Abstract
Abstract The formation mechanism of the hydrate phases in magnesium oxychloride cement (MOC) pastes is discussed. The major reaction products of MOC pastes (MgO-MgCl 2 -H 2 O system) has long been revealed to be four crystalline phases: 5Mg(OH) 2 · MgCl 2 · 8H 2 O (5 phase); 3Mg(OH) 2 · MgCl 2 · 8H 2 O (3 phase); 2Mg(OH) 2 · MgCl 2 · 4H 2 O (2 phase); and 9Mg(OH) 2 · MgCl 2 · 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 formation of these phases occurs neither through Mg(OH) 2 and MgCl 2 , nor through simple ions Mg 2+ , Cl − and OH − , etc. It was suggested that the formation of these phases probably occurs through the polynuclear complexes [Mg x (OH) y (H 2 O) z ] 2 x−y and Cl − , OH − ions. The hydrolyzing experiments for the system Mg 2+ -OH − -H 2 O show that the hydrolysis of Mg 2+ in MgCl 2 solutions with a certain concentration can yield [Mg x (OH) y (H 2 O) z ] 2 x−y . In the system MgO-MgCl 2 -H 2 O, the action of MgO is to increase the concentrations of Mg 2+ and OH − ions that induce and promote the hydrolysis of Mg 2+ . It can be concluded that the formation of the hydrate phases in MOC pastes involves the neutralization and dissolution of MgO powder; the hydrolyzing-bridging of Mg 2+ ions in MgCl 2 solutions to form a number of polynuclear complexes [Mg x (OH) y -(H 2 O) z ] 2 x−y with uncertain composition; and the conversion of the amorphous hydrogel consisting of [Mg x (OH) y (H 2 O) z ] 2 x−y complexes, Cl− and OH− ions and H 2 O to the crystalline hydrate phases.
Published Version
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