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The inhibition synergism of some plant extracts and common inorganic inhibitors to enhance the corrosion control of the embedding steel bars in concrete

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Resilient concrete is green concrete

The deterioration of steel embedded in solid concrete structures due to reactive substances is a key problem in civil engineering. Corrosion inhibitors from plant products offer a solution.

Industrial Technologies

Concrete is one of the most durable materials, able to outlast even grave natural disasters. However, the corrosion of reinforcements often limits the durability of reinforced concrete structures. The main causes are the impact of chloride ions or the procedure of carbonation. This effect can be controlled through corrosion inhibitors without significant modifications to the concrete properties. Green corrosion inhibitors have been qualified as more appropriate than synthesised corrosion inhibitors due to their lower number of toxic compounds. The corrosion inhibitors extracted from plant products combine low cost, availability and sustainability. Thanks to their composition, most parts of plants, such as the leaf and the stem, could be ideally used as corrosion inhibitors for various substrates in different electrolytes. Funded by the Marie Skłodowska-Curie Actions programme, the NATCON project aimed at developing synergistic organic natural inhibitors and inorganic ones for construction applications. The NATCON solution forms an absolute novelty, as there is still plenty of research to be conducted on inhibitors obtained from plant extracts in strongly alkaline environments such as the ones formed in the pores of the concrete. Apart from the synergy with traditional inorganic inhibitors, nitrites, the project tried to evaluate the performance of natural inhibitors both in solution and in mortar.

Liquorice and nettles as a means of corrosion control

The NATCON team has carried out a comprehensive analysis of the possible use of plant extracts as green corrosion inhibitors for steel. The research included both studies in solution and in mortar, evaluating the efficiency and possible mechanisms of the inhibitor as well as the effect of plant extracts on mortars, both on embedded steels and on mortar properties. Moreover, the synergism between the traditional inhibitor sodium nitrite and a selected plant extract, liquorice, has been demonstrated. The simultaneous use of the materials allows the reduction of the total inhibitor amount.

The potential of green concrete admixture

“The usage of plant extracts is the main green aspect of NATCON. The studied plants can be found everywhere, and the extract procedure is very easy,” explains Francisco Velasco, project coordinator. “Their demonstrated ability for this application, and their synergism with traditional nitrite inhibitors, reduce the employment of nitrites that often cause environmental problems.” The plant extracts are completely safe in use and are added together with water during concrete manufacturing through well-established procedures. The application field relies on the steel reinforcements typically used in construction. “We feel that the project results can be used in specific infrastructures; it would be difficult to use them everywhere,” notes Velasco. “In fact, applications are the same as those that can be found for nitrites. Companies dealing with additives for concrete can easily manage this technology, and supply constructors.”

Findings bring more queries

The NATCON team aims to continue the relevant research as there are two pivotal questions to be answered: Which is the chloride threshold that these plant extracts provide, i.e. under which corrosion conditions do they perform properly? Which would be the spectrum of their use in concrete, given that all typical constructions use large aggregates that were not considered in NATCON? Another direction for future research might be the combination of different plant extracts to achieve an inhibition synergism.

Keywords

NATCON, concrete, corrosion, plant extract, corrosion inhibitors, nitrite, liquorice

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