Mechanical behavior, durability, thermal performances and microstructure of GGBFS – Modified MPC solidified dredged sludge

Abstract

Solidification is an effective way to solve the poor mechanical properties of sludge. In order to improve strength and reduce the cost, the influence of the ground granulated blast furnace slag (GGBFS) content on the properties of solidified sludge (SS) with magnesium phosphate cement (MPC) was investigated. The mechanical behavior, durability, capillary water absorption, microstructure, thermal conductivity (λ) and interaction mechanism between GGBFS-MPC and sludge were analyzed. Finally, microscopic analysis methods (TG-DTG/DSC, XRD, SEM) were used to examine the SS. Results showed that GGBFS-MPC (MgO:GGBFS = 1:1, H3) has excellent mechanical properties (unconfined compressive strength (UCS) splitting tensile strength (STS)) and low shrinkage rate. H3 has the highest UCS and the highest STS with the largest failure displacement, showing good tensile properties. After adding GGBFS, the water resistance increased by about 21.03%-24.05%. XRD detects new hydration products in GGBFS-MgO-ADP, such as Ca(H2PO2)2, CaP2O6, Ca2P2O7·2H2O, CaH2P2O7, Ca4H(PO4)3·2.5H2O, NH4MgPO4·6H2O, NH4MgPO4·H2O, Mg(OH)2, AlPO4, which cemented sludge and synergistically improved the mechanical properties and water resistance. The water sorptivity coefficient of H3 and H5 was small, which attributes to chemical effect and filling effect by hydration products. As the increase of apparent density, the λ increased. The λ does not change much with temperature under dry state. As the water content increased, the λ increased non-linearly. TG-DTA/DSC showed that the weight loss was caused by the loss of free water and bound water. SEM revealed that a small amount of Mg(OH)2 was formed, and struvite cemented the sludge to form agglomerates.