Resumen de Intercalation mechanism of polycarboxylate-based superplasticizers into montmorillonite clays
The presence of clays in the sands used for concrete production interferes with the development of the fluidity of concrete, producing an instantaneous slump loss just after batching and the premature loss of fluidity. This interference occurs with all types of additives and clays but is especially problematic when combining sands containing expansive clays such as montmorillonites with new-generation high water-reducer/superplasticizer admixtures based on polycarboxylate polymers (PCE).
Water-reducers based on PCE polymers offer much better performance than traditional superplasticizers based on sulfonated naphthalene polymers (BNS) and sulfonated melamine polymers (MNS), making great advances in concrete technology, both from the technical and economical point of view as in reducing the environmental impact associated to concrete. However, these advantages are inhibited when sands contain clays of the expansive type in their composition.
All the preventive or corrective measures to mitigate the harmful effect of clays contained in sands result in increases of production costs and in greater environmental impact. For this reason, during the last years it has been tried to develop polymeric structures that offer the same benefits than polymers PCE but with improved tolerance against clays of the expansive type, such as montmorillonite clays, but without getting to reach solutions with guarantees of success, due to the complexity of the interaction process between PCE polymers and montmorillonite clays.
This doctoral thesis aims to deepen knowledge about the mechanism of interaction between PCE polymers and montmorillonite clays, assuming that the deep understanding of the interaction mechanism is the essential previous step to finally develop high-performance, clay-insensitive superplasticizers for concrete. For this, the research is structured in three parts, motivated by the discrepancies that the current model of interaction proposed shows with the experimental results of sorption and fluidity loss.
In the first part, it is intended to develop a test method that allows to observe the real expansion profiles of the clays in such a way that the mentioned discrepancies can be clarified.
Secondly, with the proposed test method, it is intended to identify how the structure of PCE polymers, as well as the dosage used, influences on the expansion of montmorillonite clays produced by the absorption of polymer. And, thirdly, to identify how the properties of clay affect the interaction process.
The first phase of the research campaign has made it possible to propose an improved test method for the d-spacing determination that revealed the real intercalation behavior, by which the number of PCE side chains intercalated into the interlaminar space of montmorillonite clays is up to ten times higher than that deducted from the traditional analytical method. And from the new test method proposed it has been possible to clarify the role of the different structures of PCE polymers and the properties of montmorillonite clays in the intercalation mechanism, in agreement with the experimental results of fluidity loss and of sorption behavior.
Based on the mentioned achievements, an extended model for the intercalation mechanism has been proposed, whereby montmorillonite clays inhibit the dispersing capacity of PCE polymers, being able to identify the parts and properties of both the clay and the PCE polymers that control this process.
With this contribution, the knowledge of the intercalation mechanism is extended to understand how the interaction between PCE polymers and montmorillonite clays is developed, which is the main objective of this research.
