Strength and Stiffness Degradation Mechanisms of Stabilized/Solidified Sediments by Freeze–Thaw Cycles

A big amount of dredged sediments from oceans, lakes, rivers, and ditches for navigation and environmental purposes have been disposed in oceans or on land annually (Pinto et al., 2011; Park et al., 2016). These disposed sediments have brought environmental pollutions and been limited or prohibited by many countries and regions gradually (Pinto et al., 2011; Zentar et al., 2012; Fu et al., 2022). Meanwhile, solidification/stabilization technology used worldwide to improve weak soils by mixing solidification additives into weak soils is effectively applied for large-scale resource utilization of dredged sediments (Chiang et al., 2016; Cerny et al., 2017; Wu et al., 2018; Zentar et al., 2021; Guo et al., 2022; Han et al., 2022). In production process of stabilized/solidified (S/S) sediments, additives such as cement, lime, fly ash, epoxy resin, MgO-bearing binder, and geopolymer ,etc. are added into and mixed with sediments (Zentar et al., 2012; Wang et al., 2013, 2021; Anagnostopoulos, 2015; Chen et al., 2021; Zhu et al., 2021). Stable structures including solidification skeletons and sediment aggregates are produced by solidification reactions between solidification additives and sediments (Zentar et al., 2012; Lemaire et al., 2013). Compared with traditional filling materials, S/S sediments present higher strength, higher stiffness, and lower cost (Park et al., 2016), and are widely applied in land reclamations, dikes, and embankments (Zentar et al., 2012).

However, S/S sediments will suffer erosions from freeze–thaw cycles (FTs) when they are applied in construction in cold regions (Kamali et al., 2008; Boz and Sezer, 2018). Macro performance of S/S sediments will be degraded by freeze–thaw cycles (FTs) (Boz and Sezer, 2018; Wang et al., 2018). For example, Lake et al. (2017) have successfully produced S/S sediments with high-quality performances by adding cement as solidification agents, but test results showed that hydraulic conductivity increased by 4.5 times and UCS decreased by 40%, when sediments solidified by 3% cement only were exposed to 3 FTs. Boz and Sezer (2018) have effectively applied lime, polypropylene fiber, and basalt fiber to produce S/S sediments with high resistance to FTs, but test results showed that the mass loss of S/S sediments still increased up to 10% by 10 FTs, unconfined compression strength (UCS) decreased 50% at most, and ultra-pulsive velocity decreased nearly 50% at most, when lime is absent. It follows that S/S sediments will suffer hydraulic conductivity increase, strength losses, and mass losses from FTs. However, stiffness change principles under FTs are still not clear enough by now, while stiffness parameters including elastic moduli and deformation moduli are as important as strength parameters in engineering designs and constructions. Zentar et al. (2012) applied siliceous–aluminous fly ash and cement to solidify marine sediments, studied stiffness parameters such as elastic modulus of S/S sediments during curing, and UCS of S/S sediments under FTs, but have not studied elastic modulus changes under FTs. To explore the stiffness changes of S/S sediments in FTs, authors of the present study conducted odometer tests on sediments solidified by cement, lime, and metakaolin under FTs in previous studies (Wang et al., 2019), captured evolution principles of void ratio, compression modulus and consolidation coefficient, and revealed the relative mechanisms by microanalysis. To move forward a single step, the present study is going to explore the evolution principles of stress–strain curves, elastic modulus, and deformation modulus by unconfined compression strength tests.

Moreover, studies have not given enough importance to microstructure evolutions of S/S sediments in FTs, to reveal degradation mechanisms of strength and stiffness. The microstructure of S/S sediments is composed of solidification skeletons, sediment aggregates, and pores, generated by solidification reactions including hydration reactions, ion exchange reactions between hydrates and sediments, pozzolanic reactions between hydrated Ca(OH)2 and sediments, and carbonation reaction between hydrated Ca(OH)2 and CO2 in the air (Bell, 1996; Zentar et al., 2012; Ahmed, 2015; Du et al., 2016; Pu et al., 2019a; Ke et al., 2019). Current studies (Liu et al., 2017; Wu et al., 2017) have indicated that water–ice transformation in the freezing stage of FTs will produce expansion force to damage the inner structure, ice–water transformation in the thawing stage of FTs will produce shrinkage force to damage the inner structure in an inverse direction, and the microstructure of S/S sediments will be destroyed gradually when the two forces work alternatively. But with regard to degradation mechanisms of strength and stiffness by FTs, the following issues remain to need further explorations: 1) Which microstructure components are influenced, the solidification skeletons or sediment aggregates? 2) How are these microstructure components changed, expanded or shrunk? 3) How do microstructure component changes affect the strength and stiffness—what is the damage model?

Studies applied various microanalysis methods such as scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP) to reveal the microstructure and substance compositions of S/S sediments (Bell, 1996; Ahmed, 2015; Pu et al., 2019a; Ke et al., 2019). But the most effective methods to analyze the degradation of S/S sediments in FTs are microstructure analyses such as microscopy and porosimetry, rather than material composition analyses such as EDS, XRD, or thermal gravimetric analysis (TGA), since freezing and thawing are physical reactions rather than chemical reactions, no new chemical substances will be produced, but microstructure changed during FTs, (Moon et al., 2009; Wang et al., 2015; Hamidi and Marandi, 2018). For example, Lake et al. (2017) have employed transmitted light optical microscopy and MIP to study the morphology changes and micropore changes of S/S sediments in FTs, to reveal the mechanism of hydraulic conductivity increase, and they captured microcracks generated by FTs, and concluded that hydraulic conductivity increase during FTs was primarily a result of cracking in S/S sediments. Commonly, SEM and MIP are commonly combined to analyze the microstructures of S/S sediments (Wang et al., 2013, 2015). As pointed out by Lake et al. (2017), “significant changes in the damaged areas are not captured via the porosimetry scale” by MIP. The reason is that smaller micropores (˂100 nm) will be damaged by high pressure during the measurement process of MIP (Alderete et al., 2017; Li et al., 2018). Fortunately, nitrogen adsorption porosimetry (NAP) which is maturely used in many industries including shale gas production and air purification can measure smaller pores (˂100 nm) accurately (Pan et al., 2016; Abbaslou et al., 2017). Therefore, NAP is applied in the present study together with SEM and MIP to explore the microstructure of S/S sediments in FTs.

According to the aforementioned analyses, the scope of the present study includes the following: 1) investigate strength and stiffness changes of S/S sediments in FTs by UCS tests and direct shear tests; 2) investigate the microstructure changes of S/S sediments in FTs by SEM, MIP, and NAP; and 3) derive the damage model of S/S sediments in FTs by comprehensive analysis based on the aforementioned test results. These studies will be very conducive to promote the durability of S/S sediments facing the risk of damage from FTs.