Research On CO₂ Absorption Ability And The Reverse Action Mechanism Of Fresh Cement Paste

CO₂ curing concrete is the main mean of the CO₂ absorption of concrete.Nevertheless,the concrete capacity and efficiency of CO₂ decline significantly due to the concrete’compact structure,the long time for CO₂ diffusion and the continuous packing of products out of the concrete as the reaction continues.Therefore,realising the high-efficiency CO₂absorption of concrete is an international scientific challenge.Fresh concretes were prepared to absorb CO₂to solve the above challenge.First,the influencing laws of the stirring rate,waster-cement ratio,water reducing agent and adding sequence on the CO₂ absorption rate and absorption limit of fresh cement paste under mechanical stirring were discussed.On this basis,ultrasonic vibration was introduced.According to its characteristics and working principle,the CO₂ absorption capacity of concrete can be further increased.Meanwhile,the three-dimensional microstructural model,molecular structural model and improved model were constructed based on the rheology of cement paste to disclose the action mechanism of CO₂ absorption of fresh concrete.A systematic theoretical analysis and experimental study were carried out to realise the above objective.Some conclusions could be drawn:（1）Study on the CO₂ absorption performance of fresh cement-based materialsA mechanical stirring device for the CO₂ absorption of fresh cement was developed and manufactured independently.The influencing laws of multiple factors（e.g.,stirring rate,water-cement ratio,water reducing agent and adding sequence）on the CO₂ absorption rate and absorption limit of fresh cement paste were discussed.SEM,XRD and EDS analysed the variation laws of the morphological features and components of cement paste.Meanwhile,a microstructural model of the fresh paste under different reducing agents was constructed,and the collaborative action mechanism of water reducing agent and CO₂ in cement paste was disclosed.（2）Study on improving CO₂ absorption efficiency of cement pastes via ultrasonic vibrationThe mechanical stirring device was reconstructed to further improve the absorption efficiency.A new stirring device based on ultrasonic vibration was developed and manufactured.The optimal ultrasonic frequency was determined through an experiment.The variation laws of the CO₂ absorption rate,absorption capacity,fluidity and porosity under ultrasonic vibration were analysed.The influencing laws of ultrasonic vibration on working performances and mechanical performances of cement paste were further disclosed.Finally,the influencing laws of ultrasonic waves on the flocculating constituent of cement and the grain size of the solution were analysed deeply by combining micro/macroscopic structural characteristics.A micro molecular structural model was constructed to disclose the hydration mechanism of cement under ultrasonic vibration.（3）Study on the rheological performances of cement paste after CO₂ absorptionThe influencing laws of cement paste with different CO₂ absorption capacities on rheological performances under the collaborative effect of ultrasonic vibration and high-efficiency water reducing agent were discussed through rheological parameters,such as yield stress,plastic viscosity and the initial setting time of the suspension liquid.Besides,a ultrasonic Rheological Model of cement-based materials after CO₂ absorption under ultrasonic vibration was constructed.The microstructural characteristics of cement paste with a water reducing agent were discussed through SEM.The action mechanism of ultrasonic vibration and the water reducing agent on rheological performances of cement paste was disclosed,and a rheological model was built.Finally,a microstructural model under the molecular scale was built.The variation of the intermolecular repulsion,internal kinetic energy,internal potential energy,internal pressure and radial distribution function laws in the CO₂-water reducing molecular structure,C-S-H gel with CO₂ and CO²⁺C-S-H gel structure were investigated.On this basis,the action mechanism of rheological performances of fresh cement paste was further disclosed.（4）Study on carbonation resistance of cement paste with CO₂ absorptionThe carbonation depth of cement mortar pieces with different curing ages was measured.The carbonisation depth at different curing ages and carbonisation laws in the p H range under ultrasonic vibration and mechanical stirring after different CO₂ adsorption capacities were discussed systematically.Besides,microstructural characteristics,component variation laws and carbonisation laws of cement paste were disclosed by combining XRD and SEM.Finally,a carbonisation resistance mechanism model was built to disclose the variation mechanism of carbonisation resistance.The results show that the absorption of CO₂ by fresh slurry has no adverse effect on the carbonation resistance of hardened specimens.Compared with the pure cement paste（without CO₂ absorption）,the carbonation resistance of the cement paste with CO₂absorption by mechanical mixing has no obvious change.However,the carbonation resistance of cement specimens is significantly improved under ultrasonic vibration.The more CO₂ the newly mixed slurry absorbs,the better its carbonation resistance.