Abstract: The reactive MgO-fly ash cementing materials were innovatively introduced into the improvement of dredged sludge from East Lake, Wuhan by the combined technology of carbonation-solidification. Through unconfined compression strength, scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests, the effect of CO2 carbonation on the mechanical properties and microstructure was investigated under different carbonation modes, carbonation time, ratios of MgO to fly ash and dosages of binding agent. The results indicate that the strength of solidified sludge is evidently increased due to carbonation, accompanied by a narrowed compaction stage of stress-strain curve. The sludge specimens with different binding agents have different optimal pressurization modes, which determines the CO2 intake amount of solidified sludge for the same carbonation period, and affects the strength gain of carbonated samples. Low amount of reactive MgO leads to relatively low compressive strength, which firstly increases and then decreases as the carbonation time increases. The strength of carbonated sludge with relatively high amount of reactive MgO reaches rapidly certain higher value with carbonation time and then increases slowly. The microscopic experiments demonstrated that the formation of magnesium carbonate (e.g. hydromagnesite, dypingite and nesquehonite) is the main reason for enhancing the compressive strength of specimens by the combined carbonation-solidification technology. The expansibility and cementation of the magnesium carbonate promote the transformation of pore in aggregates into inter-granular pore, which makes the soil more compact and increases its compressive strength of carbonated-solidified sludge.

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