Effects of citrate acid, solidum silicate and their compound on the properties of magnesium oxysulfate cement under carbonation condition

Abstract

Additives including sodium silicate (SS) and citrate acid (CA) have been used to modify magnesium oxysulfate (MOS) cement. To improve the durability of modified magnesium oxysulfate (MMOS) cement and widen its application under carbonation environment, this paper reports experimental investigations into the mechanical properties, microstructural characteristics, and porous structure of MOS cement modified by CA, SS and their compound of CS by accelerated carbonization test. The properties were examined by mechanical strength tests, evaporable water test, X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetry (TG). Comparing to uncarbonated MOS and MMOS cements, 28 days of carbonation reduced the flexural strength by 20%-50%, and the compressive strength of cements modified by CA or CS was increased by 5%. Both CA and SS significantly improved the strength of carbonated or uncarbonated MOS cement, and CA required fewer time to achieve the mechanical strength similar to CS. Carbonation grated more MgCO3 and refined the porous structures of cement paste by reducing total pore and large pre. CA or CS at least reduced the brightness MOS cement paste by three times, whether they were carbonated or not. Microstructural analysis of samples collected in different carbonation depths further explained the modification mechanism from chemical composition perspectives. Finding of this study provide insights into the design of high-performance and sustainable MOS cement-based building materials under carbonation environment.