Renovating a 1930s terraced property in the UK has been a big gamble. One of a long list of jobs was to lay paving to the front of the property. Whilst preparing it became obvious that new pointing was required below the damp proof course.
Black ash mortar was used in the construction of 1930s properties in the UK. Crumbling black mortar can be seen in the mortar joints to the front of the property.
How can you tell if you have black ash mortar?
It’s black and softer than cement mortars. Modern bricks are likely fired at higher temperatures making them harder allowing harder mortars to be used in modern buildings. Whereas older properties benefit from less dense and more flexible mortars. If Black Ash mortar is disturbed by using an angle grinder or mechanical action like scraping and gets in the air is smells and tastes like ash.
Due to the colour and chemical composition black ash mortar is not compatible with cement based mortars. Cement mortars will not look very good and weather more quickly.
A throw back to the United Kingdom’s industrial past from Victorian times to the late 1920’s and early 1930’s black ask was taken from collieries and added to mortar. Utilising a by product of industry by using it in construction solved a problem and was cost efficient. A good solution to the problem of getting cheap building materials when the country didn’t have a supply chain of commercial cement available. Furnace ash was commonly used in black ash mortar and ash lime mortars.
Portland cement was being developed at the same time but wide spread commercial use had not been initiated, probably due to the high barriers to entry for manufacturers, and was only used in ad hoc projects. One of the first uses of Portland cement was Eddystone Lighthouse. John Smeaton, the engineer, experimented with traditional methods of creating mortar.
Without sending a sample of my house mortar for testing it’s difficult to know if it is a cement, sand, black ash mortar or a lime, sand, and black ash mortar mix. Based on reports of other 1930’s properties I suspect it’s a lime, black ash and sand mix.
Black ash mortars have attracted some criticism because the ash used often produces weak sulfuric acid. Speeding up the corrosion of ferrous metals often used as wall ties for cavity walls.
What are the options for pointing with black ash mortar?
Getting black ash is one option but seems like too much work for such a simple job. This was eliminated as an option.
Using cement and a black die was eliminated becuase cement based mortar is not likely to bind well to the black ash mortar and cement dyes have been known to fade and wash out of cement mortar.
Lime based mortars work better with black ash mortar due to chemical composition and physical properties. Soft and porous properties make lime mortar work well with black ash mortor.
Lime plaster was used in the pyramids of Giza roughly 6000 years ago. Containing lime, aggregates such as sand (perfect for the Egyptians) mixed with water, it’s still the same basic mixture used today.
Black Ash, specifically fly ash, is actually one of the additives to hydraulic lime mortars used today in conservation projects. Fly ash is one additive that will produce a stronger mortar providing better compression strength which increases protection from weathering.
Hydraulic lime has a better compression strength than non hydraulic lime making it preferable.
A mortar that is sympathetic to the existing mortar beds is required exhibiting maximum durability.
NHL 3.5 was required. NHL stands for natural hydraulic lime and the number relates to compressive strength. Natural lime is obtained from limestone which contains all the right chemical components.
NHL 3.5 is good for medium density masonary and good for repointing. Mortar of houses built in the 1930s exhibit moderate to high permeability so NHL 3.5 fits the bill. Bricks of properties of this era are more likely to expand under heat and contract during cold so the mortar needs to have less compressive strengh to accomodate this movement. Also permeability is important so that moisture can evaporate from the walls.
NHL 2 is too soft and slow setting whereas NHL 5 is too hard and more appropriate for dense masonary like granite, also where weather exposure is expected to be extreme.
NHL 3.5 hydraulic lime mortar options
Natural hudraulic lime can be purchased in bags from various producers. For instance, Blue Circle by Tarmac is a well known name, and available from large building suppliers like Travis Perkins. Smaller bags are available from manufacturers like Conserv.
Making lime mortar for pointing with black ash mortar using natural hydraulic lime.
Clean and well graded sharp sand should be used. Washed sand removes clay and silt which can cause excessive shrinkage.
Batch by volume using the same container and mix dry. Mix thoroughly.
1 part NHL.
2 to 3 parts clean and well graded sharp sand depending on the existing mortar.
Add water until the required consistancy is acheived. It needs to be just workable and not sloppy.
Allow to stand for 10 minutes before a final mix is done before application.
Matching the colour of NHL lime mortar to black ash mortar.
A natural coloured pigment can be added to NHL to achieve the right shade of black to match the existing mortar. Celtic sustainables do a range of natural coloured pigments that will fit the bill.