There is a growing concern among concrete users and producers about the scarcity of high-quality coal fly ash, arguably the most common supplementary cementitious material used in concrete to enhance its durability and prevent or mitigate alkali-silica reactivity. Electric power plants are increasingly using gas in place of coal, leading to a decreased supply of coal ash. Environmental regulations for air pollution keep on tightening, adversely impacting the quality of the ash coming out of the coal-fired power plants. The situation will almost certainly exacerbate with time, leaving the concrete industry with no other option but to rethink about ashes that, until now, it considered unacceptable. Such ashes include marginal ashes that do not meet the current specification as well as those disposed of in landfills or impoundments. There is now a growing realization that these ashes should not be rejected simply because they do not come up to the historical specifications adopted when high-quality coal ash was available in abundance; rather, they should also be evaluated and considered if concrete made with them meets the requisite performance. However, to be able to do this, the current specification will need to be revisited to include marginal and unconventional-source coal fly ashes. More importantly, any new specification should be based on measuring and evaluating ash properties that impact the performance of the concrete.  

Under NCHRP Project 10-104,“Recommendations for Revision of AASHTO M 295 Standard Specification to Include Marginal and Unconventional Source Coal Fly Ash,” South Dakota School of Mines and Technology was tasked to propose how current specification could be revised so that off-specification coal ashes could also be considered for use in concrete without any negative impact on concrete performance. These off-specification ashes included ashes that are currently discarded in landfills and impoundments. Following a critical literature review on the state of practice with particular reference to marginal and unconventional-source coal fly ashes, a new specification, based on the impact of ash on concrete performance, was proposed. An extensive testing program involving a large suite of standard and unconventional coal ashes was subsequently developed and executed to evaluate the proposed specification and demonstrate its applicability. Based on this evaluation, draft language is proposed for consideration by AASHTO to incorporate the research results in the next update of the AASHTO M 295 Standard Specification to include marginal and unconventional-source coal fly ash. The report includes a step-by-step guide for state DOTs and other users on a plan of action, including application of mitigation strategies for any negative effects, to allow the use of coal ashes hitherto considered non-compliant with specifications for the intended construction applications.  

NCHRP Research Report 1105: Use of Marginal and Unconventional-Source Coal Ashes in Concrete, recommends changes to currently used methods for selecting coal fly ash for concrete construction so that ashes, presently deemed unacceptable, could also be considered and evaluated for use in concrete. The proposed changes focus on  ash properties that impact concrete performance rather than other factors or characteristics. The findings and recommendations of this report should be particularly useful to state departments of transportation (DOTs) and others involved in selecting concrete materials for their highway infrastructure projects.