Selleys SLC50 Leveller

What is Selleys SLC50 Leveller, and where should you use it?

Selleys SLC50 Leveller is a self-smoothing cementitious levelling underlayment, formulated specifically for preparing concrete substrates before timber flooring is installed. It forms a smooth, hard surface for a range of floor finishes including timber, carpet and ceramic tiles, and it can be used internally and externally, on both new construction and refurbishment work.

Its defining capability is depth. SLC50 can be applied in thicknesses of up to 50mm in a single pour without adding aggregate, and it can also be tooled to a feather edge — so one product handles both a slab that is 50mm out and a floor that just needs the last millimetre taken out of it.

The limitation to know before you specify it: SLC50 is designed as an underlay material only, and NOT a wearing course for traffic. It goes under a floor covering. It is not the floor.

Technical specification

Property	Typical result
Technology	Cementitious mortar
Colour	Grey
Packaging	20kg paper bag
Mix ratio	1 x 20kg bag to 4.4L of clean water for a free flowing self-smoothing consistency, or 4.0L for ramping up to 50mm in one pour
Coverage	Approximately 12m² at 1mm thick, or 4m² at 3mm thick
Application temperature	10 – 35°C
Working time	Approximately 15 minutes at 20°C
Cure time	Normally walked on after 3–4 hours; ready for floor coverings after 24 hours at 23°C
Compressive strength	1 day 15–18 MPa; 7 day 18–22 MPa; 28 day 30–32 MPa
VOC content (ASTM D3960-05)	1 gram per litre
Shelf life	12 months stored correctly

These figures are indicative data only and are not intended for the preparation of specifications.

What the key numbers mean in practice

Coverage: 12m² at 1mm from a 20kg bag. That figure scales linearly with depth, which makes estimating straightforward and sobering. At 3mm you get 4m² from a bag. At 10mm you are looking at roughly 1.2m² per bag — so a 20m² room that needs an average 10mm of levelling is a pallet of product, not a few bags. Work out your average depth before you order, not after.

Working time: approximately 15 minutes at 20°C. This is the number that governs how you organise the job, and the data sheet is explicit that you must not attempt to rework material after this time. It also tells you to pour and level within 5 to 15 minutes. Fifteen minutes is not long to mix, pour, screed and let a batch self-smooth — which is why the instruction is to mix no more than can reasonably be laid within that flow time, while having subsequent mixes ready so pouring is continuous until the area is complete. On anything beyond a small room, that means a second person on the mixer.

Compressive strength develops over 28 days. The 15–18 MPa at one day is what lets you walk on it and continue working; the 30–32 MPa at 28 days is the eventual strength. The gap between those two matters because early strength is not final strength — a floor that takes foot traffic at four hours is not yet a floor that should take heavy point loads.

Cure time: walked on after 3–4 hours, floor coverings after 24 hours at 23°C. Note the temperature qualifier on the 24 hours. Cementitious products cure more slowly in the cold, so in winter conditions treat 24 hours as a minimum rather than a schedule.

The two water ratios are a deliberate choice, not a tolerance. Use 4.4 litres per bag for a free-flowing, self-smoothing mix; drop to 4.0 litres when you are ramping up to 50mm in one pour, because a stiffer mix holds a build-up that a free-flowing one would simply run out of. What you must not do is add water beyond the stated ratio: over-watering promotes bleed and separation, with a reduction in bond, tensile and compressive strength.

Where to use it

• Preparing concrete substrates before timber flooring is installed
• As a heavy duty underlay for timber, carpet and ceramic tile finishes
• Interior and exterior applications
• New construction and refurbishment projects
• Over concrete slabs prior to using Selleys Liquid Nails flooring adhesives
• Over Selleys VBS moisture barrier products, after application of Selleys VP30 Primer

Surface preparation

This is where a levelling job succeeds or fails, and the requirements are specific:

• New concrete should be at least 28 days old.
• Excess laitance should be removed by light scabbling or captive shot blasting, followed by vacuuming to remove dust and debris.
• The substrate must be clean, sound and free from loose material and contamination — curing compounds, oil, grease, waxes, sealers, previous coatings or adhesives that may inhibit bonding. Complete removal must be carried out wherever contamination is suspected or evident.
• Diamond grinding is the best preparation, because it removes high spots as well as laitance. Shot blasting, sand blasting and captive abrasive blast cleaning are also suitable. Acid etching is not recommended, as the concrete then has to be neutralised and dried afterwards.
• All concrete sub-floors must be fully cured and dry in accordance with AS 1884 — less than 75% relative humidity when measured in accordance with ASTM F2170.

Priming — do not skip this

SLC50 goes onto a primed substrate, and the reason is mechanical rather than adhesive. The stated object of priming is to seal the substrate to prevent air release from the subfloor rising and forming bubbles and pinholes on the surface of the SLC50. A porous slab breathes as the levelling compound goes over it, and that air has to come out somewhere.

