A green ultra-high performance geopolymer concrete containing recycled fine aggregate: Mechanical properties, freeze-thaw resistance and microstructure

The Dispersion and Hydration Improvement of Silica Fume in UHPC by Carboxylic Agents

Silica fume (SF) is an essential component in ultra-high-performance concrete (UHPC) to compact the matrix, but the nucleus effect also causes rapid hydration, which results in high heat release and large shrinkage. In this paper, the carboxylic agents, including polyacrylic acid and polycarboxylate superplasticizer, were used to surface modify SF to adjust the activity to mitigate hydration at an early time and to promote continuous hydration for a long period. The surface and dispersion properties of modified SF (MSF), as well as the strength and pore structure of UHPC, were studied, and the stability of the modification was also investigated. The results demonstrated that, after treatment, the carboxylic groups were grafted on the SF surface, the dispersion of SF was improved due to the increased negative pentanal of the particle surface and the steric hindrance effect, the early hydration was delayed about 3-5 h, and the hydration heat release was also mitigated. The compressive strength of UHPC with MSF reached a maximum of 138.7 MPa at 3 days, which decreased about 3.7% more than the plain group, while flexural strength varied insignificantly. More pores and cracks were observed in the matrix with MSF, and the hydration degree was promoted with MSF addition. The grafted group on SF fell off under an alkali environment after 1 h.

Designing low-carbon fly ash based geopolymer with red mud and blast furnace slag wastes: Performance, microstructure and mechanism

In order to tackle the environmental problems induced by Portland cement production and industrial solid wastes landfilling, this study aims to develop novel ternary cementless fly ash-based geopolymer by recycling red mud and blast furnace slag industrial solid wastes. The fresh-state properties, mechanical strength, water permeability, phase assemblage and microstructure were systematically investigated to evaluate the performance variation and reveal the hydration mechanism for geopolymers with different mixing proportions. The results showed that a higher slag content or a lower red mud content could result in the higher fluidity and shorter setting time for fresh mixture. The existence of slag promoted the transformation of N-A-S-H to C-A-S-H gel, which contributed to higher compressive strength and better resistance to water penetration. However, an excessive incorporation of 30% red mud may impede the generation of N-A-S-H gel and form more flocculent-like loose hydrates, thus to mildly degrade the mechanical strength and anti-permeability. The synergetic utilization of red much and blast furnace slag in fly ash-based geopolymer led to much less CO2 emission compared with the condition that red much or slag was singly added, which demonstrated prominent environmental advantages for such kind of ternary cementless geopolymer with equivalent mechanical strength.

Study on Frost Resistance and Interface Bonding Performance through the Integration of Recycled Brick Powder in Ultra-High-Performance Concrete for Structural Reinforcement

This study aims to address the issues posed by frost damage to concrete structures in cold regions, focusing on reinforcement and repair methods to increase the service life of existing structures instead of costly reconstruction solutions. Due to the limitations of conventional concrete in terms of durability and strength, this research focused on ultra-high-performance concrete (UHPC) by replacing part of the cement with recycled brick powder (RBP) to strengthen ordinary C50 concrete, obtaining UHPC-NC specimens. Mechanical tests investigated the bonding performance of UHPC-NC specimens under various conditions, including interface agents, surface roughness treatments, and freeze-thaw after 0, 50, 100, and 150 cycles with a 30% replacement rate of RBP. Additionally, a multi-factor calculation formula for interface bonding strength was established according to the test data, and the bonding mechanism and model were analyzed through an SEM test. The results indicate that the interface bonding of UHPC-NC specimens decreased during salt freezing compared to hydro-freezing, causing more severe damage. However, the relative index of splitting tensile strength for cement paste specimens showed increases of 14.01% and 14.97%, respectively, compared to specimens without an interface agent. Using an interface agent improved bonding strength and cohesiveness. The UHPC-NC bonding model without an interfacial agent can be characterized using a three-zone model. After applying an interfacial agent, the model can be characterized by a three-zone, three-layer bonding model. Overall, the RBP-UHPC-reinforced C50 for damaged concrete showed excellent interfacial bonding and frost resistance performance.

Testing and Analysis of Ultra-High Toughness Cementitious Composite-Confined Recycled Aggregate Concrete under Axial Compression Loading

In order to analyze the axial compressive properties of ultra-high-toughness cementitious composite (UHTCC)-confined recycled aggregate concrete (RAC), a batch of UHTCC-confined RAC components was designed and manufactured according to the requirements of GB/T50081-2002 specifications. After analyzing the surface failure phenomenon, load-displacement curves, scanning electron microscope (SEM), and parameter analysis of the specimen, the result shows that UHTCC-confined RAC is an effective confinement method, which can effectively improve the mechanical properties and control the degree of surface failure of RAC structures. Compared with the unconfined specimen, the maximum peak load of the UHTCC confinement layer with a thickness of 10 mm and 20 mm increased by 44.61% and 79.27%, respectively, meeting the requirements of engineering practice. Different fiber mixing amounts have different effects on improving the mechanical performance of RAC structural. The specific rule was steel fiber (SF) > polyvinyl alcohol fiber (PVAF) > polyvinyl alcohol fiber (PEF) > no fiber mixture, and the SF improves the axial compression properties of UHTCC most significantly. When there are strict requirements for improving the mechanical properties of the structure, SF should be added to UHTCC. On the contrary, PVAF should be added to UHTCC.