The formation mechanism of the hydrate phases in magnesium oxychloride cement

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 2O system) has long been revealed to be four crystalline phases: 5Mg(OH) 2 · MgCl 2 · 8H 2O (5 phase); 3Mg(OH) 2 · MgCl 2 · 8H 2O (3 phase); 2Mg(OH) 2 · MgCl 2 · 4H 2O (2 phase); and 9Mg(OH) 2 · MgCl 2 · 5H 2O (9 phase). All of the phases are the basic salts whose formal formula can be written as: Mg x (OH) y · Cl · nH 2O. 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 2O) z ] 2 x−y and Cl −, OH − ions. The hydrolyzing experiments for the system Mg 2+-OH −-H 2O show that the hydrolysis of Mg 2+ in MgCl 2 solutions with a certain concentration can yield [Mg x (OH) y (H 2O) z ] 2 x−y . In the system MgO-MgCl 2-H 2O, 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 2O) z ] 2 x−y with uncertain composition; and the conversion of the amorphous hydrogel consisting of [Mg x (OH) y (H 2O) z ] 2 x−y complexes, Cl− and OH− ions and H 2O to the crystalline hydrate phases.