We have just had a serious occurrence of dusting of a vertical concrete surface, even though this was very carefully and thoroughly cured. Could the form oil we used be at fault?
This is very unlikely, if a reputable product was used. Dusting can be due to chemical and physical factors, such as: 1. Earth dust on forms. With subgrade or grade-level walls and columns, excessive amounts of ordinary ground dust build up and accumulate, especially in the corners on heavily oiled forms. Later, when the concrete is placed, this dust soaks up too much of the surface water so that the necessary silicates cannot be formed. Especially bad in this respect is soil containing high amounts of certain types of clays such as bentonite which are capable of absorbing up to 5 times their weight of water. 2. Use of porous timber. Sometimes when certain soft woods such as brown (shingle type) cedar, soft pine and other soft woods are used in forming, a sufficient amount of water is absorbed to prevent proper surface curing of the concrete. 3. Excessive heat. Sun temperatures above 100 degrees F. beating down on wall forms, especially steel forms, will tend to induce the surface water to move inward to the cooler portion of the concrete, as well as upward, to impair proper surface curing. 4. Excessive water. Water from rains or surface flooding will tend to dilute or carry away sufficient amounts of the calcium and aluminum hydrates from the bases of walls and piers before they can convert chemically to the necessary silicates. 5. Chemical contamination. Muriatic acid, and patented lime-solvent-type chemical cleaners are sometimes used to clean wood forms. The wood soaks up enough acid to affect the concrete surface on the first re-uses of the forms.