Troubleshooting Guide - Crumbling or Failing Mortar

Tŷ-Mawr ‌‌ posted this on 1 Apr 2016

Crumbling or failing mortar can be caused by a number of reasons:

  • Failing mortar can usually be attributed to a lack of carbonation of the lime within the mortar meaning that the mortar will have little if any, strength. Lack of carbonation can be caused by exposure to freezing temperatures causing expansion of the lime as it is absorbing CO2 and prohibiting a coherent mortar being formed. It can also be caused by the mortar drying too fast by either not wetting the background or lack of tending and protection afterwards
  • The use of an aggregate that is too fine or poorly graded can restrict the movement of CO2 into the mortar, vastly reducing the speed at which it carbonates thereby leaving it vulnerable to attack from frosts.
  • If the mortar is exposed to heavy rain soon after application lime can be washed out of the mortar leaving the affected areas devoid of a binder and as such crumbly to the touch.


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, use or protection of materials.

We start to issue winter weather warnings with all of our lime based products from the middle of August as lime products continue to gain strength markedly up to and beyond 90 days. So you must think well ahead and consider what the temperature is likely to be over the coming months when undertaking work with lime. If the temperature falls below 5ºC for any length of time before the mortar has gained sufficient strength then there is likely to be problems in the future.

Protection of new work from extremes of weather is vitally important in ensuring the longevity of the mortar.

The mortar should be raked out and reapplied in accordance with the manufacturer's instructions. Cement and modern admixtures must be avoided at all times as these will have a deleterious effect on the mortar.

As well as providing free telephone support to clients, training courses are also available on-site or in our training centre near Brecon. We also produce the instructional ‘The Lime Handbook’ which is available to buy and have lots of advice available via our website. 

Categories: Lime Mortar