Floor Levelling Sydney Over Anhydrite Screed: Why a Cement-Based Pour Needs a Different Preparation Plan

Anhydrite, or calcium-sulphate, screed should not be treated like ordinary concrete before a cement-based floor-levelling pour. In Sydney renovations, the correct sequence typically involves identifying the screed, checking its condition and moisture, mechanically removing surface laitance, controlling dust and using a primer or barrier system specifically approved for the substrate and levelling product. Missing that distinction can turn a routine flooring programme into a bonding, moisture and rework problem.

The complication is easy to miss because a stripped floor does not arrive with a label.

Once carpet, vinyl, timber or tile has been removed from a Sydney apartment, hotel, office or residential development, the remaining grey surface may be casually described as "concrete". In reality, the structural slab may sit beneath another layer entirely. One possibility is an anhydrite or calcium-sulphate screed, a material with different moisture behaviour and different preparation requirements from conventional cement-based screed.

That distinction becomes particularly important when the proposed correction is another cementitious material.

Manufacturer guidance illustrates why. ARDEX warns that cementitious material should not simply be placed in direct contact with calcium-sulphate material because of potential adverse interaction, and specifies preparation and priming requirements before compatible levelling products are installed. Sika likewise publishes different priming arrangements and waiting periods for calcium-sulphate substrates depending on the levelling system being applied.

ARDEX guidance on levelling anhydrite and gypsum-based screeds

Sika's calcium-sulphate substrate guidance

Both demonstrate that substrate type changes the preparation specification.

The Critical Decision Happens Before Anyone Mixes a Bag

Most levelling problems are discovered only after the room has been cleared and the previous floor has disappeared. The temptation is to move immediately into grinding, priming and pouring because every additional inspection appears to threaten the installation date.

With anhydrite, that pressure can produce the wrong sequence.

The first question should not be, "How many bags will this floor require?" It should be, "What exactly are we levelling over?"

Positive substrate identification affects several downstream decisions:

• whether mechanical preparation is required and how aggressive it should be;
• whether surface laitance must be removed;
• which moisture assessment is appropriate;
• which primer, barrier or bonding system is permitted;
• the allowable levelling depth;
• the waiting period between preparation stages;
• compatibility with adhesives and the final flooring;
• whether the manufacturer will support the proposed assembly.

That makes the substrate check a project hold point rather than a minor flooring detail.

Anhydrite Is Not Simply Another Name for Cement Screed

Conventional sand-cement screeds and calcium-sulphate screeds can perform a similar practical job: creating a usable floor layer above a structural base. Chemically, however, they are not interchangeable.

Anhydrite screeds use calcium sulphate as the principal binder. Cementitious levelling compounds use a different binder system. That is why reputable manufacturers publish substrate-specific instructions rather than treating all mineral floors as equivalent.

The practical lesson for Sydney project teams is straightforward: colour and texture are not an adequate specification.

A surface that looks like a conventional screed should not automatically receive the same primer or preparation method used on a concrete slab.

Why the Interface Matters

The risk is not limited to whether the fresh levelling material appears to stick on the day of installation.

The interface must remain stable after the floor is covered, occupied and subjected to environmental changes. Depending on the products involved, inappropriate contact between calcium-sulphate and cementitious materials can create an unfavourable reaction zone. Moisture can make that interface more consequential.

This is why the primer should not be reduced to the phrase "prime before levelling".

The specification needs to answer four different questions:

1. What substrate has actually been identified?
2. What condition and moisture state is that substrate in?
3. Which primer or barrier does the levelling manufacturer permit over it?
4. Does the specified preparation remain compatible with the final flooring and adhesive system?

If those questions cannot be answered, the pour is premature.

The Surface Laitance Problem

Calcium-sulphate screeds can develop a weaker surface layer or laitance during curing. Leaving that layer in place can mean that an otherwise strong levelling system is bonded to the weakest part of the floor.

Manufacturer guidance commonly calls for mechanical preparation to remove laitance, loose material and contamination and expose a sound surface before subsequent flooring work. ARDEX, for example, specifically calls for mechanical preparation of calcium-sulphate screeds before its subsequent priming and levelling sequence.

Operationally, this changes the work package.

A quote based simply on "prime and level" may need to become:

• protect the apartment or work zone;
• mechanically prepare the screed;
• remove generated dust;
• inspect the exposed substrate;
• repair local defects where necessary;
• verify moisture and substrate suitability;
• apply the specified primer or barrier;
• observe the required drying or waiting period;
• install the approved levelling layer;
• verify the resulting surface before flooring begins.

That is materially different from treating an exposed concrete slab as automatically ready for an ordinary primer.

