reaction boundary

reaction boundary

[rē′ak·shən ‚bau̇n·drē]
(physical chemistry)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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Based on the theory of reaction boundary layer, this paper presents a sulfuric acid corrosion model for concrete combined with the authors' previous research results [24].
However, [H.sup.+] (hydrogen ion) and S[O.sub.4.sup.2-] diffuse into the inner concrete through the corrosion layer from the soaking solution under the action of concentration gradient thereby forming the reaction boundary layer.
In the process of sulfuric acid corrosion for concrete, OH- diffuses from interior to exterior of concrete under the action of concentration gradient and reacts with [H.sup.+] in the soaking solution to form the reaction boundary layer.
Reactions occurring in the reaction boundary layer formed by the sulfuric acid corrosion for concrete can be represented as
Assuming that the concentration change of O[H.sup.-] in the reaction boundary layer is consistent with the diffusion process of homogeneous reaction; then the whole diffusion process can be expressed as
where x is the distance of a point within the reaction boundary layer from the inner boundary (m), and t is the soaking time (s).
where [mathematical expression not reproducible] is the concentration of O[H.sup.-] in the saturated Ca [(OH).sub.2] solution (mol x [L.sup.-1]), [mathematical expression not reproducible] is the concentration of H+ in the soaking solution (mol x [L.sup.-1]), and S is the thickness of the reaction boundary layer (m).
Assuming that concentration distributions in the reaction boundary layer for O[H.sup.-] and [H.sup.+] are satisfied, the following functions are derived as
where [phi]([theta]) and [psi]([theta]) are the shape functions of concentration distributions in the reaction boundary layer for O[H.sup.-]and [H.sup.+], respectively.
On the inner boundary within the reaction boundary layer, the relationship between the acid consumption rate of concrete and time can be expressed as
At the same time, CaS[O.sub.4] is constantly dissolving out of the concrete to reduce the CaS[O.sub.4] concentration in the reaction boundary layer, resulting in the dissolution of the boundary layer, which increases the reaction rate.
A sulfuric acid corrosion model for concrete was established based on the reaction boundary layer theory and was solved by applying the separation of variables.