In Situ Measurements of the Chemical Stability of a Cast Aluminum Alloy Embedded in a Cement Paste with a High Amount of Supplementary Cementitious Material

Abstract

In traditional reinforced concrete, the alkaline pore solution which passivates the steel rebars will get neutralized with time in an exposed environment. Therefore, to prevent corrosion initiation, the permeability of the concrete is reduced and extra-thick concrete covers the steel rebars. Aluminum is passive in the neutralized environment, but the calcium hydroxide formed during the cement hydration will dissolve the aluminum. By substituting 55% of the cement in traditional cement paste with fast reactive supplementary cementitious material (SCM), aluminum will be compatible over time. In the initial state however, before the SCM consumes the hydroxide formed during the rapid cement hydration by the pozzolanic reaction, aluminum may corrode. Hydrogen gas then develops, resulting in a porous cement region enclosing the rebars with potentially reduced bond strength. In the present work, the chemical stability of a sand-cast aluminum lattice embedded in a paste where cement is replaced by 55% calcined kaolinitic clay is investigated by gas chromatography and open-circuit potential during the cement hydration. The hydrogen gas development stagnated for all measurements, indicating that aluminum is compatible with the novel cement paste. Two stable potentials were observed for the non-heat treated samples, indicating the formation of a metastable complex. Being able to use aluminum-reinforced concrete constructions would result in an extraordinary long service life with low cement consumption, which will potentially result in a substantial reduction in the third-largest CO2 emitting industry.

Compatibility of Different Aluminium with a New Environmental-Friendly Concrete

“…Both aluminium as extruded and after aging heat treatment are tested and compared. The hydrogen gas generation curves clearly show that heat treatment can significantly impede the between aluminium and cement, which agrees with the observation in [6]. The hydrogen gas generation for as-extruded aluminium can be three times as high as for aluminium after aging on average.…”

“…For as-extruded aluminium, the most severe reaction is seen from AA5052, and lowest is from Al-9Si-2Cu. It was discussed in the study with the Open-Circuit Potential (OCP) measurement results that silicate complex may form an aluminium surface, which blocks the further reaction [6]. It can explain the better performance for casting aluminium candidates with higher Si content.…”

“…After the submersion in 6% NaCl solution for 1 year, no sign of corrosion was shown, and a dense binder-zone formed near the aluminium surface was found to stop further hydrogen gas evolution after an initial very small one and account for a longterm stability [5]. In [6], Runningen et al examined the chemical stability of a cast aluminium alloy in the DARE2C concrete via measuring hydrogen gas production. Heat treated aluminium shows a prolonged and reduced gas development in the concrete.…”