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Mechanical Performance and Microstructural Mechanism of a Ternary Silicate–Ferroaluminate–Fly Ash Composite Cementitious System
2026-08-06 17:13:08
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0 Introduction
Ordinary Portland cement (OPC) remains the dominant hydraulic binder in global construction due to its excellent mechanical performance, wide availability, and well-established production technology. However, with the rapid expansion of global infrastructure development, several limitations of OPC systems have become increasingly apparent, including relatively slow early-age strength development, concentrated hydration heat release, significant drying shrinkage, and durability challenges in demanding environments such as marine structures and rapid repair projects.
Meanwhile, the cement industry is under growing pressure to reduce greenhouse gas emissions. Cement manufacturing accounts for approximately 7–8% of global CO₂ emissions, mainly resulting from high-temperature clinker production and limestone decarbonation. Therefore, the development of low-carbon cementitious materials with reduced clinker consumption, improved mechanical performance, and enhanced durability has become a key research direction worldwide.
Ferroaluminate cement (FAC), a specialized hydraulic cement with a high content of ferroaluminate phases, has attracted increasing attention because of its rapid strength development, low shrinkage characteristics, corrosion resistance, and relatively lower clinker production energy demand compared with traditional Portland cement. Its major clinker phases include calcium sulfoaluminate (C₄A₃S), dicalcium silicate (C₂S), and calcium ferroaluminate (C₄AF).
However, similar to other rapid-hardening cement systems, pure FAC may suffer from insufficient later-age strength development or strength reduction due to limited long-term hydration products. To overcome this limitation, supplementary cementitious materials (SCMs), especially fly ash (FA), have been widely studied.
Fly ash, a globally available industrial by-product, improves workability through particle packing and ball-bearing effects. Meanwhile, its pozzolanic activity consumes calcium hydroxide (CH) and produces additional calcium silicate hydrate (C–S–H) gel, contributing to long-term strength enhancement and carbon reduction.
Previous studies have mainly focused on binary cement systems such as OPC–SCM or OPC–sulfoaluminate cement blends. However, the hydration mechanisms, mechanical evolution, and microstructural characteristics of OPC–FAC–FA ternary composite systems remain insufficiently understood.
This article presents a global perspective on the development of a ternary composite binder combining Portland cement, ferroaluminate cement, and fly ash. Through analysis of mixture design, mechanical properties, shrinkage behavior, and microscopic characterization using X-ray diffraction (XRD) and scanning electron microscopy (SEM), the synergistic hydration mechanism of this low-carbon cementitious system is discussed.
1 Experimental Design Overview
The experimental program was conducted with a constant water-to-binder ratio of 0.36 and sand-to-binder ratio of 0.5.
A 42.5-grade OPC was used as the reference binder. Ferroaluminate cement was introduced by replacing OPC at levels of 0–10% by mass, producing a series of OPC–FAC binary mixtures.
Based on the optimized FAC content of 8%, fly ash was further incorporated by replacing OPC at levels ranging from 0–60%, forming OPC–FAC–FA ternary systems.
The experimental evaluation included:
Setting time measurements of cement pastes;
Mortar flowability testing;
Flexural and compressive strength testing at different curing ages;
Drying shrinkage measurements up to 90 days;
XRD phase analysis and SEM microstructural observation.
2 Evolution of Macroscopic Properties
2.1 Setting Behavior and Workability
The incorporation of FAC significantly accelerated cement hydration.
For pure OPC, the initial and final setting times were approximately 147 minutes and 225 minutes, respectively. When 10% FAC was introduced, the initial setting time decreased to 40 minutes, representing a reduction of 72.7%, while the final setting time decreased to 80 minutes, a reduction of 64.4%.
This accelerated setting behavior is mainly attributed to rapid hydration of ferroaluminate phases. During OPC hydration, calcium hydroxide increases system alkalinity and promotes the formation of ettringite (AFt) from C₄A₃S in FAC. The rapid crystallization of AFt creates an early rigid framework, accelerating the loss of plasticity.
The effect of FAC on workability showed a typical "increase followed by decrease" trend. At lower FAC dosages, particle packing improved flowability, while excessive FAC accelerated hydration and increased water consumption, reducing fluidity.
In the ternary OPC–FAC–FA system, fly ash addition produced a non-linear effect. Low fly ash contents accelerated AFt formation and shortened setting time, while higher fly ash contents increased setting time and improved flowability due to dilution effects and improved particle packing.
