Mechanical strength and water resistance of magnesium oxysulfate cement based lightweight materials

Abstract

One novel lightweight building material was successfully manufactured using magnesium oxysulfate (MOS) cement as binder and foam and/or rice husk as inert fillers in this paper. It behaves much higher specific strength and lower environmental load than normal concretes prepared by Portland cement, natural sand and aggregates. To improve the mechanical strength and water resistance of MOS matrix, three kinds of weak acids were incorporated at low levels. On the basis of the modified MOS matrix, foam, rice husk, and the mixture of foam and rice husk were added at variable dosages as fillers to prepare MOS cement based lightweight materials. Mechanical strength, bulk density and compressive strength/volume deformation of these materials were evaluated after variable wetting-drying cycles. At the same time, phase composition and microstructure of typical samples were studied using X-Ray Diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR), Opto-digital Microscope (OM) and Scanning Electron Microscope (SEM) tests. Experimental results indicated that the addition of 0.5% citric acid by MgO weight can greatly enhance compressive strength and water resistance due to the formation of finer needle-like crystals of 5 × 1 × 7 phase instead of 3 × 1 × 8 phase. The higher incorporation of foam and rice husk decreases bulk density and mechanical properties of MOS cement pastes. With a density of as low as 1000 kg/m3, the compressive strength of MOS cement based lightweight materials can reach higher than 12 MPa. The wetting-drying treatment leads to a negative effect on both compressive strength and volume stability of these materials, due to the transformation of unreacted MgO into Mg(OH)2 crystals upon water immersion. On the other hand, the coupling effect of water-swelling and drying-shrinkage induces the formation of micro cracks in specimens. A proper addition of foam bubbles and rice husk presents the better resistance to wetting-drying cycles. Therefore, the MOS cement based lightweight material could be widely applied to inner partition walls and structural component, and its long-term service behavior should be further assessed.