Troubleshooting Guide - Lime Mortar Cracking

Cracking of Lime Mortar has several possible causes:

1.  Lack of moisture control – Drying and carbonation are two different processes, just because a mortar is dry it does not follow that it has carbonated. Drying is just the process of moisture leaving the mortar, whereas carbonation is the absorption of Co2 from the atmosphere and this is how the mortar gains its strength. In carbonation of lime can only take place in the presence of water. It can years for a new mortar bed to reach its full strength. If pointing dries before sufficient carbonation has taken place then cracking can occur. Therefore regular misting with water and protection from the wind and the sun are crucial in tempering this process.
2. mortar too wet – Pre-mixed fat lime mortars will rarely require the addition of any extra water. Lime is thixotropic, this is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. If a mortar is inadequately mixed or ‘knocked-up’ the addition of water can have the effect of making the mortar more fluid, however, this causes the mix to be overbulked. Lime reduces in volume when it dries and carbonates, as a result over bulking with water will exacerbate this effect. 
3. lack of protection from wind/sun – This will cause the same effects as point 1 (above).
4.  inappropriate aggregate choice – The aggregate in the mortar improves strength and reduces the effects of shrinkage. It is recommended that the aggregate in the mortar is no smaller than 1/3 of the mortar joint. So for a 6mm graded aggregate, it can be used to point a joint up to 18 mm before pinning stones need to be employed. If the aggregate is too small relative to the size of the joint being pointed excessive cracking will occur. We recommend that beach sands are not used as even after washing they can still contain salts that can be detrimental to the mortar.  We produce a sand that we call ‘Beach', it only had the appearance of a beach sand and is not sourced from the marine environment. 
5. not properly compressed -  In the first few days after pointing is applied as it carbonates a degree of shrinkage will occur and this is quite normal. The mortar joints can be beaten back with a stiff churn brush into the joint to consolidate them; this must be done prior to carbonation whilst the mortar is still plastic. 
6. too much mortar used, more than 25mm of mortar in joint – If the joints that have been raked out are deeper than approx 25mm, then the pointing must be built up in layers of 25mm leaving sufficient time between each application for shrinkage and carbonation to begin.
7. not adequately mixed – For hydraulic limes leave to mix for at least 20 mins after all water has been added to aggregate and lime to ensure that all of the lime is fully hydrated and that the sand is evenly distributed throughout the mortar, longer mixing times will result in less shrinkage and cracking of the resultant mortar.  Fat lime mortars require ‘knocking-up’ before use to eliminate the need for the addition of water.

Resolutions

Ty-Mawr products use the highest quality British quicklime and are produced under the strictest of conditions in accordance with ISO9001:2000. It must be remembered that the carbonation of lime is a very simple chemical reaction and there is very little to go wrong with it, problems are usually caused by the inappropriate selection or use of materials.

If cracking does occur it can provide a route for water to infiltrate the body of the wall.  Therefore careful assessment of the mortar needs to be made. Chemical analysis can be made to ascertain if carbonation has taken place. Defective pointing may need raking out and replacing.

Ty- Mawr does provide an on-site supervision and advice service that can be requested.