Experimental Study On Basic Mechanical Properties And High-temperature Spalling Mechanism Of Geopolymer Concrete

Cement concrete is one of the most widely used building materials with largest usage,but the cement used in the production of concrete is an important factor that contributes to global warming.Geopolymer concrete(GC)is a new type of environmentally friendly building material,which exhibits good mechanical properties and high temperature resistance.In order to further study workability,basic mechanical properties and high-temperature spalling of metakaolin-fly ash based GC systemly,the following work was carried out in this paper,and some innovative achievements were gained:1.Through tests on 105 cube and 9 cylinder specimens of GC,the optimum formula of GC was obtained.Based on the formula,workability(slump,slump flow,setting time)of GC was tested,and mechanical properties of GC,such as compressive and splitting tensile strength under different curing conditions and ages as well as elastic modulus,and poisson’s ratio under different strength grades,were studied.Test results showed that:(1)GC with water-cement ratio of 0.4,sand ratio of 0.3 and aggregate binder ratio of 3.5 possesses compressive and splitting tensile strength of 59.7MPa and 3.27 MPa respectively,and it also exhibits good workability;(2)GC has early strength property,4h and 1d compressive strength of GC at room temperature is 18.5MPa and 48.5MPa respectively,corresponding to 31%and 81%of 28d compressive strength;(3)At room temperature of 25 ℃,initial and final setting time of GC are 4.1 h and 4.6h respectively,and the time difference between them is 0.5h,which are much shorter than those of ordinary concrete in the literatures;setting time and time difference of initial and final setting time of GC will decrease with increasing temperature,indicating its high sensitivity to temperatures;(4)Calculation values of compressive and splitting tensile strength at different ages from proposed formulas are in good agreement with test results of this paper,thus the proposed formulas can be used to calculate strength of GC with different strength grade at different ages;(5)At room temperature,the ratio of splitting tensile strength to compressive strength for GC is 1/15～1/18,which is close to ordinary concrete and high strength concrete;relationships between splitting tensile strength,elastic modulus,poisson’s ratio and compressive strength were established,and were consistent with test results.2.Compressive and splitting tensile strength of 30 GC cubes,and uniaxial compressive strength of 15 GC cylinders were tested after exposure to elevated temperatures,the effect of temperatures on compressive strength,splitting tensile strength,deformation properties,peak strain,elastic modulus,poisson’s ratio and stress-strain full curves were studied.Test results showed that:(1)compressive and splitting tensile strength,elastic modulus of GC will decrease with increasing temperature,and decrease significantly at the temperature of 3 00～500℃;Poisson’s ratio and peak strain both increase with increasing temperature,but after 500 ℃,the growth slows down.The calculation formulas about mechanical properties(compressive and splitting tensile strength,elastic modulus and poisson’s ratio)of GC varing with temperatures were established by regression analysis;(2)compared with ordinary concrete,GC prepared in this paper exhibits better high temperature resistance;(3)the relationship between compressive and splitting tensile strength of GC after exposure to elevated temperatures was proposed in this paper,which agreed well with test results;(4)by observing GC stress-strain whole curves,the shape of stress-strain curves will vary with temperatures when the temperature is over 100℃,the nonlinearity of curves increases gradually;at the temperature of 300～700 ℃,the shape of stress-strain curves gradually gets flat,peak stress decreases rapidly,and peak strain increases obviously.Through formula derivation and parameters analysis,stress-strain whole curve equation of GC after exposure to elevated temperature was put forward.3.Through the tests,the effect of compressive strength level,moisture content,temperature increasing rate and the temperature on spalling characteristics and internal pore structure of GC at high temperature were studied,and spalling mechanism of GC was analyzed along with results of XRD tests.The results show that:(1)GC exhibits good spalling resistance.Effect of moisture content and temperature gradient on spalling properties of GC are not significant,even if temperature gradient is as high as 463.5 ℃,GC of 40 and 60 MPa didn’t exhibit apparent spalling phenomenon,just a small number of spalling bulges were founded on the surface;one of 15 specimens of 80 MPa appeared serious local explosive spalling damage with saturated moisture content(concrete spalling),and the starting temperature of explosive spalling is 500 ℃;(2)compared with cement concrete of same strength,GC has larger water permeability coefficient at room temperature,that is,GC has more highly developed continuous pore structure.When subjected to high temperature,highly developed continuous pore structure is beneficial for water vapour to spread outward,which prevents the form of excessive pore pressure within GC,so that the probability of explosive spalling greatly reduces;water permeability coefficient of GC increases with increasing temperature,but the growth of water permeability coefficient of cement concrete with increasing temperature is greater than that of GC with same strength,that is,the influence of temperatures on pore structure of cement concrete is more significant than that of GC;(3)under the condition of hydrothermal curing,polymer diffraction peak can be observed by XRD analysis,such as polysilicic acid(K)-silica(K-PSS);GC contains a lot of SiO2 and a certain amount of mullite,while SiO2 and mullite both belong to high temperature resistant material,so GC has good high temperature resistance,and the possibility for explosive spalling of GC reduces;(4)the theory coupling steam pressure and thermal stress can explain spalling rules of GC,that is,when pressure values under the action of thermal-mechanical"coupling" exceed tensile strength of GC,water vapor will appear an unsteady diffusion along original micro cracks,and then,cracks expand rapidly and develop unsteadily,so spalling phenomenon of GC may appear.