PaperElectrical conductivity, diffusion, and permeability of Portland cement-based mortars
Introduction
Characterization of the developing microstructure within cementitious systems represents an important area, and a variety of techniques have now been exploited in this respect. These include, for example, scanning electron microscopy, mercury intrusion porosimetry, magic angle spinning and small angle X-ray scattering. The continuously evolving pore network is critical in all aspects of the science of cement-based materials including strength, permeability, diffusion, sorptivity, leaching, creep and shrinkage. Electrical property measurements, as applied to cementitious materials, represent an additional and still developing investigative technique in the study of these materials both at the micro- and macro-scale (see, for example, [1], [2], [3], [4], [5], [6], [7], [8]). From an engineering point of view, there is a need to be able to characterize the capillary pore network using easily measured properties and, to this end, electrical measurements could be exploited.
The current work investigates the electrical conductivity of a range of Portland cement-based binders over a 450-day time scale; in particular, the influence of thermal cycling on electrical measurements of mature cementitious systems formed an important aspect of the investigation. Regarding the latter, there is a dearth of information in this area. At the end of the test period, a more limited series of tests was undertaken on water permeability and chloride diffusion.
Section snippets
Test specimens
The oxide analysis of the materials employed within the experimental program is presented in Table 1. Ordinary Portland cement (OPC) was used throughout. The pozzolanic materials comprised ground granulated blast-furnace slag (GGBS), metakaolin (MK), and micro-silica (MS). Mortar samples were used throughout and are presented in Table 2; the fine aggregate (i.e. <5 mm maximum size) conformed to grading zone M as defined in BS882: 1992 [9]. Materials were initially dry-mixed in a Hobart
Bulk conductivity (at 20°C)
The decrease in bulk conductivity, σbulk, over the 450-day test period is presented in Fig. 1a. In comparison with the plain OPC mix (Mix 1), the conductivity of the mixes with pozzolanic additions decreases more rapidly over the initial 28-days (particularly Mixes 3 and 5) and, in the longer term, achieve a lower value than Mix 1. This reflects the influence of the pozzolanic reaction. The decrease in conductivity of the saturated specimens will result from two sources,
(a) a reduction in
Concluding comment
In the current work, the electrical conductivity of a range of cementitious binders was studied over 450 days. Bulk conductivity measurements on mortars and their respective pore fluids indicate that over the initial 28-days hydration, changes in the pore structure exerted a greater influence on the measured conductivity than changes in pore-fluid conductivity. At the end of the test period, Arrhenius plots were established for the conduction process under a thermal cycling regime and
Acknowledgements
The Authors wish to thank the Engineering and Physical Sciences Research Council (EPSRC) for financial support (Grant Reference GR/K65089). Thanks are also expressed to Dr. H. Ezirim and Mr. H. Barras for their assistance in the experimental work.
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