Effect of alkali-activated slag/cement additive on geotechnical properties of fine-grained soils

This study investigated the stabilization performance of high-plasticity clayey soil treated with alkali-activated hybrid slag/cement. Ukrainian clay was used as the clay soil, Ground Granulated Blast Furnace Slag (GGBFS) as the aluminosilicate source, and Portland cement (OPC) was added to enhance the binding properties. Sodium hydroxide (SH) and sodium silicate (SS) were preferred as the alkali activators. Twenty-seven mixtures with different contents were used in the stabilization of the clay soil. The total proportion of GGBFS and OPC used together was determined to be 20% of the dry weight of the clay. Alkaline activators were used at a rate of 10% of the dry weight of the clay. The activator system was prepared as a mixture of SS and SH, and the SS/SH ratio was determined as 1, 3, 5, and 7. First, the physical and geotechnical properties of the untreated clay were determined. Sieve analysis and a hydrometer test were performed to determine the grain distribution of the clay. Accordingly, no gravel was observed in the clay, 3% sand, 29% silt, and 68% clay. It was observed that the fine content in the soil was quite high. Liquid and plastic limit tests were carried out to determine the plasticity properties of the clay. Because of these tests, the liquid limit of the clay was 57% and the plastic limit was 26%. According to the plasticity card, the clay belongs to the high-plasticity (CH) clay group. Standard Proctor tests were performed to determine the compaction properties of the clay. According to these tests, the optimum water content (OWC) value of the clay was 21.6% and the maximum dry unit weight (MDD) value was 15.7 kN/m3. The OWC values of the additive soil varied between 21.6% and 19.2%, and the MDD values varied between 15.7 kN/m3 and 16.5 kN/m3. The Unconfined Compressive Strength (UCS) value of the clay was 346.6 kPa and the Ultrasonic Pulse Velocity (UPV) was measured as 1131 m/sec. Standard Proctor tests were performed on the treated clay soil for each mixture, and the OWC and MDD values of the treated soils were determined. The OWC and MDD values of the mixture were used in the samples prepared to determine the stabilization performance. The UCS values of the clay and stabilized clay were obtained under air-drying and wet-curing conditions at 0, 3, 7, 28, and 90 days of curing time. The air-dried and wet-cured samples were kept in closed cabinets to protect them from sunlight. In addition, the UPV test was applied to samples with 90 days of curing time under both curing conditions. Linear regression analyses were performed to evaluate the relationship between the UCS and UPV values. Within the scope of the regression analysis, three separate regression models were developed for the air-dried samples, the wet-cured samples, and samples covering both curing conditions. Empirical equations were generated to describe the relationship between the data. Using these equations, parameter estimation can be performed between the data. Fourier transform Infrared Spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) tests were performed on selected samples in order to effectively evaluate alkaline activation and/or hydration after stabilization. According to the results of the standard Proctor tests, the OWC values of the treated clay decreased, whereas the MDD values increased, compared with the untreated clay. The highest decrease in OWC value and the highest increase in MDD value occurred in the mixture containing 20% GGBS. No significant increase in UCS values was observed in specimens stabilized with alkaline activator alone compared with the untreated specimen. These results indicate that a mixture of SS and SH alone is not effective for stabilizing clay soil stabilization. In the samples stabilized only with OPC, an increase in the UCS values was observed due to the increase in the amount of OPC. In the samples where the clay was stabilized with 20% GGBS, a limited increase in the UCS values was observed compared with the untreated clay. In the samples stabilized with GGBS and OPC, an increase in the UCS values was observed due to the increase in the amount of OPC. The highest UCS value in this group was obtained in the samples stabilized with 12% GGBS + 8% OPC under both air-drying and wet curing conditions. Samples stabilized only with OPC gained strength faster and higher strength at early curing times, while the strength increase in the samples with the alkaline activation process became evident at longer curing times. The SS/SH ratio seems to affect the stabilization performance. Among the samples stabilized with GGBS, OPC, and the alkaline activator, the highest UCS values were observed in the samples with an SS/SH ratio of 5. This ratio was followed by 7, 3, and 1, respectively. The highest UCS value was observed in the sample with 16% GGBS + 4% OPC + SS/SH=5. This was followed by samples prepared with 18% GGBS + 2% OPC + SS/SH=5, 16% GGBS + 4% OPC + SS/SH=7, 20% GGBS + SS/SH=5 mixtures, respectively. In the sample treated with 15% OPC, the strength increase compared to natural clay at the end of the 90-day curing period was determined to be 240.1% in the air-dried samples and 427.7% in the wet-cured samples. In the samples stabilized with 16% SFC + 4% OPC and an SS/SH ratio of 5, the increase was 298.4% in air-dried specimens and 536.7% in wet-cured specimens. Clay soils stabilized with hybrid slag/cement obtained by alkali activation exhibited better stabilization performance than soils stabilized with 15% OPC. This was observed in 10 specimens under air-dried curing conditions and 9 specimens under wet-curing conditions. Crystallization was observed in the specimens with an SS/SH ratio of 1 and the air-dried specimens. Excess SH absorbed moisture due to its unreacted portion and subsequently reacted with carbon dioxide (CO2) to form sodium carbonate monohydrate (Na2CO3·H2O) crystals. This phenomenon had a negative impact on the stabilization process. According to the linear regression analyzes of the UCS and UPV data, the determination coefficient (R2) value was found to be 0.87 for air drying, 0.88 for wet curing and 0.89 for the overall analysis. These results show that there is a positive and significant relationship between the two parameters. A Comparative analysis was performed between the untreated and treated clay samples using FTIR, XRD, and SEM-EDS methods. Within the scope of the study, FTIR spectroscopy was performed on 22 samples, XRD on 20 samples, and SEM-EDS on 18 samples for material characterization, morphological analysis, functional analysis, phase analysis, surface imaging, and elemental analysis of both the clay and stabilized samples. The difference between the FTIR spectra of the untreated clay and the treated clay was taken. Thanks to the difference spectra obtained, the structural changes occurring in the clay were revealed more clearly. In the difference spectra, new phases formed because of the geopolymerization were observed. In the XRD diffraction patterns, new peaks indicating geopolymerization and hydration were observed in three different regions when the clay and stabilized clay samples were compared. In the treated clay samples, new phase formations (C-S-H and N/C-A-S-H), chemical bond structures, functional groups, amorphous structures, morphology and microstructural distributions of the gel structures formed because of hydration and geopolymerization were observed. In this study, the SS/SH ratio is effective in the geopolymerization and it was observed that stabilization with a ratio of 5 and 7 gave better results. The combined use of GGBS and OPC has been defined as a hybrid. The stabilization of clayey soil using alkali-activated hybrid slag/cement binders demonstrated a notably good performance.