Bonding agents are one of those construction chemicals that rarely get mentioned until something goes wrong. A plaster coat peels off after six months. A repair patch on a column face cracks along its edges and falls away. A floor screed separates from the substrate. In most of these cases, the material used wasn't the problem — the bond between old and new was.
What is a bonding agent and what does it actually do?
A bonding agent is a chemical applied to an existing concrete, masonry, or plaster surface before a new layer of mortar, plaster, or concrete is placed on top. Its job is to create a mechanical and chemical link between the two surfaces so they behave as one — not as two separate layers that can move independently and eventually separate.
Concrete and masonry surfaces are porous but also dusty, carbonated, and often contaminated with curing compounds, paint, or efflorescence. A new mortar layer placed directly on such a surface doesn't bond reliably — it bonds to the surface contamination, not the substrate. Over time, thermal movement and moisture cycling widen the gap between the two layers until delamination occurs.
A professional-grade bonding agent solves this by penetrating the substrate, improving surface wettability, and creating a chemically active interface that the new layer bonds to properly.
Why standard cement slurry often isn't enough
Many sites use a neat cement slurry as a bonding layer before plastering or patching. It works acceptably on fresh, clean concrete. But on older, carbonated, or contaminated surfaces it provides very limited improvement — and if it dries out before the next layer is applied, it actually acts as a bond-breaker. Polymer-based bonding agents like SBR latex or acrylic co-polymer emulsions maintain their tack longer, penetrate deeper, and provide significantly stronger and more consistent bond strength across a wider range of substrate conditions.
Where professional-grade bonding agents make the biggest difference
Repair work on existing structures is where bonding agents earn their place most clearly. When you're applying a repair mortar to spalled concrete on a beam or column, the substrate is old, possibly contaminated, and the repair patch is small — meaning it has limited contact area to develop bond. A high-performance bonding agent like ADT Flex SBR NPU or Fosroc Nitobond AR, applied as a slurry coat before the repair mortar, significantly increases pull-off strength and reduces the risk of the repair patch debonding under structural or thermal loads.
For plastering over old surfaces, acrylic co-polymer bonding agents improve adhesion on smooth, low-suction substrates where plaster would otherwise struggle to key in. For tile fixing over existing tiles or dense substrates, polymer bonding agents improve the contact area and bond strength of the tile adhesive, reducing hollow spots and long-term debonding.
Waterproofing applications also benefit — a properly bonded waterproofing membrane on a roof or bathroom surface is far less likely to bubble, crack at edges, or lift under hydrostatic pressure than one applied over a surface with inadequate preparation.
What to look for in a bonding agent
Not all bonding agents are the same. The key variables are polymer type (SBR, acrylic, epoxy), dilution ratio, open time before the next layer can be applied, and compatibility with the repair or plaster system being used. Using a bonding agent from the same manufacturer as the repair mortar or waterproofing system ensures chemical compatibility and gives you documented performance data for the application.
ADT Industries supplies bonding agents from Fosroc, Sika, Dr. Fixit, and Zydex — covering SBR latex, acrylic co-polymer, and deep-penetrating polymer systems for concrete repair, plastering, waterproofing, and tile fixing applications across India.
