Reaction Mechanism And Applications Of Alkali-based Geopolymers At Low Temperature And Vacuum Conditions

Alkali-based geopolymer is a type of three dimensional structural aluminum silicate inorganic polymer composed of[AlO4]and[SiO4]tetrahedrons,mainly prepared by aluminosilicate minerals(natural mineral,synthetic powder and industrial wastes)in alkali activated conditions at room temperature or high temperature.Alkali-based geopolymers possess excellent property;preparation process is simple and it is low energy consumption and environmentally friendly compared with other inorganic polymers.So far,many studies have been carried out on the behavior of alkali-based geopolymer paste either at room temperature or under high temperature conditions.Very few studies have examined the geopolymerization behavior of alkali-based geopolymer material at low temperature(<25 ?)and vacuum conditions.Due to the fact that alkali-based geopolymer reacts very quickly at room temperature,it is difficult to study some aspects of geopolymerization at this temperature.However at low temperature,it is possible to separate the depolymerization and polycondensation;this could be beneficial for the study the geopolymerization mechanism.The properties of alkali-based geopolymer were explored under the condition of vacuum and freeze-thaw cycles;the water was recycled and reused.In this paper,we studied the chemical composition of raw materials,coordination of silicon and aluminum,different types of alkali-activators,alkalinity,water content and retarder influence on gel structure of geopolymer,which started from the nature of the chemical reaction.Moreover,we studied the curing temperature effect on the dissolution rate of ions and solidification rate,and the influence of gel content.A new way to obtain an environmentally friendly and high value construction material from waste by alkali-activated technique was presented.(1)The gel structure in the process of geopolymer reaction was affected by different types of anionic and silicate ions;solid water glass,liquid water glass and sodium hydroxide were used to activated slag in this paper.The results showed that the initial setting time of three kinds of alkaline reagent was consistent and decreased as the amount of alkaline reagent reduced;the final setting time of samples using liquid water glass was different from the others.Through the compressive strength,shrink ratio,SEM and DSC analysis results,it was shown that the gel structure with solid water glass and liquid water glass were better than the one with sodium hydroxide;the main product of sodium hydroxide with slag is zeolite X while the main product of solid or liquid water glass with slag is amorphous.The FT-IR results of the three kinds of alkaline reagent were consistent.Finally,the solid water glass was selected as alkaline reagent,and it was observed that the good mechanical performance can be obtained when slag/solid water glass mass ratio=5:1.(2)In this experiment,we explored water content influence on the mechanical properties of alkali-based geopolymers,with fixed slag/solid water glass mass ratio=5:1;the results showed that the samples have better mechanical properties when the water content is 25%or n(H2O)/n(Na2O)=21.16.The exploration of reaction process at low temperature,through the gel conent and SEM/EDX analysis showed that the gel content of the sample first increased and then decreased with the decreasing of the curing temperature,reaching a maximum value of 37.8 wt%in the samples cured at 0 ?.The 27Al,29Si MAS NMR spectra showed that 6-coordinated Al(?)aluminum can be observed at low temperature.The results are consistent with gel content characterization in the previous section.The study of the geopolymerization reaction kinetics through ICP and impedance analysis at low temperature was found to be in agreement with the Arrhenius theorem and showed apparent activation energy of reaction of-17.55 kJ/mol.(3)In this experiment we explored fly ash and retarder influence on the mechanical properties of alkali-based geopolymers,which fixed slag/solid water glass mass ratio=5:1 and water content 25%;the compressive strength of samples with different solid water glass content,water content,fly ash content and retarder content in the condition of vacuum and freeze-thaw cycles was tested.Through the orthogonal experiment analysis,it was found that the content solid water glass is the primary factor,while the retarder content is unimportant.The optimum formula was determined as follow:m(SL)/m(WG)=5:1,retarder 7%,fly ash 10%,water 28%.(4)Lunar building materials that use tektite simulated lunar soil was prepared and characterized,explored its alkali-activated activity;it was found that the alkali-based geopolymers using sodium hydroxide as activator has better stability after vacuum and freeze-thaw cycles in liquid nitrogen compared to the one using sodium silicate;the compressive strength of the tektite powders with 42%of 5M sodium hydroxide was 26.50 MPa,the compressive strength fully meets the construction conditions on the lunar surface.The water recovery was 99.14%,and the water was recycled and reused.The impedance analysis indicated that the reaction is mainly divided into depolymerization and polycondensation process,in agreement with the Arrhenius theorem,showing apparent activation energy of reaction of 40.52 kJ/mol and 59.17 kJ/mol,respectively.(5)Volcanic ash was found to have a close resemblance to lunar regolith and have attracted interest for the development of lunar soil simulants.Sodium hydroxide was used as alkali-activator to prepare lunar building materials in this experiment.The 7-day compressive strength of the alkali-based geopolymers was 50.36 MPa when the Na2O/Al2O3 molar ratio was 1.25,water/ash 0.27 and curing temperature 90 ?;the compressive strengths of the alkali-based geopolymers were 45.53 and 44.95 MPa after vacuum and freeze-thaw cycles in liquid nitrogen,respectively.Comparatively to tektite,the alkali-based geopolymers using volcanic ash has better compressive strength,and the water recovery was 98.61%.Volcanic ash was activated by sodium hydroxide to form a sodium aluminosilicate hydroxide hydrate:Na8(AlSiO4)6(OH)2·4H2O observed by XRD.The impedance analysis indicated that the reaction is mainly divided into depolymerization,polycondensation and dehydration process.The use of the Arrhenius theorem showed apparent activation energy of reaction of 20.63 kJ/mol,59.66 kJ/mol and 3.99 kJ/mol,respectively.