Abstract
Characterization studies conducted on Class-F fly-ash specimens gathered from different producers in the southeastern United States confirm general trends reported for fly ash worldwide. Additional tests and detailed analyses explain the spread in specific gravity (interparticle porosity cenospheres), highlight the tendency to segregation and layering, and show marked ferromagnetism. Furthermore, data show that early diagenetic cementation—within days after wetting—hinders densification and produces a fabric that is prone to collapse. New procedures are specifically developed to diagnose and characterize early diagenesis, including (1) pH measurements as an indicator of diagenetic potential, (2) test protocols to assess early diagenesis using oedometer tests and shear-wave velocity, and (3) procedures to determine realizable unit weights as reference values for the analyses of contractive or dilative tendencies and instability. In the absence of early diagenetic cementation, dilative fly-ash behavior is expected in the upper ≈20 m under monotonic shear loading. Flow instability may follow the failure of the containment structure if the ponded ash is saturated and has experienced hindered densification.
Highlights
Fly ash is a fine-grained by-product of coal combustion
Results obtained from several studies conducted on fly ash produced in the southeastern United States are summarized
The tests were conducted on freshly pluviated saturated and dry specimens, remolded specimens recovered from ponds, and undisturbed specimens kept inside Shelby tubes
Summary
Fly ash is a fine-grained by-product of coal combustion. The United States alone produces over 50 million t per year. The wide range of tests (Table 2) included (1) general characterization tests (grain-size distribution, plasticity, pH, imbibition, specific gravity, electrical and magnetic properties, and scanning electron microscopy), (2) depositional characteristics (tendency to segregation and layering and realizable densities), (3) hydraulic conductivity, (4) mechanical response (oedometer tests, small-strain stiffness, and both drained and undrained triaxial tests), and (5) tendency to early diagenetic cementation. The tests were conducted on freshly pluviated saturated and dry specimens (fly ash was gently allowed to fall in air or in a water-filled mold), remolded specimens recovered from ponds, and undisturbed specimens kept inside Shelby tubes (tube slices were carefully cut; the tube was used as the oedometer cell, and instrumented loading caps were placed on both ends). Oedometer-wetting tests and shear-wave measurements were conducted on samples recovered from different plants in southeastern United States to explore this effect.
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