Effects Of Curing Regimes On The Chloride Binding And Migration Behavior Of Cementitious Materials

Manufactured cement and concrete are now a new direction in the cement industry,and the challenges encountered in its service environment are also hot research directions.Most of them have been used in the moist heat curing system,which can improve their mechanical properties,shorten the curing period and increase the economic effect in the early stage.However,while accelerating the hydration of the cement,the hygrothermal curing also brings new problems to the durability of the material,such as high brittleness,easy cracking,poor toughness,uneven distribution of hydration products,and roughness of the pore structure.Based on the behavior of chloride binding,diffusion and migration in cementitious materials under different curing systems,this paper investigates the effects and causes of mineral admixtures,water-cement ratio and other factors on the binding and migration behavior of chloride ions in cementitious material systems under different curing regimes.Based on the national key research and development project"key materials and preparation technology for long life concrete products"（NO.2017YFB0310000）,the main work and innovation achievements of this paper are as follows:（1）The binding ability of cementitious materials to chloride ions under three different curing systems——standard curing,steam curing,and autoclave curing environments were investigated.The total and chemically bound isothermal adsorption patterns of chloride ions under standard and steam curing are in accordance with the Freundlich adsorption curve,while the physical adsorption pattern is in accordance with the Langmuir adsorption curve;while the materials under autoclave curing are generally more in accordance with the Freundlich adsorption curve.In general,the isothermal adsorption and binding law of cementitious materials to chloride ions is in accordance with Freundlich adsorption curve,which is more general.（2）To investigate the chloride binding behavior of different cementitious material systems with different water-cement ratios and admixtures,it was found that the binding ability of the materials to chloride ions increased with the decrease of water-cement ratio;mineral admixtures containing more Al phase could enhance the chloride binding capacity of cementitious materials,and the addition of Mg-Al LDHs type of hydrotalcite lamellar materials could also enhance the binding ability of cementitious materials to chloride ions.The steam curing can improve the physical adsorption capacity of cementitious materials to chloride ions,and the binding ability of cementitious materials to chloride ions decreases significantly under autoclave curing.（3）Microscale tests were conducted on cementitious materials prepared under different curing regimes,and the effects of the three curing regimes on the microstructure of the materials were investigated by SEM,²⁷Al MAS NMR,and TG-DSC.The results showed that after steam curing,the C-S-H gel morphology of cement hydration products was transformed from granular to honeycomb,and after aotuclave curing,the C-S-H gel was highly crystallized and transformed to fibrous.After steam curing,the AFm content in the Al-phase hydration products of cement hydration increased;after autoclave curing,AFm and AFt were transformed into TAH,a third amorphous products.（4）The effects of different curing regimes on the pore structure distribution and chloride ion migration behavior of cementitious materials were tested by Low Field-NMR and RCM,etc.The results showed that steam and autoclave curing enhanced the porosity and coarsened the pore size of the materials,while steam curing enhanced the chloride ion migration coefficient of the materials.The reduction of water-cement ratio and the addition of mineral admixture can reduce the depth of chloride ion penetration in the material in the curing regimes of steam and aotuclave curing and improve its resistance to chloride ion attack.