Abstract: The treatment process of sludge dredged from East Lake, Wuhan is improved by innovatively introducing the reactive MgO-fly ash cementing materials, based on the combined technology of carbonation-solidification. Through the unconfined compressive strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests, the effect of CO2 carbonation on the mechanical properties and microstructure was investigated under water immersion, dry-wet cycles and freeze-thaw cycles. The test results indicate that solidified sludge by reactive MgO-activated fly ash has higher strength and better water stability after CO2 carbonation. After 20-day water immersion, the compressive strength of carbonated samples is about 29% higher than that of non-carbonated solidified samples. The strength of carbonated samples increases slowly with the increase of wet-dry cycles, while the strength of non-carbonated samples first increases and then decreases. The evolutions of compressive strength of carbonated and non-carbonated solidified samples agree well with each other as the freeze-thaw cycles extend, i.e. the strength increases initially, then decreases and finally keeps constant. The microscopic analysis demonstrates that the elongated prismatic hydromagnesite (skeleton construction), flower/bone-like and flaky dypingite and nesquehonite (filling and cementing) are the main carbonation products. In addition, the combined skeleton-filling-cementing effect makes the carbonated samples achieve high strength, good water stability, and strong resistance to dry-wet and freeze-thaw cycles. The continuous water immersion leads to an increasing macropores and transformation of dypingite and nesquehonite to hydromagnesite. The dry-wet cycles also induce the transformation of dypingite and nesquehonite to hydromagnesite, but there is no obvious phase transformation in carbonated samples under freeze-thaw conditions.

Key words: dredged sludge, reactive magnesia-fly ash, carbonation-solidification, durability, micro-mechanism