You should add sand as an amendment to clay soils. Remember, clay is sand in advanced age. Clay is tiny, tiny particles of sand, and is in your soil as a result of time, glaciers, and you not realizing the importance of a soil test before you bought your house.
Plants roots suffocate and rot in it.
The more organics you add, the better, up to fifty percent. If you want to bring in some pulverized black dirt, that's good too. Just keep the sand in the sandbox, where it belongs.