Moisture Becomes a Scheduling Issue, Not Just a Test Result

Calcium-sulphate screeds are moisture-sensitive materials. A project team therefore needs to know whether the screed is sufficiently dry for the proposed system rather than relying on appearance, elapsed time or a generic assumption about concrete.

Product manufacturers establish their own requirements. ARDEX guidance, for example, describes a preparation pathway for calcium-sulphate screeds deemed dry below a specified relative-humidity threshold, while also identifying different systems for other moisture conditions. These figures should not be copied indiscriminately across brands. They demonstrate why the selected product documentation must govern the actual installation.

For builders and flooring contractors, this can affect:

Demolition completion

• Why anhydrite changes it: The newly exposed layer still needs to be identified.
• Operational consequence: Do not release levelling immediately after floor removal.

Mechanical preparation

• Why anhydrite changes it: Surface laitance or weak material may require removal.
• Operational consequence: Add grinding or sanding and dust-control time.

Moisture verification

• Why anhydrite changes it: Calcium-sulphate substrates have system-specific limits.
• Operational consequence: A test result may become a formal hold point.

Primer selection

• Why anhydrite changes it: Ordinary concrete primer assumptions may be wrong.
• Operational consequence: Confirm the complete manufacturer-approved system.

Levelling depth

• Why anhydrite changes it: Primer and leveller requirements can vary with application thickness.
• Operational consequence: Survey the floor before ordering material.

Flooring installation

• Why anhydrite changes it: The final adhesive and floor covering remain part of the assembly.
• Operational consequence: Coordinate levelling and flooring specifications before work starts.

Why Sydney Apartment Renovations Are Particularly Exposed

The technical issue is universal, but Sydney renovation logistics make the consequences more expensive.

Apartment programmes frequently involve tightly sequenced access windows, lift bookings, strata restrictions, flooring deliveries and other trades working around the same rooms. If anhydrite is discovered only after demolition, the programme may already have installers booked to begin within days.

A preparation change can then affect:

• loading-dock and lift bookings;
• dust-producing work approvals;
• neighbour notifications;
• material deliveries;
• flooring installer mobilisation;
• cabinetry and joinery sequencing;
• skirting installation;
• door and threshold heights;
• overall handover dates.

Elyment's existing analysis of floor levelling over new concrete makes a related project-management point: a calendar date is not the same thing as substrate readiness. Anhydrite creates a different technical problem, but the delivery principle remains relevant.

A Better Sequence for an Anhydrite-to-Cement Levelling Package

A disciplined Sydney renovation programme can treat the substrate transition as a series of controlled decisions.

1. Expose and identify the substrate. Review project documentation where available and inspect the floor after removal. If the material cannot be confidently identified, stop treating it as generic concrete.
2. Survey the existing floor. Establish the required finished datum, high points, low points, thresholds and expected levelling depths before specifying the material system.
3. Assess structural and surface integrity. Cracking, hollowness, friability, contamination or debonding should be resolved before a broad levelling pour is used to conceal them.
4. Mechanically remove unsuitable surface material. Prepare the screed to the level required by the selected system, including removal of laitance where required.
5. Control preparation dust. Vacuum extraction and site isolation need to form part of the work method, particularly in occupied buildings and strata environments.
6. Verify moisture suitability. Use the method and acceptance criteria applicable to the proposed flooring system and manufacturer requirements.
7. Approve the primer or separating system. Confirm that the primer, barrier and levelling compound are approved as one compatible build-up rather than selecting each product independently.
8. Observe specified waiting times. Sika, for example, publishes different waiting requirements for its primer when calcium-sulphate substrates subsequently receive calcium-sulphate versus cement-based materials, reinforcing that curing time is part of the specification.
9. Complete the levelling pour. Install within the specified thickness range, environmental conditions and application limits.
10. Release the floor to the finishing trade only after verification. Flatness, condition, curing and flooring-specific requirements should be checked before adhesive or floor covering is installed.

Grinding Is Part of Preparation, Not an Automatic Cure

Mechanical preparation often plays an important role, but more grinding is not automatically better.

The purpose is to remove unsuitable material and create the surface condition required by the specification. It is not to blindly chase every floor deviation through the screed.

This distinction mirrors the cost and datum issue explored in Elyment's article on when concrete grinding can reduce unnecessary floor-levelling build-up. On an anhydrite project, however, mechanical work also has a substrate-preparation function.

Grinding and sanding operations also need appropriate dust controls. SafeWork NSW identifies airborne crystalline silica as a serious workplace hazard when silica-containing materials are cut, drilled, ground or otherwise mechanically processed, making effective dust management important where relevant materials are being disturbed.

SafeWork NSW silica guidance should be considered when developing the applicable work method.

The Existing Screed Should Not Be Preserved at Any Cost

Recognising an anhydrite screed does not automatically mean it should remain.

