What's so special about Lime -
A Permanent cure for damp stone walls
The new owners of the old farmhouse at Stenhill on the outskirts of North Petherwin in Cornwall in 1999 quickly discovered that the more exposed south west walls were so damp that moisture continually ran down the interior in cold weather. The house had been dated as being over 500 years old with newer additions as young as 200 years old. Examination of the structure of the walls immediately revealed the cause of the problem. The walls were of stone and lime mortar construction and had been rendered both inside and out with modern cement and plaster. Some of the walls had up to five layers of finishing plaster applied, presumably as the walls became damp another layer of plaster was added.
The mixing of modern and old materials and methods was at the root of the dampness problem. Modern materials such as Portland Cement work well with modern building techniques but cause severe damage when used on old buildings. Modern building methods require materials that come together easily and quickly and hence cheaply and today extend to the assembly of factory manufactured timber modules on a concrete base on site. Prior to the advent of timber framed houses brick or stone and mortar wall construction was the norm. In the 1850s and more so at the turn of the century Portland Cement Mortar replaced Lime Mortar as the joining material for brick and stone as it hardened quickly, was easier to work with and above all was cheaper. Unbeknown to builders up to a few tens of years ago and still unknown by the vast majority of constructors today is the fact that modern cements and plasters behave in a completely manner to the old lime mortar they replaced despite both being derived from Limestone. The sedimentary rock, Limestone, is mined widely throughout the world and is used not only as a source of building materials but is a valuable chemical with many uses from the purification of water to removing sulphur in flue gases.
The mined Limestone can contain impurities depending on where it is mined. The pure form which was, and still is, used to make the old Lime Mortar is Calcium Carbonate with less than 10% impurities. The conversion of the pure Limestone to a building Lime and the hardening, known as setting, of the material is one of the oldest known chemical reactions and is a natural cycle were the Limestone reforms within the walls. The setting can take centuries to complete as it depends on reaction with Carbon Dioxide in the atmosphere. This beautiful old material is self-healing in that any cracks that do form disappear as the material regrows as air and moisture pass through it. The passage of air and moisture through the Lime Mortar is the distinct difference between it and modern Portland Cement Mortar and is crucial to the principle of construction of old stone walled buildings.
If the impurities in the Limestone, which are mostly Silicates, are greater than 10% the predominant chemistry involved in its setting is different and a Lime Mortar with different properties is produced. The Silicates present react with water to set resulting in a material that sets more quickly and is less porous. This material was classified by the French engineer Louis Vicat in the 1840s as a Hydraulic Lime because it is capable of setting under water. Nowadays Hydraulic Lime is further classified into three levels depending generally on the Silicate content and on added setting agents such as volcanic and brick dust. The classes are HL2, HL3.5 and HL5 which is the strength the Lime Mortar attains in MaP after 28 days. This arbitrary classification is only useful for Hydraulic Limes that set quickly by reaction with water. The purer Lime was therefore classified as Non-Hydraulic Lime and as it sets by carbonization which is a long slow process the strength achieved after 28 days setting is much less than 2 MaP. After centuries of setting its strength may well outstrip that of the other materials.
If the Silicates present in the Limestone are greater than 20% then it is used as the base material for Portland Cement. The other ingredients are Calcium Sulphates, mainly Gypsum, which controls the setting time and other minor constituents depending on the purity of the Limestone used. Portland Cement is a Hydraulic material which sets mainly by reaction with water.
Mortars are a mix of the Limestone materials and aggregate which is commonly sharp sand and water. The colour of the aggregate determines the colour of Lime Mortar and in restoration work the original Lime Mortar is typically analyised to match the type, colour and grain size of the original aggregate used. The addition of small amounts of Portland Cement to non-Hydraulic Lime Mortar to reduce its setting time is to be avoided as the cement segregates and blocks the pores thereby robbing the material of its main attribute, breathability. It also reduces the self-healing qualities and the increases the modulus, stiffness, of the mortar. Stiffness of a mortar is important in that low stiffness allows the mortar to flex as a stone or brick wall moves with movement in the under lying earth. The addition of Hydraulic Lime to non-Hydraulic Lime mortar to produce a "hybrid Lime Mortar" is also to be avoided for similar reasons.
Non-Hydraulic Lime which is generally known as "Lime Putty" is delivered to site wet in plastic tubs or bags with a slight excess of water to exclude air and prevent premature carbonization. Hydraulic Lime is generally delivered to site dry in waterproof bags.
Quick fix remedies such as the application of Silicone compounds to stone walls are to be avoided as the chemical acts by capillary action to block the surface pores in both the stone and the mortar thereby preventing the wall from breathing. Another quick fix remedy to be avoided if possible is "under pinning" which is the careful removal of earth at the base of a wall which is exhibiting signs of movement such as severe cracking and filling the hole with Portland Cement concrete. Most old walls do not have foundations in the modern sense, the walls were built on either rock or firm hard earth. As the under lying earth moved the whole wall, generally, moved in unison and remained intact over the centuries. Under pinning portions of a wall creates areas of greater stiffness at the base of the wall preventing the whole of the wall to move in unison.
Non-Hydraulic Lime mortar was used to point the interior walls at Stenhill and Hydraulic Lime Mortar to point the exterior. The preference was to use Non-Hydraulic lime mortar throughout but protecting the exterior while it set sufficiently to withstand the Cornish driving rain and sun was impractical.
What's so special about Lime Mortar : it breathes, it is relatively flexible and it is self-healing
© 2007/to/2015 S.E.Reddock, M.Sc., C.Eng.
Lime pointed stone and (left) Lime plastered cob walls
These notes and web pages are for information and general guidance only and are not to be considered as a complete technical description of the subjects covered. Furthermore, due to the nature of old constructions and the large number of variables that need to be considered it would not be possible to form a judgement based solely on these pages. The author and anyone else representing Reddock Consultancies cannot accept liability for the consequences of anyone acting on the information here given.