Used centuries ago, lime mortar still finds a use in restoring historic property. Heat and pulverisation converts limestone into lime which is essentially calcium. The product is also known as quicklime and calcium oxide. These products, when combined with water, form a putty or mortar which hold stones, blocks or bricks in place. The process of making and using lime mortar requires some understanding of chemistry but is doable by most do-it-yourselfers.
Mix the dry materials of hydrated lime and sand at a ratio of one part lime to three parts sand.
Add water to the dry mixture to form a putty consistency mortar. The mortar stays flexible longer than other mortars and, according to Masonry Magazine, becomes more pliable and workable as it ages.
Store the lime mortar in a sealed container until use. The lime mortar remains soft and workable as long as it isn't exposed to the carbon dioxide in the air.
Modern lime mortar is identical to the mortars used in masonry projects prior to the advent of Portland cement in the late 1800s. The chemical reaction between the carbon dioxide in the air and the lime leads to a hardening of the lime. Over years this chemical process returns the lime to the stone of its original form.
Keep lime mortar damp or wet for about a week after application. If the mortar dries out too soon the chemical reaction with the carbon dioxide is slowed, resulting in a weaker bond. Using hydraulic lime--a lime product that has been wetted and allowed to partially begin the chemical reaction before being dried and pulverised--speeds the setting process. Do not use a lime mortar in an outdoor project that will be exposed to freezing temperatures in less than 30 days.