If investigation shows that the existing screed is extensively weak, debonded, damaged, contaminated or otherwise unsuitable to support the new floor assembly, the more rational decision may be removal or substantial remediation rather than constructing another layer over it.

That decision is fundamentally different from the preparation problem addressed here. Elyment's discussion of when an existing screed bed should be removed before floor levelling examines the failure and removal side of that decision.

The important point is that a levelling compound should correct geometry. It should not be expected to rehabilitate an unsound substrate by hiding it.

Wet Areas Need a Separate Compliance Review

Anhydrite identification also matters where the project crosses into bathrooms, laundries or other wet areas.

Waterproofing should not be treated as a decorative layer that can simply be added over whatever preparation system is convenient. The substrate, levelling layer, waterproofing membrane, falls, penetrations and finished surface form a coordinated assembly.

The National Construction Code contains specific requirements for wet-area construction, including substrate and waterproofing considerations, and directs attention to compliant wet-area systems rather than isolated products.

NCC wet-area provisions should be reviewed where the proposed floor assembly forms part of regulated wet-area work.

In practical terms, the project team should establish where the waterproofing sits in the build-up and confirm that all substrate and levelling materials below it are permitted within the selected system.

What Should Appear in the Flooring Scope?

The word "levelling" is too broad for an anhydrite project.

A more useful scope identifies the assumptions and interfaces explicitly:

Existing substrate

• What should be defined: Identified or assumed substrate type and how that assumption will be confirmed.

Surface preparation

• What should be defined: Mechanical preparation, laitance removal, local repairs and cleaning.

Dust control

• What should be defined: Extraction, containment and disposal arrangements.

Moisture

• What should be defined: Testing responsibility, acceptance criteria and hold point.

Primer or barrier

• What should be defined: Specified compatible system rather than a generic allowance.

Levelling

• What should be defined: Estimated area, depth range, datum and material limits.

Interfaces

• What should be defined: Doorways, joinery, wet areas, movement joints and adjacent substrates.

Release to flooring

• What should be defined: Who confirms readiness for the final covering.

This makes variations easier to explain because the project team can distinguish an unforeseen substrate condition from a simple increase in levelling material.

Mixed Substrates Require Even More Discipline

Renovations rarely expose one uninterrupted material.

A Sydney apartment may contain anhydrite screed through the living area, concrete around a removed wall, patching mortar at service penetrations and a different substrate near a bathroom or balcony threshold.

A single broad pour can therefore cross several interfaces.

Each transition needs to be considered for:

• bonding;
• differential movement;
• absorbency;
• primer compatibility;
• existing joints;
• whether movement joints must continue through the new floor system.

Elyment's analysis of floor levelling across steel and concrete interfaces illustrates the broader rule: once a levelling pour crosses more than one substrate, the preparation specification must accommodate the interfaces rather than assuming the whole room behaves like concrete.

The Cost Risk Usually Comes From Rework, Not the Primer

Specialist preparation can make a levelling quote look more complicated. But the relevant comparison is not between the price of two primers.

It is between controlled preparation before installation and disruptive rectification after flooring has been completed.

A failed interface may require some combination of:

• removing the newly installed floor covering;
• removing adhesive;
• breaking out failed levelling material;
• re-preparing the original substrate;
• retesting moisture;
• re-levelling;
• reordering flooring;
• rebooking trades;
• extending the occupation or handover programme.

In an apartment building, another weekend of demolition or grinding can also mean another round of access, noise and strata coordination.

That is why substrate identification can be one of the cheapest decisions in the entire flooring package.

A Small Material Difference Can Change the Whole Programme

Anhydrite screed demonstrates a recurring problem in renovation delivery: materials hidden beneath previous finishes can look familiar while behaving differently.

For Sydney builders, owners, flooring contractors and project managers, the safest response is not to assume that every grey floor is concrete and not to treat every primer as interchangeable.

The stronger workflow is to identify the substrate, establish its condition, survey the required datum, complete the specified mechanical preparation, verify moisture, confirm the compatible primer or barrier and only then release the floor for the cement-based levelling pour.

That preparation plan may add another hold point before levelling. It can also remove a much larger one before handover.

Confirm the Floor Build-Up Before the Pour Begins

Elyment can review substrate conditions, preparation requirements, levelling scope, interfaces and renovation sequencing before flooring works progress across Sydney projects.

Request a Floor Preparation Review

Sources and References

• ARDEX guidance on levelling anhydrite and gypsum-based screeds
• Sika's calcium-sulphate substrate guidance
• SafeWork NSW silica guidance
• NCC wet-area provisions
• Floor levelling over new concrete
• When concrete grinding can reduce unnecessary floor-levelling build-up
• When an existing screed bed should be removed before floor levelling
• Floor levelling across steel and concrete interfaces