Polymeric sand
Also called: poly sand, joint sand, jointing sand, paver sand
Fine sand mixed with a polymer binder, swept into paver joints and activated with water so it hardens into a flexible, weed- and ant-resistant filler that keeps the joints from washing out. $25–$40 a bag, one to two bags per 100 sq ft. The last step on a paver job, and the one most often done wrong.
Pavers interlock through their joints: sand packed between them transfers load from one to the next and keeps the surface acting as one piece. Plain sand does that until rain, ants and a pressure washer take it out. Polymeric sand does the same job and then sets — firm enough to stay put, flexible enough to move with freeze and thaw.
The application is unforgiving, which is why it’s the step that gives DIY paver jobs a bad name. The pavers and joints must be completely dry. The sand is swept in until the joints are full to 1/8 inch below the chamfer, the surface is run over with a plate compactor to consolidate it, topped up, and then every grain is swept and blown off the paver faces — any residue left on the surface hardens into a haze that’s very difficult to remove. Then a light mist of water, in stages, enough to soak the joint without flooding it, and 24–48 hours without rain or traffic. Too much water washes the binder out; too little leaves it loose.
Redone every 5–10 years as it wears, and after any repair. On a driveway it also stabilizes the joints against tire scrub, which plain sand can’t. Ask a contractor which brand and whether the surface was compacted after sanding; on a DIY job, read the bag twice.