The Hardening Behaviour Of CO₂-cured Portland Cement And Post Hydration

CO₂ cured cement-based materials refers to the use of the chemical reactions between CO₂ and cement clinkers and form the cementitious reaction products,which is completely different from the natural carbonation.Generally,natural carbonation means the reaction of hydrated products in hardened cement-based materials with CO₂.The use of CO₂ cured cement-based materials not only can obtain products with better performance,avoid the energy consumption required by steaming curing,but also can effectively use of the CO₂ emitted from factory.Moreover,it can effectively utilize the emitted CO₂ and organically combine the global topic of CO₂ recovery and utilization with the science of concrete materials,which is of great significance for the sustainable development of cement and concrete industry and the mitigation of the greenhouse.Therefore,the microstructure of CO₂ cured cement-based materials and its subsequent hydration process is investigated in this paper.Firstly,the properties and microstructure of CO₂ cured cement paste under various CO₂concentration.The results show that the carbonation depth and carbonation degree of cement-based materials increase with the increase of CO₂concentration.However,the enhancement effect is not obvious when the CO₂ concentration is above 20%.There is a liner relationship between the carbonation depth and the square root of carbonation duration,but the liner correlation coefficient becomes weaker with the increase of CO₂concentration.Three CaCO₃ crystals,including calcite,aragonite and vaterite,are produced in CO₂ cured cement-based materials regardless of CO₂ concentration,but the calcite is the most prominent crystalline.The CaCO₃content and its crystalline degree increase with the increase of CO₂concentration,which improves the decomposition temperature of CaCO₃.The morphologies of formed calcite,vaterite and aragonite show prismatic/cubic,spherical particles and spear-shaped/rod-shaped,respectively.Moreover,the size of calcite and aragonite growths with the increasing CO₂concentration.Transmission electron microscopy(TEM)and selected-area electron diffraction(SAED)images confirm that the calcite crystalline mainly grows along(1 0 4)direction,while aragonite grows along(0 2 1)direction.Increasing CO₂concentration improves the average elastic modulus of silica gel and its polymerization degree of CO₂ cured cement-based materials.The properties and microstructure of CO₂ cured cement paste under various temperature is studied.The carbonation depth and carbonation degree of CO₂-cured cement paste increased with the increasing temperature.Higher temperature results in higher CaCO₃ content and crystalline degree.The vaterite tends to form at lower temperature,while the aragonite is likely to generate at higher temperature.However,calcite is the most dominant CaCO₃ crystalline at various temperature.the increase of temperature is beneficial to the size growth of calcite crystalline,but the size and morphology of formed calcite shows no obvious change when the temperature exceeds20?.However,the changed temperature plays an important role in the morphology of aragonite.The shape of aragonite of CO₂ cured cement paste changed from spear into rod with the increased temperature.Higher temperature leads to a higher average elastic modulus of silica gel.The polymerization and average chain length of silica gel improved with the increased temperature.Furthermore,the reaction activation energy of CO₂ cured cement paste suggests that the carbonation reaction can occur instantaneous at room temperature.The properties and microstructure of CO₂ cured cement paste and its subsequent hydration process are investigated.Compared to hydrated cement paste,the CO₂ cured cement paste reveals a rapid early compressive strength development and maintains a higher long-term strength.In addition,the early and long-term strength improve with the increase of CO₂concentration.Early CO₂ curing does not hinder subsequent hydration process.The generated CaCO₃ under early CO₂ curing mainly plays the filling effect in the whole process,and shows the nucleation effect and chemical effect in the subsequent hydration process.The content of newly hydration products calcium aluminate mono-carbonate increased with the hydration duration and cured CO₂concentration.Moreover,the formed carbonation product silica gel reacts with Ca(OH)₂to produce a secondary C-S-H gel.A model of CO₂ cured cement-based materials and its subsequent hydration process is proposed.The properties and microstructure of CO₂ cured cement paste under 5? and its subsequent hydration process at 5? are investigated.Compared to hydrated cement paste at 5?,the CO₂ cured cement paste under 5? reveals a rapid early compressive strength development and maintains a higher long-term strength.In addition,the early and long-term strength is improved with the increase of carbonation duration.Early CO₂ curing at 5? does not hinder subsequent hydration process at 5?.The generated CaCO₃under early CO₂ curing mainly plays the filling effect in the whole process,and shows the nucleation effect and chemical effect in the subsequent hydration process at5?.Moreover,the formed silica gel in CO₂ curing reacts with Ca(OH)₂ to produce a secondary C-S-H gel.However,the strength development of cement paste is lower in early CO₂ curing and further water curing compared at 5? compared to the sample cured at room temperature.Finally,the effect of recycled cement powder after carbonation treatment on the properties and microstructure of cement-based materials is perspective studied.The hydration products in uncarbonated recycled cement powder react with CO₂ and formed calcite and silica gel.Compared to the paste with uncarbonated recycled cement powder,the cement paste with carbonated recycled cement powder shows a better flowability,compressive strength,water absorption and drying shrinkage,but increases the carbonation depth.The hydration products of paste with uncarbonated recycled cement powder are same to Portland cement.However,the generated calcite in carbonated recycled cement powder results in the formation of calcium alumina mono-carbonate and calcium alumina hemi-carbonate thus to a stabilization of ettringite compared with Portland cement paste,in which a part of ettringite converts to mono-sulphoaluminate.The calcium alumina hemi-carbonate was mainly less stable than calcium alumina mono-carbonate and transforms to calcium alumina mono-carbonate in about one week after its formation.The paste with carbonated recycled cement powder exhibits a denser microstructure than that Portland cement paste and paste with uncarbonated recycled cement powder.