Fly ash can effectively improve the sulfate resistance of immersed concrete by refining the pore size. However, increase of capillary pores may not be a good thing for salt crystals. In this paper, the effects of fly ash contents and environmental humidity on the sulfate resistance of semi-immersed mortars was investigated by appearance, mechnical strength, and microstructure. Capillary adsorption capacity, ingressed sulfate concentration and erosion productes were further investigated to analyze damage mechanism of mortar under semi-immersion condition. Lower environmental humidity and higher fly ash content were not conducive to the sulfate resistance of semi-submerged mortar, when the humidity was lower than 55%, the fly ash content should be further reduced.The migration of sulfate ions in capillary pores and the physical salt crystallization in the region of humidity change are the main reasons for the damage.

The use of uncalcined arenaceous rock coal gangue powder (ACP) instead of fly ash as a siliceous raw material to produce autoclaved aerated concrete (AAC) is a potential method for achieving low energy consumption and high-efficiency utilisation of ACP. The effects and mechanism of the Ca/Si ratio (C/S) on the slurry and physical and mechanical properties of ACP-based AAC (AAAC) and fly ash-based AAC (FAAC) were studied. The results show that the compressive strength of FAAC decreases first and then increases with C/S, while the compressive strength of AAAC increases with C/S increasing. When the C/S is 0.73-0.93, the compressive strength of AAAC was 89.3-165.6 % lower than that of FAAC, and the thermal conductivity was 3.1-10.9 % higher. This is mainly related to the morphology, distribution characteristics and pore structure of tobermorite crystals in different AAC, and the morphology and quantity of tobermorite crystals are mainly affected by the amount of active Si4+ 4 + dissolved in the siliceous raw material.

MICP was a modification method often used to modify recycled fine aggregates (RFAs) in recent years. The paper studied the mechanism of modifying two RFAs with different particle grading by MICP. The results showed a corresponding alteration in the proportion of internal mineralization sites and external mineralization sites within RFAs with the variation in particle size, thereby impacting the amount and proportion of internal and external CaCO3. The internal and external CaCO3 improved the properties and packing state (closer to the MAA model) of the RFAs, respectively. The use of <1.18 mm RFAs could take into account both improvement efficiency.

The low carbonization of ultra-high performance concrete (UHPC) has become a hot topic for sustainable development in the construction industry. Calcined coal gangue was used as aggregate and binder to prepare ultra-high performance coal gangue geopolymer concrete (UHPGC) in this paper. The hybrid reinforcement effect of steel fibers, calcium carbonate whiskers (CW), and carbon nanotubes (CNTs) on UHPGC was studied in the experiment. The results indicated that when the mixing ratio of straight steel fibers and hooked-end steel fibers was 2:1, the 28 d-compressive strength of UHPGC reached 145.94 MPa. However, excessive hooked-end steel fibers deteriorated the reinforcement effect of adjacent fibers and reduced the mechanical properties of UHPGC. CW and CNTs had a more significant improvement in the flexural properties of UHPGC. The bridging effect of CNTs and CW dispersed the stress on the matrix and delayed the formation and development of micro-nano cracks. Meanwhile, CNTs and CW improved the fiber bridging energy dissipation, thereby improving the flexural properties of UHPGC. The UHPGC had the highest flexural strength and flexural deflection of 13.59 MPa and 4.12 mm, respectively. The UHPGC prepared in this paper had lower carbon emissions and economic costs compared to traditional UHPC. Overall, this study provided valuable references for the resource utilization of coal gangue and the multi-scale strengthening and toughening of UHPGC.

Sulfate attack is one of the most important factors affecting the durability of concrete, and the corrosion sources of internal sulfate attack (ISA) mostly come from the raw materials of concrete. Adding corrosion resistance admixtures to concrete is one of the most effective ways to prevent internal sulfate attack. By wrapping barium salt with SA gel, the modified barium chloride admixture (MB) is obtained which can exist stably in concrete and release barium ions to form barium sulfate in a sulfate environment. The modified barium salt has little effect on the cement workability and hydration process and even can improve the mechanical properties slightly. When the MB content reaches 6% (MB-6), the strength improvement effect is most obvious. Compared with the P-0, the 7d compressive strength of MB-6 is increased by 9.14%, and the 28d compressive strength is increased by 5.77%. It can be seen by SEM that the amount and crystal size of ettringite in the cement mixed with admixtures are reduced. The corrosion resistance of the matrix can be further improved by using a modified barium chloride-silica fume composite admixture (MB-F). After 90 days of internal sulfate attack, the strength of the MB-F even improves slightly. When the modified barium chloride to silica fume ratio is 4:6, the cement-based material achieved the best resistance to internal sulfate attack. In addition to MB, which can make sulfate ions fixation and inhibit the formation of ettringite, silica fume can also consume calcium hydroxide and generate more C-S-H gel, which compacts the weak part of the matrix and greatly improves the resistance to internal sulfate attack of cement-based materials. It provides the possibility for the application of modified barium salt in external sulfate attack.

The rusting and cracking of reinforced concrete structures cost billions of dollars around the world every year, and there is a lack of targeted repair methods. In this study, we first put forward an electrophoretic deposition repair method using a high-performance colloid solution containing water-based cationic epoxy resin. The effect of electrophoretic deposition on rust-cracked reinforced concrete was studied. Then, the mesoscopic and microscopic morphologies of the repaired concrete surface, cracks, and defects at the interface of reinforced concrete were examined. Finally, the mechanism of electrodeposition was discussed, and the related reactions were summarized (ionization, electrolysis, electrophoresis, electrodeposition, electroosmosis, and curing reaction). The results show that electrophoretic deposition successfully increased the internal compactness, unconfined compressive strength, and split tensile strength of reinforced concrete, while greatly reducing the water absorption and the accelerated carbonation depth.

