| The magnesium oxysulfate cement is a new type of ecological non-hydraulic cementitious material.However,its application scope is limited greatly due to the poor mechanical properties and durability for complex composition system,insufficient hydration and instability of hydration products.In this study,the strength and structure of magnesium oxysulfate cement with chemical additives were studied by the method of X-ray diffraction,scanning electron microscope and mercury injection experiment.Therefore,the kinetic rules of microstructural volution in the hardening process of magnesium oxysulfate cement and the effect of additive on the kinetic rules are illustrated.And the reinforcement mechanism of additive on the magnesium oxysulfate cement is revealed.The mechanism of corrosion is put forward by studying the changes of hydration products and microstructure of magnesium oxysulfate cement after immersing in water,acid solution and sulfate solution.The toughness reinforcement of magnesium oxysulfate cement is realized through addition of polymer and glass fiber.The chemical additive of citric acid has chelation action on Mg2+ ion of magnesium oxysulfate cement paste.This chelation action restrained the formation of Mg(OH)2 phase and the ion of Mg2+,OH-,and SO42-in the system avail to crystallized to 5Mg(OH)2·MgSO4·7H2O phase.The strength of magnesium oxysulfate cement is reinforced by the structure of 5Mg(OH)2·MgSO4·7H2O phase.Compressive strength of magnesium oxysulfate cement is mainly depend on phase composition and pore structure.The early strength derived from generation and development of phase composition,later strength resulted from the decrease of porosity and finer pore radius.The poor water resistance of magnesium oxysulfate cement without chemical additive is due to the unreacted MgO hydrating to Mg(OH)2 phase upon immersion in water that cause cracking for expantion reactivity.The citric acid helps the formation of 5Mg(OH)2 ·MgSO4·7H2O and gives magnesium oxysulfate cement a perfect structure.Due to these two effects,the magnesium oxysulfate cement with citric acid has higher water resistance than that without citric acid.However,water resistance of magnesium oxysulfate cement with citric acid is influenced by molar ratio of MgO/MgSO4 and H2O/MgSO4 also.Higher molar ratio of MgO/MgSO4 and lower molar ratio of H2O/MgSO4 can improve the water resistance of magnesium oxysulfate cement.Fly ash can function as filler for pores in the paste,thus decreasing the porosity and enhancing the water resistance of the magnesium oxysulfate cement modified by citric acid further.The acid corrosion resistance and sulfate corrosion resistance of magnesium oxysulfate cement mixed with slag and fly ash are increased significantly.Magnesium oxysulfate cement with slag has stronger corrosion resistance than that with fly ash.The mechanism is that fine particles of slag and fly ash filled the voids in magnesium oxysulfate cement which restrained the solution penetrating into the harden cemeng and increased corrosion resistance.The results show that incorporation of polymer decreased the viscosity of magnesium sulfate cement and showing a good mobility.Meanwhile the polymer also improved the toughness,shrinkage,corrosion resistance and adhesion strength of cement.The polymer formed thin film net in the cement system and interlaced with cement hydration products constituting entirety network structure.The structure strengthened the cohesion strength and improved the toughness and shrinkage of hardened cement.These limited the inner crack spread and prevented the harmful substance invade that the strength and anti-corrosion properties were improved.The proper molar ratio of MgO/MgSO4/H2 O in magnesium oxysulfate cement reinforced with glass fiber,optimal glass fiber paving pattern and reasonable fiber fraction are determined by experiments of mechanical properties.The numerical analytical model of magnesium oxysulfate cement panel reinforced with glass fiber is developed by the finite element software.By numerical computing,the flexural strength of magnesium oxysulfate cement panel reinforced with glass fiber are obtained and compared with experimental results.The influences of fiber fraction and paving pattern on the mechanical properties are discussed. |
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