Lime

Hydrated lime is calcium hydroxide, Ca(OH)₂, produced by adding water to quicklime. When used in mortar, it gradually hardens through carbonation, whereas natural hydrated lime is made by burning naturally occurring limestone containing clay and other minerals. Those minerals give it hydraulic properties, meaning it can set in the presence of water.

Natural Hydrated Lime (NHL) 3.5 is a moderately hydraulic natural lime used to make mortar, render and plaster. NHL 3.5 is  a popular product for brickwork, stonework and the repair of traditional buildings because it combines useful strength with breathability and flexibility.

The main reason you would want to use lime mortar instead of cement mortar in an older building is compatibility. Typically, older buildings have been constructed with lime mortar because it works differently from modern cement mortar. Replacing the existing lime mortar with a hard cement mortar can cause problems for the original brickwork or stone.

There isn’t one universal lime mortar ratio. The correct mix will heavily depend on the type of lime, the sand, and what you’re using the mortar for. For lime mortar, well-graded sand is preferable to uniformly fine sand. You want a mixture containing different particle sizes so that the sand particles pack together effectively.

Hydrated lime is often added to cement mortar to make the mortar more workable for the bricklayer. The mortar becomes easier to spread with improved adhesion, water retention and strength.

Yes, you can make repointing mortar with hydrated lime instead of natural hydrated lime, but it behaves differently, and the choice depends heavily on the building.

Lime mortar can be used in cold weather, although it may cause complications. Do not use lime mortar in conditions below 5 degrees celsius as it is particularly vulnerable to frost which can disrupt the developing structure.