The hydroxide formed during cement hydration in traditional cement paste leads to an alkaline pore solution, which passivates the steel reinforcement. The solution will, however, neutralize with time, resulting in rebar corrosion. Aluminum is passive in the neutralized environment, but the hydroxide formed during the hydration will dissolve the aluminum in traditional cement paste.

By substituting 51% of the cement in traditional cement paste with supplementary cementitious material (SCM), aluminum will be compatible, but in the initial state, before the SCM consumes the hydroxide from the cement hydration, aluminum will corrode. Hydrogen gas then develops, resulting in a porous cement region enclosing the aluminum reinforcement with reduced bond strength.

In the present work, three ram extruded aluminum profiles of alloy 6082, Al-9Si-2Cu, and Al-9Si-0.3Mg are investigated as potential reinforcement in concrete construction using gas chromatography for measuring the chemical stability during the cement hydration. The cement paste used are two different mixtures with a dry content of 51% SCM, 41% cement, 5% gypsum, and 3% limestone powder by weight, where calcined kaolinitic clay from Portugal and alkali-reduced bauxite residue (AR-BR) from the Bayer process have been used as SCM.

From the gas chromatography measurements, the hydrogen gas development was significantly reduced for all heat-treated alloys to T6 condition compared to T1 and T5. Cement pastes with AR-BR as SCM lead to less hydrogen gas development than calcined clay. The Al-9Si-2Cu alloy, imitating a recycled engine block, was the most chemically stable alloy.

The results are promising for utilizing secondary Al-9Si-2Cu alloy as reinforcement in a cement paste with 51% AR-BR. When used in concrete constructions, it will result in extraordinary long service life, as aluminum will be passive in the exposed environment over time, unlike steel. With low cement consumption and recycled aluminum rebars, this will potentially result in a substantial reduction in the third-largest CO₂ emitting industry.