  We investigate the fast-reaction asymptotics for a one-dimensional
reaction-diffusion (RD) system describing the penetration of the carbonation
reaction in concrete. The technique of matched-asymptotics is used to show that
the RD system leads to two distinct classes of sharp-interface models, that
correspond to different scalings in a small parameter \epsilon representing the
fast-reaction. Here \epsilon is the ratio between the characteristic scale of
the diffusion of the fastest species and the one of the carbonation reaction.
We explore three conceptually different scaling regimes (in terms of \epsilon)
of the effective diffusivities of the driving chemical species. The limiting
models include one-phase and two-phase generalised Stefan moving-boundary
problems as well as a nonstandard two-scale (micro-macro) moving-boundary
problem -- the main result of the paper. Numerical results, supporting the
asymptotics, illustrate the behavior of the concentration profiles for relevant
parameter regimes.
