Tong Gao

Fly ash (FA) recycling into cementitious binders offers a promising route to simultaneously reduce the environmental burden of high-impact construction materials, divert industrial waste from disposal pathways, and generate engineering value from an existing aluminosilicate residue. FA has therefore been widely incorporated into ordinary Portland cement (OPC), calcium sulfoaluminate (CSA) cement, magnesium potassium phosphate cement (MKPC), and alkali-activated/geopolymer systems, where it can improve workability, mechanical strength, durability and promote hydration reactions, depending on the binder chemistry. However, these benefits are not always successful: FA addition may also reduce early-age strength, delay setting, limit reactivity, impair fluidity, or produce under-reacted matrices when ash properties are mismatched with the chemistry window of the target binder. Rather than revisiting these systems as isolated application categories, this review develops a property-guided framework for interpreting and selecting FA across major cementitious routes. It first highlights why FA should not be treated as a single material. Key descriptors include glass content, fineness, calcium level, carbon residue, mineralogy, and beneficiation state. The review then compares four representative binder environments to clarify the role of FA shifts from pozzolanic contributor to filler-dominated hydration modifier to functional regulator, and finally to reactive precursor, including the calcium hydroxide (CH)-rich Portland systems, CH-poor CSA systems, phosphate-bonded MKPC systems, and alkali-activated/geopolymer systems. The central conclusion is that the key question is not simply where FA can be used, but which FA is best matched with which binder chemistry and for what performance objective.

Alkali-activated materials (AAMs) are widely regarded as promising alternatives to ordinary Portland cement for marine engineering because of their low carbon footprint, efficient utilization of industrial by-products, and potentially favorable mechanical and durability performance. However, their long-term application in marine environments remains challenging, as the original advantages of AAMs can be progressively weakened by the individual and coupled actions of aggressive seawater ions, particularly chloride (Cl⁻), sulfate (SO₄²⁻), and magnesium (Mg²⁺). These ions affect AAMs through distinct but interconnected mechanisms, including chloride binding and transport, competitive ion interactions, phase transformation, destabilization of reaction products, pore-structure evolution, and the subsequent degradation of macroscopic properties. Meanwhile, the response of AAMs to marine exposure is highly system-dependent, since precursor chemistry, activator design, reaction-product assemblage, and pore structure strongly govern their resistance to ion attack. In recent years, considerable efforts have been devoted to improving the marine durability of AAMs through composition and phase design, pore-structure refinement, and transport control. Nevertheless, current understanding remains fragmented, particularly regarding the coupled effects of multiple seawater ions and the links between microstructural evolution and long-term performance. The primary purpose of this review is to provide a systematic overview of the marine durability of AAMs from the perspectives of multi-ion threats, material-dependent responses, and existing mitigation strategies. Particular emphasis is placed on the roles of Cl⁻, SO₄²⁻, and Mg²⁺, the controlling effects of precursor and activator chemistry, and the translation of micro-mechanisms into macroscopic durability evolution. By integrating these aspects within a unified framework, this review aims to support the design and application of AAMs for reliable long-term use in coastal and offshore engineering. Future research should prioritize standardized multi-ion exposure protocols, coupled transport–reaction models, long-term field validation, and durability assessment of reinforced AAM concretes under realistic marine conditions.

It has been proven that silica fume (SF), which is a by-product from the manufacturing of single-crystal silicon, is beneficial for enhancing the mechanical properties, durability, and workability of geopolymers, as it can be quickly dissolved and form silicate-based cementitious phases in alkaline environments. However, the reinforcement mechanism of SF on geopolymer remains unclear due to the chemical complexity of geopolymer and the variety of SF types. Additionally, the solubility of calcite in an alkali environment is quite limited, and thus the formation of the amorphous calcium-based gels will be thwarted due to the lack of soluble calcium ions. Most importantly, with the development of the single-crystal industry, the amorphous silica content, crystallinity, and trace elements of SF itself have changed, which blocks the understanding of the activation mechanism of geopolymers combined with SF and insoluble calcite. To unveil the underlying modification mechanisms of SF on geopolymer materials along with insoluble calcite, in this study, two types of SF were used as the fly ash replacement in a fly ash/limestone system to prepare geopolymer materials. The reinforcement effect significantly depends on the SF types even with similar particle size and chemical compositions. The results indicate that the mechanical properties of geopolymer materials modified with SFs are not only governed by the ratio and contents of Si, Ca, Al, and Mg in SFs but also depend on the crystallinity and activity of the SFs. The hydration products could be varied according to the reaction environment. The research results not only contribute to the optimization design and application of geopolymer materials but also pave new pathways for the upcycling use of solid wastes such as SF, low-grade fly ash, or even other aluminosilicate solid wastes to achieve sustainable development.