The role of metakaolin in the protection of concrete against the deleterious action of chlorides

Resumen

Metakaolin has been increasingly employed in Portland cement concrete as a substitute for part of the cement, hoping to be able to play an important role in the durability of structures, especially in marine environments where there is a prominent presence of chloride ions. This study evaluates the effect of replacing part of cement by metakaolin (10%) in a commercial concrete composition of the metropolitan region of Goiania, with fck = 30 MPa. From previous works, it is known that the use of this mineral addition (metakaolin) increases the concrete mechanical strength. Thus, besides the reference concrete mix (without replacement), two more mixes were prepared: one with the same water/binder ratio of the reference concrete and the other by increasing this ratio in order to observe the influence of metakaolin at the same level of mechanical strength. In all three mixes, consistency of the concrete was maintained within the same range of values, namely: slump equal to (100±10) mm. To evaluate the mechanical properties of the different types of concrete, axial compressive strength tests were performed. The porous structure of the mixtures was evaluated by open porosity tests. Specimens attacked by chlorides were sprayed with silver nitrate to obtain an indication of the chlorides advance in the cement matrix. Finally, X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were made to check the potential of metakaolin to fix chlorides in the form of Friedel's salt and to see the differences in the resulting microstructure. It was thus concluded that the replacement of cement by metakaolin reduces the diffusivity of chloride ions, to the extent that this mineral addition produces physical refining of the concrete pores structure and also because it induces the formation of calcium chloroaluminate, which makes it a very effective agent in preventing reinforcement corrosion in chloride-rich environments.