No! Adding builders or any other sand to clay soil will make it as hard as concrete because the finer clay particles will adhere to the larger sand particles any any airspaces that you had in the clay soil will be obliterated. This mixture is good for making bricks though. I have very heavy clay soil and I gave up trying to amend it because I finally realized that this could not be accomplished in my lifetime. I turned to the lasagna the gardening method and the results were terrific and accomplished in no time at all.

The other problem with this kind of sand, which has presumably been bought with some sort of building project in mind, is that they often contain salts and lime, neither of which you want on your garden.

Humus rich materials (which I expect you're already adding) such as composted manures, garden compost and the like, are useful on such soils, and horticultural grit - good garden centres sell that in larger bags.

Thanks, glad I asked. The LASAGNA method ? Pl explain.

Google 'lasagne beds' or lasagne gardening, Hank - it works well, but attractive it ain't... usually used by people growing veggies.

My method of lasagna gardening can be found in my 14 June 2014 answer to Shahida upon a question about clay soil. The question was: " Hi, I have very sticky hard clay soil...".
Sorry for the delay to your question to me.

Have now looked up Lasagna gardening. Might just try it on one of my small raised beds. Thanks.

Sorry to disagree but sharp sand would be ok for your soil (especially as it was for free) as long as you don't have lime-haters. If you are worried about any salt content then give it a soak before using. Builders' or soft sand is something else, yes, very small particles which will not nothing to improve. I have used sharp sand regularly, especially on lawns. The horticultural sharp sand is expensive as it contains no calcium.

Thanks Jim, that's great. Just in case, I'll soak it first.