ABSTRACT

In this study, the effect of the addition of a thermally activated coal mining waste (CMW) on the chloride permeability of blended cements in amounts from 10 to 50% was assessed by salt ponding test. The parameters involved in the infiltration of chloride have been evaluated by XRD, TG and MIP techniques, as well as by electric resistivity measurements. A remarkable improvement in chloride diffusion was found as the CMW content increased, which has resulted in a decrease of one order of magnitude in the chloride diffusion coefficients of specimens containing 50% of CMW (2.7 x [10.sup.-12] [m.sup.2]/s) with respect to OPC samples (20 x [10.sup.-12] [m.sup.2]/s). The chemical analyses have shown that immobilization of chloride ions as Friedel's salt takes place as a result of their reaction with the abundant hydrated aluminate phases formed as a consequence of the high alumina content in CMW. Moreover, it has been observed a refinement of the capillary network with the addition of CMW that has been manifested through a decrease in pore size and a subsequent increase in electrical resistivity.

Keywords:

Coal mining waste

Blended cement

Chloride diffusion

Salt ponding

Friedel's salt

Electrical resistivity

1. Introduction

Concrete is usually regarded as a highly durable material. Nevertheless, most applications of concrete require the use of embedded steel reinforcing bars and this can generate a weakness in the system since steel is known for its susceptibility to deterioration in environments containing water and oxygen (both present in the pore network of concrete). Fortunately, in strong alkaline solutions such as those found in concrete pores, a passivating thin layer of iron oxide is formed around the steel bar, preventing it from being corroded. However, under certain circumstances, the integrity of this protective layer can be compromised, leading to corrosion of the steel and the subsequent cracking and spalling of the concrete, due to the expansion of the corroding bar. One of the major causes of destruction of the passivating film is the introduction of chloride salts in the concrete [1,2].

Corrosion problems in modern concrete caused by chloride ions arise primarily from exposure to aggressive environments, especially those in which concrete is in contact with seawater or deicing salts [3], In these cases, additional measures should be taken in order to improve the chloride resistance of the material. The use of Supplementary Cementitious Materials (SCM) is often the preferred course of action.

The reason behind the effectiveness of many SCMs in the enhancement of chloride resistance is due to two factors. Firstly, the addition of pozzolans to cement implies a modification in the microstructure of the hydrated material as a result of the pozzolanic reaction, which usually leads to a more refined pore network. Consequently, the permeability of the system is reduced and the introduction of external damaging agents into the material is minimized. On the other hand, chloride ions can be retained by cement hydration products, thus reducing the degree of free chloride infiltration in the specimens. This process is known as chloride binding...