Ardex X 7 G Plus vs Mapei Adesilex P9 for Fixing Ceramic Over Anhydrite Screed

Calcium sulphate screeds retain moisture in a way that creates problems for cement-based tile adhesives. The usual residual moisture limit before tiling is 0.5 percent CM by the carbide method; on heated anhydrite screeds the common limit falls to 0.3 percent CM. If tiling starts above those figures, the interface between cement adhesive and gypsum screed can develop an ettringite reaction. The result is a weak crystalline layer, sometimes showing up months after grouting as tiles begin to debond.

Ardex X 7 G Plus and Mapei Adesilex P9 sit on the same side of that chemistry because both are cement-bound. Neither product rescues an anhydrite screed that has been covered while too wet. The useful comparison begins after the screed has been measured, prepared and isolated with the correct primer.

Preparation before the bag is opened

The anhydrite surface has to be mechanically abraded first. That removes the laitance skin, which is the weak, dusty layer left at the top of many calcium sulphate screeds. After abrasion, the floor is vacuumed thoroughly and then primed.

Ardex specifies Ardex P 51 diluted for suitable dry anhydrite surfaces. Ardex P 82 is used as a two-component epoxy barrier where residual moisture sits close to the accepted limit. Mapei directs installers toward Primer G or Eco Prim Grip for the same type of background.

The primer has two separate jobs. It consolidates the friable top layer of the screed, giving the adhesive a firmer surface to grip. It also separates the cement adhesive from the gypsum, which suppresses the sulphate-to-cement reaction at the interface.

When the CM reading is above the limit and the programme cannot wait for further drying, the moisture-control layer becomes the controlling specification item. A single-coat dispersion primer at 0.6 percent CM is too light for that duty. A broadcast-sanded epoxy barrier is the system that deals with the risk.

Once that barrier has cured and has been sanded to provide a mechanical key, the tile adhesive above it behaves much more like it would over a stable cementitious base. At that point the practical differences between X 7 G Plus and P9 are mainly handling, open time, slip resistance and deformability.

What Adesilex P9 gives on site

Mapei Adesilex P9 is classified C2TE under EN 12004. That means an improved cementitious adhesive with reduced slip and extended open time. Installers often choose it for large-format porcelain because of that extended open time.

Under standard conditions, P9 has an open time of around 30 minutes. A baseline C2 mortar is closer to 20 minutes. On a 600 by 600 mm porcelain tile fixed with a 10 mm notched trowel, that extra working time can decide whether the adhesive ridges still transfer fully to the tile back or have already skinned over at the surface.

P9 mixes to a creamy consistency and holds a combed ridge well. Its pot life is around eight hours in the bucket, although the time available on the wall or floor matters more during setting. On a wet room floor in porcelain, the reduced slip rating helps tiles remain in position on a slight fall without wedges.

The creamier body needs care. If it is over-trowelled, it can slump into the notch valleys, making coverage harder to read. With porcelain above 900 mm on any edge, back-buttering is advisable so the bed reaches the 90 percent contact expected on a shower floor under EN 12004 solid-bed rules.

Why X 7 G Plus is different on heated anhydrite

Ardex X 7 G Plus is sold as a flexible, low-dust C2TES1 mortar. The S1 part of that classification is the important addition when the screed contains underfloor heating. Under EN 12002, S1 deformability means transverse deformation between 2.5 and 5 mm in the bending test.

Heated anhydrite moves as the heating system ramps up and cools down. A plain C2 mortar is less able to accommodate that movement, so stress is more likely to reach the grout line and the tile. An S1 bed has enough deformation capacity to absorb part of the cycle across the adhesive layer.

That is where X 7 G Plus earns its place. For a herringbone parquet-effect porcelain laid over heated anhydrite, every plank can cross two or three heating loops. The S1 rating then becomes a specification issue, especially where the tile format and the heating pattern increase movement across the floor.

The G Plus formulation is gel-modified. In use, the gel binder gives a smoother comb and a longer correction window than older Ardex X 7. It combs cleanly with a 10 mm notch and transfers well to porcelain backs without the same slump concern that can arise with a creamier adhesive.

Standard Adesilex P9 does not carry an S1 classification. Mapei has deformable products for that duty, including Keraflex Maxi S1 and Ultralite S1. On heated anhydrite, those products are the closer Mapei comparison to X 7 G Plus.

Coverage for X 7 G Plus is usually around 2.5 to 3.5 kg per square metre with an 8 to 10 mm notch, broadly comparable with P9. The low-dust claim also has a visible effect at mixing, reducing airborne silica during whisk mixing on occupied refurbishment sites.

Cost, primer and coverage on a 20 square metre floor

Take a 20 square metre heated bathroom floor in 600 by 600 mm porcelain. At 3 kg per square metre in a solid bed, the floor needs 60 kg of adhesive, equal to four 15 kg bags.

Ardex X 7 G Plus and a comparable Mapei S1 adhesive usually sit in a similar retail range, about £22 to £30 per 15 kg bag. That puts the adhesive cost near £88 to £120. Standard Adesilex P9 typically comes in around 20 to 30 percent cheaper per bag, which explains why it appears in specifications where S1 deformability is unnecessary.

The primer can change the bill more sharply than the adhesive. A two-component epoxy barrier such as Ardex P 82 covers around 350 to 400 grams per square metre per coat over two coats. On 20 square metres, that consumes about 14 to 16 kg, and reactive primers are a meaningful cost line. If the screed is dry enough for a single dispersion primer, the primer cost falls to a fraction of the barrier system.

Large-format porcelain also exposes flatness errors quickly. On any adhesive bed, lippage becomes likely if the base is outside SR1 tolerance. A wedge-and-clip lippage control system such as Raimondi RLS can hold adjacent tile edges coplanar while the adhesive cures, although it cannot make an uneven screed flat.

If the anhydrite shows more than 3 mm deviation under a 2 metre straightedge, a self-levelling compound belongs in the specification before tiling. It goes over the prepared and primed surface first. The adhesive decision then follows the corrected surface.

Practical selection

Both Ardex X 7 G Plus and Mapei Adesilex P9 can bond porcelain to correctly primed anhydrite, provided the screed has been abraded, cleaned and matched to the right moisture-control system. The unresolved tension is commercial as much as technical: a cheaper bag of standard P9 can be workable on a dry, stable surface, while heated anhydrite keeps pulling the specification toward an S1 adhesive and, in wetter cases, a reactive barrier.