• For porous concrete, prime with Selleys VP30 Primer diluted 1:1 with water.
• Extremely porous substrates may require two priming or sealing coats.
• Primer coats must be allowed to dry before applying SLC50.
• Where Selleys Liquid Nails VBS or VBS Rapid has been used, a coat of undiluted VP30 Primer must be applied and allowed to dry before the SLC50 goes down.

Notes on dampness

Selleys recommends applying a vapour barrier coating such as Liquid Nails VBS or VBS Rapid on all ground floor concrete slabs, to limit moisture transmission to less than 15g/sqm/24hrs. Slabs known to be prone to high moisture levels may require two applications of VBS.

If the slab is below grade and there is a possibility of the water table being high enough to pressurise the concrete, perimeter drains would need to be installed. That is a construction issue, and no amount of levelling compound addresses it.

How to mix and apply it

1. Prepare and prime the substrate as above, and allow the primer to dry.
2. Pour the mixing water into a clean mixing vessel first — 4.4 litres of clean, cool water per 20kg bag, or 4.0 litres if you are ramping up to 50mm in one pour.
3. Add the powder slowly to the water, mixing continuously. Always add powder to liquid, never the reverse.
4. Mix with a heavy duty drill capable of 600rpm, fitted with a suitable paddle, for at least 2–3 minutes until a smooth, creamy consistency is obtained. Do not over-water.
5. Pour the SLC50 onto the primed substrate and use a screed bar or trowel to give the required thickness, then allow it to self-smooth. Pour and level within 5–15 minutes.
6. Do not rework the material after the approximately 15 minute working time.
7. For a second coat, allow the SLC50 to dry for at least 12 hours, then prime and repeat the application.
8. Wait 24 hours at 23°C before laying floor coverings. The surface can normally be walked on after 3–4 hours.

Limitations — where it should NOT be used

• SLC50 is an underlay material only, NOT a wearing course for traffic. It must be covered by a floor finish.
• It must not be used in areas subject to permanent wetting, or where back water pressure is likely.
• Not to be used in immersed situations such as swimming pools or spas.
• Maximum thickness in any one application is 50mm. Deeper build-ups need a second coat, dried for at least 12 hours and primed first.
• Work within the 10–35°C application temperature range, and avoid applying in very hot, dry or windy conditions.
• Mix only with clean water.

Cleaning and storage

Remove SLC50 from tools and equipment with clean water immediately after use. Once hardened it can only be removed mechanically — which, on a mixing paddle or a screed bar, means chipping it off. Cleaning as you go is far quicker.

Store the bags in a cool, dry place above ground level, out of direct sunlight. Under those conditions the product has a 12 month shelf life. Storing paper bags of cementitious powder on a damp concrete floor is the usual way that shelf life gets shortened.

Hazard classification

Classified as hazardous under Safe Work Australia GHS 7, with the signal word Danger.

• Carcinogenicity — Category 1. H350: may cause cancer.
• Serious eye damage/eye irritation — Category 1. H318: causes serious eye damage.
• Skin corrosion/irritation — Category 2. H315: causes skin irritation.
• Specific target organ toxicity, single exposure — Category 3, respiratory tract irritation. H335: may cause respiratory irritation.

Not classified as Dangerous Goods by the criteria of the Australian Code for the Transport of Dangerous Goods by Road & Rail or New Zealand NZS 5433.

What is in it

Chemical entity	CAS number	Proportion (w/w)
Cement, portland, chemicals	65997-15-1	30–60%
Crystalline silica (contains >0.1% respirable)	14808-60-7	<1%
Ingredients determined to be non-hazardous or below reporting limits	—	Balance

The carcinogenicity classification belongs to the crystalline silica, and the Safety Data Sheet singles it out for a reason. It is present at under one percent, but the entry is annotated as containing more than 0.1% respirable — the fraction of particles fine enough to reach the deep lung rather than be caught in the nose and throat. Category 1 is the highest carcinogenicity classification, reserved for agents where the evidence of human carcinogenicity is established rather than suspected. Respirable crystalline silica is a well-recognised occupational lung hazard in Australia, and a cementitious powder is one of the ordinary ways of encountering it.

The hazard lives in the dry powder, not in the poured floor. Once the compound is mixed with water, there is no dust; once it has cured, the silica is locked into a solid. Every moment of real exposure on this job is a moment when powder is airborne, and the directions above name them all: opening a 20kg paper bag, adding the powder slowly into the mixing water, and running a 600rpm drill and paddle in an open vessel for two to three minutes. On a large floor that sequence repeats bag after bag, because the 15-minute flow time forces continuous mixing.

