Quicklime

A white, caustic powder made by burning limestone or chalk in a kiln, from simple turf-covered clamps to stone flare kilns. Mixed with water ('slaked'), it became the basis of mortar and limewash, both needed in great quantities for the castles, churches and monasteries built after the Conquest. Lime was also used to loosen hair from hides before tanning and parchment-making. Spreading lime on fields to improve soil became common only later.

Sources: historicengland.org.uk