Strength gain and microstructure of carbonated reactive MgO-fly ash solidified sludge from East Lake, China

The reactive MgO-fly ash-CO2 cementing materials are innovatively introduced to improve the engineering property of sludge dredged from East Lake, China by the combined technique of carbonation-solidification. Based on the unconfined compression strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests, the influence of CO2 infiltration on the mechanical properties and microstructure of MgO-fly ash solidified sludge was systematically investigated under different carbonation modes, carbonation periods, mass ratios of MgO to fly ash and dosages of binding agents. The test results demonstrated that the compressive strength of MgO-fly ash solidified sludge is significantly raised owing to the CO2 carbonation, accompanied by a narrowed compact stage of stress-strain curves. The carbonation process exerts a slight but perceptible influence on the relationships of UCS similar to epsilon(f) and UCS similar to E-50. The solidified sludge samples with different mass ratios of reactive MgO to fly ash have different optimal pressurization modes, which determine the total CO2 intake amount for the same carbonation period and influences the strength gain of carbonated samples. The compressive strength of carbonated sludge is greatly affected by the amount of reactive MgO, which might change the variation trend of strength as the carbonation period increases. A series of mineralogical and microscopic tests, including XRD, SEM and MIP, demonstrated that the formations of magnesium carbonates (hydromagnesite, dypingite and nesquehonite) are the intrinsic mechanisms why the strength performance and microstructure of carbonated samples is largely improved by the combined technique of carbonation-solidification. The cementation and expansibility of chemical reaction products can effectively make the soil structure more compact and fill the interparticle and intraparticle pores, finally increasing the strength properties of carbonated-solidified sludge.