Microstructural evaluation of the cement stabilization of hematite-rich red soil

Abstract

Red soil, composed of a clayey bed containing hematite, is used as pavement material in southern Iran. As an effective and economical technique, soil improvement with cement is common practice for improving the engineering properties of different soil and pavement layers. The formation of nanostructured calcium silicate hydrate (C-S-H) is the primary factor contributing to this improvement. On the other hand, the presence and dissolution of hematite in the soil structure can also influence C-S-H formation. The present study investigates the effects of cement on the engineering properties of the hematite-rich red soil from a microstructural point of view, with particular emphasis on changes in the C-S-H nanostructure. For this purpose, red soil specimens were stabilized with 2, 4, 6, 8 and 10 wt% cement for different curing times. The soil's engineering properties were evaluated by macrostructural testing, namely by soil particle size analysis and slake durability, water absorption, Unconfined Compressive Strength (UCS), and Ultrasonic Pulse Velocity (UPV) tests. Furthermore, the soil stabilization process and the effects of hematite on the C-S-H microstructure were evaluated by pH and Electrical Conductivity (EC) tests, X-Ray Diffraction (XRD), energy-dispersive X-ray analysis (EDX), and Scanning Electron Microscope (SEM) imaging. It was shown that adding cement to hematite-rich soil promotes Calcium Ferrite Hydrate (C-F-H) and ilavite (C-F-S-H) formation and, consequently, improves the red soil's stability and compressive strength. In fact, using 6 wt% cement, the presence of hematite, and the formation of Fe-containing nanostructures helped the red soil specimen develop a compressive strength of 2.04 MPa (17 times that of the natural soil) in seven days.