Excess AlCl3 was reacted with Mg(OH)2 and Ca(OH)2 in suspensions of Wyoming bentonite and the nature of the reaction products formed and their effect on the cation-exchange capacity (CEC) of the clay was determined. Reaction of Mg(OH)2 and AlCl3 with the clay produced marked decreases of the CEC in the bentonite, whereas much smaller decreases were observed in the Ca–Al–clay preparations. The decreases in the CEC were attributed to the formation of mixed Mg–Al and Ca–Al hydroxide clay complexes. The greater stability of the mixed Mg–Al hydroxide complexes with the clay appeared to account for the marked reduction of CEC in these systems.

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