When concrete cures, it forms long crystals that grab onto the sand and gravel that forms the rest of the concrete. The inclusion of large amounts of organic matter and/or salt will weaken the bonds. For this reason sand containing salt or organic matter needs to be washed clean before it is used.
Jan 15, 2009 · Going down to the beach and getting a trailer full of sand? If so these are not the best to use..so what should I use? You want to get some clean (washed), sieved to 2 to 4 or .5mm to 1mm, and preferably a rough dry sand. If you use dry sand you then can control the water required to add to your concrete mix.