Effect Of MgO On The Sintering Of Portland Cement Clinker And Volume Stability Of The Consequent Paste

To alleviate the problems related to the exhaustion of high grade limestone resources(traditional raw material cement production), it is of great significance to use low grade limestone to partially or completely replace high grade limestone for the sustainable development of cement industry. However, the high content of magnesium oxide in low grade limestone leads to the high Mg O content in producted cement, which may reduce its volume stability in engineering application, signaficantly limiting the application of low grade limestone in cement industry. In the present studies,fly ash was normally admixed in high magnesia cement to improve its volume stability. Although this method is efficient, the utilization efficiency of fly ash is not high and the improvement mechanism remains unclear. In this thesis, Fly ash and blast furnace slag silica fume are classified accurately into three particle size ranges which are fine, middle zise and coarse frations, differnet range of which is mixed in high magnesia cement paste to improve its volume stability. Besides, it is illustrated that the particle size of supplementary cementitious materials is a big contributor for the improvement of volume stability of high magnesia cement paste and its micro mechanism is revealed.The main research in this study can be summarized as follows:According to the physical and chemical properties, four industrial raw materials: low grade limestone, high grade limestone, clay and iron tailings were utilized for the design of the mineral composition of clinkers based on three modulus values: KH, SM and IM. By choosing the appropriate sintering temperature and cooling type, high magnesia cement clinker with Mg O content from 4% to 12% was prepared. For the cement clinkers prepared in this study, when Mg O content was lower than 8%, the content of f-Ca O was lower(0.94%~1.29%), indicating a better burnability of raw material, the quality of clinkers was also better evidenced by the perfect and compact growth of silica minerals crystal. While when Mg O content was higher than 8%, the content of f-Ca O was higher(1.51%~1.81%) indicaing a worse burnability of raw material, the quality of clinkers was also worse evidenced by the bad growth of silicate minerals crystal.The alterations of periclase content in clinkers with different was discussed. When Mg O content was(8%~14%)high in clinker, more of Mg O existed in glass state and solid solution state, and the ratio converted to periclase was reducd(from 74.0% to 67.1%).The volume stability of high magnesia cement paste and its influential factors was studied in this thesis. From the angle of autoclave test, the volume stability of high magnesia cement was evaluated. When Mg O content was higher than 8%, the expansion ratio of the harden paste will be over 0.8% or destroyed directly.By contrast, the long-term expansion ratio of cement paste was tested and evaluated through the angle of accelerated curing. When Mg O content is higher than 8%, the long-term expansion was far less than the autoclave expansion ratio and there was high possibility that the cement paste’s volume stability may be eligible. It turned out that the latter is a more reasonable method.The influence of supplementary cementitious materials including fly ash, blast furnace slag and silica fume on the volume stability of high magnesia cement paste were investigated. Silica fume presented the strongest ability to inhibit the volume expansion, followed by blast furnace slag, and fly ash. The dosage and size of the supplementary cementitious materials were the main factors influencing the expansion of high magnesia cement paste.Furthermore, the mechanisms related to the improvement of volume stability of high magnesia cement by fly ash, blast furnace slag and silica fume was investigated. Physical dilution: the mixed supplementary cementitious materials equivalently replaced partial high magnesia cement, reducing the amount of the expansion source(periclase) in cement. The secondary hydration of supplementary cementitious materials: on one hand, hydration products produced by the secondary hydration constantly filled the pores of the hardened cement pastes and transfered large pore into harmless micro pore, which is beneficial for absorbing the expansion stress generated by the hydration of periclase. On the other hand, secondary hydration consumes the Ca(OH)2 in cement paste and weakens the hydration degree of periclase. Considering the ability of inhibiting periclase reaction degree, silica fume is the best, followed by blast furnace slag and fly ash.