Modification Of Quarternary Phase By Mineral Admixture And The Corresponding Mechanisms

With the proposal of“maritime power”strategy,the development of Marine economy and the protection of maritime frontier have become the important development direction of China.In the harsh marine environment,Portland cement concrete is facing the problems of poor corrosion resistance and short service life,while calcium aluminate cement（CAC）exhibits good cementitious property,fast hardening and high early strength,good resistance against seawater erosion and abrasion,making it suitable for application in marine construction and repairing engineering.However,the traditional CAC is expensive and its metastable hydrates have the risk of phase transformation.The phase conversion mechanism of CAC metastable hydrates and the modification mechanism of CAC by mineral admixtures are not adequately investigated.Hence,it is difficult to effectively guide the preparation of high-performance aluminate cement based material（HAC）.In this paper,the differences in sintering and hydration properties between Q phase(Quarternay phase,C₂₀A₁₃M₃S₃)and CA were comprehensively studied.On this basis,Q phase was proposed to replace CA,the main cementing phase in the traditional CAC,as the cementitious component in the preparation of HAC.The phase conversion process of the synthetic CAH₁₀ and C₂AH₈ was studied to reveal the conversion mechanism of the metastable hydrates formed in CAC paste.The effects of metakaolin,silica fume and slag on the mechanical properties of Q phase matrix,the composition and distribution of hydration products and the microstructure evolution were comprehensively studied.Furthermore,these mineral admixtures were reacted with synthetic metastable hydrates to reveal the differences in their modification effects on the Q phase matrix and the corresponding mechanisms,to guide the preparation of HAC and provide the guidance for the efficient modification of traditional CAC.The main work and innovative achievements in this dissertation are as follows:（1）The difference in sintering and hydration properties between Q phase and CA were investigated.The conversion mechanisms of the two metastable hydrates(CAH₁₀ and C₂AH₈)were revealed.The results will provide theoretical support for the HAC preparation.It was found that the sintering temperature of Q phase was significantly lower than that of CA and the mechanical properties of Q phase were better than that of CA under normal curing.The hydration products of Q phase were CAH₁₀ and C₂AH₈,while that of CA was C₂AH₈,which was prone to phase conversion.The phase conversion process of the metastable hydrates formed in CAC paste were simulated by the synthetic CAH₁₀and C₂AH₈.It was found that the two metastable hydrates were directly transformed to C₃AH₆,but the conversion rate of CAH₁₀ was significantly lower than that of C₂AH₈.Based on the“solution-precipitation”mechanism of the metastable phase conversion,the ion product of C₃AH₆ was calculated by monitoring the changes of p H value and ion concentration during the conversion of metastable hydrates.It was proved that the high supersaturation coefficient of C₃AH₆ was the main reason for the fast conversion during the phase conversion of C₂AH₈.Besides,the supersaturation coefficient of C₃AH₆ was the main factor controlling the nucleation and growth process of C₃AH₆.（2）The influence of metakaolin,silica fume,and slag on the mechanical properties,composition and distribution of hydration products,and microstructure evolution of Q phase paste were comprehensively studied.The influence of metakaolin on the mechanical properties of Q phase paste and the composition of hydration products was mainly studied.It was found that the modification of the paste by metakaolin was closely related to its dosage and the curing temperature.When cured at 40℃water bath,the dissolved amorphous aluminum from metakaolin can stabilize CAH₁₀ and significantly inhibit the formation of C₃AH₆.Meanwhile,the dissolved aluminum inhibits the dissolution of silicon.Therefore,the formation of C₂ASH₈ is inhibited with the increase of metakaolin content.However,increasing the curing temperature accelerates the dissolution of silicon,thus promoting C₂ASH₈ formation.When cured at 50℃for 120d,the strength of the blended Q phase paste with 15%metakaolin still increases stably under the synergistic effect of amorphous silicon and aluminum contained in metakaolin.However,it is difficult for silica fume to effectively inhibit the formation of C₃AH₆ by forming C₂ASH₈,therefore a strength reduction in the blended paste also occurs in the later age.The slag itself has weak cementitious property,and amorphous silicon belongs to Q⁰,which has high activity and is easy to be dissolved.The formation of C₂ASH₈ can be significantly promoted by increasing the slag content,thus effectively inhibiting the formation of C₃AH₆.（3）The differences of modification effects of mineral admixtures on Q phase paste and the correspondidng mechanisms were revealed,which provided theoretical support and practical guidance for the preparation of HAC.According to the reactions of three mineral admixtures with synthetic CAH₁₀ and C₂AH₈,it is proved that the reason of the influence of mineral admixtures on the properties of Q phase paste is the reaction between the active component dissolved and the metastable hydrates formed in the early stage.Therefore,the enhancing effect of mineral admixture on the performance of Q phase or traditional CAC is closely related to the type of metastable hydrates,the type and amount of mineral admixtures,and the curing temperature.Compared with CAH₁₀,the conversion of C₂AH₈ into C₃AH₆ is faster,therefore its conversion is difficult to be inhibited.Both amorphous silicon and aluminum contained in metakaolin can affect the conversion of CAH₁₀.Based on the solution equilibrium theory of hydrates,the ionic composition of metakaolin-CAH₁₀solution was determined.By calculating the ion product and supersaturation coefficient of CAH₁₀,it was found that the dissolution of amorphous aluminum increased the concentration of Al（OH）₄^-in the pore solution,decreased the solubility of CAH₁₀ and thus improving the stability of Ca H₁₀ under the action of"common-ion effect".The dissolved silicon reacts with CAH₁₀ to form C₂ASH₈.The amorphous silicon from both slag and silica fume reacts with the metastable hydrates to form C₂ASH₈,thus achieving the purpose of reducing C₃AH₆.However,due to the difference in the activities of amorphous silicon contained in the two minerals,the effects of amorphous silicon in inhibiting metastable phase conversion are different.