Effect Of High Temperature Carbonation Curing And Pre-carbonized Steel Slag On The Material Properties Of Steel Slag-Cement Binding Materials

Basic oxygen furnace steel slag (SS) is the by-product of steel production and constitutes 15～20% of steel output. In 2014, China’s crude steel production more than 800 million tons, and global steel production over 1.6 billion tons. Production of SS as a by-product has reached more than 100 million tons. The large amount of SS storage not only occupies land resources but also pollutes the environment. Therefore, it is imperative to find effective ways to utilize SS. Research shows that SS and Portland cement clinker have similar chemical and mineral compositions and can be used as a cement admixture. However, if the addition amount reaches 30%, the early strength of the concrete decrease drastically, and there will be other problems, such as poor stability. Therefore, applications containing more than 30% SS will be limited. Currently, the excitation mode used to improve the early strength of SS cementitious materials can be divided into three categories:1. Physical excitation, which refers to the use of physical methods to improve the fineness of the ground slag; 2. Chemical excitation, which is the addition of a chemical excitation component to the early strength agent; 3. Thermodynamic excitation, which improves the temperature of the SS hydration to improve the cement performance.CO2 emissions from global fossil fuel combustion increased again in 2013 up to 36 billion tons. CO2 emission reduction and fixation has become a popular issue. In this paper, through high temperature carbonation curing, CO2 can quickly cure and be permanently stored in the SS. CO2 can also stimulate the activity of the SS. Through pre-carbonized SS, than adding pre-carbonization SS to replaced 30% of the cement to prepare cementitious material, the early and late strength of cementitious materials with a large amount of SS increased significantly and achieved the P.O 42.5 cement intensity level. This strategy not only solved the problem of low early strength in the Cement Binding materials with a large volume of SS but also stored gaseous CO2, which helps alleviate the greenhouse effect. This study aspect is rarely reported. The composition and microstructural changes of SS cementitious materials after pre-carbonation were also studied.Replace 30% of cement by slag preparation of cementitious materials, high temperature and carbonation curing. Study on effect of curing conditions on the mechanical properties of steel slag-cement binding materials. Analysis of the degree of hydration of slag cementitious material by XRD and TG-DTA. Characterization of hydration products and changes in its morphology with SEM and EDS. Results showed that:the best of curing conditions is 60℃ carbonized 7h. Its 3d、7d、28d flexural compressive strength are higher than standard curing. The compressive strength of 3d increased by 63.94%,7d increased by 25.55%,28d increased by 11.79%; cementitious material surface after carbonation produced a layer of dense shell of CaCO3, the particles size of CaCO3 are 1～8 μm.The dense shell can effectively prevent the occurrence of dissolving leaching of cement-based materials, improve the durability of cementitious materials. The particles of CaCO3 are fine, and play a roles in micro-aggregates in cementitious materials, accelerated the hydration of C3S.Pre-carbonized steel slag under normal pressure, and replaced 30% of the cement with carbonized steel slag to prepare cementitious material. This was done to study the effect of adding pre-carbonization steel slag on the mechanical properties of steel slag-cement binding materials. The changes of mineral composition of steel slag after carbonization was analysed by XRD. The early hydration properties of cementitious materials was analysed by Hydration exothermic curve. The hydration degree of the steel slag cementitious material was analysed by XRD and TG-DTA. Characterization of the hydration products and the changes in the cementitious material morphology was investigated using SEM and EDS. Determination of pore structure of steel slag-cement binding paste by mercury intrusion method. The results showed that the best pre-carbonization condition was 75℃, the quality ratio of water is 10%, the carbonization time is 2h. After carbonization, Some carbonate minerals appeared and RO phase peak almost disappeared in carbonated steel slag powders, and the expansion rate of pre-carbonization specimen decreased by 60.0%. The hydration degree of pre-carbonization steel slag cementitious material 3d sample is 2.2 times the standard sample, and the non-evaporable growth rate is 18.18% at the age of 28d. The Ca(OH)2 crystal growth direction was more diverse after carbonized, and the total porosity decreased by 29.55%, so the crystal structure will be more dense. Compared to the standard specimens, the compressive strength of 3d,7d, 28d were increased by 25.51%,21.55% and 5.83%; flexural strength of 3d,7d,28d were increased by 18.90%,16.01% and 10.91%. The 28d compressive strength is 44.27MPa, reached the P42.5 cement strength grade.