Investigations On Reinforcing, Toughening And Durability Of Alkali-activated Fly Ash-based Geopolymer

Geopolymers, as a novel type of green building structural materials for realizinghigher-value recycling of industrial solid wastes, become one of the central issues inmaterials engineering currently. Based on the research status of alkali-activated flyash-based geopolymer, contraposing the defects of low compressive strength and poortoughness, strengthening and toughening of fly ash-based geopolymer were conductedused novel alkali-activators, mineral admixtures, styrene-acrylic emulsion and severalkinds of fiber in this paper; the durabilities of geopolymer were investigated, thecharacterizations were conducted by modern analysis methods including mechanicalproperties, mineral phase and microstructure respectively. It aims to investigate theeffects of different ions during geopolymerizations, the styrene-acrylic emulsionreinforced fly ash-based geopolymer with excellent weatherability and slag/fly ashgeopolymer embedded fiber with high temperature resistance were successfullyprepared respectively. The main research contents and the results are as follows:(1) Investigating the mechanical property of alkali-activated fly ash-basedgeopolymers with pure alkali-activator including LiOH, KOH, NaOH, Na2SiO3,Na2CO3and K2CO3, the activation efficiency of them could be identified as thefollowing: K2CO3<Na2CO3<LiOH<KOH<NaOH<Na2SiO3. The highest activationefficiency of binary activator consisted of Na2SiO3·9H2O and KOH, the3dcompressive strength of specimen after85℃curing for9h achieved54.08MPa. Itdemonstrates that synergistic effect occurs between Na+and K+: Na+with betterhydration ability boosts the formation of Si(OH)4, doping K+favors and accelerates the reorganization between SiO4and AlO4tetrahedron, forming more Q44of Si units,leading to a denser microstructures.(2) Geopolymers were prepared with different Si/Al molar ratio with silica fume assilicon source, Al(OH)3as aluminium source, the effect of Al(OH)3minerals at differentcalcination temperatures on the mechanical property were investigated. It found that thereaction potential of gibbsite in geopolymerizations was the highest of the three typesincluding gibbsite, boehmite and-Al2O3.(3) It is found that silica fume and slag are efficient to reinforce and toughen flyash geopolymer, the more activated Si(OH)4formed with an optimum substitution of10wt%silica fume, leading to an increase in the volume of pores (<20nm) and highercompressive strength, but lower freezing-thawing resisting performances. It reached77.5MPa when incorporating30%slag due to the reaction between Si(OH)4involvedin fly ash and the vitreous derived from slag. The nano-gels form firstly and thendendritic gels grow, finally geopolymeric gels with a denser structure develop. It isproposed that the tortuosity of pores could be used to evaluate the mechanicalperformance, the results demonstrated that the higher of the tortuosity, higher strengthof specimen.(4) A novel method to reinforce and toughen fly ash geopolymer was proposedfirstly, the compressive and flexural strength of the specimen with doping1wt%styrene-acrylic emulsion were42.11and6.30MPa, improved by62%and115%,respectively. The mechanism might be the following: the Zeta potential decreases afterdoping emulsion and it promotes the condensation of Si(OH)4monomer; the emulsioncould react with the geopolymeric gels and develop into interpenetrated polymernetworks; it inhabits the vaporization loss of water and maintains a continuation ofgeopolymerizations, leading to an enhancement of mechanical properties.(5) Toughening effects of several fibers on slag/fly ash geopolymer have beenexplored including basalt fiber, organic fiber, protein fiber and ceramic fiber. It foundthat basalt fiber presented the best effects. The28d flexural strength was5.9MPa forthe specimen with basalt fiber embedded, it improved by84.3%compared with thespecimen without fiber. The mechanism ascribes to the formation of inorganic silicategels between basalt fiber and geopolymer matrix under the alkaline environment. The triaxial compressive test (Mechanical Test System，MTS) was used to evaluate theperformances of steel slag/slag/fly ash (2:2:6) geopolymer mortar, the stress-strainresults showed that the basalt fiber improved the maximum bending strain and stress.The3d and28d compressive strength of geopolymeric concrete embedded basalt fiberwere30and65.3MPa, respectively.(6) High temperature (150-1200℃) experiments indicated that the mechanicalproperties of slag/fly ash geopolymer were improved with the increasing heat treatmenttemperatures, but they decreased when the temperature was above500℃, due to theformations of gehlenite and labradorite, and a larger number of holes and poresappeared on the morphology. Based on the theory of thermal stress, the thermostabilityof slag/fly ash geopolymer was improved by embedding basalt fiber under900℃, dueto an increase in the fracture resistance factor.(7) The weather resistance (36months) of emulsion/fly ash geopolymer wasexamined, it found that it was difficult to efflorescent with the prolonged aging, andcontinuing reactions occurred, leading to an enhancement of mechanical properties.