Quick lime
from the EPA's Mid-Atlantic Integrated Assessment:
"This chemical is highly reactive, relatively inexpensive, and apparently easy to handle in 50lb bags (it can also be bought in bulk). In order to apply pebble quicklime, a water wheel called the Aquafix machine is typically used and placed above a channel of water. Here, quicklime is dispensed into the AMD water where it dissolves over time. Depending on the level of acidity and volume of water flow, appropriately sized feeder bins or silos are place above the machine. These bins hold the quicklime and feed it into the machine, powered by the flow of water."