Sand is a necessary building material for interlocking brick projects, for two different parts of the installation. Two kinds of sand are used for the properties that produce the best results. A sand that hardens has been developed as an alternative to the traditional jointing sand.
Interlocking bricks are segmental pavers laid on a compacted surface and kept from moving with sand swept into the joints between them. This is a pervious type of paving; water can infiltrate into the ground instead of running off the surface. Water collecting in the subgrade will cause shifting eventually. Sand is used to level the surface for the brick and for sweeping between the brick once its laid to lock the bricks together.
Once the subgrade is prepared, about 2 inches of sand -- the laying course -- is installed. Grit sand, or sharp sand, is used for this course. Grit sand has a low clay content, not more than 3 percent clays or silts, and larger grain sizes than the soft building sand. Building sand is very fine and can become waterlogged, causing the brick pavers to settle. Some builders lay the bricks on a crushed stone sub-base instead of sand.
The last step in installing the brick patio is sweeping jointing sand between the joints. The jointing sand creates frictional interlock between the pavers, a process that keeps the bricks at surface level. It's very difficult to remove a jointed block. Kiln-dried silica sand is used for jointing; it's a very clean sand selected for grain particle sizes that increase the friction between the bricks. Building sand, play sand, soft sand and grit sand are not recommended.
Polymeric sand is sand injected with a chemical binder to harden it. This sand consists of quartz and crystalline silica, and a water-activated polymer or organic ingredient that binds the sand particles together. Polymer sand prevents weeds and doesn't wash out; its stability keeps water from collecting and causing soil movement. Polymer sand has to be swept completely off the bricks when the project is completed. A light spray of water will activate hardening.