Coal fly ash (CFA) can be utilized as a substitute material in cement mixtures, which may affect runoff and groundwater quality. Monolith and pH dependent leaching of trace elements from CLSM (Controlled Low Strength Material) and grout mixtures containing high content of CFA were assessed to inform criteria for CFA usage in infrastructure applications in Mediterranean and semi-arid regions. A CFA with high leaching concentrations of contaminants of potential concern (COPCs) was used. Expected flux and concentrations of relevant COPCs (As, B, Cd, Cr, Mo, Sb, Se, V) were evaluated for surface runoff and a subsurface infrastructure in the vadose zone in three locations in Israel with different climate and soil properties.
The highly cautious runoff assessment comparing the leaching results to water quality standards resulted in acceptable COPCs concentrations for all conditions examined pointing to the safe usage of CFA for surface applications.
For unsaturated vadose zone conditions, annual diffusion distances were calculated to be less than 30 cm from the interface, whereby aqueous solubility, not mass transfer rates, limits the release of many COPCs. For such cases, initial screening using pH dependent leaching test results and considering the hydraulic boundary layer around the structure are sufficient to demonstrate environmental safety.
In the tunnel scenario, only B, Cr and Mo release were not limited by aqueous solubility. The resulting assessment ratios (i.e., COPC concentrations relative to threshold values) are low, and only for one location several COPCs exceeded 1. Portland cement, not CFA was the primary contributor to Cr leaching (highest assessment ratio of 4.6). Adsorption processes in the vadose zone not considered in the assessment are anticipated to reduce all these ratios to less than 1.
Results indicate that even cementitious mixtures containing high CFA and COPCs contents will not adversely impact water resources under the scenarios evaluated.