Experimental Study on Mechanical Properties and Structural Characteristics of Modified Sludge in Foundation Pit

Abstract

To explore the efficient method of sludge modification, Ultra-fine Portland cement (UPC) was introduced as a sludge modifier regarding Ordinary Portland Cement (OPC) modified sludge as a reference. The mechanical properties and microstructural changes of UPC-modified sludge with different curing time and cement content were carried out by unconfined compressive strength (UCS), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) tests. Results show that the UCS of UPC-modified sludge varies with curing time and cement content in the same way as that of OPC-modified sludge. However, compared with OPC-modified sludge, UPC has a higher sludge modification efficiency, and the UPC-modified sludge has greater compressive strength, significantly early-strength, and stronger resistance to deformation. The stress-strain curves of UPC-modified sludge present significant peak stresses, and which show a brittle failure mode. The combination of the hydration products calcium silicate hydrate (C-S-H) gels and ettringite (Aft) crystals are the essential reason for the improvement of the macroscopic strength of the modified sludge. In contrast to OPC, the UPC hydrates faster and more fully. The UPC-modified sludge can generate more hydration products under the same conditions, this is why that has high efficiency and early-strength. The conclusions obtained in this study can provide a reference for the similar engineering application of ultra-fine cement in modified sludge.