Choosing the right mortar is vital when repairing old walls. Cement is a real problem as most builders will not understand the need to use the right mortar. Even choosing the right lime mortar can be problematic.
Cement mortar is less breathable and hence tries to lock in moisture or makes the moisture pass through the stone or brick. This weakens the stone and brick and hence causes more erosion. Cement should therefore be avoided when repointing older solid walled buildings. Where cement mortar has been used in the original building then using a standard mix is not a major issue.
So on buildings that use softer materials like stone you will need to find the right mortar mix that is weaker than the main structural elements. The main way of doing this is to use lime, however lime comes in a variety of forms: Hydraulic and Putty. Hydraulic also comes in a variety of strengths (generally NHL 2, NHL 3.5 and NHL 5 with 5 being the strongest) whereas putty is the softest. For most re-pointing we would recommend using putty, but you might want to reassurance of a stronger lime and so using 2 or 3.5 is also very common.
In areas where the mortar is likely to be in constant contact with water we would also recommend using a lime putty made from hard limestone rather than one made from chalk.
The aggregate is important as well, so normally we would recommend using stone dust and a pozzalan (brick dust, PFA, GGBS), but most builders will use sand. Sand is not ideal as it is not porous in itself and certainly lime putty does not work well with just a sand mix. Hydraulic lime, though is commonly used with just sand (but we would still recommend stone dust).
The existing mortar needs to be scraped out so that you have room for at least 1cm of new mortar. The wall (and especially the recesses) then needs to be made damp before applying the mortar. The mortar should not protrude beyond the stone / bricks.
For more advice and products please call the Eco Home Centre on 02920373094.
Rounded Developments Enterprises Ltd have endeavoured to ensure that the information contained in this report is accurate. However, Rounded Developments Enterprises Ltd. accepts no liability for the use of this information.
Statement of Vested Interest
Rounded Developments Enterprises Ltd are a well-recognised supplier of a range of sustainable building products and as such have a commercial interest in some of the recommendations contained within the report. In some cases, cost estimates have been given on the basis of current quotations for similar equipment supplied by Rounded Developments Enterprises Ltd, and may not be the only equipment available. However, it is our opinion that the study offers an appropriate level of detail in view of the resources available and information provided. The authors have no expectation of any order being placed with them and would welcome questioning of the choice and costs of any equipment.