Some time ago an article in
Concrete Construction warned against the use of unvented heaters for protecting concrete during cold weather construction. It was stated that the exposure of plastic concrete to carbon dioxide produced soft, chalk-like surfaces. How can this be reconciled with the fact that carbon dioxide is sometimes used to harden precast concrete?
The article "Watch Those Unvented Heaters" appeared in the April 1956 issue of C/C, pages 2 to 4. It was based on a study by Huron Portland Cement Company which indicated that if carbonation due to carbon dioxide takes place when the cement gel is unstable, the surface hardness of the concrete is permanently impaired. Although it seems contradictory, carbonation which takes place after the cement gel is in a stable condition may increase both the hardness and the strength of the concrete. Carbonation has been used to some extent as a means of case hardening the surface of precast concrete products, but the treatment is always carried out at least 24 hours after casting.