Resumen de Evaluación de mezclas proyectadas: comportamiento y durabilidad
Sprayed cementitious materials are widely used in the construction industry, from buildings to infrastructure. The process of construction consists of spraying a concrete mix that consolidates once it reaches the substrate. The sprayed mix must have an adequate consistency and setting time to enable the build-up of layers with proper thicknesses. To reduce the setting times and increase the early strength of the matrix, accelerators are commonly employed. These chemicals alter cement hydration, affecting the mechanical properties at short and long term.
Despite the recent advances on the field of sprayed cementitious materials, several topics still require further studies. For instance, the properties of the sprayed material depends on the cement-accelerator compatibility. Research and industrial development have focused on the improvement of accelerators, paying little attention to the improvement of cements. Moreover, few studies are found in the literature about the durability of sprayed cementitious materials with accelerator. One of the areas of concern is the external sulfate attack (ESA). The use of sulfate resistant cements might not be enough to guarantee the durability given the particular microstructure of the sprayed cementitious material and the additional content of aluminum provided by accelerators.
This study extends the knowledge in this field through the following objectives: evaluate the compatibility cement-accelerator considering the inclusion of addition gypsum and evaluate the durability of sprayed mixes exposed to ESA. The first part of the research focuses on the study of the compatibility between typical cements and accelerators applied in sprayed mixes from chemical and mechanical standpoints. The experimental campaign performed highlights the importance of a proper sulfate content in the positive evolution of the mechanical properties.
Continuing in this line, the second part of the research focuses on the influence of the gypsum content in the hydration and in the development of mechanical properties of sprayed mixtures. Results confirm that a proper addition of gypsum to the cement provides a valid and cheap extra source of sulfates for an optimum compatibility with the accelerators. Useful criteria are proposed for the design of cements and matrices specific for spraying.
The third part of the research addresses the vulnerability of sprayed mixes when exposed to an ESA. The experimental campaign performed evaluates how the cement type, the chemical composition of accelerators and the age of the initial exposition influence the behavior of sprayed mixes subjected to ESA. Based on the results obtained, the durability of the structures may be improved by selecting accelerators with smaller aluminate-to-sulfate ratios, preferably those that are alkali-free.
The fourth part of the research evaluates the behavior of sprayed mixtures with additional gypsum against the ESA. Moreover, the contamination potential of sprayed mixes with different compositions was evaluated through leaching tests. The experimental campaign performed reveals that the inclusion of gypsum in the mixtures improves the durability of the material by reducing the potential formation of secondary products in the ESA. Also, the results confirm that the additional gypsum has no negative effect in the leaching of elements.
