Effect Of Magnesia On Expansion In High-magnesium Clinker

The expansion of periclase originated form clinker is one of important factors causing unstability of cement pastes.A strictly limitation on the content of MgO was applied in national standards and specifications.On the other hand,the expansion of periclase hydration can compensate shrinkage due to temperature stress,and decrease cracks of mass concrete,effectively.Meanwhile,the scarcity of high quality limestone resources will be alleviated by using high magnesium limestone to production of clinker.In response to these resource and environment problems in cement industry,combining the research results of cement mixed with MgO,the influence of MgO on the expansion of cement was investigated,in order to provide the theoretical basis for the efficient utilization of high magnesium limestone.Pastes and mortars were prepared with different MgO content clinker.Samples were cured in different temperatue water（20 ℃,50 ℃ and 80 ℃）,in different temperatue air（20 ℃,50 ℃ and 80 ℃）and in different natural region（Shenzhen,Xi’an and Wulumuqi）.Hydration and hydrates of cement prepared by different MgO clinker were characterized by means of X-ray diffraction（XRD）,derivative thermogravimetry analysis（DTA）,scanning election microscope（SEM）.Shrinkage of mortars cured in different circumstances wre measured.The effect of MgO on the content of periclase and other mineral,the influence of SO₃ on the content of periclase and impact of SO₃,H3BO3 on belite polymorph was analysized.The study results showed that:（1）Shrinkage of mortars prepared with higher content of MgO content have lower shrinkage for curing in given temperature air,and shrinkage will be increased when temperature is raised.Shrinkage of mortars prepared with higher content of MgO content have higher expansion for curing in given temperature water,and the expansion will be increased when temperature is raised.（2）the higher content of magnesium in clinker,the more periclase in clinker and the harder clinker is to grind.（3）The dehydration reaction of magnesium hydroxide take place at 340 ℃ to 410 ℃,and the dehydrating valley of Mg（OH）2 appeared at about 360 ℃.The higher the temperature,the faster the hydration rate of magnesium oxide,the more magnesium hydroxide produced.（4）The magnesium-containing aluminosilicates is found at about 5 μm from unhydrated periclase in pastes cured in 80 ℃ water for 7 days.After 130 d,periclase from clinker almost completely hydrate,and Mg（OH）2 and C（M）-S-H coexist in the magnesium cement.（5）The incorporation of SO₃ can effectively reduce the periclase content in high magnesium clinker.The incorporation of H3BO3 can stabilize β-C2 S,and Ca7MgSi4O16 is formed.MgO is decompounded from Ca7MgSi4O16 with the incorporation of SO₃ and periclase formed.（6）When β-C2 S is simultaneously stabilized by SO₃ and H3BO3,the optimal proportion of SO₃ and H3BO3 is that SO₃ content is 4% and H3BO3 content is 0.5%.