Super sulfate cement (SSC) serves as a sustainable alternative to ordinary Portland cement, offering lower carbon emissions and superior performance. Magnesium oxide (MgO) and building gypsum (BG) were utilized as activators for granulated blast furnace slag (GBFS), and together they formed SSC, which was employed to stabilize the waste soft clay (SC). The mechanical strength development characteristics of solidified clay and the types of its hydration products were investigated through mechanical experiments, including unconfined compressive strength (UCS) tests as well as microscopic experiments, such as X-ray diffraction tests and scanning electron microscopy tests. The mass ratios of GBFS, MgO, and BG were 8:2:0 (A2) and 6:2:2 (B1), respectively; these ratios were employed to stabilize the clay, resulting in solidified clay samples designated as S-A2 and S-B1. The UCS of S-B1 increased by 36.5% to 49.3% compared to S-A2 at the curing time from 7 to 91 days. The strength residual coefficients were 34.5% and 39.1% for S-A2 and S-B1, respectively, after ten wet–dry cycles. After soaking in sodium sulfate solution, the UCS of S-A2 and S-B1 decreased by 49.1% and 29.8%, respectively, compared to the unsoaked condition. The results of microscopic tests showed that the hydration products of S-B1 mainly included needle-like calcium silicate hydrate (C-S-H) gel, flaky hydrothermal gel, and ettringite (AFt) crystals. BG promoted the formation of AFt, while MgO facilitated the generation of C-S-H gel. In this study, SSC was used to stabilize the waste clay, which provided a way for the application of waste SC and SSC.

Steel slag powder (SS), ground granulated blast-furnace slag (GGBS), and flue gas desulfurization gypsum (FDG) are environmentally friendly and cost-effective substitute materials for ordinary Portland cement (OPC). This study investigated the use of industrial solid wastes, including SS, GGBS, and FDG, as auxiliary materials in OPC to stabilize pretreated recycled concrete aggregate (pretreated RCA). The use of pretreated RCA, mixed cementitious materials, and water at the optimum content created a mixture designated recycled cement-stabilized macadam (RCSM). A series of mechanical tests were conducted to clarify the performance of the RCSM, and microscopic tests were performed to elucidate the microcharacteristics of the mixed cementitious materials. With a curing time from 3 days to 28 days, the unconfined compression strength (UCS) of the mixed cementitious materials (A4) composed of SS, GGBS, FDG, and OPC increased by 5.94–10.79% compared with that of the cementitious material of OPC (A0). The UCS of the mixture composed (C4) of SS, GGBS, FDG, OPC, and pretreated RCA was greater than that of the mixture composed (C0) of OPC and RCA from 7 days to 90 days, increasing by 4.26–8.35%. The total drying shrinkage coefficient of C4 was lower than that of C0, whereas the temperature shrinkage coefficient of C4 was higher than that of C0, indicating that the use of A4 can effectively reduce drying shrinkage cracking in C4. The hydration products of A4 primarily consisted of flocculent calcium silicate hydrate (C-S-H) gel, fibrous calcium aluminate hydrate gel, and needle-like ettringite crystals. The interlocked growth of C-S-H gel and ettringite crystals continued and promoted an increase in the UCS of the cementitious system. The test results provide a reference for the application of similar materials.

In order to reuse granulated blast furnace slag (GBFS) and low-strength soft clay (SC), this study developed a curing material using magnesium oxide (MgO) as an alkali activator to excite the GBFS and basalt fiber (BF) as reinforcing material to prepare the SC. The mixing ranges of GBFS, MgO, and BF were established as 9.48%~14.52%, 0.48%~5.52%, and 0%~1.00454% of the dry clay mass, respectively, and the mixing ratios of the three were optimized using the central composite design (CCD) test. Through the analysis of variance, factor interaction analysis, and parameter optimization of the CCD test, the optimal mass ratio of GBFS, MgO, and BF was determined to be 13.35:4.47:0.26. The curing material of this ratio was named GMBF and mixed with SC to prepare GMBF solidified clay. An equal amount of ordinary Portland cement (OPC) was taken and formed with SC to form OPC solidified clay. The mechanical properties, durability, and hydration products of GMBF solidified clay were clarified by the unconfined compressive strength (UCS) test, freeze–thaw cycle test, X-ray diffraction (XRD) test, and scanning electron microscopy (SEM) test. The UCS of the GMBF solidified clay was 1.08 MPa and 2.85 MPa at 7 and 91 days, respectively, which was 45.9% and 33.8% higher than that of the OPC solidified clay (0.74 MPa and 2.13 MPa) at the same curing time. After ten freeze–thaw cycles, the UCS of GMBF and OPC solidified clay decreased from the initial 2.85 MPa and 2.13 MPa to 1.59 MPa and 0.7 MPa, respectively, with decreases of 44.2% and 67.1%, respectively. By XRD and SEM, the hydration products of GMBF solidified clay were mainly calcium silicate hydrate gel and hydrotalcite. The interface bonding and bridging effect formed between BF and SC or hydration products, indicating that these interactions contributed to the solidified clay enhanced structural integrity. This study demonstrates that the CCD approach provides solution for recycling SC and GBFS. Laboratory tests confirm the potential of the optimized GMBF formulation for practical engineering applications.