Study On Carbonation Reaction Process Of Non-Hydraulic Calcium Silicate

Global warming caused by CO₂ emissions and environmental pollution caused by industrial solid waste residues are major issues facing countries around the world.studies have show that calcium-containing industrial solid wastes can be used to store CO₂ because the calcium-containing minerals in these wastes are used to absord CO₂and then are hardered into useful block,this would be a promising way to treat these wastes and CO₂ simultaneously.Based on the above assumptions,the carbonation reaction process of two typical non-hydraulic minerals of 3CaO·2SiO₂?C₃S₂?and?-2CaO·SiO₂??-C₂S?were firstly studied in this work,secondly,using?-C₂S containing steel slag and CO₂ as reactunts,the preparations and properties of steel slag carbonated blocks were also investigated.The findings of this work would provide a theoretical basis for effectively and simultaneously using waste slag and gas.Using chemical respectively reagents CaCO₃ and SiO₂ as raw materials,pure C₃S₂ and?-C₂S minerals were first prepared after adjusting their proportion and sintering.Then the effects of carbonation time,CO₂ partial pressure and temperature on the carbonation process of C₃S₂ and?-C₂S minerals were studied.The results showed that the carbonation degree of both C₃S₂ and?-C₂S minerals increased with carbonation time.For the carbonation of C₃S₂,the carbon fixation was 12.7%at 3h,and the corresponding compressive strength of the test block with size of?26mm×30mm can reach 21MPa.The partial pressure also significantly influence the carbonation rate and when the partial pressure was lower than 0.4MPa,the carbonation rate of minerals growed fast,while the CO₂ partial pressure continued to increase,and the carbonation rate increased slowly.For?-C₂S samples at the partial pressure of 6MPa,the compressive strength of carbonation 7d is 37MPa,only 11.4%higher than that the samples under the partial pressure of 0.2MPa,when the carbonation temperature is 60?-90?,the carbonation rate of both minerals were faster.X-ray diffraction?XRD?,fourier transformed infrared?FTIR?,nuclear magnetic resonance?NMR?and scanning electron microscope?SEM?were adopted to analyze,the reaction degree,product type,microscopic morphology and growth position.The Rietveld refinement method was used to quantitatively analyze the product and determine the carbonation reaction equation.The results showed that the calcite-type CaCO₃ crystals and the C_xSH_y?CSH?after Ca²⁺elution were the main carbonation products of both C₃S₂ and?-C₂S.The size of CaCO₃ was generally 2-10?m,and the particles were closely packed,mainly filled in the voids of the sample matrix which was reducing the porosity of the samples and improving the mechanical strength of the hardened test block.When the CO₂ partial pressure was 6MPa,the morphology of intermediate product CSH changed into a fibrous or needle-like,the CaCO₃ changed into cubic and elongated shape with the pores.As the carbonation reaction progressed,the Ca/Si ratio in CSH gradually decreased.When the carbonation times was within the range of 10min-7d,the measurement relationship of CSH was C_0.35-1.3SH_0.03-0.3.Taking steel slag brick as the research object,two steel slag block bricks were cast with a fixed mixproportion of steel slay:stone:sand ratio=2:1:1,its mechanical strength,product type,micromorphology and stability after carbonation were systematically analyzed,and a hardening model was also established.The results showed that the 7-carbon retention of hollow brick and solid brick were 7.02%and5.2%respectively,and the compressive strength meeted 27.5MPa and 47.1MPa respectively,Among them,the steel slag solid brick has meeted the MU40 level of the national solid brick load-bearing brick standard.Under the same conditions,the strength of the solid brick is 70%higher than that of the hollow brick.C₃S,C₂S,f-CaO and MgO in the steel slag will react with CO₂.However,Ca₂?Al,Fe?₂O₅,FeO·2MgO did not participate in the reaction.the produced carbonation products can connect the stone and sand to form a dense structure and reduce the porosity.Additionally,the steel slag brick also presented a good volume stability under the autoclaving pressure of 3MPa.