Enteroliths, hard crystals that form in the colon, can cause an impaction colic. It is more common in Arabians and female horses.
Enteroliths are composed of mineral salts. They cling to foreign bodies like wood shavings or gravel in the colon. Slowly, these accretions build up and can cause a blockage.
Alfalfa hay is a suspected cause, as is a diet high in magnesium, phosphate and/or calcium. Confinement and lack of pasture may contribute, too.
Reduce the amount of alfalfa you feed, test your water for hardness and add a cup of apple cider vinegar to your horse's water to make your horse's colon a bit less enterolith-friendly.