A binder is a substance that binds other materials together to form a cohesive whole. In construction, cement, lime and gypsum are the main binders, used in mortar, concrete and plaster to hold aggregates, sand and other components firmly in place once the mixture has hardened.
How binders work
Binders, referred to as “bindmiddelen” in Dutch, undergo a chemical or physical reaction that causes them to set and harden. Hydraulic binders such as Portland cement react with water in a process called hydration, forming strong crystalline structures that lock the surrounding aggregate particles together. This reaction continues for weeks and even months, which is why concrete keeps gaining strength long after it is poured.
Non-hydraulic binders, such as air lime, harden through carbonation: they absorb carbon dioxide from the air and gradually revert to calcium carbonate. This process is slower and produces a softer, more flexible material, which is why lime mortars are preferred in historic restoration work where slight building movement must be accommodated.
Gypsum is another common binder, used mainly for interior plaster and plasterboard. It sets quickly by recrystallising when mixed with water, giving tradespeople a short working time but a fast turnaround before follow-on trades can proceed.
Choosing the right binder
The choice of binder depends on the required strength, setting time, flexibility and exposure conditions. Cement-based binders deliver high compressive strength and water resistance, making them the standard for structural concrete and external mortar. Lime-based binders offer breathability and flexibility, which are valuable for older masonry that needs to move without cracking. Gypsum binders are ideal for smooth interior finishes but must be kept away from sustained moisture.
Blended binders, such as cement-lime mixes, combine the strengths of both materials. Many modern mortars use these blends to achieve a balance of workability, strength and durability.
Related terms
- Cement
- Lime
- Mortar
- Concrete
- Mix
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