| Different curing temperature favors the formation of different types of calcium aluminate cement(CAC)hydration products including CAH10,C2AH8,C3AH6 and AH3.Additionally,the related research indicated that a new hydration product,C4AcH11,could be generated,at the condition of normal temperature,high humidity and long curing period.However,the correlational studies regarding the formation mechanism of this new hydrate,as well as the influence of the new hydrate formation on the performance of CAC bonded castables,are limited.Therefore,it is needed to make a systematic study on the preparation of the new hydrate-containing CAC bonded castables and their structures and performances.In this thesis,a method to maintain the high curing humidity and raise the curing temperature was adopted,hoping to obtain CAC bonded castables involving the new hydration product in a short period of curing time.Afterwards,the effect of different curing temperature and humidity on the early-stage hydration phases,matrix microstructure and castable demolding and drying strength was investigated.Comparative studies were also made to detect properties,phases assemblage and micro-structure of the various temperature heated castables including of excluding the new hydrate phase.(1)The influence of curing humidity on the early hydrate phases of CAC paste and castable matrix,the matrix micro-structure and castable demolding and drying strength has been studied.The CAC bonded castables and matrix are cured at temperature of 50℃ and relative humidity(RH)of 80%and 30%.A new hydration product C4AH19 is found but C4AcH11 is not found at RH of 80%,while the hydrates only include C3AH6 and AH3 when RH is 30%.The demolding strength of the castables after 3 h of curing at RH of 80%achieves 5.22 MPa which has fulfilled the demoulding and is much higher than the demolding strength of castables cured at RH of 30%.After curing at 50℃ and RH of 80%for 3 h,the formation of granular C3AH6 and plate-shaped C4AH19 and AH3 is beneficial to the early strength of the castables,compared to the role of formation of C3AH6 and AH3 at low humidity.After drying at 110℃,the C4AH19 in the high-humidity cured samples vanishes,and C4ACH11 appears,which indicates the metastable C4AH19 is easy to absorb carbon dioxide in the air and transforms to C4ACH11.The microstructure analysis indicates there are a mass of micro-pores on this hydrate C4 AcH11,which lowers the compactness of the castable matrix and increases the apparent porosity.However,the pores of the new hydrate may favor the discharge of the free water and bound water during thermal treatment,and is hopeful to improve the explosive spalling resistance of the castables.(2)The influence on the phase composition and microstructure of castable matrix and properties of CAC-bonded corundum castable cured at 30%and 80%RH after heating at 110-1450℃ has been investigated.After fired at 200-800℃,the CMOR of the CAC-bonded corundum castables cured at 30%RH is slightly higher than that of the castables cured at 80%RH.When the CAC is cured at 30%RH,the main phases are C3AH6 and AH3.Compared with that,when the CAC is cured at 80%RH,the main hydrates are C4AcH11,C3AH6 and AH3.During the heated-treating process,the decompression of different types of hydrates is various,which leads to the difference of strength.Moreover,with increasing temperature from 200℃ to 800℃,the CMOR of castables(cured at 30%and 80%RH)decreases,because the hydrates(including C4AcH11,C3AH6 and AH3)decompose into C12A7,and the structure of C12A7 collapse.After fired at 800-1200℃,the CMOR of the CAC-bonded corundum castables cured at 30%RH is higher than that of the castables cured at 80%RH,because there are a mass of micro-pores on this hydrate C4AcH11 which would decompose to form more pores during heating process and resulted in the decrease of CMOR.Additionally,with increasing temperature from 800℃ to 1200℃,the CMOR of castables(cured at 30%and 80%RH)improves.After heated at 800-1200℃,C12A7 would react with Al2O3 to form CA、CA2 和 CA6.Moreover,the increasing heating temperature could promote the sintering of micro-powders and improve the sintering degree,which are both beneficial for improving the CMOR.After fired at1450℃,the physical properties of the CAC-bonded corundum castables(including CMOR,apparent porosity and bulk density)cured at 30%RH and 80%RH are similar,because the main phase of castable matrices cured at different relative humidity and fired at1450℃ is both CA6.(3)The influence on the phase composition and microstructure of castable matrix and properties of castable after heating at 110-1450℃ of various CAC contents has been investigated.With the increase in hydrate amount by increasing the added CAC level,the strength of castable cured under different humidity was improved after firing at 300600℃.This is because the hydrates could keep their morphology during heattreatment,thereby providing a certain level of strength of castable.The higher amount of hydrates,the higher strength of castable.During heat-treatment at temperatures ranging from 300-600℃,the decrease rate of castable cured under 80%is less than that under 30%.This is because the "new"hydrate of C4ACH11 has a plate-like morphology and could keep this morphology during heating.When castables were fired at temperatures ranging from 800℃ to 1450℃,the strength of castables cured under different humidity and fired at the same temperature is almost identical.This is because hydrates in castable were totally decomposed and the strength of castable is mainly related to the sintering and solidstate reactions occurred in the castables.This thesis mainly contributes to study the formation of new hydration product C4AH19 at curing temperature of 50℃ and RH of 80%,and its positive effect on the hardening of castables which could shorten the curing time and enhance the demolding strength.The influence of the two kinds of hydrates systems on the phases and structure evolution and the properties of the castables in the heating procedure is compared.The effect of hydrates content on the castable performance is further investigated,which reveals the relationship between different hydrate systems and the phases,micro-structure and the properties of the castables during thermal treatment.The research results are expected to enrich the hydration theory of CAC,and provide theoretical support for the curing mechanism of large CAC-bonded castable fabricates,which is conducive to improving the production efficiency and economic benefit. |
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