Gangue is a worthless waste generated from coal mining. It looks similar to coal and takes up around 20% of the total coal output. In the past, coal gangue was often piled on the ground. It took up land and contaminated the environment in different ways. But now, the waste is processed into paste and placed back in the mined underground areas to sustain geological layers. That's what's happening in Lu'an Chemical Group's Gaohe coal mine in Changzhi City, north China's Shanxi Province.