The reaction of a Wyoming-type bentonite with pH 13.5 solutions was investigated experimentally at 35 and 60°C for periods of 1 to 730 days. Some crystal properties of the starting montmorillonitic clay remain unchanged, i.e. stability of the octahedral sheet, total cation exchange capacity (CEC) and CEC after neutralization of the octahedral charge, full expandability in the Ca-saturated state, and size distribution. Other properties are changed, e.g. there is an increase in the expandability after octahedral charge neutralization; a slight increase in the average layer charge; a decrease of the total surface area; and a particle morphological change from flakes to hexagonal shape.

The composition and the structure of the smectite layers did not change significantly during the reaction. The increasing number of expandable layers after octahedral charge neutralization is attributed to modifications in the stacking sequence. The number of interlayers surrounded by two charged tetrahedral sheets increases with reaction time.

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