Research On The Carbonation Behavior And Mechanism Of Hardened Alkali-Activated Slag Cement Pastes

Alkali-activated slag cement is a kind of environmental friendly green binding material, which has the advantages of superior properties, effective utilization of industrial by-products, simple producing progress, low energy, low natural resource consuming and greenhouse gases emission. So it is praised as one of most exploration potential cementitious materials in the 21st century. From the practical application angle, improving the durability of materials can adapt well to extend service time of concrete structures and to the sustainable development of soc-economy. Carbonation and steel corrosion has been proven to be the main form of damages to the concrete structures durability. Although alkali-activated slag concrete has noticeable excellent properties, previous studies have demonstrated that its carbonation rate is high, which especially be reflected in the low strength grade concrete. Overall,there is little study on its carbonation resistance and there is dispute on its carbonation resistance evaluation, there is no common agreement on the essence of carbonation behavior and the proper technical approach to improve the carbonation resistance of alkali-activated slag concrete has not been established, which is the weak link and is one of the key technologies to restrict its development. Therefore, the investigation of carbonation behaviour and microscopic mechanism of alkali-activated slag cement stone(AASS) would lay a foundation to scientifically evaluate the ability of alkali-activated slag concrete to carbonation resistance and to guide the durability design and the application and development of alkali-activated slag concrete structure.In this project, alkali-activated slag cement stone(AASS) activated by water glass and NaOH was used to investigate the effect of main factors such as alkali activator type, Na₂O equivalent, water glass module(Ms), water binder ratio(W/B), curing condition and pretreatment method on the carbonation depth and shrinkage behavior. The effect of carbonation on the pH value, Na⁺, K⁺ and Ca²⁺ content of pore solution of AASS was also studied. The products of carbonation and their microscopic structure were identified by using XRD, SEM, TG-DSC, FT-IR analysis and so on. The effect of carbonation on the pore structure of AASS was analysed by using nitrogen absorption measurement. Based on the technical measures of increasing the alkalinity of solid dispersive phase and reducing the drying shrinkage and soluble alkali content of AASS, technical approach to improve the carbonation resistance of AASS was studed. Chief laws and results are summarized as follows:In the range of 3%～6%, carbonation depth of AASS decreases with the increase of Na2O equivalent, although the extent of reduction decreases with the increase of Na2O equivalent. At the same Na2O equivalent, the carbonation depth of AASS activated by NaOH is larger than that of AASS activated by water glass. In the range of 1.0～2.0, with the increase of module, carbonation depth of AASS activated by water glass first decreases, then followed by an increase. When Na2O equivalent is 4%⁶% and the module of water glass is 1.2¹.5, AASS possesses excellent capability to carbonation resistance. Besides, carbonation depth of AASS increases when W/B increases and decreases when standard curing ages extends. Water curing can increase the carbonation depth of AASS.Alkali-activated slag cement mortar is more easily carbonated than Portland cement mortars when water-binder ratio is the same. While the consistency of mortars is the same, alkali-activated slag cement mortar used water glass as alkali activator is less carbonated than Portland cement mortar. After being carbonated, the compressive strength of Portland cement mortar increases, while that of alkali-activated slag cement mortar shows the tendency of decline with the increase of carbonation ages within 60 days.The study on the characteristic of carbonation shrinkage of AASS shows that carbonation doesn't increase the shrinkage of AASS. In the range of 3%～6%, carbonation shrinkage of AASS decreases with the increase of Na2O equivalent. When water glass is used as the activator, carbonation shrinkage is relatively small with the module of 1.2¹.5. In the range of 0.30～0.35, while W/B increases, carbonation shrinkage of AASS increases and that decreases when standard curing ages extends. Carbonation shrinkage of AASS under water curing is larger than that of standard curing at the same age.Before carbonation, Na⁺ content is more, K⁺ content is less, while Ca²⁺ content is much lower in the pore solution of AASS. The pore solution pH value of AASS activated by NaOH is larger than that of AASS activated by water glass. After being 60d carbonated, pH value of pore solution reduces to about 10 and Na⁺ content increases, while K⁺ and Ca²⁺ content decreases.When water binder ratio is 0.30, in the AASS activated with water glass, the diameter of main pores is less than 20 nm which volume percentage touches 78%. While in the AASS activated with NaOH, capillary which diameter more than 20nm is more and its volume percentage is 68%. Carbonation increases the amount of the specific surface area of AASS and reduces the average and the most probable distribution pore diameters. C-S-H gel of AASS is an amorphous gel which is more close to tobermorite. C/S ratio of C-S-H gel of AASS activated with water glass is smaller than that of AASS activated with NaOH. Carbonation causes decalcification of C-S-H gel ,so C/S ratio of C-S-H gel decreases, polymerization degree increases. As carbonation products, the main form of CaCO₃ is calcite. With the process of carbonation, vaterite and aragonite come into being. Besides, gaylussite and nahcolite have also been detected. The existence of more alkali (Na⁺,K⁺) in the pore solution, no existence of Ca(OH)₂ crystal in hydration product and the larger drying shrinkage of alkali- activated slag cement stone are the important reason for its serious carbonation.Alkali-activated slag cement stone with different activator has its own proper technical approach to improve the carbonation resistance. Increasing the alkalinity of solid dispersive phase is suitable for AASS activated with water glass. Right amount of Ca(OH)₂,Ba(OH)2 and ZnCl₂ can effectively reduce the carbonation depth of AASS. While to AASS activated with NaOH, decrescence of drying shrinkage is useful. Right amount of air entraining agent and Na2SO4 do well to reduce the carbonation depth of AASS.