| In recent years,problems such as sea level rise and local climate anomalies caused by the greenhouse effect have threatened the human living environment.And carbon dioxide is one of the main gases.Reducing carbon emission is of great significance for human living environment.The " double carbon " strategy was formally proposed by China at the 75 th United Nations General Assembly.Carbon emissions strive to peak by2030 and strive to achieve carbon neutrality by 2060.Therefore,it is imperative to carry out low-carbon emission reduction research in various fields.As one of the most used building materials,carbon emissions of concrete are particularly serious.Improving durability can effectively reduce carbon emissions of concrete.In addition,active capture and storage of carbon dioxide is another effective way to reduce emissions.Internal curing technology can reduce the possibility of cracking and improve durability of concrete.Concrete properties can improved by carbonization curing while absorbing carbon dioxide.Molecular sieve is a kind of porous material,its special pore structure can not only store water,but also can absorb a certain amount of carbon dioxide.In this paper,water and carbon dioxide were introduced into concrete with molecular sieve as the carrier to realize hydration internal curing and carbonization internal curing.Two kinds of internal curing effects based on molecular sieve were studied,and the following conclusions were obtained:In cement mortar,after adding saturated granular molecular sieve,the mechanical properties and shrinkage properties of cement-based materials was optimized and the hydration reaction of cementitious materials was also promoted.When 10 % of the volume of fine aggregate was replaced by the saturated molecular sieve,28 d compressive strength and flexural strength of mortar increased by 5 % and 10 % respectively.At the age of 28 d,the shrinkage value decreased by 17.5 %.In ultra-high performance concrete,internal curing effect of two kinds of saturated molecular sieves showed differences.Under the influence of granular saturated molecular sieve,28 d strength of the corresponding samples decreased to varying degrees.And 28 d strength of the sample were improved with the influence of the powdered saturated molecular sieve.And when content was 10 %,the UHPC strength was the highest,which was 6 % higher than that of the blank group.Different water release rates and particle sizes were the main reasons for this difference.According to microscopic test,both of two forms of saturated molecular sieves improved hydration degree and pozzolanic reaction of UHPC,especially the powdered saturated molecular sieve.The increase of reaction degree of cementitious materials led to decrease of porosity,which was one of the reasons for increase of UHPC strength,but it also aggravated self-drying shrinkage.According to shrinkage curve,shrinkage reduction effect of granular saturated molecular sieve on UHPC was better than that of powder saturated molecular sieve.When replacement amount of granular saturated molecular sieve was 15 %,shrinkage value of UHPC was the lowest,which was 22 % lower than that of the blank control group.On the whole,UHPC had better shrinkage performance and better durability under the influence of granular saturated molecular sieve.The powdered saturated molecular sieve had a greater improvement in mechanical properties of UHPC.When granular carbon dioxide saturated molecular sieve was mixed into paste,free water would be quickly absorbed and then carbon dioxide would be released,so that carbonation reaction occurred rapidly,making paste lose fluidity quickly.According to microscopic test analysis,carbonization occurred inside paste at the initial stage of the reaction,which accelerated hydration reaction and made paste rapidly solidified.In order to meet working performance requirements,the surface hydrophobic modification of the granular molecular sieve was carried out.Silicone resin was coated on surface of particles to reduce its water absorption,and then CO2 emissions were under control.Silica fume and retarder were added to regulate the carbonation reaction rate.When silica fume replaced 10 % of cement mass and content of retarder was 0.1 %,fluidity was the best.Carbonized internal curing improved early paste strength.And strength test showed that7 d strength was increased by 18 %,28 d strength was comparable to samples in conventional curing.It was caused by filling effect of carbonization reaction.Shrinkage test showed that self-drying shrinkage value was reduced by 24 % and drying shrinkage value was reduced by 2 %.Durability of UHPC had been greatly improved. |
