Patching concrete with ordinary cement (OPC) leads to high shrinkage, poor bonding, and moisture penetration — it will flake off again before long. Spalling repair mortar contains polymer modifiers that provide shrinkage compensation, chemical bonding, and moisture barrier protection for the rebar. Its service life far exceeds that of ordinary cement, depending on environmental conditions, workmanship, and ongoing maintenance. The material costs more, but you save on re-scaffolding and rework — significantly cheaper in the long run.
⚠️ Important Notice: Concrete repair is structural maintenance work that must be carried out in accordance with the product Technical Data Sheet (TDS) and relevant codes. Where rebar is exposed or spalling is extensive, consult relevant professionals.
What's Wrong with Patching Concrete Using Ordinary Cement?
When tradespeople (or property owners doing DIY) see concrete falling from the ceiling or external walls, the first instinct is to "mix up some cement and trowel it back on." Ordinary Portland Cement (OPC) is cheap and available everywhere — seems like the most convenient option. But this approach has several fundamental problems:
1. Shrinkage Cracking
Ordinary cement shrinks as it hardens. The larger the repair area, the greater the shrinkage stress. The result is a ring of fine shrinkage cracks at the junction between the patch and the original concrete. Once cracks form, water finds its way in, and the repair starts loosening and peeling from the edges.
2. Poor Bond Strength — Mechanical Interlock Only
The bond between ordinary cement and old concrete relies mainly on the "mechanical interlock" from surface roughness. There is no chemical bond with the existing concrete, so inherent bond strength is low. Combined with the tensile stress from shrinkage, the bond is easily exceeded, causing the entire repair patch to lift off in one piece.
3. High Permeability — Moisture Gets Through
The porosity of an ordinary cement patch is high, allowing moisture and chlorides (salts) to pass through freely. This means even if the repair looks fine initially, moisture and harmful agents continue reaching the rebar surface. The rebar keeps rusting and expanding, and eventually blows the concrete apart again — this is the so-called "repeated spalling cycle."
The Result: You'll Be Doing It Again Soon
Patching concrete with ordinary cement saves on material cost up front, but the same problem typically recurs in short order. For external wall or ceiling work, the cost of re-scaffolding far exceeds the price difference in materials.
Why Is Spalling (Repair Mortar) Different?
"Spalling" is the English origin of the Cantonese term — it refers to concrete delamination and flaking. So "spalling repair" means repairing spalled concrete, and the material used is purpose-made repair mortar.
The key differences between repair mortar and ordinary cement lie in several critical modified components:
Thixotropic — Won't Flow, Won't Fall
The repair mortar formulation is thixotropic — after mixing, it has a thick, paste-like consistency that does not flow when applied with a trowel. Even when applied to the underside of a ceiling (overhead position), it stays put. You can build up to 25–40 mm in a single pass without slumping. This makes it ideal for ceilings, beam soffits, column faces, and other vertical or overhead surfaces — exactly the locations where concrete spalling most commonly occurs. In contrast, ordinary cement mortar is too fluid and tends to fall off ceilings.
Polymer Modifiers
Repair mortar contains SBR or acrylic polymer emulsions. During hardening, these polymers form a continuous film that binds cement particles together. The effects are:
- Chemical bond — instead of relying on mechanical interlock alone, the mortar forms a chemical bond with the old concrete surface, dramatically increasing bond strength
- Low permeability — the polymer film seals capillary pores, blocking moisture and chlorides, thereby protecting the rebar
Shrinkage Compensation
Repair mortar is precisely formulated with expansive agents and fibres to minimise shrinkage during hardening, or even achieve slight expansion. The result: no shrinkage cracks at the repair edges, and the entire repair patch stays tightly bonded to the old concrete.
Corrosion Inhibitors
Some high-performance repair mortars (particularly Class 40) also contain corrosion inhibitors that form a protective layer on the rebar surface, slowing the rate of corrosion. When combined with appropriate anti-corrosion primers, the protection is even more effective.
High Alkalinity — Restoring the Rebar Passivation Layer
This point is often overlooked: repair mortar is highly alkaline, serving not just as filler but as chemical protection.
- Neutralises carbonation — As concrete ages, carbonation lowers its pH and rebar loses its protection. When you apply the highly alkaline repair mortar, alkalis migrate into the surrounding old concrete, raising the pH back up.
- Restores the passivation layer — The stable high-alkalinity environment enables the rebar surface to re-form a passivation layer, providing long-term corrosion inhibition.
Although ordinary cement also starts with high alkalinity, its porous structure allows rapid carbonation, so the alkalinity drops over time and protection does not last. Repair mortar's polymer-densified structure maintains alkalinity for much longer, providing more reliable chemical protection for rebar.
