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

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.

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CO2 mineralization of demolished concrete wastes into a supplementary cementitious material – a new CCU approach for the cement industry

RILEM Technical Letters ◽

10.21809/rilemtechlett.2021.141 ◽

2021 ◽

Vol 6 ◽

pp. 53-60

Author(s):

Maciej Zajac ◽

Jan Skocek ◽

Jørgen Skibsted ◽

Mohsen Ben Haha

Keyword(s):

Silica Gel ◽

Cement Paste ◽

Cementitious Material ◽

Cement Industry ◽

Recycled Concrete ◽

Recycled Aggregates ◽

Supplementary Cementitious Material ◽

Co2 Mineralization ◽

Concrete Recycling ◽

Composite Cements

This contribution discusses the carbon capture and utilization (CCU) approach based on CO2 mineralization of cement paste from recycled concrete as new approach to capture CO2 and significantly contribute to the reduction in CO2 emissions associated with cement production. The current literature suggests that all CO2 released from the decomposition of limestone during clinker production can be sequestered by carbonation of the end-of-life cement paste. This carbonation can be achieved in a few hours at ambient temperature and pressure and with a relatively low CO2 concentration (< 10 %) in the gas. The carbonation of cement paste produces calcite and an amorphous alumina-silica gel, the latter being a pozzolanic material that can be utilized as a supplementary cementitious material. The pozzolanic reaction of the alumina-silica gel is very rapid as a result of its high specific surface and amorphous structure. Thus, composite cements containing carbonated cement paste are characterized by a rapid strength gain. The successful implementation of this CCU approach relies also on improved concrete recycling techniques and methods currently under development to separate out the cement paste fines and such. Full concrete recycling will further improve the circular utilization of cement and concrete by using recycled aggregates instead of natural deposits of aggregates. Although the feasibility of the process has already been demonstrated at the industrial scale, there are still several open questions related to optimum carbonation conditions and the performance of carbonated material in novel composite cements.

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