Calcium sulphate screed (CA)
Calcium sulphate screed (CA / anhydrite screed): The modern screed for your building project
Calcium sulphate screed, classified as CA (calcium sulphate screed) in accordance with DIN EN 13813 (previously often referred to as anhydrite screed), represents the modern standard in screed construction, especially indoors. Its special material properties, such as low shrinkage, excellent thermal conductivity and often faster readiness for covering, make it the technically superior solution for many applications in new buildings and renovations, especially for heated screeds and high demands on evenness. As a specialized company, we have mastered the planning and precise execution of calcium sulphate screeds for demanding floor constructions - a core competence of our company.
Composition and material properties
Calcium sulphate screed is a mineral screed mortar whose performance depends on the quality of its components and the exact formulation. It is mainly delivered to the construction site as a premixed dry mortar or as a flowing screed. The main components are
- Binder: Calcium sulphate (in accordance with DIN EN 13454-1), usually in the form of synthetic anhydrite (from flue gas desulphurization plants), natural anhydrite or alpha hemihydrate. This largely determines the strength development and the characteristic properties.
- Aggregate: Quality-tested sand or special aggregates (in accordance with DIN EN 12620) with optimized grading curve for a dense structure and good processing properties.
- Mixing water: In precisely defined quantities to control hydration and consistency (especially critical for self-levelling screeds).
- Additives: Used specifically to optimize specific properties such as flowability (superplasticizer for self-levelling screed), setting time (accelerator/retarder) or to improve processing.
Curing is based on the hydration of the calcium sulphate to gypsum dihydrate (), which results in a firm, dimensionally stable and virtually stress-free screed structure. Its characteristic features are low shrinkage (reduces cracking and cupping), high early strength and excellent thermal conductivity. A key feature, however, is its sensitivity to permanent exposure to moisture.
Areas of application for calcium sulphate screed
The technical advantages allow calcium sulphate screed to be used in many segments of building construction:
- Residential construction: Ideal basis in single and multi-family homes, especially with underfloor heating and high demands on indoor climate and evenness.
- Commercial and commercial construction: load-bearing and flat surfaces in offices, sales rooms, hotels, administration buildings. Choosing the correct strength class is crucial.
- Public buildings: Durable and comfortable solutions for schools, hospitals and kindergartens.
- Heated floor constructions: The first choice as a heating screed thanks to optimum enclosure of the heating pipes (especially with flowing screed) and high thermal conductivity for energy-efficient operation.
- Refurbishment: Low dead weight and low installation heights (conditionally possible) can be advantageous.
- Universal substrate: Compatible with almost all common floor coverings once ready for covering - from tiles and parquet to resilient coverings and coatings.
- ❌ Restriction: Not suitable for permanently damp or wet areas (e.g. commercial kitchens, outdoor areas, unheated cellars with high residual moisture) without special protective measures and sealing.
Advantages of calcium sulphate screed
- 💨 Fast readiness for covering: Under optimum drying conditions, often ready for covering faster than cement screed.
- 🧘 Low shrinkage: Minimizes the risk of cracking and cupping of the edges, ideal for large areas with few joints.
- 🌡️ Excellent thermal conductivity: Predestined for use as a heating screed, ensures efficient heat dissipation and fast control behavior of the underfloor heating.
- 📐 High evenness: Particularly self-leveling as a flowing screed (CAF-F), creates perfect surfaces for demanding coverings.
- 🌱 Ecologically beneficial: Frequent use of FGD gypsum (from flue gas desulphurization) as a resource-saving recycling product.
- 🏠 Good indoor climate: Is considered to be harmless from a building biology perspective and can have a moisture-regulating effect.
- 💪 Good strength development: quickly achieves high early strength, construction site can be walked on earlier.
- 🔥 F ire protection: Meets the highest requirements as a non-combustible building material (class A1).
Technical properties and classification
The performance of a calcium sulphate screed is clearly defined by its compressive strength (C = Compressive Strength) and flexural strength (F = Flexural Strength) in accordance with DIN EN 13813: CA-C[value]-F[value].
- CA: Calcium Sulfate Screed
- C[value]: Characteristic value of the compressive strength class (e.g. C25 ≥ 25 N/mm²)
- F[value]: Characteristic value of the bending tensile strength class (e.g. F5 ≥ 5 N/mm²)
The selection of the standard-compliant strength class according to the subsequent use and load is a fundamental planning task.
