Feasibility of using fly ash as binder in coarse and fine aggregates for bases (FHWA-OK-86-8) 2125

Abstract

Aggregate-fly ash mixes consisting of fine aggregate bases (FAB), coarse aggregate bases (CAB) and Class C fly ash additions of 15, 25 .and 35% were tested in compressive and flexural strength for curing periods of up to six months. X-ray diffraction (XRD) was employed to study the mineralogical composition of the mixes and identify the fly ash hydration products. Scanning electron microscopy (SEM) was used to verify the XRD findings and study the microstructural developments in the mixes. The compressive strength of the mixes varied considerably during the first week of curing but exhibited better uniformity in later ages. The variation was attributed to the massive formation of ettringite. Later, when ettringite was transformed to monosulfoaluminate and the CAH, CASH and CSH hydration phases form, the strength of the mixes gained uniformity and magnitude. After a month's curing the mixes presented little resistance to flexural failure but in 90 days the flexural strength level was substantial. The mixes were characterized as stiff but structurally flexible and ?? flexural design approach was devised. SEM observations indicated packing and densification of the mix matrix with time, as the hydration products form skeletal matrices in the mixes, resulting in better strength. The study establishes the dual role of the fly ash as a filler and a chemical agent (binder).