This gives a second, health-based reason for an instruction the page already gives for mixing quality. "Pour the mixing water into a clean vessel first" and "add the powder slowly to the water, never the reverse" are stated as the way to get a smooth, lump-free mix. They are also the way to keep powder out of the air. Tipping a bag into an empty bucket, or adding water to powder, raises a cloud; feeding powder into standing water does not. The same applies to the surface preparation step above: light scabbling and captive shot blasting of laitance are dust-generating operations in their own right, which is why the instruction to vacuum the debris matters more than it looks.

The Safety Data Sheet's stated exposure controls are safety shoes, overalls, gloves and safety glasses, with natural ventilation adequate under normal use conditions. Read alongside a Category 1 carcinogenicity classification driven by respirable dust, "normal use conditions" should be taken to mean use that does not raise a cloud of powder in the first place — not a reason to skip the dust control.

Once it is wet, the hazard changes rather than disappears. Portland cement at 30–60% is strongly alkaline in water, and that is what the Category 1 eye damage classification is about — a splash of fresh compound in an eye is a chemical burn, not grit. The skin entry in the first aid section carries the warning that effects may be delayed, which is the signature of a cement burn: wet compound on a knee or inside a boot may not hurt while you are laying the floor, and the damage shows up afterwards. Do not kneel in it, and do not work on with a bootful.

Protection

• Eye protection — safety glasses, with a Category 1 eye damage hazard once the compound is wet.
• Gloves — nitrile rubber for intermittent contact. Glove construction varies, so make your own assessment.
• Overalls and safety shoes, changed if they become wet with compound rather than worn through the day. Wash contaminated clothing before reuse.
• Ventilation — natural ventilation is adequate under normal use conditions; keep the powder out of the air rather than relying on it.
• Wash hands before smoking, eating, drinking or using the toilet.

First aid

Exposure	What to do
Eyes	Hold the eyelids apart and flush the eyes continuously with running water. Continue flushing until advised to stop by the Poisons Information Centre or a doctor, or for at least 15 minutes, then transport to a doctor or hospital.
Skin	Effects may be delayed. Remove contaminated clothing immediately and flush the skin and hair with running water. If swelling, redness, blistering or irritation occurs — including some time after the contact — seek medical assistance.
Inhalation	Remove the person from exposure without becoming a casualty yourself. Remove contaminated clothing and loosen the rest. Let them rest in the most comfortable position and keep them warm until fully recovered. Seek medical advice if effects persist.
Swallowed	Rinse the mouth with water. Do not induce vomiting. Give a glass of water to drink. Never give anything by mouth to an unconscious person. If vomiting occurs, give further water. Seek medical advice.

If poisoning occurs, contact a doctor or the Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand — with the bag to hand. Selleys emergency line: 1800 220 770 in Australia, 0800 220 770 in New Zealand.

Common questions about Selleys SLC50 Leveller

How thick can I pour it?

Up to 50mm in a single application, without adding aggregate. It can also be tooled to a feather edge at the other extreme.

How much water per bag?

4.4 litres of clean, cool water per 20kg bag for a free-flowing self-smoothing mix, or 4.0 litres when ramping up to 50mm in one pour.

What if I add a bit more water to make it flow better?

Don't. Over-watering promotes bleed and separation, and reduces bond, tensile and compressive strength.

How much will a bag cover?

Approximately 12m² at 1mm thick, or 4m² at 3mm thick.

How long do I have to work with it?

Approximately 15 minutes at 20°C. Pour and level within 5 to 15 minutes, and do not attempt to rework the material after that.

When can I walk on it?

Normally after 3–4 hours.

When can I lay the floor covering?

After 24 hours at 23°C.

Do I have to prime first?

Yes. Priming seals the substrate so air escaping from the subfloor does not form bubbles and pinholes in the surface. Porous concrete takes VP30 diluted 1:1 with water; over VBS or VBS Rapid, use undiluted VP30. In both cases the primer must be dry before the SLC50 goes on.

Can I use it as the finished floor?

No. It is designed as an underlay material only and is not a wearing course for traffic.

Can I use it outside?

Yes, it is suitable for interior and exterior use — but not in areas subject to permanent wetting, or where back water pressure is likely.

Can I use it in a pool or spa surround that stays wet?

Not in immersed situations such as swimming pools or spas.

How old does the concrete need to be?

New concrete should be at least 28 days old, and all concrete sub-floors must be fully cured and dry in accordance with AS 1884 — less than 75% relative humidity when measured per ASTM F2170.

Can I acid etch the slab to prepare it?

Acid etching is not recommended, because the concrete then has to be neutralised and dried afterwards. Diamond grinding is the best preparation.

What if I need more than 50mm?

Apply a second coat: let the first dry for at least 12 hours, then prime and repeat.

How do I clean my tools?

With clean water immediately after use. Hardened material can only be removed mechanically.

How long does an unopened bag keep?

12 months, stored in a cool dry place above ground level and out of direct sunlight.