Ever been called back to a job by a client where your work has been acid cleaned but there are still residual mortar stains and smears? Ever had the client suggest that they acid clean the work again?
Well tell them not to – what they probably have are clay smears, not cement.
Cement is an alkaline that correctly applied acid cleaning must attack. Clay, however, is neutral and not affected by acid washing. Clay is a water-reducing agent which takes in water and expands on drying. So, when clay sits on block or brickwork and dries, it holds on like a barnacle to a boat. So how does clay get into your sand and how much should be in there?
Sources of Sand
Bricklaying sands come from two main sources Dune Sands and Depositional Sands. There are also river sands produced from dredging but they are not as common as the other two. Dunes sand come from coastal areas and are usually quite angular in particle sizes and are known as “Sharp” sands. Sharp sands usually have very little to no clay in them.
Depositional sands usually come from inland sources and are more oval shaped particles. They have flowed down into deposits at the foothills of mountains mixing with clays along the way as they wash downhill. They are most commonly known as “Bush Sands”. When “bush sand” is washed once twice or, in some cases three times it is to wash out the clay particles. Trouble is, when this happens it washes the fine sand out also which is actually needed to make a fatty dense mortar.
Best Mix: Washed + Unwashed Sand + Sieve Test
So, many times suppliers will mix washed sands with slightly unwashed sands – you may have heard of a “fifty-fifty” mix? This refers to 50% washed and 50% unwashed to keep a fine particle proportion of the mix. Be aware though that it isn’t an exact science especially if the work is being done by front-end loaders based on weight.
Quality suppliers mix washed and unwashed sands then pass them through a sieve test. AS3700 (the Australian standard for masonry and reference manual for the BCA) states that bricklaying sands must past a sieve test known as AS1141.1. It states “that a bricklaying sand must not have more than 9% of weight of the sand passing through a 75um sieve. This means that not more than 9% of your total sand has particle sizes of .75 of one mm in size.
To ensure your sand is compliant to AS3700 you should request a test certificate showing the sieve test AS1141.1. If your supplier can’t give you that certificate it does not mean necessarily that the sand doesn’t meet the standard but it can pose a risk.
Here are a couple of quick ways to test your sand for clay:
- Get an empty water bottle with a screw on lid, fill it up about two thirds of the volume with your on site sand and fill the rest of the bottle up with water. Then screw the lid on and shake the whole lot for about a minute. Leave the bottle to stand somewhere for two hours. The larger particles of sand will sink to the bottom the clay and fine particles will remain floating before settling on the top. By putting a small ruler at the side of the bottle you can gauge roughly the relative percentages of the mix that are clay and fine particles, or
- Take a handful of sand and rub it together like you’re washing your hands till all the sand is gone. Look at your hands and see if you have a beige or brownish coloured residue on them – this will almost certainly be clay.
How to remove a clay stain from brickwork after it has been cleaned?
Definitely not with more acid! The way to clean a clay stain is with water, a stiff nylon brush and elbow grease. However if you make sure that a high amount of clay aren’t in your sand in the first place this will be a process you will never have to concern yourself with!
Treasurer, MASONRY CONTRACTORS ASSOCIATION (MCA) of NSW & ACT