Calcium sulfoaluminate cement (CSA) was used to stabilize a type of marine soft soil in Dalian China. Unconfined compressive strength (UCS) of CSA-stabilized soil was tested and compared to ordinary Portland cement (OPC); meanwhile the influence of amounts of gypsum in CSA and cement contents in stabilized soils on the strength of stabilized soils were investigated. X-ray diffraction (XRD) tests were employed to detect generated hydration products, and scanning electron microscopy (SEM) was conducted to analyze microstructures of CSA-stabilized soils. The results showed that UCS of CSA-stabilized soils at 1, 3, and 28 d firstly increased and then decreased with contents of gypsum increasing from 0 to 40 wt.%, and CSA-stabilized soils exhibited the highest UCS when the content of gypsum equaled 25 wt.%. When the mixing amounts of OPC and CSA were the same, CSA-stabilized soils had a significantly higher early strength (1 and 3 d) than OPC. For CSA-stabilized soil with 0 wt.% gypsum, monosulfate (AFm) was detected as a major hydration product. As for CSA-stabilized soil with certain amounts of gypsum, the intensity of ettringite (Aft) was significantly higher than that in the sample hydrating without gypsum, but a tiny peak of AFm also could be detected in the sample with 15 wt.% gypsum at 28 d. Additionally, the intensity of AFt increased with the contents of gypsum increasing from 0 to 25 wt.%. When contents of gypsum increased from 25 to 40 wt.%, the intensity of AFt tended to decrease slightly, and residual gypsum could be detected in the sample with 40 wt.% gypsum at 28 d. In the microstructure of OPC-stabilized soils, hexagonal plate-shaped calcium hydroxide (CH) constituted skeleton structures, and clusters of hydrated calcium silicates (C-S-H) gel adhered to particles of soils. In the microstructure of CSA-stabilized soils, AFt constituted skeleton structures, and the crystalline sizes of ettringite increased with contents of gypsum increasing; meanwhile, clusters of the aluminum hydroxide (AH₃) phase could be observed to adhere to particles of soils and strengthen the interaction.

Although amounts of steel slag are left as waste, advances in treatment technologies have made steel slag a potential low-carbon supplementary cementitious material. Replacing cement with steel slag will drastically reduce carbon footprint, especially, in the engineering field where there are efforts to improve the geological properties of soft soil using cement-based cementitious materials. However, the influence of the degree of fineness, precarbonation treatment, and substitution ratio of steel slag on the mechanical properties of stabilized soils with a cement-steel slag blend and the corresponding influence mechanism are still unclear. Herein, four types of steel slag powders (i.e., coarse steel slag (CSS), fine steel slag (FSS), fine steel slag under precarbonation treatment for 2 h (FSS-C-2h), and fine steel slag under precarbonation treatment for 18 h (FSS-C-18h)) were prepared in a laboratory and used as a substitute to cement at ratios of 10%, 20%, 30%, and 50%. Tests were conducted to evaluate the strength and performance of the cement—steel slag blend stabilized soils (SCSs), and the changes in the phase and morphology of the hydration products were observed. The results showed that the SCS with 10% FSS-C-18h had the highest unconfined compressive strength after curing for 60 d at 110% of stabilized soil with pure cement. The SCSs with CSS and FSS showed no expansion failure during the continuous boiling test over 60 d. The main hydration products for the SCSs were C—S—H and ettringite. Concerning long-term strength and stability, FSS-C-18h at a substitution ratio of 10% or 20% is recommended for achieving green and low-carbon stabilized soils.

The condition of an offshore wind turbine (OWT) should be monitored to assure its reliability against various environmental loads and affections. The modal parameters of the OWT can be used as an indicator of its condition. This paper combines the Kalman filter, the random decrement technique (RDT), and the stochastic subspace identification (SSI) methods and proposes an RDT-SSI method to estimate the operational frequency of an OWT subjected to ambient excitation. This method imposes no requirement on the input/loads; therefore, it is relatively easy for field application. An experimental study with a small-scale OWT was conducted to verify the accuracy of the proposed RDT-SSI method. The test results implied that the frequency estimated by the RDT-SSI method is close to that estimated by an impact hammer test. Moreover, the small-scale OWT was buried at different embedment depths to simulate the influence of the scouring phenomenon, and the frequency of the OWT decreased with decreasing embedment depth. Additionally, the bolts at the root of the turbine blades were also loosened to investigate their influence on the frequency. As more blades were loosened, the identified frequency of the OWT also decreased, indicating that the proposed RDT-SSI method can be employed for the health monitoring of an OWT.