Study On The Accelerated Carbonation Curing And Its Impacts On The Properties Of Dry-hard Cement Paste

In recent years,dry-hard cement-based products prepared with low water to cement ratio（w/c）and high moulding pressure,have gradually replaced some traditional casting cement-based products with a relatively high w/c due to their low energy consumption,low cost,and high performance.Different from traditional cement-based products,dry-hard cement-based products can be demoulded without any setting time,thus can be carbonated in early age before hydration,which is critical for shortening curing time and improving physical performance in industrial applications.However,the diversities of fabrication conditions,such as w/c and moulding pressure,lead to the differences in the internal structure,thus,require different accelerated carbonation conditions.Besides,the existing research about the carbonation regularity of traditional cement paste which has already hydrated is not applicable for the extrapolation to the carbonation of dry-hard cement paste.Therefore,to optimize the accelerated carbonation of dry-hard cement paste and its application in engineering practice,this study explores the early-age carbonation process based on the physical properties and the microstructure changes of short-term carbonation（2h）of dry-hard cement paste,considering the carbonation curing conditions of CO₂concentrations（0.04%,1%,3%,10%,and 20%）and moulding conditions including water-cement ratios（0.125,0150 and 0.175）and moulding pressure（10 MPa,15 MPa and 20 MPa）.In this study,the compressive strength,pore water saturation,carbonation depth,and p H value are investigated.Thermogravimetric analysis（TGA）,electron microscopy（SEM）,X-ray diffraction（XRD）,and nuclear magnetic resonance（NMR）are adapted to identify microstructure change.The main findings are as follows:（1）The differences of CO₂concentrations in cement system promotes the carbonation reaction,thus increasing CO₂concentrations can accelerate the carbonation rate and carbonation degree.The strength of sample carbonated with 20%CO₂for 2h（C-20-2h）is about 7 times higher than that of sample carbonated in air（N-0.04-2h）.Besides,the increasing CO₂concentrations accelerate the hydration reaction by the continuously consuming of Ca²⁺.NMR results reveal that the decalcification degree and polymerization in C-S-H increase as the CO₂concentration increases from 0.04%to 20%,and the average chain length increases from 2.72 to3.37,which indicates a higher hydration degree.The improvement of carbonation reaction and hydration reaction can significantly enhance the strength development ofdry-hard cement paste.The strength of sample C-20-2h（4.22 MPa）is about 4 times greater than that of sample N-0.04-2h（1.10 MPa）.（2）Increasing CO₂concentration can promote the size growth of Ca CO₃and Ca（OH）₂without affecting its crystal form.It is observed in SEM that the crystal morphology of Ca CO₃and Ca（OH）₂fully develop as the CO₂concentration increases,and the increased content shows a better filler effect.However,only calcite is detected by XRD in carbonated sample regardless of altering the CO₂concentrations.（3）The coupling of w/c and moulding pressure affects the carbonation process by the diffusion rate of CO₂.The initial pore water saturation(k_initial)is the reflection of the coupling effect of w/c and moulding pressure,which dominates the gas-liquid diffusion limit in the cement system.The k_initialyields an increase with the increasing w/c and molding pressure.Sample prepared with lowest w/c of 0.125 and moulding pressure of 10 MPa（A-0.125-10）gives the lowest k_initialvalue of 50.58%,while sample prepared with highest w/c of 0.175 and moulding pressure of 20 MPa（C-0.175-20）achieves a maximal k_initialvalue of 91.93%.The enhancement of water filling effect and the capillary water absorption effect caused by the increasing of w/c and moulding pressure are responsible for the increase in initial pore water saturation.（4）The higher initial pore water saturation(k_initial)compresses the diffusion space of CO₂and magnifies the difficulty in CO₂diffusion,thus hinders the carbonation reaction,and deceases the carbonation efficiency.Therefore,sample A-0.125-10 with lowest k_initialobtains high carbonation strength of 36.88 MPa,and exhibits the maximum carbonation depth of 7 mm.In addition,it possesses the lowest p H value at the range of 9.2～10.9 as well as the highest Ca CO₃and Ca（OH）₂contents in each sliced layers,compared with that in corresponding layers of other sample.However,the carbonation strength of sample C-0.175-20 with highest k_initialis only4.36 MPa and there is a lowest carbonation depth of 0 mm as well as the highest p H value of sliced layers ranges from 12.4 to 13.1.（5）Moreover,regardless of the atmospheric carbonation or pressurized carbonation,subsequent water curing of carbonated specimens is found to compensate the inadequate strength development resulted from the short-term carbonation in early age,which is associated with the further hydration of uncarbonated C₂S and C₃S for strength development.Early age carbonation can be developed as a pre-curing method to shorten the curing time and achieve the higher early strength as well as promoting further strength development.Meanwhile,it is vital to control w/c between 0.125 to0.150 for better carbonation strength and the further hydration strength in industrial applications,while the corresponding moulding pressure is within 15 MPa to 20 MPa.