Study On Mechanical Performance And Mechanism Of Geopolymer-based Organic-inorganic Composite Cementitious Materials

Slag based geopolymer is one of the most promising new green cementitious materials due to its advantages of low energy consumption,small expansion,and strong corrosion resistance.However,the excessively fast hardening speed,low flexural strength and low fracture toughness of slag based geopolymer limit its wider application in the construction field.In this study,slag was used as the raw material,and the hydrophilic organic polymer was used as the modification additive.To improve the mechanical properties of slag based geopolymer,the geopolymer-organic composite cementitious materials were prepared by the ordinary molding process and the mixing process.The mechanical properties of the composite cementitious materials were investigated.The micro-composition and structure of the hybrid cementitious materials were characterized by using FTIR,XRD,NMR,SEM,MIP.The relationship between microstructure and mechanical properties of the composite materials was established.The experimental results are summarized as follows:?1?The sodium hydroxide solution with different alkali content and the water glass solution with different modulus were used as the alkali activator.The effect of the parameters of alkali activators on the mechanical properties and polymerization degree distribution of slag based geopolymer were studied.The quantitative relationship between the mechanical properties,polymerization degree distribution of geopolymer and the parameters such as content and the modulus of alkali activator was established.When the alkali activator with content of 6%and modulus of 1.6,the slag based geopolymer exhibited the best mechanical properties,and the 28-day compressive strength and the 28-day flexural strength was 91.0 MPa and 7.5 MPa,respectively.Combined with the ²⁹Si NMR analysis of the polymerization degree of the material,it was found that the cumulative strength of the Q³ silicon oxygen tetrahedron and the average chain length in the slag based geopolymer at this ratio was 11.19%and 8.36,respectively,and were higher than those of other polymers.?2?On the basis of the preferred activator,9 types of hydrophilic organic polymers,i.e.,PVA,PAA,CPAM,HT-3000,HT-3288A,HT-3288B,EP1#,EP2#,EP3#were introduced as the organic composite component.Three types of organic-inorganic composite cementitious materials were prepared by the ordinary molding process.Among the 9 kinds of organic-inorganic composite cementitious materials,PVA-slag based geopolymer composite cementitious material and EP3#-slag based geopolymer composite cementitious material showed better mechanical properties.When the content of PVA was 0.5%,the 28-day compressive strength and the 28-day flexural strength of the composite cementitious materials were 98.3MPa and 9.2MPa,respectively,increasing by 8.02%and 29.33%compared with the reference sample;when the content of EP3#was 1.0%,the 28-day compressive strength and flexural strength of the composite cementitious materials were100.6MPa and 9.5MPa respectively,increasing by 10.55%and 26.67%compared with the reference sample,respectively.It concluded that the two hydrophilic organic polymers PVA and EP3#could significantly strengthen and toughen the slag based geopolymer.?3?According to the infrared spectrum results of PVA-slag based geopolymer composite cementitious material,an absorption peak with wavenumber around 1180 cm^-1?representing C-O-Si?Al?bond vibration?appeared,indicating the bonding reaction between organic phase and the inorganic phase;combined with XRD analysis,it was concluded that neither PVA nor EP3#had an obvious effect on the composition of reaction products in the geopolymer,and the main phases were C-S-H gel and C-A-S-H gel;.NMR results of the composite cementitious material showed that the PVA and EP3#reduced the reaction degree,the polymerization degree and substitution rate of Al atoms for Si atoms.?4?The geopolymer organic-inorganic composite cementitious materials with PVA and EP3#as the composite component were prepared by mixing and pressing molding process.The addition of PVA and EP3#and the curing condition on the flexural strength of composite cementitious materials were studied.The results showed that when the content of PVA is 3%,the 28-day flexural strength of the composite cementitious material was12.6MPa;when the content of EP3#was 7%,the 28-day flexural strength of the composite cementitious material reached 16.7MPa;Under the dry heat?80?,24h?curing condition,the flexural strength of PVA-slag based geopolymer composite cementitious materials increased significantly,and the 28-day flexural strength increased significantly to 19.0MPa;EP3#-slag based geopolymer composite cementitious was more suitable for steam curing?80?,24h?condition,the 28-day flexural strength reached 25.1MPa.?5?Under the mixing process conditions,absorption peaks at 1514cm^-1,1249cm^-1 and1297cm^-1?representing vibration of C=C and C-O bond?appeared in the infrared spectrum of EP3#-slag based geopolymer composite cementitious materials,indicating that chemical bonding reaction occurred between EP3#and the geopolymer;Combined with the NMR results,it was found that the content of Q³ silicon oxygen tetrahedron in composite cementitious materials was increased greatly under the high-temperature curing condition,indicating the layer structure or double chain structure were increased significantly in the system.The observation of the microstructure of composite cementitious materials revealed that the organic phase was tightly bound to the matrix,and the EP3#emulsion effectively bridged the two sides of the crack.The results of the pore size distribution of composite cementitious materials showed that the pore size of the material was refined via the mixing-pressing process and the high-temperature curing system.The EP3#-slag based geopolymer composite cementitious material prepared with the steam curing had the smallest pore size of 3.29nm and 6.03nm,and its porosity was decreased to 4.88%.