Comparison Table
| Property | Ordinary Cement (OPC) | Spalling / Repair Mortar |
|---|---|---|
| Bond to old concrete | Mechanical interlock only, low bond strength | Chemical bond + mechanical interlock, high bond strength |
| Shrinkage | High shrinkage, prone to shrinkage cracks | Shrinkage-compensated, sealed edges |
| Permeability | High — moisture and chlorides penetrate freely | Low — effectively blocks moisture |
| Rebar protection | Essentially none | Low permeability + corrosion inhibitors |
| Alkalinity protection | Initially high, but drops rapidly with carbonation | High and sustained — neutralises carbonation, restores passivation layer |
| Thixotropy | Fluid — falls off overhead surfaces | Thixotropic formula — stays firm on overhead surfaces |
| Compressive strength | Moderate (25–35 MPa) | High (25–60+ MPa, depending on Class) |
| Expected service life | Shorter (repeated spalling common) | Far exceeds OPC (depends on environment and workmanship) |
| Material cost | Lower | Higher (roughly 5–10x) |
| Total cost (incl. labour & scaffolding) | Repeated repairs make it more expensive long-term | Done right once — cheaper in the long run |
How to Repair Spalled Ceiling Concrete
Ceiling concrete spalling is one of the most common problems in older Hong Kong buildings. The correct repair procedure is as follows:
Step 1: Hacking — Thorough Removal
Use a cold chisel or power chisel to remove all loose and hollow-sounding concrete. Don't just remove the obviously spalled sections — tap around the area and remove anything that sounds hollow, down to sound concrete. All edges must be cut to a square edge with a minimum depth of 10 mm. Avoid "feather edges" — thin edges will crack and debond due to thermal movement.
Step 2: Rebar Cleaning
Clean exposed rebar using a wire brush or sandblasting to Sa 2.5 grade (near-white metal finish). Also hack behind the rebar to leave approximately 10–15 mm clearance, ensuring the primer and mortar can wrap around 360 degrees without leaving blind spots. This step is critical — if rebar corrosion is not addressed, the rebar continues to rust and will blow the concrete apart again.
Step 3: Rebar Protection — Zinc-Rich Primer
Apply zinc-rich primer to the cleaned rebar. Once dry, this primer forms a dense metallic zinc layer providing three levels of protection:
- Barrier effect — The dense zinc layer physically blocks moisture (H2O), oxygen (O2), and chloride ions (Cl-) from reaching the rebar surface.
- Cathodic protection — Zinc acts as a "sacrificial anode": when both zinc and steel are exposed to moisture, zinc oxidises preferentially, protecting the rebar. Even if the primer coat is slightly damaged, the zinc continues to sacrifice itself until fully consumed.
- Potential equalisation — The zinc primer equalises the potential difference between the repair zone and surrounding old concrete, reducing the chance of a halo effect (翻發) at the boundary, extending the repair's service life.
Recommended products: Fosroc Nitoprime Zincrich, Sika Armatec 110 EpoCem, or equivalent zinc-rich primers. Wah Hing generally keeps these in stock.
Step 4: Bonding Primer
Apply a coat of bonding primer to the cleaned concrete surface. The primer creates a chemical bonding bridge between the old concrete and the new repair mortar, significantly increasing bond strength. Strictly follow the wet-on-wet method — apply the mortar while the primer is still tacky. Applying mortar after the primer has fully cured will actually cause delamination. Do not skip this step — see the "Is Primer Important?" section below.
Recommended products: Unicell Primer, Fosroc Nitobond AR, or equivalent bonding primers. Wah Hing generally keeps these in stock.
Step 5: Mortar Application
Mix the repair mortar according to the water-to-powder ratio specified in the TDS. Press it firmly into the repair area with a trowel to expel air and ensure full contact with the substrate — leave no voids. Each layer should not exceed 40 mm. If the repair is deep enough to require multiple layers, score the first layer (keying) to provide mechanical key for the next layer.
Recommended products: Unicell 40, Unicell 25, Renderoc HB40, Renderoc HB25, Optimix RM740, Optimix RM725, SikaRep 25 HB, Mapei HB25, Mapei HB40, or equivalent repair mortars. Wah Hing generally keeps these in stock.
Step 6: Curing
After completion, keep the repair moist (water spray or curing compound) to prevent the surface from drying out too quickly and developing shrinkage cracks.
Is Primer Important?
Extremely important. Many failed repairs can be traced to skipping the primer step.
The bonding primer (also called bonding agent) serves three purposes:
- Seals capillary pores in old concrete — Prevents the old concrete from sucking moisture out of the fresh mortar (if moisture is drawn out, the mortar cannot fully hydrate and its strength is greatly reduced)
- Creates a chemical bonding bridge — The primer bonds to both the old concrete and the new mortar, joining them into a single unit
- Boosts bond strength — Bond strength with primer versus without can differ by 2–3 times
Wah Hing stocks Unicell Primer, most products typically in stock, specifically designed for use with repair mortar.
What's the Difference Between Class 25 and Class 40?
