Title: Effects of Material Ratio, Fly Ash, and Citric Acid on Magnesium Oxysulfate Cement

Author(s): Chengyou Wu, Hongfa Yu, Jinmei Dong, and Lina Zheng

Publication: Materials Journal

Volume: 111

Issue: 3

Appears on pages(s): 291-298

Keywords: citric acid; compressive strength; fly ash; magnesium oxysulfate cement; material ratio

DOI: 10.14359/51686723

Date: 5/1/2014

Abstract:
The effects of material ratio, fly ash, and citric acid on the compressive strength and phase composition of magnesium oxysulfate (MOS) cement have been studied. From these results, when the molar ratio of active MgO/MgSO4 increases from 3 to 5, the compressive strength of magnesium oxysulfate cement increases. When the molar ratio of active MgO/MgSO4 is 3, the main crystal phase contributing to the mechanical strength is 3Mg(OH)2•MgSO4•8H2O; when the molar ratio increases to 5, the main crystal phase is a new magnesium subsulfate that can be expressed as yMg(OH)2•MgSO4•zH2O (Y phase). The compressive strength of MOS cement by adding fly ash decreases from 56.89 to 33.75 MPa (8.251 to 4.895 ksi) by 39.29% when the dosage of fly ash increases to 40%, because SiO2 in fly ash can form magnesium silicate hydrate gel, which can adsorb onto the surface of the Y phase nucleus and inhibit its growth. In the presence of 0.5% weight citric acid (CA), the compressive strength of MOS cement increases, because CA may facilitate the formation of yMg(OH)2•MgSO4•zH2O (Y phase).