We have had trouble with a white substance that forms on exposed aggregate walls made with regular gray cement. A retarder had been painted on the forms. After stripping, the aggregate was exposed and the walls were cleaned with muriatic acid. How can the problem be prevented, and how can we clean up our discolored walls?
The deposit is probably silica gel, an insoluble substance that forms when muriatic acid reacts with portland cement paste. Deposits are likely to be more dense if concentrated acid is used, so it is recommended that acid washing be done with five- to ten- percent muriatic acid. Although silica gel forms even with dilute acid, it can be removed if water washing is done as soon as the acid has stopped bubbling. It is also advisable to moist-cure the concrete as long as possible before the acid washing. Moist curing should then be continued as long as possible after the acid washing, but at least three or four days. It is most important to delay acid washing for at least a week and preferably two weeks. The discolored walls can possibly be cleaned enough to make them acceptable by washing the surface with a cleaning solution containing hydrofluoric acid. This should be done by a company experienced in building cleaning operations and familiar with the special dangers of this acid. Such cleaning should definitely not be done by the concrete contractor's employees.