Modified Hydrotalcites as Smart Additives for Improved Corrosion Protection of Reinforced Concrete

Corrosion of reinforcing steel is a major culprit to durability and serviceability of concrete structures. This problem is highly relevant for civil engineering structures in the transport sector, such as bridges, tunnels, harbour quays and parking structures. The dominant aggressive external influence is the chloride load from de-icing salts or sea water, penetrating the concrete and destroying the natural (high pH) passivation of the steel. The direct and indirect costs of reinforcement corrosion are substantial, as it entails additional repair, rehabilitation, and monitoring activities to ensure the safety, functionality and aesthetics of concrete structures and components. In addition, many repairs have a short working life, necessitating repeated repairs within the use life. Consequently, the construction industry is in need of improving the corrosion protection of reinforcing steel, preferably by low-cost measures. Presently available corrosion preventive measures are either too costly or technically too complicated to be applied on a wide scale. Stainless steel reinforcement is 5 or 10 times more expensive than reinforcing (carbon) steel. Cathodic prevention and protection may be effective but both are a special niche expertise and are thus not applied on a wide scale. Coatings on the concrete surface normally do not last long enough (10-20 years), which causes a maintenance cycle of its own. Corrosion inhibitors seem to be attractive owing to their low cost and the ease and flexibility of application. However, there are conflicting opinions about the reliability of the inhibitors for corrosion protection in concrete in terms of long-term efficiency; some are toxic, such as nitrites. A possible promising solution to overcome this problem is the encapsulation/immobilization of desired inhibitors within the molecular structure of a host compound. The immobilized inhibitor then can be slowly released in a controllable way by an external stimulus (e.g. chloride ions) and therefore provide a relatively long-term corrosion protection. Owing to the unique fine tunable molecular structure and high anionic exchange capacity, modified hydrotalcites (MHTs) have the potential to be used for the immobilization of a desired inhibitor. Hydrotalcite is one representative of large mineral group of Layered Double Hydroxides (LDHs), in general formula [MII1-x MIIIx (OH)2]x+ [(An-x/n)]x-·mH2O, where MII and MIII are di- and trivalent metals respectively, and An- is an interlayer charge-balancing anion with valence n. The x value is in the range 0.20-0.33. Although the most common anion found in naturally occurring hydrotalcites is carbonate, in practice however, there is no significant restriction to the nature of the interlayer charge-balancing anions. The MHT structure can accommodate various cations in the hydroxide layers with varying MII/MIII ratios as well as a wide range of anionic species in the interlayer regions. Within the MHT family, a class of materials with emerging importance is that constituted by MHTs intercalated with organic species. In addition, increasing awareness of the health and ecological risks has drawn much attention to amino acid-based inhibitors because they are nontoxic, environmentally friendly, relatively cheap and easy to produce with higher purity. Therefore, the marriage of the two kinds of materials is expected to not only offer an improved inhibiting effect than using the inhibitor alone but also to impose less impact on environment. Recently a study on the application of amino acid modified hydrotalcites in cementitious materials has formed the basis of a patent (WIPO Patent, WO 2011/065825 A1). However, its scale was relatively small and further work was considered necessary by the applicants and their organisations. In this research, four different types of sodium salts of amino acids (i.e., Glycine, 6-aminocaproic acid, 11-aminoundecanoic acid, and p-aminobenzoic acid) were proposed as potential candidates for the modification of hydrotalcite. Sodium nitrite was also chosen as a modification candidate for comparison purposes due to its well-recognized inhibition performance in concrete. Based on the anti-corrosion performance evaluation in chloride contaminated simulated concrete pore solution (Chapter 3), sodium nitrite, sodium salt of p-aminobenzoic acid (pAB) and sodium salt of 11-aminoundecanoic acid (11AUA) were selected as the most promising candidate modifiers for synthesis of MHT. Subsequently, six MHTs (two Mg/Al atomic ratios of 2.2 and 2.7, which were denoted as 2 and 3 respectively) were synthesized through the modification of two commercially available carbonate Mg-Al hydrotalcites PURAL® MG 63 HT (Mg/Al atomic ratio 2.2) and PURAL® MG 70 HT (Mg/Al atomic ratio is 2.7) by NaNO2, pAB and 11AUA (Chapter 4). They were characterized by means of X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermogravimetry (TG), Differential scanning calorimetry (DSC) and relevant elemental analysis. The ion exchange characteristics of the six