Electrochemical lithium migration to mitigate alkali-silica reaction in existing concrete structures

Abstract

Alkali-silica reaction (ASR) is a deterioration process that affects the durability of concrete structures worldwide. During the reaction, hydroxyl and alkali ions present in the pore solution react with reactive silica from the aggregate, forming a hygroscopic ASR gel. Alternatively, the silica structure is attacked by hydroxyl ions and the degraded structure acts as a hygroscopic silica gel. Either way, the gel (or gel-like structure) swells, which may cause expansion and cracking of the concrete element. It is a slow reaction – it usually takes from 5 to 15 years for its symptoms to be noted. Therefore, even though preventive measures have been known for decades, it is still possible to have newly diagnosedcases.
Methods to prevent ASR development in new structures have been successfully applied for decades. Nevertheless, when ASR is detected in existing structures, the currently available intervention options to stop further ASR expansion are far more limited. Current procedures, such as limiting the access of external water by the application of sealants, have limitations and their effectiveness has shown to be variable. In this framework, the development of effective intervention methods for ASR in existing concrete structures is still necessary and the application of lithium compounds has been proposed as a possiblemitigation method.
For decades, the addition of lithium based admixtures has been known to successfully reduce or prevent deleterious ASR expansion in new concrete structures. Numerous works have reported the beneficial influence of various lithium compounds (such as LiNO3, LiOH and others) on the expansive behavior of concretes and mortars composed with different types of reactive aggregates. In new concrete structures, lithium compounds can be incorporated to the fresh mixture. In the case of existing structures, however, such incorporation is no longer possible and lithium ions need to be transported into the concrete.
Research has been done on potential lithium-based treatment methods, such as topical application and vacuum impregnation, and results have shown that, in those cases, the penetration depth was limited. Some experimental studies compared different techniques such as immersion, vacuum impregnation, wet-dry cycles and electrochemical migration, and the latter has shown to be the most suitable to drive lithium ions into concrete, providing deeper penetration and higher concentration.
Electrochemical lithium migration is a technique that uses an electric field to transport lithium into concrete. Under the action of the electric field, positive ions migrate towards the negative electrode (cathode) and negative ions move in the opposite direction. The same principle is applied in treatments such as electrochemical chloride extraction (ECE). The use of electrochemical lithiummigration as a treatment against ASR has been investigated by several authors. However, there is no agreement on its effectiveness against ASR expansion.
This thesis aims to contribute with further insights on the feasibility of lithium migration as a treatment against ASR. For that, the basics of lithium migration oriented towards application and its possible effects on ASR affected concrete were investigated. The influence of different parameters during lithium migration was evaluated with standard mortar specimens in two-chamber migration set-ups (with external electrodes).
The influence of the type of lithium compound and concentration used in the anolyte solution was studied. Results pointed out that the concentration of the solution, rather than the type of lithium salt, played a role in the experiments. The anolytes with the highest concentrations provided the highest levels of lithium in the specimens. The effect of the duration of the experiments was also investigated. Interestingly, under the investigated conditions, longer testing periods did not lead to much higher lithium transport.
During electrochemical treatments such as ECE, the reinforcement is often used as cathode. Therefore, the use of an embedded cathode during lithium migration was also evaluated. After testing, it was observed that this type of configuration led to the accumulation of alkalis in the region close to the embedded cathode and not many lithium ions were able to reach that area. If a ASR affected concrete structure would be treated with this configuration, the accumulation could lead to further ASR development (as long as there is still reactive silica, moisture and calcium ions). The alkali accumulation was not observed when the cathode was placed externally to the specimens. In those cases, sodiumand potassium were removed from the pore solution.
Numerical models are useful tools which may provide further understanding of the mechanisms during an experiment. In this thesis, a mathematical model for multi-ionic transport in concrete under the effect of an electric field was presented. The model was numerically implemented for some of the migration experiments with mortar specimens. When comparing to the experimental results, the model predicted well sodium and potassium concentrations in the catholyte as well the final average lithium concentration in the specimens. The model indicated that the removal of all sodium and potassium ions from the pore solution was necessary before lithium ions could reach the catholyte chamber.
The influence of different expansion levels (and possibly different cracking levels) on lithium migration was investigated. ASR-reactive specimens were cast and placed under ASR accelerating conditions (60°C and R.H. 100%) for three or ten weeks prior to migration testing. In this work, cracks were not visually observed on the surface of the specimens, regardless of the expansion level. In fact, the resistivity of the specimens increased with the pre-testing time. The increase in resistivity led to a decrease in passing currents. Therefore, in this work, the specimens with the highest initial expansion had the lowest final lithiumconcentrations.
Finally, the effects of different treatments on ASR expansion were analyzed. For that, ASR-reactive specimens were placed under ASR accelerating conditions prior to testing. In two-chamber set-ups, the specimens were submitted to migration treatments with LiOH or Ca(OH)2 solutions as anolyte, or to diffusion with the same solutions. After treatment, the specimens were placed again in the ASR reactor and their expansions were monitored. Overall, the lithiummigration treatment led to lower expansion values than the other treatments.