining to the nature of the aggregate-cement paste interface were obtained. Extensive experimentation indicates that the $\theta$-parameter is a powerful interfacial microstructural descriptor. It is also a useful parameter for characterizing chemical processes at interfaces in cement-aggregate systems. A hypothesis for interfacial microstructure formation is proposed in concert with observed features of the $\theta$-parameter. Electrical conductivity models of both the interfacial zone and the bulk paste are developed to formulate the hypothesis. The hypothesis stresses the significance of the water film on aggregates at mixing for the interfacial microstructure formation. A relationship between interfacial bond strength and interfacial microstructure is developed. It was deduced that reducing the thickness of the water film and using low w/c ratio are two possible methods of enhancing interfacial bond strength. A method of enhancing interfacial microstructure, i.e. coating aggregate surfaces with silica fume, was developed. The experiments indicated that the interfacial microstructure can be effectively densified by aggregate surface treatment. The consequence of interfacial densification is that compressive strength and sulphate resistance of mortar are increased; bending strength is however increased only at later hydration times. A new tool, an a.c. impedance spectroscopy technique, has been utilized recently to study hydrating cement systems. Its further application was however limited due to lack of a fundamental explanation of the a.c. impedance behavior of the hydrating cement systems. A theoretical model for a.c. impedance spectroscopy of hydrating cement systems was developed. A fundamental understanding of the a.c. impedance behavior of hydrating cement systems has been obtained through application of the proposed model, and the validity of the a.c. impedance technique in investigating hydrating cement systems has, therefore, been strongly enhanced. The established link between interface phenomena and ingress of sulphate ions into cement mortar prompted a re-examination of durability related to sulphate expansion. Sulphate expansion is an important research topic dealing with durability of concrete. The mechanism of sulphate expansion, however, remains controversial. A thermodynamic theory of sulphate expansion is proposed after a careful analysis of the physico-chemical processes concerning the sulphate expansion. The theory is validated by extensive experiments. It appears that most sulphate expansion phenomena in the previous work can be explained by the proposed theory.
Date: 1992
URI: http://hdl.handle.net/10393/7672

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