How to Glue Cement to Cement | Bonding Repair Rules

For cement-to-cement repairs, the right bond comes from surface prep plus an epoxy bonding agent or neat cement paste, matched to the patch size.

Getting new concrete to stick to old is less about picking a glue and more about following a procedure. The reason patches fail is almost always a dirty surface or the wrong bonding material. Whether you need a thin bonding layer or a two-component epoxy depends on how large the repair is and whether steel is showing.

Do You Need Epoxy or Cement Paste?

The honest answer is: it depends on the size of the patch and whether rebar is exposed. The distinction comes straight from California DOT’s concrete repair guidance.

• Small, non-structural patches under roughly 1/2 square foot and up to 3 inches deep — with no exposed rebar — can use neat cement paste as the bonding layer.
• Larger patches or any repair with exposed reinforcing steel require a two-component epoxy-resin bonding agent that meets ASTM C881/C881M.

That ASTM standard covers epoxy systems made for Portland-cement concrete, including types that cure in humid conditions and bond to damp surfaces. Single-component products marketed as epoxy don’t qualify — California DOT notes these are not effective as structural concrete bonding agents. If your patch is bigger than a dinner plate or you can see metal, skip the paste and buy a proper two-part epoxy.

Surface Preparation Is the Real Secret

Concrete bonding fails when the old surface isn’t ready. Dust, loose material, and foreign matter must be removed completely — California DOT warns that even a thin layer of dust will prevent bonding. The U.S. Bureau of Reclamation’s repair guide says the surface should be cleaned down to sound concrete before any bonding agent goes on.

For cementitious repair materials, the old concrete should be brought to saturated surface dry (SSD) condition: soaked thoroughly, then allowed to dry until the surface is only slightly damp. The surface shouldn’t look wet, but the pores should still hold moisture. For epoxy bonding agents, dampness is handled differently — ASTM C881 systems are formulated to bond to damp surfaces, so follow what your specific product’s label says rather than assuming one rule applies to everything.

Applying the Bonding Agent Step by Step

Once the surface is clean and at the right moisture level, application is straightforward but timing-sensitive.

For a two-component epoxy, mix only what you can use within the manufacturer’s stated open time. Spread a thin coat over the contact surface and work it into the concrete with stiff brushes. Avoid pooling — puddles of epoxy weaken the bond. Place the patching material before the epoxy begins to set; if it skins over, the bond is already compromised.

For a cementitious bonding agent, a latex-modified mix works well. The standard recipe is 1 part cement to 2 parts sand, with enough latex admixture added to reach a creamy paint consistency. Brush this slurry onto the prepared surface, then apply the repair mortar while the bonding agent is still wet. Letting it dry before placing the patch is one of the most common mistakes — the bond forms while the slurry is workable.

When mixing any cement-based product, wear waterproof gloves and safety goggles. Cement is caustic and contact with skin or eyes causes real burns.

Common Bonding Mistakes to Skip

Most failed repairs trace back to the same handful of errors. Knowing them saves you a second trip to the hardware store.

• Using single-component epoxy where a two-component system is required — it simply won’t hold for structural repairs.
• Skipping the cleaning step — dust and loose particles silently destroy adhesion even when everything else is done right.
• Letting the bonding layer dry before placing repair mortar when the product requires immediate placement.
• Using neat cement paste on large patches or over exposed rebar, where epoxy is the specified material.
• Assuming dampness is always acceptable — some guidance calls for a dry surface at application; check your product’s label instead of guessing.

If you’re weighing which bonding product fits your repair, our tested roundup of the best cement glues on the market breaks down the options by patch type and strength.