Study On The Thermal Stability Of New Phosphoaluminate Cement

Phosphoaluminate cement（PAC）is a new type of hydraulic cementitious material,which has a unique mineral composition of clinker.The clinker mainly consists of three mineral phases:calcium aluminate（CA）,tricalcium phosphate（C₃P）,and calcium aluminophosphate（CAP）.Compared with ordinary Oortland cement,the cement has better durability and is a new type of hydraulic cement with great potential.As phosphoaluminate cement material is a new type of inorganic material,many properties have not been developed,so it is of great significance to carry out systematic research on its various properties.In this paper,the thermal stability of PAC was studied.Firstly,the effect of anhydrous gypsum on the properties and hydration process of PAC was studied.Under the condition of normal temperature water curing,the compressive strength of PAC past tested after hydration 28d reached 83.6MPa,and PAC hydration products were C₂AH₈and hydrated C-A-H,C-A-P-H and A-P-H gel.Pore analysis showed the hardened PAC paste has a dense microstructure.Under normal temperature curing conditions,ettringite was formed in the hydration products of PAC when 15%gypsum was added,and the compressive strength of 28d reached 98.3MPa.Under the condition of 45℃hydrothermal curing,the compressive strength of PAC increases rapidly in 3 and 7 days,but it shrinks in 28 days.The microstructure analysis shows that the hydration products transform into C₃AH₆,resulting in more micropores.However,under the condition of45℃,adding anhydrous gypsum can improve the performance of PAC and prevent the strength from shrinkage.When 25%gypsum is added,the compressive strength of PAC is the highest at 28d,reaching 89.5MPa.The main reason is that the hydration products of anhydrite and PAC react further to form ettringite,which prevents the conversion of hydration products,keeps the PAC structure compact,and improves the compressive strength.Secondly,the properties and microstructure of hardened PAC paste after effect of high temperature were studied.The high temperature resistance data of PAC hardened paste at 300℃,600℃,900℃,1200℃and 1500℃show that the compressive strength of PAC sample decreases gradually（from 83.0MPa to 41.6MPa）with the temperature rising from room temperature to 1200℃,and it still maintains high compressive strength at 1200℃.After 1500℃,the compressive strength increases to96.9MPa,16.7%higher than that at room temperature.Microstructure analysis showed that different mineral phases were formed in PAC samples under different temperature,and the density of microstructure was consistent with compressive strength.Because of the sintering reaction at 1500℃,the microstructure is the densest and the compressive strength is the highest.The results show that PAC can be used as a potential refractory.