Abstract
Internal sulfate ions are the main cause of delayed ettringite formation, which leads to expansive
damage of cement concretes. On the other hand, the exposure of cementitious systems to chloride
ions causes corrosion of reinforcement bars that finally seriously shortens the service life of
reinforced concrete structures. It has been found that, when concomitantly present in cementitious
systems, chlorides interact with sulfate ions. Particularly, for a given concentration of sulfate ions
present, low to moderate concentrations of chloride ions bring about serious deterioration of
concretes due to high amount of ettringite formed, while higher contents of chlorides tend to reduce
and even completely eliminate ettringite formation. The objective of the present project is to
mathematically formulate the interaction between chloride and sulfate ions in cementitious materials.
Such knowledge is useful for accurate consideration of the action of chlorides on concretes used in
sea environments, thus predicting more precisely their service life (this being a far important step in
design).
Kinetic analysis with pure reactants involved in individual steps implicated in the whole
mechanism that pertain to the formation of, or destruction of ettringite in saline solution can be
made. Monitoring each of these chemical reactions may help in establishing reaction rate equations,
from which rate constants may be obtained. In the whole mechanism, it then possible to express
reaction rates of individual compounds involved, chlorides ions and ettringite included. This finally
gives a system of partial differential equations of compound concentrations as a function of time,
one solution of which can be put in a form of an equation of concentration of ettringite formed as a
function of concentration of chlorides.
This article is the first part of a three-part study: In this part is formulated the approach (from
reaction kinetics theory) followed for establishment of the mathematical equation; in the second part
will be presented and discussed lab kinetics results and their use down to the mathematical model;
the third and last part deals with applicability and limitations of the developed model, assessed in
terms of expansion observed on steam-cured mortars made with cements used in the South African
building industry.