Use of ZnAl-Layered Double Hydroxide (LDH) to Extend the Service Life of Reinforced Concrete

Abstract

This work investigated the use of ZnAl-layered double hydroxide (LDH) intercalated with nitrate or nitrite ions for controlling the corrosion of steel in reinforced concrete. The work started by analyzing the stability of the powder in the 1-14 pH range and the capacity for capturing chloride ions in aqueous solutions of different pH. The effect of the ZnAl-LDH on the corrosion of steel was studied in aqueous 0.05 M NaCl solution and in mortars immersed in 3.5% NaCl. It was found that the LDH powders dissolved partially at pH > 12. The LDH was able to capture chloride ions from the external solution, but the process was pH-dependent and stopped at high pH due to the partial dissolution of LDH and the preferential exchange of OHions. These results seemed to imply that ZnAl-LDH would not work in the alkaline environment inside the concrete. Nonetheless, preliminary results with mortars containing ZnAl-LDH showed lower penetration of chloride ions and higher corrosion resistance of the steel rebars. dioxide leads to the carbonation of concrete, which is accompanied by a decrease in pH. When the front of low pH reaches the steel surface, the passivity is lost and uniform corrosion starts. Chloride ions, on the other hand, can disrupt the passive layer even at high pH, if a threshold concentration is surpassed [5]. In these conditions, the corrosion is localized in the form of pitting, crevice, or stress corrosion cracking. The iron corrosion products are more voluminous than steel, which creates expansive stresses, leading to cracking at first and spallation of the concrete cover at the end. After that, steel becomes directly exposed to the atmospheric environment, and corrosion proceeds at a much faster rate.Since the corrosion of steel rebars dramatically limits the service life of reinforced concrete, many forms of corrosion control are being explored [2][3][4]6,7], including the use of stainless steel or galvanized steel rebars, application of epoxy coating on steel rebars, cathodic protection, and addition of corrosion inhibitors to concrete (e.g., calcium nitrite, sodium benzoate, chromates, phosphates, polyphosphates, silicates, polycarboxylic acids, fatty acids emulsions, and alkanolamines). Corrosion can also be delayed by painting the surface of the concrete structure or using a thicker layer of concrete, separating the rebars and the environment. Less porous concrete (lower water/cement ratio), high quality cement, and the use of water and aggregates without soluble salts also contribute to extending the durability of the structure.The direct addition of corrosion inhibitors to concrete may affect the curing process or the mechanical properties of the hardened material. Because of this possibility, the encapsulation of inhibitors in nano-or micro-reservoirs to be released only when needed (either with the onset of corrosion or in the presence of aggressive species) is a line of investigation worth pursuing.Layered double hydroxides (LDHs) are one example of such nanostructured reservoirs. The LDH structure...

Recent Advances on the Application of Layered Double Hydroxides in Concrete—A Review

“…On the other hand, LDH can also decelerate or retard cement hydration. Recent studies by Gomes et al [25] reveled that addition of ZnAl LDH had a decelerating effect on hydration processes and early strength development. They used a dosage of 2-5% LDH to cement with an average particle size of 25 µm in cement pastes.…”

“…The presence of Zn 2+ ions in the early stages of cement hydration can lead to formation of Zn-Ca complexes such as Ca(Zn(OH 3 ) 2 )·2H 2 O which in turn reduce the concentration of Ca 2+ and OH − in the pore solution thereby retarding the hydration reactions [114]. Gomes et al [25] also observed that particle size matters because bigger agglomerates ∼125 µm did not affect the hydration. This can be understood considering the reduced surface area of bigger agglomerates which do not suffer so significant dissolution as compared to finer particles.…”

“…[25] used LDH loaded with NO 2 − ion with the aim of capturing Cl − ion and releasing NO 2 − ion and reported positive results. Many authors [25,100] have conducted natural diffusion test on concrete with added LDH in order to ascertain the enhanced chloride capture in such mixes. Yang et al [100] conducted a rapid chloride migration tests on concrete with and without LDH.…”

“…In recent times, layered double hydroxides (LDHs) have emerged as a new class of engineering materials [24,25] which can aid in the corrosion control of concrete structures and potentially prolong their service life. LDHs are clay-like powdered materials which are often referred to as nano-containers or nano-reservoirs and have the ability of entrapping ions from the environment e.g., Cl − ions.…”

Numerical and Experimental Analysis of Self‐Protection in Reinforced Concrete due to Application of Mg–Al–NO2Layered Double Hydroxides

“…However, the capacity to capture chloride ions can be influenced by the amount of LDH itself as well as pH of the surrounding environment. [52] The chloride capture is very fast and a thermodynamic heterogeneous equilibrium is established almost instantly.…”