Binding and setting of kaolin based materials with natural organic acids

In the present study, the correlation between adsorption experiments and the setting process of “geomimetic” materials prepared with humic-like solutions (HS) and a kaolin were investigated. Two HS were compared: a humic acid solution (HA, pH = 13) and a fulvic acid solution (FA, pH = 3). The adsorption experiments revealed a higher capacity of adsorption of the kaolin towards FA than HA, which could be attributed to the charge effects on the clay surface and to the organic molecule conformation depending on the pH. Materials elaborated, with high content of HA was consolidated after 4 days of curing whereas no setting was observed for materials prepared in the same conditions with FA. The hypothesis of inter/intra-molecular H-bonding between carboxylate groups of HA molecules, taken in evidence by FTIR analyses, and carboxylic or hydroxyl groups, was proposed to explain the material setting. Indeed, the characterization of HA revealed a highly aromatic structure favorable to mesomeric effects which led to the apparition of negative and/or positive charges and thus increased the reactivity of HA molecules. The structure of FA rather aliphatic was not favorable to mesomeric effect. Moreover, goethite was “mechanically” added in the kaolin powder because this iron phase was identified to be favorable to material setting. In this study, the addition of iron hydroxides, as powder, did not improve the material setting.