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Two kinds of layered double hydroxides (LDHs), MgAl-NO2 (N-LDH) and MgAl-CO3 (C-LDH), were incorporated to study the durability of mortar and concrete. The LDH contents of mortar were 1%, 2%, and 4% by mass and the LDH contents of concrete were 0.5%, 1%, 2%, 4%, respectively. The effect of LDHs on sulfate resistance of mortar was studied through dry-wetting cycle test, compressive strength test, and flexural strength test. In addition, the effects of LDHs on pore structure, chloride resistance, carbonation resistance, shrinkage, and creep of concrete were investigated by SEM, mercury injection test, XRD, chloride ion diffusion coefficient test, chloride salt corrosion depth test, carbonation depth test, shrinkage test, and creep test. The results showed that LDHs can improve the ability of resisting ion corrosion, carbonization, shrinkage, and creep, reduce the pore content, and optimize the pore structure of mortar and concrete to some extent. Moreover, 4% LDHs had a better effect on improving the durability of mortar and concrete compared to 0.5%, 1%, and 2% LDHs, and the effect of C-LDH was better than N-LDH. [ABSTRACT FROM AUTHOR]

• The compressive strength of CNTs/cement composites was predicted successfully • The prediction accuracy of XGB was higher. • The strengthening effect of CNTs in paste and mortar was more significant. There are many factors that affect the compressive strength of carbon nanotubes/ cementious composites. However, there is a lack of comprehensive research on the effect of various properties of CNTs on the compressive strength of cement-based materials. In this paper, machine learning was used to predict the compressive strength of single system and multi-system of cement-based materials mixed with CNTs, and the optimal parameters of CNTs properties were also determined. Linear Regression (LR), Support Vector Regression (SVR), Random Forest (RF) and Extreme Gradient Boosting (XGB) were used to predict the compressive strength of the cement paste, mortar and concrete(single system) respectively. The results showed that the R2 of prediction of RF and XGB models was greater than 0.9 for all three systems, and the accuracy of XGB was higher. The data of the three systems were combined into a new dataset(multi-system), RF and XGB were used for its prediction. The results indicated that the same prediction errors were achieved by RF and XGB, and the R2 was 0.93. SHAP model was utilized to explain the prediction results. The analysis of single system showed that the content and diameter of CNTs had obvious effects for all the three systems, and the influence degree of CNTs in concrete was less than that of paste and mortar. Some optimal parameters of CNTs were determined by the multi-system SHAP results: the optimal values of the length and diameter of CNTs were 20 μm and 25 nm, and the content should be within 0.1 %. [ABSTRACT FROM AUTHOR]

• The microstructure and mechanical properties of pumping concrete was investigated. • The finite element model can simulate the uniaxial compression of pumping concrete. • The established method can predict stress–strain relationship of pumping concrete. At present, it is unclear how the microstructure and multi-scale mechanical properties of concrete change after ultra-high-rise pumping. To this end, this study applied BSE image analysis method and nanoindentation test to investigate the influence of ultra-high-rise pumping on the porosity, thickness and elastic modulus of interface transition zone (ITZ) in concrete, firstly. The results showed when increasing the pumping height, the elastic modulus and uniaxial compressive strength of concrete and mortar increased slightly, the thickness and porosity of ITZ decreased, and the elastic modulus of ITZ increased. Secondly, based on the porosity test results of ITZ and paste, the volume fractions of pores, mortar and equivalent body (aggregate + ITZ) in concrete with different pumping heights were determined, and then a 3D model of concrete was established based on the X-ray Computed Tomography test results. Thirdly, according to the mechanical properties of the mortar before and after ultra-high-rise pumping, the constitutive relationship of the mortar in the ultra-high-rise pumping concrete was modified, and the mechanical parameters of the equivalent body in the model considered different pumping heights were calculated by the homogenization method. Finally, the uniaxial compression of ultra-high-rise pumping concrete was accurately simulated using the finite element method. On this basis, according to the relationship between pumping measurement index (PMI) and the performance of ultra-high-rise pumping concrete, the uniaxial compressive stress–strain curve of ultra-high-rise pumping concrete was precisely predicted. [ABSTRACT FROM AUTHOR]