The formation mechanism of the hydrate phases in magnesium oxychloride cement

The formation mechanism of the hydrate phases in magnesium oxychloride ceme nt (MOC) pastes is discussed. The major reaction prod ucts of MOC pastes (M gO-MgCl2-H2O system) has long been revealed to be four crystalline phases: 5Mg(OH)(2) . MgCl2 . 8H(2)O (5 phase); 3Mg(OH)(2) . MgCl2 . 8H(2)O (3 phase ); 2Mg(OH)(2) . MgCl2 . 4H(2)O (2 phase); and 9Mg(OH)(2) . MgCl2 . 5H(2)O ( 9 phase). All of the phases are the basic salts whose formal formula can be written as: Mg-x(OH)(y) . Cl . nH(2)O. It can be considered that the forma tion of these phases occurs neither through Mg(OH)(2) and MgCl2, nor throug h simple ions Mg2+, Cl- and OH-, etc. It was suggested that the formation o f these phases probably occurs through the polynuclear complexes [Mg(OH)(y) (H2O)(z)](2x-y) and Cl-, OH- ions. The hydrolyzing experiments for the syst em Mg2+-OH--H2O show that the hydrolysis of Mg2+ in MgCl2 solutions with a certain concentration can yield [Mg(OH)(y)(H2O)(z)](2x-y). In the system Mg O-MgCl2-H2O, the action of MgO is to increase the concentrations of Mg2+ an d OH- ions that induce and promote the hydrolysis of Mg2+. It can be conclu ded that the formation of the hydrate phases in MOC pastes involves the neu tralization and dissolution of MgO powder; the hydrolyzing-bridging of Mg2 ions in MgCl2 solutions to form a number of polynuclear complexes [Mg-x(OH )(y)-(H2O)](2x-y) with uncertain composition; and the conversion of the amo rphous hydrogel consisting of [Mg-x(OH)(y)(H2O)(z)](2x-y) complexes, Cl- an d OH- ions and H2O to the crystalline hydrate phases. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.