synthesized MHTs and their anti-corrosion performance were investigated in chloride-rich simulated concrete pore solution (Chapter 5). The results showed that ion exchange occurred between free chloride ions in the simulated concrete pore solution and the inhibitive anions intercalated in MHT, thereby reducing the free chloride concentration which is equivalent to increased binding of chloride in mortar/concrete. Moreover, the simultaneously released anions, in particular -pAB, were found to exhibit a notable inhibiting effect and caused shifting of the corrosion initiation of steel to a higher chloride concentration level. This evidence manifested the dual role protecting function that MHT (in particular, Mg(2)Al-pAB) offers to the steel: capturing chlorides as a chloride scavenger and providing of a beneficial release of corrosion inhibitors in parallel as an inhibitor reservoir further protecting reinforcing steel from corrosion. The effects of two MHTs, i.e., Mg(2)Al-NO2 and Mg(2)Al-pAB, were investigated in both plain and reinforced mortar specimens with a focus on their interaction with chloride ions in plain mortar (Chapter 6) and in reinforced mortar, mainly focusing on their inhibition influence on corrosion of the reinforcing steel (Chapter 7). In plain mortar, the two MHTs were incorporated at two dosage levels replacing 5% and 10% mass of cement. A testing programme including workability test, strength test, porosity test, and rapid chloride migration and diffusion test was employed to investigate the effect of the two MHTs on chloride penetration in mortar. The results indicated that the incorporation of Mg(2)Al-pAB at 5% dosage in mortar produced a notably improved chloride diffusion resistance with no remarkably negative influence on the development of mechanical strength and workability of fresh mortar, which therefore validated that the Mg(2)Al-pAB could be a promising alternative in hindering the chloride transport in mortar when an appropriate mixing dosage is adopted. In reinforced mortar, the two MHTs were applied in two different ways: (1) as one of the mixing components in bulk mortar at two dosage levels replacing 5% and 10% mass of cement; (2) as a surface coating on the reinforcing steel in a cement paste replacing 20% of the cement mass. Three test methods including electrically accelerated chloride migration, cyclic wetting-drying and natural chloride diffusion test based on chloride exposure were adopted to custom designed reinforced mortar specimen. Although no corrosion was detected after 30 weeks natural diffusion testing, the results obtained from accelerated chloride migration and cyclic wetting-drying test revealed that when an appropriate mixing dosage is adopted and applied in a proper way, the application of MHT either incorporation of a small amount (in particular, Mg(2)Al-pAB to replace 5% weight of cement) in mortar or as a surface coating of the reinforcing steel (Mg(2)Al-pAB or Mg(2)Al-NO2 to replace 20% weight of cement in paste) resulted in delayed corrosion initiation and increased chloride threshold responsible for initiating corrosion. The effects on service life of structures in chloride contaminated environment is estimated, which shows a significant improvement. In general, the research work presented in this thesis met the expectations and goals formulated at the start of the project. As the first exploration on a wider scale into the application of MHT in cementitious materials for corrosion protection purposes, a new type of smart concrete additive based on amino acid modified hydrotalcites (in particular Mg(2)Al-pAB) aiming to combat chloride-induced corrosion has been developed and documented. The results demonstrated that by using such a material, a longer service life of reinforced mortar/concrete structures can be expected. While realizing that more research is still needed for maximizing the beneficial effect of MHT as a functional additive of cementitious materials, some recommendations for further research are given in the last chapter of this thesis (Chapter 8). MHT has a very rich interlayer chemistry and can participate in anion exchange reactions with great facility. Therefore, the scope of application for MHT with combination of different kinds of host metal hydroxides and various interlayer anions with desired specific function in cement-based materials could be significantly expanded. For example, a controlled release formulation based on MHT can be made by encapsulation/immobilization of a desired functional compound within the layered molecular structure of hydrotalcites. This functional compound could be a superplasticizer, a shrinkage reducer, an ASR inhibiting compound, an air-entraining agent, a pore solution viscosity adjuster, a setting accelerator/retarder and probably other any concrete property adjusters. In this respect, we are confident that future work on applications of new types of smart functional concrete additives based on MHTs will expand rapidly and contribute greatly to the effort of searching for effective measures to improve the durability or other properties of reinforced concrete