Correct aggregates for repointing where lime mortar is concerned

Firstly, let us take the ordinary and mundane subject of sand. As we all know, that which is used to lay bricks and stone is mostly sold from quarries and local builders’ merchants. It's called screened sand, soft, or building sand and is made to a particular recipe where different types of fines are combined with larger grains. But what if you want a soft sand for building or pointing with lime mortar? Can you throw a few bags in your boot and get going with a project? In short, the answer is yes, of course you can, but after reading this you might not consider it an option. The main reason for this is that building sand is too soft and this means the lifespan of your mortar will be reduced if you use it. Care must also be taken where colour choice is concerned as soft sand comes in a multitude of colours and if you or your contractor pick the wrong one then you’re in trouble.  You may even devalue your property.

At this point I need to be careful otherwise I’ll run the risk of revealing trade secrets so it’s probably best to keep this brief: for repointing with lime mortar you need a combination of two types of sand and both have to be washed. However, most trade and DIY outlets do not supply what's necessary. So if you use unwashed sand then impurities which have not been removed will rise to the surface of newly applied lime mortar.

Sharp sand, on the other hand, has very little influence on colour. But it will have a direct affect on texture and therefore finish. Ordinary sharp sand from a quarry or builders yard is a fairly uniform texture with aggregates large enough to make it qualify as sharp. In sum, it’ll do the job but it’s nothing special. It’s the type of aggregate which is perfect for laying patios or putting down a base for a shed. However, if you want sharp sand which is going to make the mortar in the joints of a period property look sublime then you need to extend your search beyond the average home improvement retailer. This is because when lime mortar is brushed, the larger particles of sand are brought to the surface and only a top quality purposely blended mix of sharp and soft sand will recreate the depth of shade and texture needed to replicate the highly desirable effect which is so enticing where beautifully maintained period properties are concerned.

I use what I consider to be the perfect blend of sand. It consists of a mixture of soft and sharp sand to the specifications which my experience dictates is correct. It looks superb, there’s no colour leaching and the aggregate sizes and colours go together to create the ideal tint which complements natural stone. Not only this, but it also conforms to BS EN 13139:2002 Aggregates for Mortar which is the current standard.

Secondly I wish to highligh the issue of exacting standards in the use of natural hydraulic lime (NHL), quicklime and lime putty as all are obviously a priority of mine. They're flexible and porous and need nothing added to them except the appropriate sand. But unlike many other contractors I do not adhere to the widespread practise of adding cement to make my mortar cure more quickly as this pollutes the mix and simply turns good quality lime mortar into plasticised cement mortar. This is the last thing any owner of a stone built property needs – specially one which is listed. 

And finally there’s the finish. Lime must be brush finished and should never be trowel finished. However, it takes time to dry sufficiently before any  brushing should take place. Brushing prematurely not only results in cracks but also in the appearance of what look like damp patches. They're not really damp patches, it's just that the brush forms the calcination coat and brushing too early creates a mottled effect in places. Although over time this does fade, it usually takes a couple of years. 

Furthermore, omitting any of the brushing stages - or brushing improperly - not only results in a poor finish, but any lapse also means that imperfections remain as these are never revealed and therefore never attended to. None of this may matter to a potential customer if price is the only issue but employing a contractor who fails to attend to mortar correctly during its drying stages results in a job which will not stand the test of time.

In sum, lime takes time and the process of lime mortar repointing is very labour intensive. Mess up on something as simple as wetting up joints before applying mortar and things start going downhill rapidly. Leave it too long before brushing and things look even worse.

Recently I was discussing the merits of lime after I’d removed a wall full of horrible grey cement mortar and mentioned the fact that you’ve got to fight with it to know it and only when you’ve fought with it for long enough will you be its master. But mastery is  not about thumping it into place and being boss. It's about the relationship you have with it and how you must allow it to be what it is and work with it  and not against it. The reason for this  is that it's a natural material and these need comprehending. If I were ever to use an analogy to describe it then I'd equate working with lime to training a dog. At some stage you realise that many of your ways aren't productive but you have to get to that point before a lifelong relationship forms.

Anyway, I hope what’s written above has been informative in some way and that you can now see at least some of the reasons why the Conservation Department requires an example square metre before allowing contractors loose on listed buildings. 

Thanks for reading.

If you would like further information concerning the use of lime mortar please see the ‘blogs links and articles’ page at michaeljamesdesign.co.uk