Preparation of Sustainable Foamed Concrete Using High-Volume Phosphogypsum and Its Hydration Mechanism Investigation

Abstract

Reducing cement usage and replacing it with solid waste in high volume is one of the most important ways for the sustainable development of the cement industry. In the present work, sustainable foamed concrete with a high content of phosphogypsum (PG) was prepared through the combination activation and synergistic strengthening of waterglass, ground blast furnace slag (GBFS), and cement. The fluidity, mechanical properties, volume stability, and durability of this sustainable foamed concrete were studied and its reaction mechanisms were also investigated using hydration calorimetry, X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The results show that the early age cohesive strength for this foamed concrete is mainly derived from ettringite formation, which is a product of the pozzolanic reaction of GBFS in the presence of sulfate ions. Cement in this sustainable foamed concrete is presented as an alkali activator. Too small addition of cement would weaken its activation effect and reduce the hydration product formation, which delays the strength development of the foam concrete. High cement content would provide sufficient Ca2+ ions, which can also retard the GBFS hydration. The unreacted PG particles are covered by ettringite and other hydration products during the reaction process to prevent its dissolution in a moist environment and help improve the water resistance and durability of foamed concrete. The sustainable foamed concrete prepared with 15% of cement and more than 50% of PG has 7 and 28 days compressive strengths of 2.86 and 3.36 MPa, respectively. In addition, it has a water-softening coefficient of 0.81 and a volume microexpansion feature in a moist environment, which is suitable for subgrade or underground filling.