Tile Grout Formulation: The Additives That Stop Cracking, Shrinking, and Blackening

Quick Answers

1. Grout Is Not Tile Adhesive

Tile adhesive and tile grout look like cousins. They are not. They do opposite jobs.

The words that matter for grout are flexibility, water resistance, and alkali resistance. Not bond strength. People mix this up all the time — and that is why grout cracks and turns black.

2. The Standards — JC/T 1004 and EN 13888

Grout has its own standards. The two that matter:

CG1 is the basic grade. Good water resistance, fine for indoor dry areas.

CG2 is the improved grade. It adds high abrasion resistance and much lower water absorption. You want CG2 for wet rooms, floors, swimming pools, and anything that faces water or heavy cleaning.

The jump from CG1 to CG2 is mostly more and better polymer — and a hydrophobic agent.

3. The Reference Formula

This is the classic grout formula. The RDP dose is not one fixed number — it scales with how flexible and water-tight the grout must be.

A few extra points from the notes:

RDP scales by grade. 10 kg/t is the low end. 15 is already a high dose on the market. 20 is high performance. 30 is the top for a very flexible grout.

Cement 350 is strong, 270 is flexible. Pick by what the joint must do. A floor grout that gets walked on leans to 350. A grout on a moving wall leans to 270.

HPMC viscosity is medium-low — around 60,000 mPa·s, always lower than tile adhesive. In dry, cold northern China many plants use 100,000 mPa·s. Adjust to your climate and raw-material cost.

Active metakaolin can be added for extra strength and stain resistance.

Ultra-fine grout is the same formula with the fillers ground to 1,250 mesh.

4. The One Rule People Break — No Superplasticizer

This is the rule that surprises most people: grout does not take a superplasticizer.

Tile adhesive and self-leveling love a PCE. Grout does not. Why?

A superplasticizer makes the mix flow. But grout must stay put in a vertical joint. If it flows, it sags out of the joint, and you get hollow spots that later crack and fall out.

So the grout wants the opposite of flow — it wants body. That body comes from the HPMC, the starch ether, and the wood fiber, not from a plasticizer.

5. Medium-Low Viscosity HPMC — Lower Than Tile Adhesive

Grout uses a medium-low-viscosity cellulose ether — around 60,000 mPa·s. The key rule: lower than your tile adhesive.

Why lower?

Tile adhesive holds a heavy tile, so it wants a thicker, higher-viscosity HPMC. Grout fills a narrow joint, so it wants a smoother, more fluid mix that packs the gap with no air pockets. Too high a viscosity, and the grout is stiff, hard to press in, and traps air.

But this is not a fixed number. The real choice depends on your local climate, raw-material cost, and budget. In dry, cold northern China, many plants use 100,000 mPa·s. In tropical regions, pick by the actual temperature.

So: start medium-low, lower than your tile adhesive, and adjust to your market.

6. What Each Additive Does

7. Why Grout Turns Black — and How to Stop It

Grout turns black for one reason: it is porous.

Water and dirt soak into the tiny pores of the joint. They stay there. The grout stains black and starts to smell.

Two ways to stop it:

1.Add a hydrophobic agent. It repels water at the pore wall, so dirt and water cannot soak in.

2.Go to a two-component grout. A two-component grout is denser and more water-tight. It resists blackening far better.

8. CHARING Products for Grout

The pairing is simple: HP 40M / HP 60M give the smooth fill and water retention, RD 7003M gives the flexibility and water resistance, and S300 keeps the joint from soaking.

RD 7003M is our most widely used RDP — a neutral, semi-flexible grade (Tg 3°C). It is the safe, middle-of-the-road choice that works in most grouts without any surprises.

For hot climates, pick the HPMC viscosity by the local temperature — the exact grade is a climate decision, not a fixed rule.

9. Epoxy Grout — the Other Kind

There is a second kind of "grout" that is not cement-based at all: epoxy grout.

Epoxy grout is three things mixed together: coloured sand + epoxy resin + hardener. It is water-based, not oil-based.

The difference is huge:

Epoxy grout is the premium choice for pools, food areas, and anywhere that must stay clean and stain-free. Cement grout is the everyday choice for most tiling.

10. Common Problems

1. Grout cracks. Almost always shrinkage. Add or raise the expansive agent, and do not let it dry too fast — the HPMC is there for that.

2. Grout turns black. It is porous and soaked. Add a hydrophobic agent, or move to a two-component grout.

3. Colour is uneven from batch to batch. Usually the water dose changes between mixes. Measure the water the same every time.

4. Grout sags out of a vertical joint. Not enough body. Check the starch ether and do not add any plasticizer.

5. Grout falls out of the joint. Too much flow or too weak a fill. Pack it full, and keep the superplasticizer out.

11. FAQ

How much RDP should I start with? 

It scales by grade. 10 kg/t is the low end, 15 is already a high market dose, 20 is high performance, and 30 is the top for a very flexible grout.

Can I use tile adhesive as grout? 

No. Tile adhesive is for bond, grout is for filling. Tile adhesive clogs the joint and traps air.

Why no superplasticizer in grout? 

Because grout must not flow. A plasticizer makes it run out of the joint.

Low or high viscosity HPMC? 

Medium-low, around 60,000 mPa·s — always lower than your tile adhesive. CHARING recommends HP 40M or HP 60M. In cold northern China many plants go to 100,000 mPa·s.

When do I need epoxy grout? 

Pools, kitchens, food areas — anywhere that must stay clean and stain-free.