Nano-layered double hydroxides as efficient endothermic, strengthening, and flame-retardant agents for fly ash/polyurethane composite materials

Polyurethane (PU) grouting materials in coal mining must possess essential mechanical properties and exhibit low heat release and high flame retardancy to enhance the safety and efficiency of coal mining. In this study, fly ash (FA) and nano-layered double hydroxides (LDH) were utilized as fillers to prepare a series of PU/FA/LDH composite grouting materials. Results showed that incorporating 2.5 % LDH reduced the maximum reaction temperature by 26.7 °C and decreased heat release from 177.45 J/g to 108.37 J/g compared to PU grouting material. These reductions were attributed to the uniform dispersion of LDH, which enabled the endothermic removal of plentiful adsorbed water during the curing process. However, exceeding 2.5 % LDH caused agglomeration, encapsulating the water and increasing the maximum reaction temperature. The effective fraction of closed cells in the PU/FA/LDH composite grouting materials was tailored by increasing the content of LDH, leading to enhanced compression strength. Moreover, cone calorimetry revealed that LDH was more effective in reducing the heat release during the combustion process of PU grouting materials, while FA was more effective in reducing smoke production. This synergistically enhanced the flame retardancy of the PU grouting materials. This work provides an effective way to produce high-strength, flame-retardant grouting materials at lowered reaction temperatures.