Feasibility Study for Sustainable Controlled Low-Strength Material (CLSM) incorporating off-specification fly ash and bottom ash

“Controlled low-strength material (CLSM) is a self-consolidating cementitious material used in applications requiring high flowability and low strength. The primary components of this material, being cement and sand, however, negatively impact the environment. Cement is a large contributor to CO2 emissions, and the vast depletion of sand causes physical and political issues. Furthermore, over 100 million short tons of coal combustion products (CCP), including fly ash and bottom ash, are created annually in the United States, while nearly half of all CCP remain unused. No off-specification fly ash has been implemented in CLSM. This study uses coal ashes sourced from two coal power plants to fully replace the primary components of CLSM. Off-specification fly ash, a fly ash disposed in landfills, and bottom ash, a non-combustible residue, fully replace cement and sand, respectively, to create a sustainable alternative to conventional CLSM. The two coal ashes were mixed with sodium hydroxide (NaOH) to develop geopolymer CLSM mix designs. Two-part mix designs -- mixes containing both a dry mix of bottom ash and off-specification fly ash and a liquid NaOH solution -- were optimized to have a minimum of 8” spread without segregation and compressive strengths less than 300 psi for excavatability. The one-part mix designs -- mixes containing a dry combination of bottom ash, off-specification fly ash, and NaOH pellets -- improved the user-friendliness of the material by eliminating the need for corrosive solution handling. Flowability tests, compressive strength tests, and cost analyses were performed on the two-part and one-part mixes to find improvements in fresh and mechanical properties and a reduction in cost up to 94% compared to conventional CLSM”--Abstract, page iii