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Synergistic Mechanism of Mineral Admixtures and PCE Superplasticizer to Optimize Early Performance of High Strength Concrete

High strength concrete is the core structural material for super high-rises, long-span bridges and marine infrastructure projects. When mixed solely with Portland cement, High strength concrete faces significant challenges: rapid slump loss during transportation and insufficient early compressive strength for rapid construction progress. Compound mineral admixtures, including fly ash (FA), ground granulated blast furnace slag (GGBS), and silica fume (SF), when combined with a polycarboxylate superplasticizer (PCE) , can solve this dilemma through multi-component synergistic effects.

This aiticle employs an L9(3³) orthogonal experiment to investigate the main and interactive effects of three mineral admixtures on fresh concrete workability, setting time, and mechanical strength at 3d, 7d, and 28d. Microscopic SEM analysis further reveals the inner synergistic mechanism of the ternary blended gel system with PCE, providing a reliable mix design reference for stable, high-performance, high-strength concrete.

Raw Materials & Experimental Design

Raw Material Specifications

Cement: P·Ⅱ 52.5R Portland cement; 3d strength 32.5 MPa, 28d strength 58.0 MPa
Fly ash (FA): Class F Grade I, spherical glass beads, water demand ratio 93%, 45 μm residue 8.2%
GGBS (S95 slag powder): Specific surface area 428 m²/kg, 28d activity index 103%
Silica fume (SF): Compacted type, SiO₂ content 92.5%, ultra-high specific surface area 18500 m²/kg with strong pozzolanic activity
PCE superplasticizer: 40% solid content, water reduction rate 31%, recommended dosage 1.0%–2.0% of binder mass
Aggregate: 5–20 mm continuous graded granite crushed stone, natural river sand with a fineness modulus of 2.7, and mud content 0.5%
Mixing water: Tap water

Orthogonal Test Scheme

Fixed parameters: Total binder content 500 kg/m³, water-binder ratio 0.32, sand ratio 42%. Three factors (FA, GGBS, SF) with 3 mixing levels each, with the pure cement group C100 as the blank control.

Factor	Level 1	Level 2	Level 3
Fly ash (FA)	15%	20%	25%
GGBS slag	10%	15%	20%
Silica fume (SF)	3%	5%	7

Mixing sequence: Aggregates → all binders → water pre-mixed with PCE. Test standards: GB/T 50080 (slump), GB/T 50081 (cube compressive strength), SEM for 28d hydration microstructure.

Test Results & Analysis

Fresh Concrete Workability & Slump Retention

The pure cement control group C100 shows severe slump loss: initial slump 225 mm, 60-min slump 165 mm, and a loss rate of 26.7%. All mineral blended groups significantly reduce slump loss. Range analysis ranks influence intensity: Silica fume > Fly ash > GGBS. Silica fume’s ultra-large specific surface area consumes large amounts of PCE and accelerates flow loss when overdosed.

Optimal ternary mix ratio: FA20% + GGBS15% + SF5% (Group 5)

Initial slump: 235 mm
60-min slump: 215 mm
Slump retention rate: 91.5% (loss only 8.5%)
Synergy explanation of this formula:

Spherical fly ash beads produce a ball-bearing effect to cut the internal friction of the paste and stabilize the initial fluidity;
GGBS optimizes particle gradation to fill micro voids between cement and silica fume;
FA and GGBS compete for adsorption sites with silica fume, slowing PCE consumption on the high-active SF surface; more free PCE remains in the liquid phase, sustaining long-term dispersion and slump retention.

Compressive Strength Development at Different Ages

3-day strength: Pure cement C100 reaches the highest strength of 52.1 MPa, while all blended groups have slightly lower early strengths due to dilution effects and slower pozzolanic reactions.
7-day strength: Group 5 exceeds the control group, reaching 62.8 MPa, demonstrating coordinated strength development in ternary mineral blends.
28-day strength: Optimal mix FA20S15SF5 achieves 78.5 MPa, far higher than pure cement’s 71.0 MPa.

Three-stage strength evolution mechanism of the composite system:

0–3d: FA and GGBS form a compact micro-aggregate skeleton via physical filling; a small amount of SF starts pozzolanic reaction to fill tiny pores.
3–7d: GGBS is activated by Ca(OH)₂ from cement hydration, generating abundant C-S-H gel together with silica fume for rapid strength rise.
7–28d: Fly ash pozzolanic reaction proceeds fully; interwoven hydration products continuously densify the matrix and enhance long-term mechanical performance. PCE improves particle dispersion, creating sufficient space for pozzolanic reactions.

Microstructure (SEM Comparison)

Pure cement C100: Loose C-S-H structure, massive lamellar calcium hydroxide (CH), wide interfacial transition zone (ITZ, ~45 μm) full of microcracks and pores.
FA20S15SF5 blended concrete: Dense homogeneous matrix, nearly no bulk CH crystals. Interlaced fibrous C-S-H gel covers all mineral particles. ITZ thickness is reduced to ~22 μm, greatly eliminating weak zones between aggregate and paste, which is the core reason for higher late-strength and durability.

Core Research Conclusions

The influence sequence of three mineral admixtures on concrete workability: Silica fume > Fly ash > GGBS. Excessive silica fume sharply increases PCE demand and slump loss, while composite FA + GGBS can offset this negative effect.
The optimal ternary mineral blending proportion for high-strength concrete is 20% fly ash, 15% slag powder, 5% silica fume (total mineral admixture 40%, water-binder ratio 0.32). This formula balances excellent workability and outstanding long-term strength: 60-min slump retention rate 91.5%, 3d=50.2 MPa, 7d=62.8 MPa, 28d=78.5 MPa.
The multi-element synergistic effect relies on three coupled mechanisms coordinated by PCE: ball-bearing lubrication of fly ash, gradation filling of slag, pore refinement of silica fume, and competitive adsorption to stabilize PCE dispersion performance.
A ternary blended system refines the interfacial transition zone (ITZ), reduces harmful pores and CH crystals, and forms a dense hydration microstructure, thereby upgrading concrete mechanical properties and durability.

Practical Engineering Guidance

Long-distance pumped high-strength concrete: Adopt FA-GGBS-SF ternary composite admixture to control slump loss within 10%.
Projects requiring fast form removal: Select the 20-15-5 mix ratio to balance acceptable 3d strength and superior late strength growth.
Low water-binder HSC production: Avoid single high-silica-fume blending; blend fly ash and slag to reduce superplasticizer dosage and production costs.
Marine & corrosion-resistant high-strength structures: This ternary blend reduces calcium hydroxide content and optimizes the pore structure to enhance anti-carbonation and chloride-penetration resistance.

Conclusion

Matching ternary mineral admixtures of fly ash, GGBS, and silica fume with a polycarboxylate superplasticizer produces clear synergistic effects in high-strength concrete. The 20% fly ash + 15% slag + 5% silica fume mix solves the dual pain points of rapid slump loss and insufficient early strength in single-cement HSC.

The competitive adsorption between mineral particles stabilizes PCE’s dispersion capacity, while multi-stage pozzolanic reactions densify the hydration microstructure and significantly increase the 28-day compressive strength. This optimized mix design provides an economical, high-performance solution for mass production of high-strength concrete in bridges, high-rise and marine engineering.