2.2 Mechanical Performance
Pure FAC exhibited limited long-term strength development, while combining FAC with OPC effectively improved this disadvantage. The OPC–FAC binary mixtures showed higher strength than both pure OPC and pure FAC systems.
The mixture containing 10% FAC achieved the best binary performance:
7-day compressive strength: 38.3 MPa;
14-day compressive strength: 42.8 MPa;
28-day compressive strength: 46.6 MPa.
Compared with pure OPC, compressive strength increased by 24.8%, 18.4%, and 15.6% at 7, 14, and 28 days, respectively. Flexural strength also increased by 28.4%, 22.6%, and 15.1%.
The early strength improvement resulted from:
Rapid hydration of C₄A₃S and C₄AF phases;
Accelerated hydration of OPC clinker minerals, especially C₃S and C₂S.
In the ternary system, fly ash reduced early strength at 7 and 14 days because of its slower reaction rate. However, 28-day compressive strength first increased and then decreased with increasing fly ash content.
The optimum fly ash content was approximately 20%, achieving a 28-day compressive strength of 49.4 MPa, which was 7.4% higher than the FAC8% binary system.
A balanced OPC–FAC–FA composition containing 8% FAC and 30% FA maintained excellent performance, with 28-day compressive strength and flexural strength still 15.9% and 9.9% higher than pure OPC.
2.3 Drying Shrinkage Performance
Shrinkage reduction is one of the important advantages of the ternary binder system.
At 90 days:
Pure OPC showed drying shrinkage of approximately 1055 μm/m;
Pure FAC showed only 421 μm/m;
OPC with 10% FAC reduced shrinkage to 846 μm/m.
The reduction was mainly attributed to AFt crystal expansion and higher early-age stiffness, which helped compensate shrinkage deformation.
Fly ash further improved shrinkage resistance. The FAC8%-FA30% mixture achieved a 90-day shrinkage value of 787 μm/m, representing a 25.4% reduction compared with pure OPC.
The improvement resulted from:
Reduced water demand due to fly ash particle packing;
Pore structure refinement caused by pozzolanic reactions;
Reduced hydration heat and internal stress.
3 Microstructural Evidence and Hydration Mechanism
XRD analysis confirmed the synergistic hydration mechanism of the ternary system.
After FAC incorporation:
AFt characteristic peaks increased significantly;
C₃S and C₂S peaks decreased.
These results indicate that FAC accelerates hydration reactions of OPC clinker phases.
After adding 30% fly ash:
The characteristic CH peak decreased significantly;
confirming that pozzolanic reactions consumed calcium hydroxide and generated additional C–S–H gel.
SEM observations showed:
Pure OPC contained plate-like CH crystals and limited fibrous AFt phases.
FAC8% samples exhibited dense networks of needle-like AFt crystals and reduced CH content.
FAC8%-FA30% samples showed fly ash particles coated with C–S–H gel, with filled pores and a denser microstructure.
The improved mechanical properties, reduced shrinkage, and enhanced long-term strength can be attributed to the combined effects of:
AFt framework formation + CH consumption + C–S–H gel filling.
4 Global Engineering Implications and Future Perspectives
The OPC–FAC–FA ternary cementitious system represents a promising approach for developing low-carbon and high-performance binders for global construction applications.
Its advantages come from the complementary functions of each component:
FAC provides rapid early-age strength development and shrinkage compensation;
Fly ash improves long-term strength, durability, and sustainability;
OPC provides stable hydration and mechanical foundation.
The hydration process can be summarized as:
Early-age AFt framework formation → Later-age C–S–H densification
This makes the ternary system attractive for:
Precast concrete components;
Rapid pavement and bridge repair materials;
Marine infrastructure;
Underground engineering structures.
A practical composition range may be considered:
Ferroaluminate cement: 8–10%;
Fly ash: 20–30%.
The optimal ratio should be adjusted according to project requirements, including early strength, durability, and carbon reduction targets.
Future research should focus on three major areas:
Expansion of supplementary cementitious materials by incorporating slag, steel slag, calcined clay, and other industrial residues.
Comprehensive durability evaluation, including chloride resistance, carbonation resistance, sulfate resistance, and long-term volume stability.
Engineering-scale validation through concrete applications, field testing, and international standard development.
As the global construction industry moves toward net-zero emissions, ternary composite binders that combine rapid hydration phases with sustainable supplementary materials may provide an effective pathway to achieve both high performance and significant carbon reduction.
