Screed Materials Guide: Cement, Aggregates & Storage

Screed Materials Guide: Cement, Aggregates & Storage

The quality of a finished screed depends directly on the quality of the materials that go into it. Understanding the properties of cement, aggregates, and water — and how to store them properly — is fundamental to achieving consistent, reliable results on site.

Portland Cement

Portland cement is the binding agent in traditional sand and cement screeds. First patented by Joseph Aspdin in 1824 and improved by his son William in 1843, it remains the most widely used cement type in global construction. In a standard screed mix, cement acts as the binding agent that holds the aggregate together and is the key driver of setting and hardening properties.

Cement Grades for Screeding

The key grades relevant to screeding are:

  • CEM I (OPC) — Portland cement with no additions. Provides predictable strength development and is the traditional choice for floor screeds. BS 8204 recommends CEM-I of strength class 42.5N for screeding applications
  • CEM II — Portland composite cements containing limestone, fly ash, or slag. Increasingly common and suitable for screeds, but may have slightly different setting characteristics

Important: Where cements of lower strength class are used (e.g. 32.5N), the mix should be compensated with an additional 10% cement to achieve the expected strength and performance specified in BS 8204.

Specialist Cement Variants

While CEM I is the standard for most screeding work, other variants of Portland cement may be specified for particular conditions:

  • Sulphate-resisting Portland cement — used where ground conditions contain high sulphate levels that could attack the screed from below, causing cracking, scaling, or expansion
  • Rapid-hardening Portland cement — more finely ground than standard OPC, it reacts faster with water and develops early strength more quickly. Useful where fast turnaround is needed or where early foot traffic is required
  • White Portland cement — used for decorative or architectural screeds where a lighter or coloured finish is required. Manufactured by controlling ferrite content, which gives standard cement its grey colour

Always use cement from a reputable manufacturer that complies with BS EN 197-1. Check the bag date — cement has a shelf life of approximately 3 months when stored correctly, after which its binding efficiency reduces.

Aggregates for Screeds

The aggregate in a screed (typically sharp sand) makes up the largest proportion of the mix by volume. The choice and quality of aggregate significantly affects workability, compaction, strength, and surface finish.

Natural Sharp Sand

The traditional aggregate for floor screeds is natural washed sharp sand conforming to BS EN 13139. Key properties to look for:

  • Grading — a well-graded sand (0-4mm) with a good distribution of particle sizes produces the most workable and strongest screed
  • Cleanliness — the sand should be free from clay, silt, organic matter, and other contaminants. Excess fines (material passing the 63 micron sieve) weaken the screed and increase water demand
  • Shape — angular particles interlock better than rounded ones, producing a denser, stronger screed

Manufactured Sand

As natural sand resources become scarcer and environmental concerns increase, manufactured (crushed) sand is increasingly used in screeds. Advantages include consistent grading, availability, and lower environmental impact from reduced river dredging. However, manufactured sand typically has a higher proportion of micro-fines and more angular particles, which can affect workability. Adjustments to the mix water content and potentially the addition of plasticisers may be needed when using manufactured sand.

Other Aggregate Types

  • Recycled aggregates — crushed concrete and masonry can be used in some screed applications, but quality varies and must be assessed against the project specification
  • Lightweight aggregates — used in specialist lightweight screeds to reduce dead load. Examples include expanded clay, perlite, and polystyrene beads
  • Granite chippings — used in granolithic (wearing) screeds for heavy-duty applications requiring maximum abrasion resistance

Water

Water serves two purposes in screed: it hydrates the cement and provides workability. The water-to-cement ratio is one of the most important factors affecting final screed strength. As a rule, use the minimum amount of water needed to achieve a workable mix. Excess water:

  • Reduces compressive and flexural strength
  • Increases drying time significantly
  • Creates a more porous, weaker screed
  • Increases the risk of shrinkage cracking and surface laitance

Use clean, potable water. Water from unknown sources may contain contaminants that interfere with cement hydration.

Storing Materials on Site

Cement Storage

  • Store bags off the ground on pallets, in a dry and weatherproof location
  • Stack no more than 8-10 bags high to prevent compaction of lower bags
  • Use oldest stock first (first in, first out)
  • Reject any bags that are damp, lumpy, or show signs of hardening
  • In cold weather, keep cement in a frost-free area

Sand Storage

  • Store on a clean, hard-standing area to prevent contamination from the ground
  • Keep different sand types separated and clearly marked
  • Cover stockpiles to prevent saturation from rain — excessively wet sand makes water control in the mix very difficult
  • In winter, protect from frost. Frozen sand must be thawed completely before use
  • Do not allow vehicles to drive over sand stockpiles, as this compacts the material and makes accurate batching more difficult

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Call us on 0118 370 2060 for expert advice on your project. Free delivery on orders over £600 ex-VAT.