Overview of common strength classes:
ℹ️ Note: Correct dimensioning and determination of the screed class and consideration of moisture sensitivity are essential for suitability for use and freedom from damage. They are the responsibility of qualified planners or the specialist company carrying out the work.
Processing and drying time
The quality of the finished screed is based on precise adherence to all processing steps:
- Substrate inspection and preparation: Foundation for a functioning construction (cleanliness, impermeability, edge insulation strips).
- Mixing: Precise water metering for factory dry mortar; flowing screeds are usually delivered ready-mixed in a truck mixer or silo and pumped.
- Application and spreading: Professional application to the correct height, with flowing screed using pump technology and hose.
- Leveling/compaction: With conventional CA, careful compaction and screeding; with flowing screed, self-levelling, if necessary light finishing with a buffing rod for deaeration and optimum leveling.
- Surface finish: Flowing screeds usually form a very smooth surface; conventional CA is smoothed.
- Controlled post-treatment/drying: Crucial! Protect from draughts for the first few days, then ventilate constantly to remove moisture. Avoid direct sunlight.
The drying time to readiness for covering is material-specific and strongly dependent on the ambient conditions 💨. It is influenced by:
- Screed thickness
- Ambient climate (decisive: temperature and especially relative humidity!)
- Effective ventilation (air exchange!)
- Type of calcium sulphate binder
Calcium sulphate screed often dries faster than cement screed, but requires low humidity and good air exchange. The rule of thumb (~1 week/cm) is even less reliable here. ⚠️ Approval for covering may only be given once the residual moisture required by the standard has been reached, as verified by reliable CM moisture measurement. The limit values for CA are lower than for CT (often ≤ 0.5 CM% for vapor-permeable coverings, ≤ 0.3 CM% for vapor-tight coverings or parquet on underfloor heating - observe manufacturer's instructions!) Premature covering or renewed moisture penetration (e.g. due to subsequent work) leads to serious damage!
Variants of the calcium sulphate screed
Different types of construction are used to meet the specific requirements:
- Screed on separating layer: Allows movement relative to the substrate.
- Floating screed (screed on insulation layer): Standard for thermal and impact sound insulation, structurally separated.
- Heating screed: Designed as a floating screed to accommodate underfloor heating pipes, this is where CA really comes into its own.
- Flowing screed (CAF): Self-leveling variant that is pumped - ensures high installation performance, excellent pipe containment for heated screeds and high evenness.
- Thin-layer screed: Special systems for low construction heights, often in renovation.
Professional execution - our benchmark
The durability and functionality of a calcium sulphate screed are the direct result of flawless craftsmanship, the use of high-quality materials and strict attention to its specific properties. The indispensable pillars of our quality work are:
- Strict compliance with all relevant standards and regulations (in particular DIN 18560, DIN EN 13813, DIN 18202, BEB data sheets).
- Careful selection of materials and precise adherence to the manufacturer's specifications.
- Mastery of processing techniques, especially for flowing screeds.
- Guarantee of the required evenness tolerances as a basis for the top layer.
- Consistent drying management: Ensuring suitable climatic conditions and sufficient ventilation.
- Responsible implementation and documentation of the CM measurement, taking into account the lower occupancy limits.
- Protection against moisture during the entire construction phase until the floor covering is laid.
Commissioning an experienced and specialized screed company like ours is the best guarantee that these decisive quality criteria will be met without compromise - for a modern, efficient and durable foundation for your floor.
Frequently asked questions (FAQ)
The binder (gypsum vs. cement) leads to differences in shrinkage behavior (CA hardly shrinks at all, CT more), moisture sensitivity (CA is sensitive, CT robust) and the residual moisture required for readiness for covering (CA must be drier than CT).
Yes, essentially yes. Anhydrite is the calcium sulphate binder used in this screed. CA is the official designation according to DIN EN 13813 for calciumsulphate screed.
Yes, it is very suitable as a heating screed due to its good thermal conductivity and low expansion behavior. Careful compaction around the heating pipes is important.
The screed drying time until it is ready for covering (residual moisture ≤ 0.5 or ≤ 0.3 CM-%) can take weeks to months, similar to CAF. It depends heavily on thickness, climate and ventilation. The CM measurement is crucial!
In domestic bathrooms only with perfect bonded waterproofing on top. In basements only if they are absolutely dry and no moisture is expected from the substrate or walls (functioning structural waterproofing!). If there is a risk of moisture, cement screed is the safer choice.