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How does polycarboxylate superplasticizer affect the workability of concrete?
Blog How does Polycarboxy
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High volume fly ash concrete, mixed with 30% to 60% fly ash as an auxiliary cementitious material, offers sustainable advantages (reducing cement consumption and lowering carbon dioxide emissions) and greater durability. However, the high specific surface area and slow reaction of volcanic ash in fly ash also pose challenges: increased water demand, poor workability, and delayed strength development.
High content fly ash concrete is a type of concrete that uses fly ash produced by burning coal powder in thermal power plants as a volcanic ash admixture.
Sustainability: Reduce concrete’s carbon footprint by reducing demand for high-energy cement production.
Improve durability: It has excellent resistance to chemical corrosion (sulfates and chlorides) and reduces permeability over time.
Reduction of hydration heat: Minimize thermal cracks during the pouring of large volume concrete to the greatest extent possible.
Cost effectiveness: Fly ash is usually cheaper than Portland cement.
Poor workability: The small spherical particles of fly ash have a very high surface area, which significantly increases water usage and makes the mixture stiff and difficult to construct.
Slow strength development: The volcanic ash reaction of fly ash is much slower than the hydration reaction of cement, resulting in lower early strength (1-7 days).
Increased solidification delay: The combination of high fly ash content and certain additives can excessively prolong the solidification time.
The unique performance of PCE powder meets the key requirements of high-volume fly ash concrete and solves the inherent limitations of fly ash:
Alleviation of high water demand: The fine particles of fly ash increase internal friction; PCE can reduce water consumption by 25-40%, lowering the water cement ratio to 0.35-0.45 (0.55-0.65 without PCE) while maintaining a slump of ≥ 500mm.
Workability and slump retention: The spatial hindrance effect of PCE stabilizes the matrix, maintaining 70% of the initial slump for over 90 minutes, which is crucial for long pouring windows of high-volume fly ash concrete.
Promote the hydration of fly ash: PCE evenly disperses cement and fly ash particles, accelerates the pozzolanic reaction (calcium hydroxide + fly ash → CSH gel), and counteracts strength delay.
Durability enhancement: porosity reduced by 15-25%, chloride ion penetration reduced by 30-50%, solving the potential permeability problem of high float fly ash concrete.
PCE is widely used in high-fly ash concrete engineering, including:
Large volume concrete: dams, bridge piers, and foundation slabs (reducing hydration heat by 20-30%).
Sustainable buildings: projects that meet green building standards and have a low carbon footprint.
Industrial flooring, highways, and prefabricated components (balancing processability and long-term strength).
The efficacy of PCE in high-volume fly ash concrete stems from three key synergistic mechanisms with fly ash:
The comb-like molecular structure of polyvinyl chloride (PCE) (hydrophobic main chain + hydrophilic polyether side chain) can adsorb onto cement and fly ash particles, thereby generating strong steric hindrance. This helps break down aggregates, reduce internal friction, and reduce water consumption.
Unlike naphthalene-based high-efficiency water-reducing agents, PCE can more effectively disperse fly ash, even at a 50% substitution rate, maintaining uniform particle distribution and preventing separation.
The reaction rate of volcanic ash in fly ash is slow (requiring calcium hydroxide produced by cement hydration). PCE promotes cement hydration by dispersing cement particles and increases the generation of calcium hydroxide, which provides “fuel” for fly ash reaction and accelerates the formation of CSH gel.
Additional CSH gel is used to fill pores and enhance the interfacial transition zone (ITZ) between the aggregate and the slurry, thereby improving the matrix density.
Due to uneven dispersion of fly ash, the interfacial transition zone (ITZ) of high-volume fly ash concrete typically exhibits porosity. PCE can ensure uniform distribution of fly ash particles, fill the gaps in the interface transition zone, and reduce microcracks.
ITZ shear strength increases by 15-20%, enhancing the overall strength and durability of concrete.
To maximize the synergistic effect of tetrachloroethylene and fly ash, please follow the following key practices:
Choose slump-maintaining polyether ether ketone (medium polyether side chain, MW 3000-4000 Da) to meet the long-term workability requirements of high-floating powder concrete.
Avoid using polyvinyl chloride (PCE) with high retarding components; choose products with a moderate retarding degree (initial setting time of 6-8 hours) to balance processability and strength development.
PCE dosage: 0.15% to 0.35% of the total cementitious material mass (cement+fly ash). When the fly ash substitution rate is high (50%~60%), to offset the increased water consumption, it is necessary to slightly increase the PCE dosage (0.25%~0.35%).
Fly ash substitution rate: 30-60% (high usage range). Ensure that the fly ash meets ASTM C618 F or C standards (fineness ≥ 300 m ²/kg, loss on ignition ≤ 6%).
Low water cement ratio: Maintain a water cement ratio of 0.35-0.45 to utilize the water-reducing effect of PCE and promote volcanic ash reaction.
Coarse aggregate gradation: Use well-graded aggregates (10-25mm) to reduce porosity and minimize cement slurry demand.
Compatibility with mineral admixtures: Adding 5-10% silicon powder can further enhance strength and durability (synergistically with PCE and fly ash).
High volume fly ash concrete requires a longer curing time to maximize the volcanic ash reaction:
Wet curing for ≥ 14 days (using curing blanket or film).
For cold weather (<15 ℃), preheat the material (15-25 ℃) and extend the curing period to 21 days to avoid strength delay.
Polycarboxylate superplasticizer are not only compatible with high-fly ash concrete but also enhance its sustainability and performance. PCE addresses the key challenges of high-content fly ash concrete, such as high water demand, poor workability, and delayed strength development, leading to a fly ash substitution rate of 30% to 60%. This not only reduces carbon emissions but also enhances the durability and strength of the concrete.
The key to success lies in selecting a slump-maintaining polycarboxylate superplasticizer, optimizing the dosage, maintaining a low water-cement ratio, and extending the wet curing time. As the construction industry transitions towards sustainable development, PCE’s role in high-fly ash concrete is becoming increasingly indispensable – proving that environmentally friendly concrete and high-performance concrete can be achieved simultaneously.
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