Performances Evolution And Mechanisms Of Calcium Silicate Slag-based Alkali-activated Materials In Severe Environments

Calcium silicate slag(CSS)is the solid waste of the alumina recovery from high-alumina fly ash project based on lime-soda sintering process.It is an effective utilization approach for CSS to synthesize alkali-activated materials(AAMs).The CSS-based AAMs were synthesized in this study.The performances,reaction products and microstructure evolution of CSS-based AAMs in various severe environments were investigated.Then,the effects of seawater(mixing/curing)on properties of CSS-based AAMs were explored,and the mechanisms were discussed.The results will provide technical and theoretic supports for the application of CSS-based AAMs.The main reaction products of CSS-based AAMs activated by sodium silicate are calcium(sodium)silicate(alumino)hydrate gels(C(N)-(A)-S-H).The increasing sodium silicate content is favorable to the hydration,microstructure and strength development.However,as the activator content exceeded 7.5%(by the Na₂O mass of sodium silicate to powder blend,the same below),the increasing sodium silicate content inhibited the hydration and strength development of CSS-based AAMs.The synergistic effects between CSS and ground granulated blast furnace slag(GGBFS)were detected.The hydration rate was significantly accelerated due to this synergistic effect,resulting into the strength increases of CSS-based AAMs.The Grade 52.5 CSS-based AAMs were synthesized from 52.5 wt%CSS,22.5 wt%GGBFS,25.0 wt%fly ash(FA)and 5.0 wt%sodium silicate with a modulus of 2.40.In ambient air environment(T=20±2?,RH=40%?85%),the continuous alkali-activation resulted into the increasing content of gel products and compactness of the matrix.Consequently,the compressive strength of CSS-based AAMs gradually increased to 100.0MPa at 4 years.As directly exposed to elevated temperatures(20?800?),the compressive strength of CSS-based AAMs gradually degraded to 10%of the control(20?,55.0MPa).Due to the intense increase of pore pressure caused by the quick evaporation of f-H₂O,as well as the shrinkage caused by the dehydroxylation of gel products during exposing to elevated temperatures,the increasing cracking resulted into the strength degradation of CSS-based AAMs as exposed to elevated temperatures.As exposed to acidic environments,the compressive strength of CSS-based AAMs is dependent on the combined effect of the outer acid attack and the inner alkali-activation.After exposing to 5%nitric acid,5%phosphoric acid and 5%acetic acid for360d,the compressive strength of CSS-based AAMs changed from the initial 55.0MPa to 40.0MPa,70.0MPa and 50.0MPa,respectively.The corrosion of C(N)-(A)-S-H gels induced by the H⁺from acid solution is the main reason for the strength degradation of specimens exposed to nitric acid and acetic acid attacks.In contrast,as the CSS-based AAMs were exposed to phosphoric acid attack,the corrosion of gel products induced from the H⁺,as well as the formation of brushite resulted into the expansion cracking and the strength decrease of specimens.As exposed to sulfate environments,the compressive strength of CSS-based AAMs,which were immersed in 5%sodium sulfate solution and 5%magnesium sulfate for 360d,changed from the initial 55.0MPa to 95.0MPa and 65.0MPa,respectively.The sodium silicate solution barely affected the reaction products,microstructure and the strength development of CSS-based AAMs.However,typical behaviors were observed as CSS-based AAMs were exposed to 5%magnesium sulfate solution.The strength of CSS-based AAMs increased slowly with age(?180d)due to the outer sulfate attack and inner alkali-activation.At later age(>180d),the structure degradation of C(N)-(A)-S-H gels induced by the Mg²⁺,as well as the formation of gypsum inside caused the expansion cracking and strength degradation of specimens.The main reaction products of CSS-based AAMs were barely affected by seawater mixing/curing.As mixed with seawater,the ion exchange reaction of Mg²⁺in seawater and silicate anions in alkaline solution produced nano-sized M-S-H gels and Si O₂ gels.The concomitant decrease of the p H value and the soluble silica content of alkaline solution partly hindered the further alkali-activation.As a result,the compressive strength of CSS-based AAMs mixed with seawater reached 50.0MPa at 28d,which was a little lower than that of AAMs mixed with tapwater.As cured in seawater,the strength of CSS-based AAMs cured in seawater was hardly affected due to the limited corrosion and continuous alkali-activation,and consequently reached 100.0MPa at 360d.The anions(Cl^-and SO₄^2-)from seawater had great influence on the hydration and strength development of CSS-based AAMs.As the content of Cl^-(introduced in the form of Na Cl)was higher than 1%,the inhibition of hydration induced from the Na Cl,as well as the formation of un-cementitious sodalite other than C(N)-(A)-S-H gels severely degraded the strength.As the content of SO₄^2-(introduced in the form of Na₂SO₄)exceeded 3%,the saturated Na₂SO₄ gathered together and formed interface transition area in specimens,resulting into the strength decrease of CSS-based AAMs.