Bad idea. If it's that yellowy stuff it's too full of salts and will kill your plants

thanks worthy1, thought that might be the case.

Builders sand is too fine and soft to treat clay soils. You need gritty concreting sand for this.

Builders sand doesn't come from the sea and so doesn't contain salt. It can't do because it would cause structural problems when used in the building industry. Building sand is quarried or dredged from rivers. In fact, quality control for building purposes would be far higher than what's needed for horticultural purposes, so it's perfectly fine to use in gardens and for plants.

I always buy builders sand instead of horticultural sand, especially sharp sand. Apart from price, there's probably no difference. You can also buy kiln dried sand in the builders section but I just dry it myself. I tip some sand into a large shallow tray, keep it in my greenhouse and mix it about every few days. In about a week it is completely dry and I store it in plastic buckets that emulsion paint comes in. You can use it to sprinkle on lawns to dress it in the spring or use it to mix very small seeds with to make them easier to sow or as a top cover for small seeds in trays. I just use the wet sand when I need to mix some gritty soil up for pots... Can't see the point in drying it to wet it again.

I once went into a large DiY store to get some horticultural silver sand. They had it in bags, kiln dried for what I thought was expensive. I went outside into the builders section and saw the same bags, from the same company at a quarter of the price. The only difference was that it didn't have the word "Horticultural" on the bag.

The builders sand down here is dredged from the sea but it washed to get the salt out

I am of the same opinion as Myron in that salt in builders sand would cause structural problems, eg rusting of steel re-inforcing, in buildings.

I'd always thought the particle size of builders sand was too small to help in clay soils.

Not really Steragram, builders sand, also known as red sand or soft sand will help a little but the best thing to add to break up the clay would be lots organic matter. It's just that Catkin's question was could they add some leftover sand to the clay soil to get rid of the sand without it causing problems? I'm sure that this wouldn't be a problem.

If I'd seen your opinion last year we might not have had some leftover sand in a bag on the drive for so long...

I get through quite a few bags of sand, in a year, especially sharp sand. I Always have a bag to hand and even my neighbours have started calling to give me their left-over sand.

Sharp sand is very useful to add to potting compost, well that's what my mate John Innes told me ;o).

I add lots of it especially to the type of compost that contains large amounts of peat as the peat is very light and dries out quickly so I mix it with sand up to a 1:1 ratio. The compost medium ends up nice and heavy, more like garden soil, and the gritty sand helps the roots to establish.