Belzona 4521
Magma-Flex Fluid
A fast-curing, flexible elastomeric joint sealant for horizontal expansion joints on concrete, stone, and brick.
A fast-curing, two-part flexible elastomeric sealant for construction and expansion joints on concrete, stone, brick, and other rigid substrates. 25% movement capable.
- SeriesBelzona 4000 Series
- Documents3
Overview of Belzona 4521
Belzona 4521 (Magma-Flex Fluid) is a solvent-free, two-component hybrid polymer system designed for effective sealing of construction and expansion joints. Engineered as a cost-effective and high-performance alternative to conventional mastics and sealants, the system provides joints with up to ±25% movement accommodation and excellent weather and chemical resistance.
The fast-curing system adheres strongly to most surfaces — including concrete, stone, steel, and other Belzona 4000 Series products — and shares the same chemical family as the rest of the Belzona 4000 Series Magma range. This means no additional conditioners or interim preparation steps are required when integrating with a 4000-series system, which simplifies on-site application and reduces labor cost.
Belzona 4521 will reduce future maintenance, avoid costly structural rebuild, increase expansion joint flexibility, and ensure minimal stress at the bond line to extend substrate service life.
Key Benefits
100% solids formulation reduces health and safety risks in confined spaces with no shrinkage on cure
No solvents, no shrinkage during cure
Bonds strongly to virtually any rigid surface
Application Areas
What this product handles, grouped by equipment area — each links to the full application.
Floor Problem Areas
4 applications- Expansion & building joint sealingSealing of leaking joints in secondary chemical containment areasLearn more
- Joints in floors/walkwaysExpansion joints in floors, walkways, and structural decks are designed to move under thermal and live load, and joint failure usually means the original sealant lost ad…Learn more
- Pedestrian & public infrastructure surfacesParking garage expansion and construction jointsLearn more
- Walls, floors, concrete jointsConcrete construction joints, control joints, and crack openings move with thermal cycling and substrate loading, and stiff sealants split or pull free within a single f…Learn more
Technical Data
Service Temperature Limits
- Cure Profile
- Cold-curing, manually applied — no hot work required
- Joint Movement Capability
- ±25% horizontal and shear movement
- Low-Temperature Flexibility
- Remains flexible at low temperatures
- Weather & UV
- UV resistant and weather resistant once cured
- Detailed application temperature, working life, and cure schedule
- See linked Product Specification Sheet (PSS) and Instructions for Use (IFU)
Product PDFs
Gallery
Videos
Frequently Asked Questions
What is Belzona 4521 (Magma-Flex Fluid) used for?
Belzona 4521 (Magma-Flex Fluid) is a fluid-grade flexible expansion joint sealant designed for horizontal expansion joints in concrete floors, slabs, and trafficked areas. Its self-leveling formulation accommodates structural movement while providing a smooth, hard-wearing seal that resists chemicals and traffic abrasion.
What is the mixing ratio of Belzona 4521 (Magma-Flex Fluid)?
The mixing ratio of Belzona 4521 (Magma-Flex Fluid) is detailed in the Product Specification Sheet. The Base and Solidifier are mixed thoroughly using a mechanical mixer prior to pouring into the prepared joint. Please see the Product Specification Sheet (PSS) for more information.
How do you apply Belzona 4521 (Magma-Flex Fluid)?
To apply Belzona 4521 (Magma-Flex Fluid), the joint surface must be clean, dry, and firm, free of rust, paint, and other surface coatings and contaminants. Any new concrete must be dry and cured for at least 28 days. Condition the joint surfaces using Belzona 4911 (Magma TX Conditioner) and install backer rod where required. Pre-mix the Base, then add the Solidifier using a mechanical mixer until homogeneous. Pour the fluid sealant directly into the joint, allowing it to self-level, and avoid air entrapment. Allow to cure undisturbed. Please see the Instructions for Use (IFU) for more information.
Related Products
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