Water resistance and luminescent thermal stability of SiO2 coated phosphor and self-luminous cement-based materials: View from the perspective of hydration balance

The present study investigates the effect of SiO2 coating on the properties of luminescent powder (LP) and self-luminescent cement-based composite materials (SLCCM). In order to investigate it, firstly, the water resistance of the uncoated and coated LP was analyzed by thermogravimetric analysis (TG), XRD, SEM and fluorescence microscopy (FM) tests, and the hydration mechanism in neutral water was summarized. Afterward, coated and uncoated LP were admixed with Portland cement to evaluate water resistance properties in SLCCM specimens. The heat of hydration, XRD and luminescence intensity analysis were carried out. In addition, luminescent thermal stability of SLCCM was evaluated in terms of afterglow test. Based on the results of hydration studies in neutral water, it was concluded that the luminous intensity of phosphors will decrease slightly with coating, but the water resistance will be improved. The phosphors without coating would hydrolyze and exhibit a blue shift. Moreover, the hydration heat results confirmed the hydration sequence and heat release rate of SLCCM specimens with coated and uncoated LP are different. The XRD and FM analyses confirmed that the silica continuous to sperate out from coating in alkaline environment breaks the hydration balance of SLCCM, accelerates the hydrolysis of LP and reduces luminous brightness of specimen. The brightness and afterglow intensity of SLCCM increase with the decrease of ambient temperature. Based on the results obtained, it is revealed that the SiO2 coating of LP have different hydrolysis reactions in neutral water and alkaline environment, silica encapsulated by LP disrupts the hydrolysis balance between phosphor and cement and offers a negative effect against water in SLCCM.