Effect of Active Magnesia on the Properties of Cementitious Binders

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Effect of Active Magnesia on the Properties of Cementitious Binders Jing Zhu, Nan Ye, Jianwen Liu, Yalin Li, Jiakuan Yang School of Environmental Science and Engineering, Huazhong University of Science & Technology, Wuhan 430074, China ABSTRACT  

Reactive magnesium oxide (magnesia, MgO) was produced by calcining magnesite at comparatively low temperature, less than 800 oC. The reactive MgO and fly ash were used as additives to cementitious binder. The reactive MgO-ordinary Portland cement-fly ash is referred to as MgO-OPC-FA cement in further. The hydration expansion effect of active magnesia on the properties of cementitious binder in different mixing ratio was investigated. It is known that the “dead burnt” MgO reacts with water very slowly, which causes the expansion after the solidification of cement. Therefore, the MgO content in ordinary cement is commonly restricted to less than 5%. Effects of reactive MgO on the expansion properties of the cementitious binders were studied. Hydrated products of reactive MgO cements were investigated by X-ray diffraction (XRD) and Scanning electron microscope (SEM) analysis. The MgO-OPC-FA cement was sound, although the content of reactive MgO in cement was about 8 wt. %. Reactive MgO was hydrated at early age in 24 hours, thus causing rapid expansion. Mg(OH)2 appeared on initial stage of cement hydration for active magnesia. The hydration rate of active magnesia was not equal to that of the dead burnt magnesia. The hydration of reactive MgO has a negative effect on the mechanical properties of reactive MgO-ordinary Portland cement-fly ash system, in spite of the inhibitive effect of the expansion of MgO hydration produced by fly ash. Our results shed light on the potential utilization of reactive MgO in the manufacturing of cementitious binders.   Keywords: MgO, additive, XRD, SEM, microstructure

INTRODUCTION  

Much previous work focused on preventing the volumetric deformation of cement-based materials and many corresponding strategies have been developed. The application of active magnesia in cementitious binders is a promising option for cement manufacturing [ 1 ]. The slow hydration reaction and cracking of dead burnt magnesia (heated to 1450°C) is known as a particular problem as it is believed to give rise to unsoundness in Portland cement. Chatterji [ 2 ] propose the expansion mechanism due to the dead burnt MgO based on the theory of crystal growth pressure, and the solubility of hydroxide crystals, their growth habits and diffusion Mg2+ through the electrical double layer which forms round cement hydration products. Reactive magnesium oxide cements are blends of Portland cement, reactive magnesium oxide produced by calcining magnesite at lower temperature (