Reference: D.P. Bentz, submitted to Cement and Concrete Research (2005).
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Influence of Alkalis on Porosity Percolation in Hydrating Cement Pastes

Dale P. Bentz
Building and Fire Research Laboratory
National Institute of Standards and Technology
100 Bureau Drive Stop 8615
Gaithersburg, MD 20899-8615 USA


Loss-on-ignition (LOI) measurements and low temperature calorimetry (LTC) are used to study the properties of hydrating cement pastes with various quantities of alkalis. In addition to the well-known acceleration of early age hydration and "retardation" of later age hydration, the alkalis are observed to have a dramatic effect on the percolation of the porosity in the hydrating systems. At equivalent degrees of hydration, the capillary pores in cement pastes with added alkalis may depercolate while those in the original cement paste remain percolated. A simple dissolution/precipitation microstructural model is applied to examine the effects of hydration product morphology (needles, plates or laths, and random) on pore space percolation. The model suggests that the observed experimental results are consistent with the alkalis modifying the morphology of the C-S-H gel to produce more lath-like hydration products, as has been observed previously using electron microscopy. Implications for the transport properties and durability of these materials are discussed.

Keywords: Acceleration (A), Alkalis (D), Hydration (A), Low Temperature Calorimetry (nominated B)

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