Impact of Coal Combustion Fly Ash Used as a Supplemental Cementitious Material on the Leaching Constituents from Cements and Concretes.

摘要

Using available data from Europe and the United States (US), this report compares the leaching of portland cement-based materials that have been prepared with and without coal combustion fly ash. The objective of this report is to illustrate whether there is evidence based on existing data that the use of fly ash in cement and concrete products may result in increased leaching of constituents of potential concern (COPCs) compared to cement and concrete products that do not contain fly ash. Results of pH dependent leaching and cumulative release from monolith leaching, as described in the Leaching Environmental Assessment Framework (LEAF) are used for the evaluation. LEAF data is also compared with relevant single point leaching test data. The available data suggest that the use of coal combustion fly ash in cement materials, for different combinations of fly ash source and usage rates, will not increase leaching of some constituents to levels greater than typical ranges for cement materials not containing fly ash. However, there are data limitations that preclude making broad-based claims for some usage rates and fly ashes including those resulting from changes in air pollution control at coal-fired power plants. Based on available data (31 cement mortar and concrete samples containing coal fly ash in comparison to 21 cement and mortar samples that did not contain coal fly ash), results indicate that some (and probably a large portion) of coal fly ashes currently being produced can be used in cement and concrete formulations without causing greater leaching of COPCs than observed from analogous cement materials not containing fly ash and without causing adverse environmental impacts. The range of fly ash sources and usage rates for cement materials with pH dependent and monolith leaching test data available is more limited than the full range of typical fly ash usage rates and typical fly ash leaching behavior. For example, fly ash substitution rates for cement of up to ca. 45 wt% is typical in US large commercial concrete applications, while the predominance of materials reported here have ca. 20-35 wt% fly ash substitution. In addition, the data evaluated in this report does not consider changes in leaching from cement-based materials that may occur if the characteristics of coal fly ash used in cement-based materials change in response to changes in air pollution control requirements at coal fired power plants. The LEAF methodology, specifically including pH dependent and monolith leaching test data as a basis for evaluation, provides a more robust approach than single point leaching tests for evaluating the potential environmental impacts from use of coal fly ash in cement and concrete materials because LEAF considers the likely range of leaching pH over the lifecycle and the physical form of the materials (i.e., monolithic). Leaching of individual constituents from cement materials exhibits characteristic concentration as a function of pH responses. Future work is recommended using the LEAF methodology to evaluate coal combustion residues (CCRs) that are specifically marketed for use in producing cementitious materials and reflect changes occurring in air pollution control at coal-fired power plants. The focus of future work would be to confirm the findings from this report and to identify any materials, usage rates, or other conditions that might lead to release of COPCs that could be of concern to human health and the environment. This approach would support continued use of fly ash in cement and concrete products while ensuring protection of human health and the environment. iii