Experimental Study On Compression Performance Of Calcium Aluminate Cement Based Ultra High Performance Concrete Exposed To Elevated Temperatures

In recent years,ultra-high performance concrete（UHPC）has been widely used in the field of civil engineering due to its excellent mechanical properties.However,due to its compact microstructure,it is easy to burst under the action of fire and high temperature.Calcium aluminate cement concrete（CAC）has high early strength,high temperature resistance and chemical corrosion resistance properties,has been widely concerned by scholars at home and abroad,but its hydration products are temperature sensitive,resulting in strength decrease in the later stage,which limits the application and development in engineering.If the late strength decline of calcium aluminate cement can be avoided and applied to UHPC,it is expected to make UHPC have more ideal high temperature performance.Therefore,this paper studies the effects of different curing ages and temperature on the properties of calcium aluminate cement paste mixed with different types of mineral admixtures through compressive tests under the condition of low water-binder ratio.The cementitious materials with good high temperature performance and can inhibit the decrease of intensity in the later period were selected.Then,through XRD phase analysis and SEM microstructure observation,the mechanism of the influence of the research parameters on the properties was revealed.On this basis,the effects of fire temperature and steel fiber content on the cube compressive strength and axial compressive properties of calcium aluminate cement based UHPC after high temperature were studied.The main research contents are as follows:（1）The macroscopic mechanical properties and microscopic properties of hardened calcium aluminate cement paste with different types of mineral admixtures were tested with the change of curing age and after experiencing different temperatures.The results show that all three mineral admixtures,silica fume,slag and fly ash can inhibit the late strength decrease of hardened calcium aluminate cement paste to a certain extent.At 210 days of age,the compressive strength of hardened calcium aluminate cement paste with 10%silica fume,10%slag and 10%fly ash increases by 9.5%,19.2%and 17.9%,respectively.The inhibitory effect of mineral admixtures on the strength decrease of hardened calcium aluminate cement paste is realized by blocking the crystal transformation of metastable hydration products CAH₁₀ and C₂AH₈ to stable C₃AH₆.The compressive strength of hardened calcium aluminate cement paste mixed with 10%silica fume,10%slag and 10%fly ash respectively is 126.2%,113.7%,127.9%and 164.9%of that at room temperature at600℃,and 81.9%,89.8%,95.9%and 102.5%of that at room temperature at 1000℃,respectively.（2）With the increase of simulated fire temperature,the color of the calcium aluminate cement based UHPC changed from dark gray（20～200℃）to light gray（300～400℃）,gray white（500～600℃）and light yellow（800～1000℃）.At the same time,the mass loss rate gradually increased,and after 600℃structure become loose,but none of the specimens burst during the high temperature.When the temperature is low,the calcium aluminate cement based UHPC without steel fiber is brittle failure,and when the temperature is high 500℃,it is plastic failure.The UHPC specimens with steel fibers were subjected to plastic failure at different temperatures.（3）With the increase of the simulated fire temperature,the cubic compressive strength and axial compressive strength of the calcium aluminate cement based UHPC increased first and then decreased.When the temperature was 20～600℃,the addition of steel fiber increased the residual strength of UHPC.When the temperature was 800～1000℃,the residual strength of the calcium aluminate cement based UHPC was lower than that of the specimen without steel fiber due to the oxidation of the steel fiber.At the same time,the peak strain of UHPC specimen increases gradually,the elastic modulus decreases continuously,and the Poisson’s ratio decreases first and then increases.（4）At room temperature,with the increase of steel fiber content,the cube compressive strength,axial compressive strength,peak strain and elastic modulus of the aluminate cement based UHPC increased continuously,and the Poisson’s ratio decreased gradually.After the high temperature of 1000℃,with the increase of steel fiber content,the cubic compressive strength,axial compressive strength and peak strain gradually decrease,the elastic modulus first decreases and then increases,and the Poisson’s ratio gradually increases.（5）Calcium aluminate cement base UHPC after does not exceed 600℃high temperature effect microstructure are relatively dense,there is only a small amount of micro cracks and matrix with good bonding between steel fiber,when the temperature within the range of 600～1000℃,the hydration products of decomposition,appear loose and the separation of small particles,crack development,steel fiber is oxidized,after 1000℃high temperature,already cannot detect the presence of steel fiber,matrix high temperature damage is more serious.（6）Based on the statistical analysis of the test results,the calculation models of cubic compressive strength,axial compressive strength,elastic modulus and peak strain of calcium aluminate cement base UHPC considering the influence of high temperature and fire were established respectively,and the uniaxial compression constitutive equations with the rising section being quintic polynomial and the falling section being rational fractions were proposed.