Relationship between breakthrough pressure and contact angle for organo-silane treated coal fly ash

Abstract

Abstract One method to minimize contaminant leaching from soil or industrial byproducts such as coal fly ash (CFA) is through engineered water repellency. Breakthrough pressure (BP) experiments were conducted for five CFA types modified using three organo-silane (OS) products at different mix ratios. Results confirm that CFA can be modified and made sufficiently water resistant (up to 10 m pressure head) to infiltration by using OS. Such resistance can prevent infiltration of water into CFA, thereby eliminating the environmental concern of leachate generation. The experimental results of this study show that no water infiltration occurs until a positive water head threshold is reached. This value is referred herein as the breakthrough pressure. Once the breakthrough pressure is exceeded, subsequent infiltration depends less upon pore size and more upon type of OS, type of CFA, and the mix ratio used in the OS treatment. The Washburn equation was applied to predict the BP of the OS modified CFA. The Washburn equation requires an estimate of the pore radius and three approaches were used for this purpose. BP estimates using pore diameters derived from the CFA water characteristic curve and from an assumption of tetrahedral packing were reasonably close to experimental values. In this paper we propose using a relationship that is based on a revised Washburn equation that can better predict BP experimental results. The revised relationship for BP is a Washburn relationship adjusted with two variables, one to account for linear changes in surface energy, and another to account for differences between apparent and pore contact angles (CA). The apparent CA measured on a flat surface and the prevailing CA that manifests between individual pores of a compacted matrix of particles (i.e., as calculated and labeled as the pore CA) is related linearly up to approximately 140, beyond which the relationship follows an exponential function. The proposed relationship between measured CA and BP fits well the experimental data, and also correctly reflects the expected change in behavior at the line of demarcation between hydrophilic and hydrophobic systems (i.e., the line defined by CA = 90°). It was shown that the CA and BP relationship is a function of the chemical properties of the OS and the CFA.