The Hong Kong Housing Authority (HKHA) classifies repair mortars by compressive strength into two main grades:
Class 25 (25 MPa Compressive Strength)
- For general non-structural repairs
- Examples: ceiling plaster delamination, wall surface patching, cover restoration
- Relatively easy to work with, suitable for larger-area thin-layer repairs
Class 40 (40 MPa Compressive Strength)
- For structural repairs
- Examples: beam soffit spalling with exposed rebar, column repairs, load-bearing element restoration
- Higher strength — better matched to original structural concrete
How to choose? In short: if the damage is surface-level only with no structural load involvement, Class 25 is sufficient. If the repair involves structural elements (beams, columns, load-bearing walls) or exposed rebar requiring cover restoration, use Class 40. If unsure, consult relevant professionals.
Common Mistakes
The most frequent errors made by tradespeople and property owners:
1. Using Ordinary Cement Instead of Repair Mortar
This is the biggest mistake, and the core message of this article. Ordinary cement patches crack, debond, fail to protect rebar, and need to be redone in short order.
2. Skipping the Primer
Thinking "just apply the mortar directly" — without a bonding bridge between the mortar and old concrete, bond strength is severely compromised.
3. Not Removing All Loose Concrete
Rushing the job by only patching surface cracks without addressing hollow areas underneath. The new repair and the loose concrete below it will fall off together.
4. Not Treating Rebar Corrosion
Seeing rusted rebar and simply covering it over. The rebar continues to rust and expand, and blows the concrete apart again within a few years.
5. Using the Wrong Class
Using Class 25 for structural repairs (insufficient strength), or Class 40 for simple surface repairs (over-engineered, wasted cost).
Wah Hing Recommended Products
Below are products related to spalling repair. Wah Hing generally keeps these in stock:
Zinc-Rich Primers (Rebar Protection)
- Fosroc Nitoprime Zincrich — by Fosroc, solvent-based zinc-rich primer
- Fosroc Nitoprime Zincrich WB — by Fosroc, water-based zinc-rich primer
- Sika Armatec 110 EpoCem — by Sika, epoxy-based anti-corrosion coating
Bonding Primers
- Unicell Primer — by Sika Ronacrete
- Fosroc Nitobond AR — by Fosroc
- Fosroc Primer 7 — by Fosroc, epoxy-based primer
Repair Mortar — Class 40 (Structural Grade)
- Unicell 40 — by Sika Ronacrete, PCCS-RM certified
- Renderoc HB40 — by Fosroc
- Optimix RM740 — by Optimix, PCCS-RM certified
- Mapei HB40 — by Mapei
- SikaRep 40 HB — by Sika
Repair Mortar — Class 25 (General Maintenance Grade)
- Unicell 25 — by Sika Ronacrete, PCCS-RM certified
- Renderoc HB25 — by Fosroc
- Optimix RM725 — by Optimix, PCCS-RM certified
- SikaRep 25 HB — by Sika
- Mapei HB25 — by Mapei
→ Browse all spalling repair mortars
Related Articles
Complete Guide to Concrete Spalling and Repair
Frequently Asked Questions
What is spalling?
Spalling refers to the surface delamination and flaking of concrete. Spalling repair uses dedicated repair mortar to restore the damaged concrete, recovering the protective cover and structural strength.
Why can't I just use ordinary cement to patch concrete?
Ordinary Portland Cement (OPC) has high shrinkage, low bond strength to old concrete, and high permeability that allows moisture and chlorides to continue attacking the rebar. The result is that repairs fail and re-spalling occurs quickly. Repair mortar (spalling mortar) contains polymer modifiers that provide shrinkage compensation and low permeability, with a service life far exceeding OPC — depending on environmental conditions, workmanship, and ongoing maintenance.
What's the difference between Class 25 and Class 40?
Class 25 (compressive strength 25 MPa) is for general non-structural surface repairs, such as ceiling plaster delamination and wall surface patching. Class 40 (compressive strength 40 MPa) is for structural repairs, such as beam and column rebar exposure and load-bearing element restoration. If unsure which to use, consult relevant professionals.
Can I repair spalled ceiling concrete myself?
For small-area, non-structural ceiling surface spalling, you can carry out the repair yourself following the correct steps (hack off loose concrete → clean rebar → zinc-rich primer → bonding primer → apply mortar → curing). However, for large-area spalling, severely corroded rebar, or damage to structural elements, have the work done by relevant professionals.
Is bonding primer really necessary for repair mortar?
Strongly recommended. Primer seals the capillary pores in old concrete, prevents moisture loss from the fresh mortar, and creates a chemical bonding bridge. Bond strength with primer versus without can differ by 2–3 times. Skipping the primer is one of the most common reasons for repair failure.
How long does a spalling repair last?
Using compliant repair mortar applied correctly per the TDS (including primer and curing steps), the repair's service life generally far exceeds that of ordinary cement. In contrast, ordinary cement patches typically fail again in short order.
