Mechanism Research On CO₂ Mineral Carbonation Reaction And Regulation Of Cement Paste

Climate change is having a sustained and profound impact on the world,reducing carbon emissions,increasing carbon removal,and gradually achieving net zero emissions have been listed as strategic goals by more and more countries,and China has also made an important commitment of"2030 to achieve carbon peak,2060to achieve carbon neutrality"Carbon capture,utilization and storage（CCUS）technology can achieve large-scale carbon dioxide emission reduction and is an indispensable technological approach to carbon neutrality.Among many carbon dioxide utilization technologies,carbonation curing cement-based materials can accelerate the formation of early strength of cement-based materials,improve mechanical properties and compactness of products,and achieve permanent storage of CO₂,showing broad application prospect.Research and development in recent years is very rapid.In this paper,cement paste with different water-cement ratio was prepared with reference cement as raw material.The key technological parameters,product characteristics,carbonation reaction and control mechanism of CO₂mineralization curing were studied.The effects of preparation,pre-conditioning and carbonation curing parameters of cement paste on the carbonation reaction process and the properties of carbonated samples were studied.The results showed that pre-conditioning time,carbonation curing time,pressure of CO₂and initial water-cement ratio had great influence on the carbon sequestration rate of samples.With the increase of pre-conditioning time,the carbon sequestration rate of carbonated samples increased first and then decreased,and reached the maximum value when the pre-conditioning time was 8 h.The carbonation curing time and CO₂pressure are positively correlated with the carbon sequestration rate of the carbonated samples.When the carbonation reaction reaches a certain level,further increasing the carbonation curing time and CO₂pressure reveals that the carbon sequestration rate of the carbonated sample increases relatively slowly.In addition,the carbon sequestration rate of carbonated samples increases with the increase of initial water-cement radio.The cement paste with initial water–cement ratio of 0.45 is pre treatment for 8 hours and then carbonated for 8 h（CO₂pressure is0.3 MPa）.The carbon sequestration rate,carbonation depth,and compressive strength of the carbonated sample reach 20.35%,12.49 mm,and 61.99 MPa,respectively.Compared with the 28 d standard curing test block strength（54.25 MPa）,the compressive strength of the test block after carbonation curing is increased by14.27%.The carbonated sample prepared under optimized carbonation conditions was taken as the research object,and the pore structure,phase composition and micro-morphology of different regions of the carbonated sample were analyzed by slicing to explore the carbonation reaction mechanism of the cement paste.The results showed that the carbonation activity of CH was higher than that of C₂S and C₃S during carbonation curing of cement paste.The carbonated products are mianly silica gel and Ca CO₃,and the main crystal forms of Ca CO₃are calcite and aragonite;The main reaction sites during the carbonation process were pores with pore size greater than 50 nm,and the proportion of pores with a pore size less than 20 nm increased significantly after filling with carbonated products.The carbonation reaction densifies the cement paste,which is beneficial for improving the mechanical properties of the carbonated sample.On this basis,the influence of nano-Si O₂and nano-Ca CO₃on the carbon fixation and mechanical properties of the carbonated sample was studied by mixing nano-Si O₂and nano-Ca CO₃in the cement paste test block,and their induced regulation mechanism of on the carbonation reaction was analyzed.The results show that:An appropriate amount of nano-materials can further improve mechanical properties of carbonated samples on the premise of ensuring the carbon sequestration ability.The improvement of mechanical properties mainly comes from the filling of the carbonated product Ca CO₃and nano-materials,and the two filling substances are interdependent.The carbon sequestration rate of the sample with 1%nano-Si O₂content is similar to that of the pure cement group,and the mechanical properties are improved by 4.19%compared to the pure cement group;The carbon sequestration rate of samples with 1%and 2%nano-Ca CO₃content is similar to that of the pure cement group,and the mechanical properties are improved by 3.60%and 2.48%compared to the pure cement group,respectively.