Types of Sand

[malcolm]

A quick ask just in case I'm making a big mistake. Much of my pointing needs to be replaced after years of water damage from the weatherproof paint. I'm planning to re-point in lime (NHL 3.5) and sharp sand. Much of the re-pointing will be localised so I want to try to match the colour and texture of the existing. Some walls need re-pointing completely (125m2) and the colour matched pre-mix bags are prohibitive expensive for that sort of area so I want to find my own mix.

Back in the day (1910 to be exact) they would have used local sand and be done with it. Many local buildings of the period are pointed in a relatively fine yellow sand with some fairly angular white larger stones. The sharp sand I bought from Buildbase is more of a light brown colour and some of the larger stones are rounded not sharp. The small bit of test pointing I've tried so far looks too rough due to too much large aggregate. It would better match a Roman building.

My next test will be to mix the Buildbase sharp sand 1:1 with their plasterer's sharp sand which has a darker but warmer yellow tone. It should improve the colour and reduce the roughness.

Does any of that sound sensible? Short of finding the local sand pit and digging out a bit of original sand under the cover of darkness is there an easier way to find a matching sand?

 

[plasticpigeon]

You wouldn't believe what strong opinions people have about this. personally I think sharp sand from builders merchants meant for making screed or alternatively forming part of the aggregate in concrete when mixed with ballast, contains aggregate that is too big for building or pointing, it is almost impossible to use for that purpose. I perservered for ages with the stuff as it was popular opinion that it was the right thing to use but I really don't think it is right for building or pointing. I use plastering sand these days, it is sharp, but finer, making it much easier to use, and usually has a few bigger bits in too, but it has little fines in it, so the resulting mortar will dry nearly white, perhaps a very light buff colour. I have found a local supply of red and yellow building sand that I mix 2 red to 1 yellow and add this to the plastering sand to get the right colour for my house. When I've crushed some mortar from my house, the sand is pretty fine anyway, I think they just used what was local and available, so I'm probably staying true to the house!

 

[malcolm]

Strong opinions aren't good. Would it be better to chat about politics instead? A match to what you have must be the important thing and it's good to know you have been mixing locally available sand (Buildbase is local to me but sadly the local sand pits have closed down).

I have some photos that completely fail to show what the pointing here is like when you actually look at it. But it does show some large bits of grit I would like to reproduce. My plastering sand seems quite fine with no grit.

Original

My effort looks not bad (hopefully) but the sunlight and camera colour correction don't reveal the differences in texture or colour.

 

[malcolm]

Thanks plasticpigeon - you've solved it! Sounds like your plastering sand is more coarse than mine, so I bagged up a couple of samples of sand and went to the builders merchant to see if there is such a thing as coarse plastering sand. It turns out there is a sand that was once very common in the area - Leighton Buzzard sand. It is not common any more as it doesn't conform to modern sand classification. I have a few bags to try!

 

[88v8]

In theory, the aggregate size should have some relationship to the gaps - the larger the gaps, the larger the aggregate, to minimise shrinkage.
In practice, this is difficult, as in all but formal work one tends to be dealing with variable gaps.

The key thing is that the sand should be predominantly sharp. Soft sand makes a weak mortar.

When matching colour, one needs to measure carefully; the process of a little of this then a tittle of that may end up with a satisfactory first batch, but is scarcely reproducible. I remember when our previous house was being rendered, the mixer-upper was sacked because his careless gauging led to a whole wall being hacked off and done again when the top bore no resemblance to the bottom.

Ivor

 

[malcolm]

That makes sense Ivor. All of the existing mortar was sharp sand, and with lime being very much softer than cement it makes a lot of sense to use 100% sharp sand to help hold it together.

It's the modern definition of sharp sand that had me confused. If you ask for sharp sand at a builders merchant you will get a sand designed for concreting with maximum grain size of 5mm. It isn't possible to apply that into joints with a mortar finger. A coarse plasterers sand with max grain size of 2mm will be much closer to what was originally used for brickwork and that is easy to work with.

I'm delighted with the match I have. The new mortar looks very much like builders sand in the photo but is a local sharp sand. When wet it matches wet original mortar perfectly. It will dry a bit white but a year of weathering should have it perfect. Just a pity the builders merchant no longer supply the sand and have only another 12 bags. I've hacked out the bit I did with sharp sand and replaced it with the new stuff.

 

[formerly known as]

the rule of thumb is - measure the joint width, divide by three and this gives you the maximum particle size of the aggregate. It is then important to get a graded aggregate from max particle size down to 150 micron. Some aggregates are more 'thirsty' than others, requiring more lime to 'coat' the aggregate. You need to get a jam jar half filled with sand, then fill with water until the water gets to the top of the sand ..... this will equate to the amount of lime that is needed to give you a starting point for a decent mortar. I find that a mix of 1 part non hydraulic lime to 2 parts of aggregate fits most jobs. Remember that the mix of 1:3 is applicable to quicklime and aggregate; quicklime almost double in volume once slaked (hence why 1:2 works well)

 

[malcolm]

Thanks formerly known as. I have duly altered my mix to 2:1 for a tiny bit of bricklaying and it does seem to be working better. Haven't had much of a chance to sit down over the last week but will try the jam jar test and perfect the mix before doing more pointing. My sharp sand (like the original) has a fair proportion of fine particles which will need more lime than a better graded sand.

 

[formerly known as]

its always worth doing the jam-jar test when using a new source of aggregate - the variety found in sharp sands within a 20 mile radius can be almost unbelievable.

 

[malcolm]

Just on the off chance, does anyone know where to find red sharp sand? The half tonne I bought will only be enough for one gable and the suppliers have run out and say the quarry has closed.

Hopefully I have attached a picture of the sand. Let me know if I haven't. It is well graded with a max grain size of about 3mm. The colour is very much like red building soft sand. It's not easy to find as it doesn't fit within the modern sand grading system so presumably is left in the quarry or fiddled with rather than sold.

 

[Zebra]

I started using a ready mixed mortar, but changed to a mix of 2 soft (building) sand; 1 sharp sand; 1 lime. I can't think where I got that recipe from, but it must have been on here. I wasn't aware of 88v8's point that soft sand makes a poor mortar. I find that nothing but sharp sand makes it very hard to work with.

 

[malcolm]

I've ended up with 1 sharp sand, 1 red builders sand and a little less than 1 lime (about 1:2.5 ratio). It seems a reasonable match and is more workable than the local red sharp sand that I can't find any more. The sharp sand has a large grit size so I'll need to make up another mix to go back and do the few very tightly jointed bricks.

The area is known for it's red sand. I'm told it was quarried nearby in Sandy, and Potton (red building sand) is just up the road. I guess modern builders have no need for a sharp pointing sand so it's not available.