Influence mechanism of alkali excitation on strength and microstructure of Pisha Sandstone geopolymer cement composite soil

Abstract: Pisha Sandstone is well known as feldspathic Sandstone, soft rock, and montmorillonite-enriched Sandstone in the Ordos region of Inner Mongolia of China. A large amount of loose Pisha Sandstone serves as a typical engineering material for roadbed base, channel lining, or earth dam slope in most road paving, field drainage, or water transmission. In this study, the alkali-excited geopolymer cement composite soil with Pisha Sandstone as the base material was developed to improve the mechanical properties of cement soil for the further resource utilization of Pisha Sandstone. A universal material testing machine was also selected to explore the effect of alkali excitation on the mechanical properties of composite soil at the various temperatures of alkali excitation and equivalence. A mercury-pressure test was carried out to clarify the time-varying characteristics of pore structure. Subsequently, X Diffraction Pattern (XRD) was utilized to identify the crystal composition, while, Fourier Transform Infrared (FTIR) spectroscopy was used to determine the changes of atomic structure in chemical reactions. The positions of different absorption peaks in XRD and FTIR were selected to evaluate the degree of chemical reactions. A Scanning Electronic Microscope (SEM) was also used to characterize the morphology of composite soil. The results revealed that the temperature of alkali excitation and alkali equivalent behaved a great positive influence on the mechanical properties of Pisha Sandstone geopolymer hydroclay. The micromechanical data show that the N-A-S-H gel content as a main factor significantly altered the pore structure and strength of Pisha Sandstone geopolymer cement composite soil. In turn, the N-A-S-H gel content depended mainly on the temperature of alkali excitation and alkali equivalent, similar to the dissolution of some montmorillonite, quartz, and metakaolin in Pisha Sandstone. In addition, the free SiO32- and AlO32- substances promoted the production of N-A-S-H gel in alkali environment. However, the transformation of N-A-S-H gel to potassium A zeolite was promoted significantly to cause the decrease of strength, when the temperature exceeded 80 ℃. Since the N-A-S-H gel content changed the percentage of <10 nm pore structure of composite soil, the filling and cementing effect caused by the porosity of composite soil tended to decrease. Meanwhile, the percentage of large pores decreased, whereas, the percentage of small pores increased. An optimum ratio of composite soil was achieved at the alkali excitation temperature of 80 ℃ with 2% alkali equivalent. The greatest number of N-A-S-H gel products were generated in the Pisha Sandstone geopolymer cement composite during this time, particularly with the largest percentage of internal <10 nm pore size of 11.67%, the lowest porosity of 23.47%, and the highest strength of 8.23 MPa. The finding can provide a sound theoretical basis for the practical application of Pisha Sandstone geopolymer cement composite soil in engineering projects in northwestern China.