Effects Of CO₂ Curing On Alkali-silica Reaction Of Glass Aggregate Mortar And Its Mechanism

As one of the main municipal solid wastes,waste glass is mostly disposed of f at landfill,which causes great burden to resources and environment.Replacing the natural sand with crushed glass cullet in concrete products can effectively improve the recycling rate of waste glass.However,the severe alkali-silica reaction（ASR）due to the presence of glass aggregate will lead to the deterioration of concrete.In recent years,the application of accelerated carbonation curing in cement-based materials has developed vigorously,and studies have showed that CO₂ curing has perfect effects on improving the durability,such as freeze-thaw resistance and sulfate attack resistance.However,the effect of CO₂ curing on ASR remains unclear.In this study,factors on ASR of mortar specimens containing 100%flint waste glass,including curing methods（CO₂ curing,standard curing）,curing time（0.5 hours to 3 days）,aggregate size（0.15-4.75 mm）and alkali sources（internal and external）,were evaluated by ASTM C1260and ASTM C227.The flowability,carbonation depth,water absorption,ASR expansion,mass gain,compressive strength,p H value,composition and microstructure of mortar specimens were compared.And on this basis,the influence of CO₂ curing on ASR reactivity of mortar specimens containing glass aggregate was analyzed.The major conclusions of this study are as follows:（1）In ASTM C1260,alkali comes from the external Na OH solution,CO₂ curing（20%CO₂,65%RH）can effectively inhibit ASR expansion.The inhibition effect is proportional to curing time and 3h of CO₂ curing can entirely mitigate the expansion during the whole test period.However,prolonged t he time of standard curing（0.04%CO₂,99%RH）had very little inhibition effect on ASR.Calcium carbonate fills the pores inside the mortar,leading to a denser microstructure.Thus,improving the compressive strength after curing,and reducing the water absorption.Compared with the standard specimens cured for 3d,the 48h water absorption of the specimens c ured for 3d with CO₂ decreased by 22.99%,and the compressive strength increased by 26%after 14d of ASR.In the CO₂-cured specimens,ASR gel was formed at the interface between the aggregate and the cement matrix,and could be accommodated by pores in the matrix around the aggregate.For standard curing,The ASR gel was generated in the microcracks of glass aggregate in the specimens.The higher the microcrack percentage,the higher the ASR gel generation and expansion rate.（2）The decrease of glass particle size leads to the decrease of flowability.Thus,the water absorption increases while the density of hardened mortar decrease.The aggregate with smaller particle size has larger specific surface area and more angles,which can absorb more mixing water and increase the friction force during the falling process of fresh mortar.The higher the porosity of the specimen is,the more easily CO₂,water and alkaline substances in the environment can penetrate into the specimen.Among the specimens without CO₂ curing,the reactivity of glass aggregate of 1.18-2.36mm is the highest,and its 14d ASR expansion rate is as high as 0.54%.After CO₂curing,ASR gel increased with the increase of specific surface area of aggregate.When the particle size of glass aggregate was less than 1.18mm,CO ₂ curing for 3d would lead to an increase in the amount of gel products.The mass gain rate of 3d CO₂ cured mortar containing glass with particle size of 0.6-1.18mm increased by 1.18%at 14d,but without harmful expansion or cracking due to accommodation of pores.（3）In ASTM C227,alkali comes from the cement（inside of the specimen）.Carbonation consumes alkaline substances such as Ca（OH）₂,reducing the p H of the pore solution.3h of CO₂ curing could effectively mitigate the ASR expansion.Therefore,reduces the compressive strength loss since the risk of expansion induced cracking.After accelerated ASR reaction,the standard curing specimens all experienced serious expansion and cracking.Compared with the control group,after12 weeks of accelerated ASR,the expansion rate of specimens cured with CO ₂ for 3d decreased by about 90%.