Electrophoretic deposition is a new method for the repair of rust-expanded cracked reinforced concrete, which needs high performance epoxy resin repair solution. Selecting an appropriate solution concentration and temperature, clarifying the molecule electrophoretic deposition rule in the repair process, and revealing the influence mechanism of solution concentration and temperature on the repair effect will help maximize the application potential of the proposed method and control repair costs. In this study, four high-performance epoxy resin solutions with different concentration and temperature were prepared. The molecule electrophoretic deposition rule during the repair process was explored via macroscopic observation, mass increment, and resistance tests. The influence mechanism of solution concentration and temperature on the repair effect is discussed using microscopic observations combined with theoretical analysis. The results show that the waterproofing, carbonation resistance, and stray current erosion resistance of repaired specimens improve as the solution concentration increases. With the increase of solution concentration, the number of molecules per unit volume and the current density between electrodes increased, so the repair speed was faster and the repair effect was better under the condition of high concentration. The high temperature reduced the viscosity of the repair solution, which was conducive to the rapid deposition of molecules in the solution on the surface of the specimen at the early stage of the repair, but was not conducive to the film formation of epoxy resin on the surface of the specimen. Therefore, the waterproof and carbonization resistance of the repaired specimen decreased after the temperature increased.

Cationic epoxy resin is an important basic material for electrophoretic deposition matching repair solution. This paper mainly studied the effects of amination rate and neutralization degree of cationic epoxy resin on elec-trophoretic deposition repairing effect of rust-cracked specimens, and analyzed the corresponding mechanism. The results show that the higher the amination rate and neutralization degree are, the faster the electrophoretic rate of molecules in the solution is, and the higher the deposition amount of epoxy resin on the specimen surface is. However, the higher the two parameters are, the waterproof performance of the repaired specimen will be worse.

The corrosion of reinforced concrete could reduce its mechanical properties and cause concrete cracking. Electrophoretic deposition is a new method for repairing rust-cracked reinforced concrete. In this study, we observed the crack surface morphology, mass growth, epoxy resin film thickness, ultrasonic rate, and epoxy resin filling depth of rust-cracked reinforced mortar specimens during the repair process and investigated internal microstructure and phase composition of the repaired mortar, and clarified the principle behind the electrophoretic deposition of cationic epoxy resin and curing agent molecules in the repair solution. The result of research shows that longer repair times decreased the carbonization depth and water absorption of the specimens. After repair, the porosity of the surface mortar decreased, owing to the filling of epoxy resin, and the newly generated epoxy resin film had a dense structure. Thus, the carbonization resistance and waterproof performance of the repaired specimens improved. The repair process of electrophoretic deposition can be divided into the immersion, repair, and curing stages, and the physical and chemical principles of each stage were explained.

This study proposed an electrophoretic deposition repair method to effectively repair and improve the durability of rust-cracked reinforced concrete structures, exploring the influence of cationic epoxy resin types on the repair process. After electrophoretic deposition repair, the carbonation resistance and waterproofing performance of the specimens improved significantly; as the repair in the solution of cationic epoxy resin molecular weight increased, the specimen surface crack healing rate and deposition rate increased; the larger the molecular weight of the cationic epoxy resin in the repair solution, the poorer the compactness of the repaired specimen, and the smaller the filling depth of epoxy resin.

An electrophoretic deposition technique is proposed for repairing rust-cracked concrete. A water-based cationic epoxy resin was used as the main base material of a high-performance epoxy resin colloidal (HERC) solution, focusing on the resulting concentration of electrophoretic deposition and post-repair performance metrics. Water and carbonation resistance of repaired specimens improved significantly with increased HERC concentration. Compressive strength and splitting tensile strength also improved. A microscopic morphology analysis showed that water and carbonation resistance, and strength of repaired specimens improved because a dense resin film structure formed on the crack surface, and resin was deposited and consolidated in the cracks, respectively.

Durability of cement-based materials for marine/coastal structures is an increasingly challenging problem. Sulfate ions in seawater can react with aluminate in cement to form erosion products, causing cracks and spalling. When cement is used to stabilize loose erodible sand in coastal areas, the resistance to sulfate attack is questionable. In this study, four cements with different aluminate contents were used to stabilize sand. Cement stabilized sands were immersed in 5% Na2SO4 solution for 300-days to simulate long-term sulfate attack process. The deterioration of engineering performance was evaluated based on expansion ratio, mass change, uniaxial compressive strength, and ultrasonic velocity. The deterioration mechanisms were analyzed through mineralogical and microstructural observations including X-ray diffraction, EDS, scanning electronic microscopy, and nuclear magnetic resonance. The results showed that the development of macro-scale mechanical performances could be divided into two stages (initial stage and erosion stage) when subjected to 300-days immersion in 5% Na2SO4 solution. Sand stabilized by low-aluminate-content cement displayed better engineering performance especially at the erosion stage. Mechanistically, more ettringite was formed in high-aluminate-content cement stabilized sand, leading to swelling and cracking. The formation of ettringite and gypsum were accompanied with the consumption of portlandite, leading to further strength loss.