The Role of Ash Mineralogy on Breakthrough Pressure and Contact Angle

One method to minimize contaminant leaching from coal fly ash (CFA) involves modifying its surface to become water repellent. Five types of CFA, CFA-1, CFA-2, CFA-3, CFA-5, and CFA-6, were modified by three organosilane (OS) products at different mix ratios. Multiple breakthrough pressure (BP) experiments were conducted for different samples at different mix ratios to identify the performance of the modified surface to resist a positive water entry pressure. The contribution of oxides and minerals in CFA to form a water-repellent surface were analyzed without considering OS effects. Results showed that semi-log–transformed regression models can effectively establish the relationship between BP and OS composition, mineral composition, and related variables. Results indicate that BP is best predicted using a mineral model in CFA based on gypsum and magnetite content, followed by one based on quartz and gypsum. The main oxide composition in CFA (SiO2,Al2O3,CaO, and Fe2O3) exerts controlling influence on water repellency. However, these results indicate that, beyond a threshold level, their content is less significant than other components. The range at which the percentage of major minerals and oxides are effective in transforming CFA to a water-repellent surface has been identified and presented. For the materials tested, no additional benefit was seen when silica content was increased beyond 61%. Results also indicate that, compared with Class F CFA, Class C CFA is less susceptible to OS modification, ostensibly through the increased presence of CaO. However, results show that if the amount of MgO in Class C CFA exceeds 2%, this type of CFA can be transformed to be highly water repellent.