Formula studied for a type of grout capable of ‘self-repairing’ cracks in large buildings

Grout is a mixture commonly used to fill joints between tiles in small domestic projects, and usually features a simple composition of water, cement and sand. However, this is not its only application: in structures that are designed to support heavy loads, such as bridges, grout is injected as a filler in post-tensioning systems, where the cables running through the conduits inside them provide a flexibility that the concrete covering them does not have.

This is very costly and complex work that requires not only careful planning and execution, but also materials specifically designed for this function, and maintenance to avoid the risks related to their deterioration.

In this type of mega-construction, the development of a smart material capable of “self-repairing” if it finds a crack might seem like something from a science fiction plot, but it is closer to reality than one might suspect. The secret lies in the incorporation of crystalline admixtures, or additives, which, by reacting with water, generate crystals capable of sealing the crack.

Their use has already been explored in concretes and mortars, and now a team from the University of Cordoba, as part of the SMARTINCS project, is studying their addition to cement-based grouts for application in post-tensioning systems.

However, incorporating these crystalline additives into the grout mix is not as simple as it may seem. As with a cooking recipe, when manufacturing building materials it is important to address the whole process right from the designing of the materials. Neither the ingredients used nor the proportions between them are chosen at random, and the mixture must be checked for its suitability when both freshly made and afterwards, as these are materials that will be in service for decades.

When designing the composition of these materials, it is also necessary to take into account how their elements react to the addition of new ingredients, and to keep them from, in any case, altering the characteristics of the final mixture.

This was the challenge taken up by researcher Suelen da Rocha, from the FQM 175 group, who, together with Mercedes Sánchez Moreno and Luis Sánchez, colleagues in the Department of Inorganic Chemistry and Chemical Engineering at the UCO, and Isabel Santacruz, from the University of Málaga, has published a study in Materials and Structures in which she discusses the design of a grout that incorporates crystalline additives.

Da Rocha explains that the addition alters the fluidity and exudation requirements that a grout should have, which makes it necessary to reformulate the composition by altering the rest of the elements until striking the ideal balance between the proportions of each one. In addition, the process requires knowledge of how these properties evolve in their transition from fresh to hardened material.

Thus, through a series of tests, this team has come up with a successful formula for a “self-repairing” grout that may be especially useful for use in large works, such as bridges and dams, where contact with water and its absorption by cement-based materials can pose risks that would be minimized by the chemical reaction produced by the crystalline additives. The next step, Da Rocha explains, is to study its response to corrosion, another cause of infrastructure deterioration.