Applicability of the formation factor for different alternative binder types investigated on mortars

Abstract

The high energy demand and associated CO₂ emissions to produce Portland cement (OPC) create the need for alternative types of binders that can be produced in a more environmentally friendly manner. These alternative binders either have a reduced OPC content or can be produced entirely without the use of OPC clinker. To allow their widespread use also in reinforced concrete components, these binders must meet the same durability requirements as types of cement that have already been standardized. This requires a thorough characterization of the materials and, in addition, a verification of the applicability of the evaluation criteria known from the traditional binders. One of the most important factors influencing the transport processes in the material and therefore the durability, is the penetration resistance of the pore structure for aggressive species, such as chloride ions. One key parameter for describing the pore structure is the formation factor (FF), which is used for various applications to evaluate durability. It is determined based on the ratio of the electrical resistivity of the concrete and of the pore solution. In this study, mortars produced with different alternative binders are characterized in terms of their formation factor and pore size distribution. In addition, chloride migration coefficients determined by the Rapid Chloride Migration (RCM) test are related to the formation factor. The test program includes nine different alternative binders and two reference cements: In addition to a CEM I and CEM III/B, two calcined clays and a modified steel mill slag were investigated as supplementary cementitious materials (SCMs), as well as a calcium sulphoaluminate (CSA) cement and a calcium silicate hydrate (C-S-H) binder. Four materials belong to the group of alkali-activated mortars: Two geopolymers and two alkali-activated slags were investigated here. It is shown that both, the AC resistances and the conductivities of the pore solutions, differ greatly among the mortars investigated. The formation factor is a suitable parameter for a simplified estimation of the chloride penetration resistance of the investigated alternative binders, provided that realistic values of the pore solution conductivities are determined and used for the calculations.