1 USING ELECTRO-TRANSPORT TO GENERATE CAVITY-FILLING NANO-PARTICLES IN POROUS STRUCTURAL MEMBERS S. W. Morefield*, C. A. Weiss, Jr.‡, P. G. Malone‡, V. F. Hock* *US Army Engineer Research and Development Center, Construction Engineering Research Laboratory, Champaign, IL 61820 ‡US Army Engineer Research and Development Center, Geotechnical and Structures Laboratory, Vicksburg, MS 39180 ABSTRACT Improving the density and cementation in porous media leads directly to higher compressive strength, and increased durability and survivability. Both nano- particles and ions in solution can be moved into non- conductive porous media to increase the density by passing a direct current through solutions and saturated porous media. The problem with using nano-particles by themselves as pore-filling materials relates to the lack of any chemical bonding to hold the nano-particles in pores and the difficulties of producing complete filling of the pores. The current project examines the feasibility of using both electrophoretic particle migration and solution electrotransport to move seed particles into pore space and to provide a precipitation process that will grow the migrated phase to fill the pore space and produce bonding from the infill to the surfaces of the pores. The goal of this work is to produce a treated concrete that has lower porosity, increased density and improved cementation by depositing nano-particles and precipitating solid phases in the original pore space in the concrete matrix. Bench scale experiments have demonstrated that significant density and strength increases can be obtained by electrophoretically moving seed crystals of selected mineral phases such as calcite, aragonite, or vaterite into the pores in concrete and then using a low-voltage DC current to migrate in calcium and carbonate ions to grow the selected crystalline phases in masses sufficient to fill up both large and small pores. This can be done in such a way as to leave even the reactive phases in the concrete such as Ca(OH) 2 unaltered. The bulk pH of the concrete is not reduced in the mineralization process as it would be in simple carbonation. Pore infilling using electrophoretic and electrotransport can potentially in producing very dense concrete panels and pipes. 1. INTRODUCTION The strength of the cement paste in concrete is thought to be related to the both the composition of the paste and the size distribution of the pore spaces. When concrete is fully compacted and cured, its strength has been found to be inversely proportional to the amount of void space in the concrete. In 1896, Feret developed the following equation for calculating the variation in strength in concrete. 2       + + = a w c c K F c (1) where: Fc = unconfined compressive strength C = volumetric proportion of cement d = volumetric proportion of water A = volumetric proportion of air K = constant This same relationship of the proportions of void and solids is the basis of Abram’s Law ( ) c w c k k F / 2 1 = (2) where: k 1 and k 2 are empirical constants and the exponent w/c is the water-to-cement ratio. The space occupied by excess water not used in chemical hydration water becomes pore space (Mehta, Monterio and Monterio, 2005; Neville, 1981) Watson (1981) worked out a relationship using the pore volume and the solid volume of hydrated Portland cement pastes to predict the strength increase as the solid volume was increased from a base-condition designated as zero strength. The Watson model is presented as: S = K [(V s / V p ) - (V s / V p ) cr where: ] (3) S = strength Vs = volume of solids Vp = volume of pores (Vs/Vp) cr K = an empirical constant = critical solid-to-pore volume ratio that corresponds to negligible (zero) strength. Das, Singh and Pandey (2008) developed addition data related to Watson’s model and showed it could be applied to concrete or mortar and not just cement paste. Mindess (1970) proposed that, for a given porosity, the strength increases with the proportion of fine pores increases. A study by Odler and Rossler (1985) found that for various water-to-cement ratios and curing times and temperatures the main common factor influencing the