Concrete Masonry Unit Mix Designs Using Municipal Solid Waste Bottom Ash

Abstract

The disposal of municipal solid waste ash has become a significant economic and environmental issue. A sample municipal solid waste bottom ash (MSWBA) was previously evaluated as a potential aggregate for concrete masonry units (CMUs). Test results indicate MSWBA could be used as an aggregate in concrete mixes to produce CMUs that meet ASTM C 90 standards when it is processed for maximum size and gradation and ferrous removal by magnetic separation. Techniques such as aggregate gradation and the addition of coal fly ash and other admixtures offer cost effective methods for performance improvements of mixes using MSWBA as an alternative to the use of additional cement. The MSWBA was found to be deficient in sand size particles and an aggregate blend of 35% sand and 65% MSWBA significantly increased both densities and strength. The addition of sand mitigated workability problems created by the very angular and irregular shape of the MSWBA, but the workability of all mixes using MSWBA was poorer than mixes with commercial aggregate. Twenty-eight-day compressive strengths of up to 2,440 psi (16.8 MPa) and a one year continuous mist cure strength of over 3,000 psi (20.7 MPa) were obtained from mixes using 10% cement, either microsilica or coal fly ash, and a plasticizer. A Class C coal fly ash and a masonry plasticizer both provided improved strengths at all ages. A 24-hour cycle atmospheric steam cure produced strength results averaging only 55% of all mixes compared with a mist cure at ages beyond 180 days. Although the sulfate content of the MSWBA exceeded recommended limits, different cement types all produced equivalent strengths.