Q: We're building a concrete home for an owner who heard that concrete emits gas as it cures. He wants to know what kind of gas is emitted and whether it's harmful to occupants. We've never heard of concrete emitting gas. Is there any data on this subject?
A: The Portland Cement Association published data from a study it conducted to determine the emission rates of volatile organic compounds (VOCs) from concretes containing different admixtures and cements, or treated with different form-release or cure-and-seal products (see reference). Researchers prepared eight 8x8x3 1/4-inch concrete prisms using a Type I cement. As a control, one specimen was prepared without any admixture or additive. The other seven specimens were prepared with one of the following:
- Water-reducing admixture
- Air-entraining admixture
- Cement processed with a waste-derived fuel
- Water-based form-release agent on the surface
- Chemical form-release agent on the surface
- Cure-and-seal product on the surface
Samples were tested in accordance with ASTM standards for small-scale environmental chamber determination of VOCs using solid phase micro-extraction and a gas chromatograph coupled to a mass spectrometer.
Six of the eight specimens produced few or no VOCs, presenting no health risk. The specimens coated with the chemical release agent and the cure-and-seal produced high initial levels of VOCs, but these levels dropped substantially during the 28-day measurement period, posing minimal health risk.
There have been isolated cases of aggregates contaminated with ammonium nitrate being used in concrete and producing a strong, lingering ammonia odor. However, these instances are rare.
Alicje Cornelissen, "Concrete's Role in the Indoor Air Environment," PCA R&D Serial No. 2098, Portland Cement Association, Skokie, Ill., 1998.