Sodium hydroxide (NaOH)-sodium silicate-GGBS (ground granulated blast furnace slag) effectively stabilises sulfate-bearing soils by controlling swelling and enhancing strength. However, its dynamic behaviour under cyclic loading remains poorly understood. This study employed GGBS activated by sodium silicate and sodium hydroxide to stabilise sulfate-bearing soils. The dynamic mechanical properties, mineralogy, and microstructure were investigated. The results showed that the permanent strain (epsilon(p)) of sodium hydroxide-sodium silicate-GGBS-stabilised soil, with a ratio of sodium silicate to GGBS ranging from 1:9 to 3:7 after soaking (0.74%-1.3%), was lower than that of soil stabilised with cement after soaking (2.06%). The resilient modulus (E-d) and energy dissipation (W) of sodium hydroxide-sodium silicate-GGBS-stabilised soil did not change as the ratio of sodium silicate to GGBS increased. Compared to cement (E-d = 2.58 MPa, W = 19.96 kJ/m(3)), sulfate-bearing soil stabilised with sodium hydroxide-sodium silicate-GGBS exhibited better E-d (4.84 MPa) and lower W (15.93 kJ/m(3)) at a ratio of sodium silicate to GGBS of 2:8. Ettringite was absent in sodium hydroxide-sodium silicate-GGBS-stabilised soils but dominated pore spaces in cement-stabilised soil after soaking. Microscopic defects caused by soil swelling were observed through microscopic analysis, which had a significant negative impact on the dynamic mechanical properties of sulfate-bearing soils. This affected the application of sulfate-bearing soil in geotechnical engineering.

Generally, high-water-content of dredged sediment (DS) tends to suffer from inferior mechanical properties and obvious shrinkage after solidification, so finding solutions to this issue is helpful for promoting the recovery and recycling of DS. In this paper, in reference to natural gypsum (NG), phosphogypsum (PG) was incorporated into DS solidified with alkali-activated slag (AAS) system. The effect of PG (0 %-20 %) on the hydration process (0-168 h), mechanical properties (3 d, 7 d and 28 d) and autogenous shrinkage (0-7 d) of DS solidified with AAS was investigated. It is found that the addition of PG not only induces the generation of ettringite to compensate for shrinkage, but also accelerates the formation of C-A-S-H by providing active calcium to promote stiffness to resist shrinkage. This results in a reduction of autogenous shrinkage by 74.3 % and an increase of compressive strength by 28.5% when PG dosage is 15%. Compared with NG, the difference in 28d-compressive strength of PG group is not more than 7.34 % under equivalent dosages. The dissolved SO4 2-from PG could be adsorbed on CA-S-H and preserved in pore solution in the form of Na2SO4. The decrease in S/Si from 0.31 to 0.09 indicates stored SO42- could be released back into system to promote the further generation of ettringite. To obtain superior mechanical properties and volume stability, appropriate PG dosage is 10 %-15 %. Compared with the control group, it increases the content of ettringite and amorphous phase by 2.4 %-4.6 % and 3.3 %-3.7 %, respectively. This research not only provides theoretical support for DS solidified with AAS to realize efficient utilization of solid waste resources (i.e., DS, PG and slag), but also gives a new insight into solidification of other high-watercontent system, such as backfill mining, grouting materials and treatment of soft soil foundations.

Marl soil is highly prone to erosion when exposed to water flow, posing a potential threat to structural stability. The common practice of stabilizing soil involves the addition of cement and lime. However, persistent reports of severe ruptures in many stabilized soils, even after extended periods, have raised concerns. In stabilized marls, unexpected ruptures primarily result from the formation of ettringite, which gradually damages the soil structure. This article aims to assess the impact of nanosilica on the formation of ettringite and the nanostructure of calcium silicate hydrate (C-S-H) during the marl soil stabilization process with lime. To achieve this, marl soil was stabilized with varying percentages of lime and nanosilica. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were collected to observe changes in mineralogy and microstructural properties. Various geotechnical parameters, including granularity, Atterberg limits, compressive strength, and pH, were measured. The results indicate that the uniform distribution of nanosilica in marl-lime soils enhances pozzolanic activities, calcium aluminate hydrate growth (C-A-H), and the nanostructure of calcium silicate hydrate (C-S-H). According to XRD and SEM experiments, the presence of nanosilica reduces the formation of ettringite. Moreover, the compressive strength of modified samples exhibited an upward trend. In the experimental sample manipulated with 1% nanosilica combined with 6% lime, the compressive strength increased by 1.84 MPa during the initial 7 days, representing an approximately 18-fold improvement compared to the control sample.

The addition of calcium (Ca)-based stabilizers to sulfate-rich expansive soils is associated with the formation of ettringite, a deleterious reactant that can cause moderate-to-severe swell-related damage to overlying lightweight infrastructures. This research study was conducted to understand the effects of combining nanosilica admixtures with a traditional Ca-based stabilizer to effectively treat high-sulfate soils with an intent to suppress the ettringite formation. Engineering and microstructural studies were thus performed to gain a comprehensive understanding of the behavior of sulfate-bearing soils treated with lime in the presence of amorphous nanosilica. The engineering studies on treated and untreated soils included strength tests before and after capillary soaking, free swell strain tests, and resilient moduli studies that were performed to study and understand the macrostructural behavior of these soils at different curing periods. Supplemental studies using scanning electron microscope imaging and energy dispersive X-ray spectroscopy, thermal analyses using differential scanning calorimetry, and X-ray diffraction studies were also conducted to determine the microstructural changes that occur within these sulfate-rich soils. The results showed that additional silica phases furnished from nanosilica suppressed the precipitation of ettringite and correspondingly increased the formation of cementitious phases. This study also provided ample evidence that the application of amorphous siliceous nanomaterials positively impacts chemical treatments and reduces the precipitation of ettringite in sulfate-rich soils, thus enhancing their engineering performance.

It is increasingly important to find solutions for the problem of the aluminium anodising industry which generates a large amount of acid and alkaline wastewater, composed of high amounts of phosphates, sulphates, nitrates and aluminium. The sulphate removal trough ettringite precipitation is a simple process and involves a low-cost operating. The ettringite can be also formed during the cement hydration in soil-cement mixtures which causes several damages such as expansion. However, the effect of ettringite on the compressive strength, tensile strength and microstructure have few studies. This paper presents a novel experimental study on the influence of the industrial effluent treatment ettringite in resistance and microstructure of soil-cement mixtures. Experimental tests were performed using natural soil, soil mixed with 5% and 6% of cement and soil mixed with 5% and 6% of cement and ettringite for each material. The resistance of the materials was evaluated by unconfined compressive strength and indirect tensile strength, after 3, 7 and 14 days of cure. Additionally, several characterization tests and microstructure analysis were performed. Regarding the experimental results, the compressive strength and tensile strength decreases about 75% and 85%, respectively, when ettringite was added in soil-cement mixtures. The microstructure of natural soil, soil-cement and soil-cement-ettringite mixtures shows that the addition of cement and ettringite, simultaneously, increases the ettringite crystal formation mainly because the cement functions as a source of sulfate ions contributing with the formation of more crystals. Experimental results indicate that the incorporation of ettringite in soil-cement mixtures is not suitable for geotechnical applications.