Alkali‐activated materials (AAMs) are binders that can complement and partially substitute the current use of conventional cement. However, the present knowledge about how AAMs protect steel reinforcement in concrete elements is incomplete, and uncertainties exist regarding the application of electrochemical methods to investigate this issue. The present review by EFC WP11‐Task Force ‘Corrosion of steel in alkali‐activated materials’ demonstrates that important differences exist between AAMs and Portland cement, and between different classes of AAMs, which are mainly caused by differing pore solution compositions, and which affect the outcomes of electrochemical measurements. The high sulfide concentrations in blast furnace slag‐based AAMs lead to distinct anodic polarisation curves, unusually low open circuit potentials, and low polarisation resistances, which might be incorrectly interpreted as indicating active corrosion of steel reinforcement. No systematic study of the influence of the steel–concrete interface on the susceptibility of steel to corrosion in AAMs is available. Less common electrochemical methods present an opportunity for future progress in the field.

The cement industry gains increasing public attention because of the high CO₂ emissions for which it is accountable. One approach to improve the carbon footprint of cement production is the development of alternative binders, which can be produced with less energy consumption. These alternative binders have a reduced clinker to cement ratio or contain no Portland cement at all. For the corrosion protection of the reinforcement, the same requirements apply as for standardized types of cement. To perform this evaluation, a basic understanding is necessary of the products formed during hydration, the composition of the pore solutions and pore structure, and their influence on the resistance to carbonation. In this study, various alternative binders, such as novel types of composite cement with calcined clays or modified steel mill slag, alkali-activated materials, C$\overline{\mathrm{S}}$A cement, and a C-S-H binder, are presented. The conducted investigations at mortars and cement pastes show the differences in hydration, microstructure development, and pH. The changes in microstructure and phase assemblage due to carbonation and the suitability of carbonation testing at an elevated CO₂ content of 1 vol.-% for the different binders are also presented.

Both the high CO₂ emissions associated with cement production and the increasing demand for concrete call for the use of binder types that can be produced in a more climate-friendly way than that of ordinary Portland cement. To ensure that these binders can also be used in reinforced concrete structures, their influence on the corrosion behavior of embedded steel reinforcement must be investigated. In the study presented here, the passivation behavior of steel in mortars made from various new types of binders is investigated. In addition to alkali-activated materials with high and low calcium contents, a calcium sulfoaluminate cement and a binder produced from calcium silicate hydrate (C-S-H) phases, synthesized in an autoclave, were investigated. While the steel clearly passivated in the alkali-activated slag and the C-S-H binder, the calcium sulfoaluminate cement showed the lowest open circuit potentials and polarization resistances, indicating a less effective level of passivation. The metakaolin geopolymer with a potassium-based activator showed an onset of passivation that was dependent on the environment of the specimens at an early age, whereas the alkali-activated fly ash with a sodium-based activator showed a delay in passivation that was not influenced by the environment of the specimens at an early age.

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.

The main cause of corrosion of reinforcing steel, especially in infrastructure structures, is the penetration of chlorides, which can lead to high corrosion rates at the reinforcement steel. A frequently used accelerated test method to evaluate the chloride penetration resistance of concrete and mortars is the rapid chloride migration (RCM) test. The test was initially developed for ordinary Portland cement (OPC) based materials and involves spraying an indicator solution onto split test specimens to determine the penetration depth of the chloride ions. The pore solution compositions of new types of binders differ from OPC‐based materials, which can affect the reaction of the indicator solution. To investigate the suitability of the RCM test, the chloride content in the depth of the color change was determined in mortars made from different types of novel binders. The results show that the test provides reliable results for the binder types with partial replacement of OPC by supplementary cementitious materials. For binder types without OPC, there is a deviation in the migration coefficient due to a differing chloride concentration that triggers the color change.