Research Of The Hydration And Hardening Mechanism Of Cement-based Materials After Absorbing CO₂ In Mixing Stage

As a high energy-consuming and high-emission industry,the cement industry has become more and more difficult to meet the requirements of green and low-carbon development in modern society.Alkaline substances such as Ca（OH）₂ produced by hydration of cement have the ability to absorb CO₂,It is a method to reduce carbon emission of cement industry by using concrete to absorb CO₂.However,only a little amount of CO₂ can be absorbed after the concrete is formed,which also causes the external carbonization of the concrete to lose the protection on the steel bars.In this paper,the fresh mixed cement-based materials are used to absorb CO₂,which greatly increases the absorption amount of CO₂ and does not reduce the ability of concrete to protect steel bars.Recycling a part of the CO₂ waste gas produced by the cement in the production process,effectively reducing the carbon emissions of the cement industry,and gradually meeting the development requirements of green and low carbon.Several exploratory and innovative works has done in the following aspects in this paper:（1）Rate and Ability of Fresh Cement Paste to Absorb CO₂The rate and ability of fresh cement paste to absorb CO₂ were studied.The results show that,the absorption rate of CO₂ was affected by agitation rate.The ability of fresh mixed cement-based materials to absorb CO₂ is related to the free water content in the suspension system.The higher the water-cement ratio,the more CO₂can be absorbed.About 10.12%by weight of the cement CO₂ can be absorbed when the water-cement ratio is 0.7.（2）Study on the Fluidity and Mechanical Properties of the Fresh Mixed Cement-based Materials after Absorbing CO₂In order to find out the changes of fluidity and mechanical properties of fresh-mixed cement-based materials after absorbing CO₂,the fluidity,mechanical strength and rheological properties of fresh cement-based materials after absorbing CO₂ were studied by experiments.The reaction mechanism was studied by SEM scanning electron microscopy and energy spectrum analysis.The results show that the fluidity of fresh mixed cement-based materials decreases with the increase of CO₂absorption,but its mechanical strength is not affected.The yield stress and viscosity of fresh mixed cement paste will increase with the increase of CO₂ absorption.The crystallization of the CaCO₃ produced by the reaction of CO₂ with the hydration product of the cement reinforced the net-flocculation structure inside the cement,and increased the force that hinders the flow inside the cement.（3）Study on Carbonization Resistance of Fresh Mixed Cement-based Materials after Absorbing CO₂In order to find out the changes of carbonation resistance of fresh-mixed cement-based materials after absorbing CO₂,the carbonation resistance of fresh cement-based materials after absorbing CO₂ was studied by carbonation test,and the mechanism of change was analyzed by XRD.The results show that the carbonation resistance of fresh cement-based materials does not change significantly when the CO₂ absorbed by fresh mixed cement-based materials was less than 2.20%by weight of cement.The results of XRD analysis showed that the content of Ca（OH）₂ in hydration products of cement paste at 28d ages did not decrease obviously when2.20%CO₂ was absorbed by cement paste.（4）Study on Compatibility of CO₂ with Water Reducer Used in Cement-based MaterialsIn order to find out the compatibility of CO₂ and water reducer,the fluidity,mechanical strength and rheological properties of fresh mixed cement-based materials before and after absorbing CO₂ were studied.The results show that CO₂ can inhibit the performance of polycarboxylic acid and aliphatic water reducer,the degree of inhibition is related to the addition time of water reducer and CO₂ absorption.The addition of water reducer to cement-based material after absorbing CO₂ is better than adding it before absorbing CO₂.When the CO₂ absorption exceeds 1.32%,the addition of the water reducer before the absorption of CO₂ adversely affects the fluidity of the cement-based materials.