We get a lot of popouts in exposed concrete. The expansive materials are shale particles found in the sand. Even though the shale particles may be no larger than 1/8-inch in diameter, a conical fracture shape makes the popouts more noticeable. Can this problem be avoided?
The popouts you describe occur primarily in Iowa and Minnesota. Shale particle expansion results from a reaction between the shale and alkalis in the cement, not freezing. The reaction occurs too fast for pozzolans to help. The Des Moines Metro Concrete Council placed test slabs to evaluate methods for reducing popouts. Most successful was a wet burlap cure, believed to leach out alkalis near the surface so they won't react with the shale. Slag cement instead of straight portland cement also reduced popouts. Use of a manufactured stone sand eliminated popouts but may have reduced the concrete's abrasion resistance. For information, contact Byron Marks, Martin Marietta Aggregates, 4554 N.W. 114th St., Des Moines, IA 50322.
The slab in the top photo was finished and then sealed with a normal sprayed sealer. The slab in the bottom photo was finished the same way but cured using wet burlap bags. Its concrete is slightly discolored from the bags.