Effect of Fly Ash and Metakaolin on Properties and Microstructure of Magnesium Oxysulfate Cement

Abstract

To improve the mechanical performance and lower the production cost of magnesium oxysulfate cement (MOSC), this article investigates the effects of single and compounded addition of metakaolin (MK) and/or fly ash (FA) on the setting time, mechanical strength, water resistance, hydration product, composition, and microstructure of the resulting cement. MOSC samples with different proportions, ranging from 0 to 30 wt.%, of FA and/or MK substituting magnesium oxide (MgO) were prepared. The microstructure was explored by scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry. The findings suggest that adding FA can delay the setting of MOSC; however, the effect of adding MK to MOSC was reversed. Furthermore, the phase composition of the MOSC hydration products was unaltered upon adding FA and/or MK, but thicker and longer 517 phase crystals were observed. FA and MK can effectively fill the large pores of MOSC through filling and nucleation effects, reduce the pore size, and form a denser microstructure, thereby improving its mechanical properties. The optimal MOSC sample was found by substituting 10 wt.% of both FA and MK, resulting in a cement that exhibited a short setting time and an incredibly high mechanical strength and density. These findings will further the development of stronger, more cost-efficient, and more water-resistant MOSC products.

Green Concrete: An Eco-Friendly Alternative to the OPC Concrete

“…Increased [46,47] Improved [63] Setting time increased [63,64] Reduced [60,61] Silica Fume Increased at lower replacement but decreased at higher replacement [50] Improved when lesser percentage is added [59] Prolonged the setting time [66] Reduced [59] GGBFS Increased [52,53] Decreased [58] Setting time delayed [56] Higher content of GGBFS increased the segregation and bleeding [58] journal.ump.edu.my/construction ◄ Decreased with the increment of metakaolin in concrete [67] Workability of concrete reduced [64,65] Setting time reduced [64,67] Reduced [67] Agriculture waste (Rice husk ash, sawdust ash etc) Decreased [48,54] Workability Reduced [54] Increased [69] Reduced [69] Domestic waste (glass, plastic)…”

Study of Using Quartz Powder as a Mineral Admixture to Produce Magnesium Oxysulfate Cement

“…In contrast, the spherical periclase particles uniformly distributed in the MQ-15 matrix, which provided more spaces for the growth of the hydration products (dilution and dispersion effects of filler Q). As a result, the short-rod-like phase 5-1-7 whisker formed between the periclase particles [53,54]. The addition of filler Q therefore could promote the generation of phase 3-1-8 and phase 5-1-7 and enhance the early mechanical capacities of MOS paste.…”