Study On Preparation And Properties Of Phosphoric Acid-based Geopolymers Chinese Fir Fiber Composites

Geopolymer,as a new type of cementitious material,has been widely used in construction and other fields because of its many advantages such as high compressive strength,low shrinkage,impermeability,fire resistance,frost resistance,and corrosion resistance.However,as an inorganic material,it has the shortcomings of brittleness and fragility.In order to overcome its brittleness defects,geopolymer was reinforced by plant fibers.In this research,phosphoric acid-based geopolymers Chinese fir fiber composites were prepared,with metakaolin as the raw material phosphoric acid as the activator,and the agricultural and forestry residual Chinese fir fiber as the reinforcing material.Single factor experiment was used to explore the influence of different factors on the properties of phosphoric acid-based geopolymers Chinese fir fiber composites,and the optimal conditions for preparing the composites were obtained.At the same time,phosphoric acid-based geopolymers Chinese fir fiber composites were prepared by replacing some of the phosphoric acid with potassium phosphate,and their properties were discussed.Finally,the high temperature resistance and durability of the composites were preliminarily studied.In the research,the geopolymer composites were characterized by FTIR,XRD,SEM and TG.The results are as follows:?1?Through single-factor experimental exploration,the optimal preparation conditions for preparing phosphoric acid-based geopolymers Chinese fir fiber composites are:H₂O/Al₂O₃molar ratio is 10,H₃PO₄/Al₂O₃molar ratio 2.0,fiber content 10%,fiber mesh 20-40,curing temperature 60?,curing system 1-3-3?Cured for 1d in the mold,cured in a sealed bag for 3d after removing the mold,and then cured for 3d after removing the bag?.The bending strength,internal bonding strength and screw holding force of the phosphoric acid-based geopolymers Chinese fir fiber composites obtained under such preparation conditions are optimal,being13.7MPa,2.17MPa and 2051N respectively,and they all exceed the requirements of superior products in China National Standard"Cement Particleboard".?2?Through the 24h water immersion experiment,it can be seen that the water absorption rate of the geopolymer composite material with different fiber content is between 10.6%and11.0%.The thickness expansion rate is between 0.35%and 1.57%,and the thickness expansion rate meets the requirements of qualified products in China National Standard"Cement Particleboard"?thickness expansion rate?2%?.Except for the samples with a fiber content of20%,the bending strength of all samples after water absorption has reached the national standard for qualified products?bending strength?5.5MPa?.?3?It is known from experiments that potassium phosphate can partially replace phosphoric acid in order to prepare such geopolymer composite materials.When the H₃PO₄/Al₂O₃molar ratio is 1.2 and the addition amount of potassium phosphate is 12%,the bending strength of the composite materials prepared is the highest,and the bending strength reaches 10.9MPa.Compared with the geopolymer composite material without potassium phosphate,the bending strength of this composite materials is increased by 40.6%,and the internal bonding strength and screw holding force at this content are also optimal,being2.07MPa and 2210N respectively,both exceeding the national standard requirements for superior products?internal bonding strength?0.5MPa;screw holding force?600N?.?4?It can be seen from the weather resistance experiment that the mechanical properties of the such geopolymer composites after 60UV aging cycles are all reduced.Among them,the geopolymer composites with higher fiber content have a higher reduction rate of in bending strength.It can be known from the high temperature resistance experiment that when the temperature is between 200?and 400?,the bending strength loss rate of the such geopolymer composites is the largest.When the temperature is above 400?,the bending strength of the geopolymer composite material is almost lost.25 freeze-thaw cycles experiments show that the prepared geopolymer composites has excellent antifreeze performance.The samples after freezing and thawing did not show missing edges,missing corners,and cracking,and the samples could maintain good strength.After the freeze-thaw experiment,the bending strength of the sample with a fiber content of 10%can still be as high as 8.4 MPa.?5?SEM images show that the fibers and matrix are tightly bonded and evenly distributed in the geopolymer composites with low fiber content and the fibers can be well embedded in the geopolymer matrix.As the H₃PO₄/Al₂O₃molar ratio increases,the particles of metakaolin powder in the matrix decreased after the reaction,indicating that the degree of geopolymer reaction increased.When potassium phosphate is added,as the amount of potassium phosphate added increases,the gel produced in the geopolymer matrix increases,indicating that it is conducive to the geopolymerization reaction.Moreover,the fiber and the matrix can still be very tightly bonded for the geopolymer composite material added with potassium phosphate after the freeze-thaw experiment.?6?Thermogravimetric analysis results show that the weight loss of geopolymer composites is divided into three stages.The first stage is between 30?218?,in which the weight loss is due to the evaporation of free water between geopolymers.The second stage is218?418?,and the weight loss is due to the loss of crystal water and decomposition of plant fibers in the sample.The third stage is above 418?,and the weight loss is mainly due to the further decomposition of plant fibers.?7?Through FTIR and XRD analysis,it can be seen that the fiber and the phosphoric acid-based geopolymers are physically compounded,and the incorporation of the fiber does not produce new chemical bonds.