Strength development of solidified dredged sludge containing humic acid with cement, lime and nano-SiO2

The disposal of dredged sludge (DS) poses an increasingly difficult problem for sediment dredging engineering. This study investigated the strength development of solidifying DS containing humic acid (HA) with cement, lime and nano-SiO2 (NS). A range of unconfined compressive strength (UCS) and pH tests were conducted to explore the roles of cement, NS, HA, lime and curing age on the solidification effect of DS. Furthermore, microstructures and crystalline phases of typical mixes were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The results showed that the addition of NS can significantly improve the strength development of cement-solidified dredged sludge (CDS). The 60-day UCS of CDS with 1.0% NS was more than twice that without NS, concurrently, the addition of NS also reduced the pH of CDS. The HA seriously affected the strength development of CDS, and the influence threshold value of HA content was in the range of 4.5-5.0%. Using lime together with cement for solidifying DS containing HA had advantages over using cement alone, and the optimum mass ratio of lime to cement is 6:9. The optimum NS content of 1.0% was determined to be the most cost-effective for improving the strength development of CDS. The microstructure and mineralogy analysis confirmed that adding NS to CDS can effectively accelerate the hydration reaction and produce more calcium silicate hydrate (CSH), leading to a significant improvement in the strength of CDS. (C) 2019 Elsevier Ltd. All rights reserved.