What Is in Type S Mortar? (Full Composition)

This is the core question — what is actually in Type S mortar. There are two accepted ways to produce it under ASTM C270: a proportion specification (a fixed recipe) and a property specification (a performance target). Here’s what goes into each.

1. Portland Cement

The primary binder. Portland cement provides the compressive strength and hydraulic set of the mortar. Type S mortar uses 1 part Portland cement as the baseline of the mix.

2. Hydrated Lime

Hydrated lime (calcium hydroxide) is added in a proportion of over 1/4 up to 1/2 part for every part of cement. Lime improves workability, water retention, and the mortar’s ability to accommodate minor movement without losing bond.

3. Masonry Sand (Aggregate)

Clean, well-graded sand — roughly 4.5 parts — forms the bulk volume of the mix. It controls shrinkage, adds body, and must meet ASTM C144 aggregate requirements for masonry mortar.

4. Water

Potable, clean water is added last, in just enough quantity to produce a workable, trowelable paste without being so wet that it weakens the cured strength.

5. Alternative: Masonry Cement or Mortar Cement Blends

Many contractors use a pre-blended Type S masonry cement or mortar cement bag (cement and lime already combined at the factory) mixed with sand and water on site. This is faster and more consistent than batching raw lime and cement separately.

Why Use Type S Mortar?

Engineers and masons specify Type S mortar whenever a project needs more than average bond and compressive strength. Its higher lime-to-cement ratio range and cement content give it a strong balance of strength and flexibility — strong enough for structural loads, yet flexible enough to resist cracking better than the highest-strength Type M mortar.

• High bond strength — critical anywhere masonry must resist lateral (sideways) forces.
• Good for below-grade and exterior work — resists moisture penetration well.
• Balanced flexibility — less brittle than Type M, so it tolerates minor settlement or seismic movement.
• Code-driven choice — often required by building codes in high-wind or seismic zones.

Types of Mortar: How Type S Compares

ASTM C270 defines five mortar types — M, S, N, O, and K — each with a different minimum compressive strength and lime content. Understanding where Type S sits helps answer “what is Type S mortar” in context.

Type S Mortar vs Type N Mortar

The most common comparison question is Type S vs Type N mortar. Type S has roughly 2.4 times the minimum strength of Type N (1,800 psi vs 750 psi) and higher bond strength, making it the better choice for structural, load-bearing, or below-grade work. Type N is easier to work with and is the standard choice for general above-grade residential masonry like brick veneer and chimneys.

How to Mix Type S Mortar

Whether batching from raw materials or a pre-blended bag, the process for mixing Type S mortar follows the same general workflow:

1. Measure proportions accurately. Use a consistent measuring container — 1 part Portland cement, 0.5 part hydrated lime, 4.5 parts sand (or follow the bag instructions for pre-blended Type S masonry cement).
2. Dry-mix the solids first. Combine cement, lime, and sand thoroughly before adding water, so the binder is evenly distributed through the aggregate.
3. Add water gradually. Pour in clean water a little at a time while mixing, in a mechanical mixer or mortar pan, until the mix reaches a workable, buttery consistency.
4. Test the consistency. The mortar should hold a peak on the trowel without slumping or crumbling — not too wet, not too stiff.
5. Use within the working time. Type S mortar should generally be used within about 2 hours of mixing, before initial set begins; do not “retemper” mortar that has begun to stiffen from hydration (as opposed to evaporation).

Is Type S Mortar Safe?

Yes — Type S mortar is safe to use when handled correctly, but it does carry real hazards that require basic precautions, both while it’s wet and while it’s being mixed dry.

• Caustic / alkaline burns: Wet mortar is highly alkaline (high pH) and can cause chemical burns to skin or eyes on prolonged contact. Wear gloves, long sleeves, and eye protection.
• Silica dust: Dry mixing of cement and sand releases respirable crystalline silica, linked to long-term lung disease. Use a dust mask or respirator (N95 minimum) and work in ventilated areas or use dust-suppression methods.
• Skin sensitization: Repeated cement contact can cause irritant or allergic contact dermatitis; wash exposed skin promptly.
• Structural safety: Once cured, Type S mortar is chemically inert and poses no ongoing health risk in a finished wall or structure.

Advantages and Disadvantages of Type S Mortar

Common Uses of Type S Mortar

• Foundations and footings in masonry construction
• Retaining walls subject to soil pressure
• Below-grade and exterior masonry exposed to moisture
• High-wind and seismic zones where lateral bond strength matters
• Structural brick and block walls carrying building loads
• Chimneys and masonry requiring extra durability
• Stone masonry where strong adhesion is required

Frequently Asked Questions