Hot lime has been used as a bedding mortar for large walls from the medieval times right up to the end of the 18th century. Hot lime mortar is when quick lime has been mixed with sand and water. Hot lime mortar after the slaking reaction has occurred; will need to be be used straight away, before it has time to cooled down. When constructing buildings in the past, large holes were dug where the hot lime would be produced on site.
One main reason for this, the construction of building was a slow process and most material were locally sourced and made at site; this was to due to haulage issues. Others have argued that unique benefits from using a hot lime mortar and it will provide a consistent level of compressive strength throughout the life of the building, when compared to other forms of limes. No real data has been provided regarding the compressive strength for the hot lime to date. Studies do suggest that hot lime mortar performs more appropriately than equivalent lime putties for traditional stone masonry construction and repairs. Hot lime mortars resulted in increased output and provided cleaner work with the fuller joints. Also, the hot lime mortar could be used with wet stones and sands in less favourable weather conditions, with a minimized risk of leaching and slumping.
Hot lime mortar does have some potential issues, one being the fact of how dangerous and potentially hazardous process it is when mixing, due to the slaking process. Burnt skin, and blinding is know throughout the industry to be a common hazard, due to lime reacting with the water and spitting due to high temperatures in excess of 100 degrees. Another issue, is the hot lime mortar has to be made on site, as it needs to be hot before it can be used. To make hot lime mortar, quick lime is required, which is not readily available in the UK. When purchasing hot lime mortar from manufacturers and suppliers, you will need it to be hot before being used; which will normally cool down after several hours after the slaking process. Which will then turn into un-mature form of lime putty. Hydraulic lime has many additional benefits, without any of the draw backs to hot lime mortar, as it provides quicker setting times, and being in a powder form it can be mixed and used in the same way as cement.