Study On Measures For Modification The Properties Of Magnesium Oxysulfate Cement

Magnesium oxysulfate cement has characteristics of good volume stability, light weight, fire-proofing, low alkalinity, good decorative effect, suppressing frost and well rebar-protection performance, which make it has more advantages on producing decoration materials. However, large-scale applications of magnesium oxysulfate cement in civil engineering have been restricted by its defects such as low strength, long setting time and low water resistance.In this study, different mineral addition and additive agents were used to investigate their influence on hydration, construction, strength and durability properties of magnesium oxysulfate cement. The chemical efficiency of mineral addition was improved by adding alkaline compound and magnesium oxysulfate cement with better components and properties was successfully prepared. The main results could be summarized as follows:?1? The molar ration of MgO/MgSO₄ and water consumption significantly effected the properties of magnesium oxysulfate cement. The fluidity of the cement paste increased, the setteing time prolonged and the compressive strength of the cement firstly increased and then decreased with the molar ration of MgO/MgSO₄ and water consumption incresed. The proper molar ratio of MgO :MgSO₄ :H₂O was 8:1:20.?2? The strength of magnesium oxysulfate cement at all the stage decreased with addition of mineral addition. Slag, cement clinker and silica fume had negative influence on water resistance of magnesium oxysulfate cement, while limestone powder had no influence on water resistance. With 2% naphthalene-based admixture the water mixed in cement paste was reduced by 12%, the compressive strength of the cement was increased to 40% in 60 d and water resistance of cement paste was also improved. The usage of sodium dihydrogen phosphate, sodium citrate and citric acid could retard the setting time of magnesium oxysulfate cement and significantly improve strength and water resistance in harden stage as well. With addition of water glass the strength was slightly increased and softening coefficient was decreased. Adding 0.5% EDTA the softening coefficient could increase to 1.01 in 28 d, although the strength of cement was lightly improved. The setting of magnesium oxysulfate cement was effectively accelerated with 4% concentration of Ca Cl2, the setting time could be decreased by 31.6% for the initial setting, and 25.9% for the finial setting.?3? The strength remained and the softening coefficient could increase to over 0.90 in 28 d by adding mineral addition and sodium dihydrogen phosphate or sodium citrate. Meanwhile, the cement setting time was shortened with the adding of clinker.Adding 0.5% EDTA and 1% citric acid, magnesium oxysulfate cement with 58.8 MPa compressive strength, 12.8MPa tensile strength and 0.99 softening coefficient was prepared.?4? Unmodified magnesium oxysulfate cement was mainly composed of Mg?OH?₂?MgCO₃?MgSO₄·nH₂O and large amount of unhydrated MgO particles. Magnesium oxysulfate cement modified with citric acid, EDTA compound with citric acid contains a large number of 5·1·7 phase. While, besides materials above, CSH gel was found in magnesium oxysulfate cement, which modified with mineral addition and sodium dihydrogen phosphate or sodium citrate.