Montmorillonite clay itself is a sheet-like structure (Fig. 1) composed of tetrahedra SiO4 and octrahedra AlO6 at a 2:1 ratio. Each of these sheets sustains a charge on their surface and edges. Clays are characterized by their ion exchange properties
Fig. 1 Idealized layered silicate structure that result in a high affinity to water, which makes the naturally incompatible for use in concrete in their unmodified form.
The advantage of the sheet-like configuration over other small particles is that the multi-layer silicate structure can be penetrated between layers by small molecules (see Fig. 2), forcing the silicate platelets apart. This process is called intercalation.
XXXXXXXXXXXX Gallery Expanded by Reactive Species
Fig. 2 Partial separation of silicate platelets (intercalation)
If the penetrating molecules reactive enough, this process may result in complete separation of the silicate layers (i.e. exfoliation), as shown schematically in Fig. 3.
As a result of exfoliation, a small mass of montmorillonite clay can result in numerous small, thin (e.g. 20-nm) platelets, with very large surface area, that are fully separated. These nano-size silicate platelets may e.g. be used directly as pozzolans in concrete or bonded with selected species of polymers which tend to bond to all of these surfaces, creating a linkage effect among the silicate platelets. This bonding can also be considered as a flocculated material since one polymer chain can link several clay particles together. The resulting composite can exhibit vastly improved properties.
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