Impedance Spectroscopy and the Role of Admixtures in the Hydration of Portland Cement Pastes
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IMPEDANCE SPECTROSCOPY AND THE ROLE OF ADMIXTURES IN THE HYDRATION OF PORTLAND CEMENT PASTES BRUCE J. CHRISTENSEN*, THOMAS 0. MASON%, and HAMLIN M. JENNINGS** *Department of Materials Science and Engineering, *Department of Civil Engineering, Center for Advanced Cement-Based Materials, Northwestern University, Evanston, IL 60208. ABSTRACT Measurements of the bulk electrical properties of cement pastes were made using impedance spectroscopy (IS) and are useful for studying hydration. Normalization of these quantities by dividing out changes in the pore fluid reveals information pertinent to the microstructural development of these In this study, observations are made on the influence of materials. accelerators, retarders and silica fume (SF) on pastes of white and ordinary portland cements (OPC). All systems show variations in the normalized electrical properties at the same degree of hydration, as compared to a control. Changes in the microstructure that are implied by these measurements are consistent with the observations of others. INTRODUCTION The use of admixtures for modifying the hydration, mechanical properties and environmental stability of cements and concretes has become commonplace. Accelerators, like calcium chloride, decrease the time to set, resulting in more rapid strength gains. Retarders, like sucrose, prolong the induction Additions of period and provide additional time for placing concretes. pozzolans, such as silica fume and flyash, improve strength and decrease permeability. Therefore, it is important to understand better how these additives alter the hydration and final properties of these materials. A number of studies have focussed on the influence of accelerators and retarders on the electrical properties of hydrating cement pastes. It has been shown that additions of calcium chloride to cement pastes increase the bulk conductivity 2-3 times over that of an equivalent neat paste [1,21, due primarily to the increase in the number of ions in the conductive pore fluid. The increase is followed by a sharp decrease as hydration is accelerated and conductive pore fluid is consumed by the reaction. Additions of retarders such as sucrose and citric or gluconic acids produce the opposite effect, decreasing This slight decrease the initial cement paste conductivity by 10-20% [3,4]. is most likely due to hinderance of the pore fluid ions by these relatively non-conductive organic species. The end of the dormant period is marked by a rapid drop in conductivity, with the length of time to this point being Similar trends for determined by the amount of admixture addition. accelerators and retarders are observed in mortars, as well [5]. Very little research has focussed on the influence of silica fume on the A number of electrical properties of cement pastes during hydration. investigations on the dielectric behavior of these materials have been reported Recent work by the [6-9], but are for very dense and/or dried materials. authors [10] has shown that a substantial increase in the bulk resistivity of c
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