Development of a multi-species mass transport model for concrete with account to thermodynamic phase equilibriums

In this study, a coupled multi-species
transport and chemical equilibrium model has been
established. The model is capable of predicting time
dependent variation of pore solution and solid-phase
composition in concrete. Multi-species transport
approaches, based on the Poisson–Nernst–Planck
(PNP) theory alone, not involving chemical processes,
have no real practical interest since the chemical action
is very dominant for cement based materials. Coupled
mass transport and chemical equilibrium models can
be used to calculate the variation in pore solution and
solid-phase composition when using different types of
cements. For example, the physicochemical evaluation
of steel corrosion initiation can be studied by
calculating the molar ratio of chloride ion to hydroxide
ion in the pore solution. The model can, further, for
example, calculate changes of solid-phase composition
caused by the penetration of seawater into the
concrete cover. The mass transport part of the model is
solved using a non-linear finite element approach
adopting a modified Newton–Raphson technique for
minimizing the residual error at each time step of the
calculation. The chemical equilibrium part of the
problem is solved by using the PHREEQC program.
The coupling between the transport part and chemical
part of the problem is tackled by using a sequential
operator splitting technique and the calculation results
are verified by comparing the elemental spacial
distribution in concrete measured by the electron
probe microanalysis (EPMA).