The Effect of Micromorphological Development on the Elastic Moduli of Fly Ash-Lime Stabilized Bentonite.

Abstract

The mineralogical and micromorphological changes occurring in fly ash-lime stabilized bentonite were observed and related to changes in elastic moduli of the stabilized mixture. Compacted fly ash, fly ash-lime, bentonite-lime, bentonite-fly ash, and bentonite-fly ash-lime mixtures were prepared and cured at 23$\sp\circ$C and 50$\sp\circ$C, for 1, 28, 90 and 180 days. The development of microstructure and cementitous crystals were observed by a scanning electron microscope, and energy dispersive spectrum analyzer and a X-Ray diffractometer. The elastic moduli and strengths were obtained from unconsolidated undrained triaxial and unconfined compressive strength tests. The physical test results were compared with changes observed by scanning electron microscopy and X-Ray diffraction. CSH gel Type I, II and III, ettringite, afwillite and tetracalcium aluminate thirteen hydrate crystals were indentified in the cured specimens. The elastic modulus of the fly ash-lime stabilized bentonite was higher than the untreated bentonite and the increase in elastic modulus corresponded to the curing times when new cementitious crystals were observed. Acicular crystals (CSH Type I and II) and ettringite crystals spanned the pores and increased the contact points where blocky aggregates of equant crystals (CSH III) engulfed the fly ash grains providing support. The compressive strength increased, and the strain at failure decreased resulting in an increase in the elastic modulus. Some fly ash grains providing support for montmorillonite aggregates dissolved and created weak spots in the matrix, causing a decrease in elastic modulus at longer curing periods. At 50$\sp\circ$C curing temperature the same cementitious crystals were observed as at 23$\sp\circ$C. However, the rate of the reactions increased considerably.