Chloride binding is a complex phenomenon in which the chloride ions bind with hydrated Portland cement (PC) phases via physical and chemical mechanisms. However, the current utilization of clays as (Al)-rich supplementary cementitious materials (SCMs), such as metakaolin (MK), can affect the chloride-binding capacity of these concrete materials. This state-of-the-art review discusses the effect of clay-based SCMs on physical and chemical chloride binding with an emphasis on MK as a high-reactivity clay-based SCM. Furthermore, the potential mechanisms playing a role in physical and chemical binding and the MK effect on the hydrated cement products before and after exposure to chloride ions are discussed. Recent findings have portrayed competing properties of how MK limits the physical chloride-binding capacity of MK-supplemented concrete. The use of MK has been found to increase the calcium silicate hydrates (CSH) content and its aluminum to silicon (Al/Si) ratio, but to reduce the calcium to silicon (Ca/Si) ratio, which reduces the physical chloride-binding capacity of PC-clay blended cements, such as limestone calcined clay cements (LC3). By contrast, the influence of MK on the chemical chloride capacity is significant since it increases the formation of Friedel’s salt due to an increased concentration of Al during the hydration of Portland cement grains. Recent research has found an optimum aluminum to calcium (Al/Ca) ratio range, of approximately 3 to 7, for maximizing the chemical binding of chlorides. This literature review highlights the optimal Al content for maximizing chloride binding, which reveals a theoretical limit for calcined clay addition to supplementary cementitious materials and LC3 formulations. Results show that 5–25% of replacements increase bound chloride; however, with a higher percentage of replacements, fresh and hardened state properties play a more pivotal role. Lastly, the practical application of four binding isotherms is discussed with the Freundlich isotherm found to be the most accurate in predicting the correlation between free and bound chlorides. This review discusses the effects of important cement chemistry parameters, such as cation type, sulfate presence, carbonation, chloride concentration, temperature, and applied electrical fields on the chloride binding of MK-containing concretes—important for the durable formulation of LC3.

When it comes to sustainability, steel rebar corrosion has always been a big issue, especially when they are exposed to harsh environmental conditions, such as marine and coastal environments. Moreover, the steel industry is to blame for being one of the largest producers of carbon in the world. To supplant this material, utilizing fiber-reinforced polymer (FRP) and hybrid FRP bars as a reinforcement in concrete elements is proposed because of their appropriate mechanical behavior, such as their durability, high tensile strength, high-temperature resistance, and lightweight-to-strength ratio. This method not only improves the long performance of reinforced concrete (RC) elements but also plays an important role in achieving sustainability, thus reducing the maintenance costs of concrete structures. On the other hand, FRP bars do not show ductility under tensile force. This negative aspect of FRP bars causes a sudden failure in RC structures, acting as a stumbling block to the widespread use of these bars in RC elements. This research, at first, discusses the effects of different environmental solutions, such as alkaline, seawater, acid, salt, and tap water on the tensile and bonding behavior of different fiber-reinforced polymer (FRP) bars, ranging from glass fiber-reinforced polymer (GFRP) bars, and basalt fiber-reinforced polymer (BFRP) bars, to carbon fiber-reinforced polymer (CFRP) bars, and aramid fiber-reinforced polymer (AFRP) bars. Furthermore, the influence of the hybridization process on the ductility, tensile, and elastic modulus of FRP bars is explored. The study showed that the hybridization process improves the tensile strength of FRP bars by up to 224% and decreases their elastic modulus by up to 73%. Finally, future directions on FRP and hybrid FRP bars are recommended.

One of the main disadvantages of steel bars is rebar corrosion, especially when they are exposed to aggressive environmental conditions such as marine environments. One of the suggested ways to solve this problem is to use composite bars. However, the use of these bars is ambiguous due to some weaknesses, such as low modulus of elasticity and linear behavior in the tensile tests. In this research, the effect of the hybridization process on mechanical behavior, including tensile strength, elastic modulus, and energy absorbed of composite bars, was evaluated. In addition, using basalt fibers because of their appropriate mechanical behavior, such as elastic modulus, tensile strength, durability, and high-temperature resistance, compared to glass fibers, as the main fibers in all types of composite hybrid bars, was investigated. A total of 12 hybrid composite bars were made in four different groups. Basalt and carbon T300 composite fibers, steel bars with a diameter of 6 mm, and steel wires with a diameter of 1.5 mm were used to fabricate hybrid composite bars, and vinyl ester 901 was used as the resin. The results show that, depending on composite fibers used for fabrication of hybrid composite bars, the modulus of elasticity and the tensile strength increased compared to glass-fiber-reinforced-polymer (GFRP) bars by 83% to 120% and 6% to 26%, respectively. Moreover, hybrid composite bars with basalt and steel wires witnessed higher absorbed energy compared to other types of hybrid composite bars.