high heat resistant refractory concrete mix design
for outdoor fireplace slab

Updated: March 7, 2025

Author: Mike Day

QUESTION:

I wish to find out whether I can use a strong mix of concrete to set my fire bricks inside my outdoor fireplace.

I am aware that fire clay is the best method but am reluctant to use this both due to the expense and difficulty of getting hold of this product.

If you are of the opinion that I could get away with using a strong concrete mix, which would be resistant to the heat, could you advise on the ratio of the mixture.

high heat concrete mix design for fireplace

Yes, you can use a strong concrete mix as a base to set fire bricks on when building an outdoor fireplace, but with some considerations:

  1. Heat Resistance – Standard concrete is not designed to handle high temperatures. While it can serve as a base, direct exposure to heat can cause cracking or spalling. A high-strength mix with added pozzolans (such as fly ash or silica fume) can improve durability, but it’s still not as heat-resistant as refractory materials.
  2. Firebrick Mortar – When setting fire bricks, use a refractory mortar (such as a mix of fire clay, sand, Portland cement, and lime) to withstand the high temperatures of a fireplace.
  3. Insulation Layer – If the concrete base is exposed to high heat, consider adding an insulating layer between the firebricks and the concrete to reduce thermal stress.
  4. Thickness & Reinforcement – A well-reinforced concrete base with proper curing will provide stability and support for the firebrick structure.

In summary, a strong concrete mix can be used as a base for setting fire bricks, but it should be protected from direct heat exposure using refractory materials and proper insulation techniques.

High-Heat-Resistant Refractory Concrete Mix Design

For a high-heat-resistant concrete mix, you need materials that can withstand extreme temperatures without cracking or spalling.

A standard Portland cement mix is not sufficient for high heat; instead, a refractory concrete mix should be used.

Below is a high-heat-resistant concrete mix design with proper ratios:

Materials & Ratios (by weight)

1. Calcium Aluminate Cement (CAC) (High Alumina Cement) – 20%

  • This is crucial for heat resistance, as it can handle temperatures above 1,800°F (980°C). Do not use regular Portland cement.

2. Crushed Firebrick (or Refractory Aggregate) – 40%

  • Use crushed firebrick or refractory grog (pre-fired, crushed clay bricks). This improves thermal shock resistance.

3. Silica Sand – 25%

  • Fine, heat-resistant aggregate that enhances strength and heat durability.

4. Perlite or Vermiculite – 10%

  • A lightweight, insulating aggregate that prevents cracking due to thermal expansion.

5. Refractory Clay (Kaolin or Fire Clay) – 5%

  • Helps with binding and improves thermal resistance.

6. Water – Just enough for workability

  • Typically 0.4 - 0.5 water-to-cement ratio by weight, but adjust based on the consistency needed.

how thick should a concrete slab be for an outdoor fireplace

For an outdoor fireplace, the concrete slab should be at least 6 inches thick, but in some cases, 8 inches or more may be necessary, depending on the fireplace size and weight.

Here are key factors to consider:

Recommended Concrete Slab Thickness

  • Standard Outdoor Fireplace (Small to Medium): 6 inches
  • Large or Heavy Masonry Fireplace: 8 inches or more

    • Additional Considerations

    1. Reinforcement – Use #4 rebar (1/2-inch diameter) in a grid pattern (12-inch spacing) to improve strength and prevent cracking. Wire mesh can also add durability.
    2. Concrete Strength – A mix with at least 4,000-5,000 psi compressive strength is recommended for long-term durability.
    3. Subgrade Preparation – Ensure proper compacted gravel (at least 4-6 inches) under the slab to prevent settling or cracking.
    4. Expansion Joints – If the slab is large, include expansion joints to allow for movement due to temperature changes.
    5. Freeze / Thaw Climate - Consider laying 2" styrofoam sheets under the slab to help prevent the slab from frost heave in the winter.

    how big should the concrete slab be versus the size of the fireplace?

    For an outdoor fireplace, the concrete slab should extend at least 12 to 24 inches beyond the fireplace footprint on all sides.

    Recommended Slab Size vs. Fireplace Size

    • Minimum Overhang: 12 inches on all sides for stability and safety.
    • Ideal Overhang: 18 to 24 inches to accommodate additional weight, foot traffic, and potential expansion.

    Why the Extra Space?

    1. Structural Support – Prevents cracking or shifting under the heavy weight of the fireplace.
    2. Fire Safety – Extra space protects surrounding areas from heat and sparks.
    3. Design & Functionality – Allows room for hearth extensions, seating, or future modifications.

    For example, if your fireplace is 5 feet wide by 3 feet deep, the slab should ideally be at least 7 to 8 feet wide and 5 to 6 feet deep to ensure proper support and longevity.

    conclusion:

    Building an outdoor fireplace on a concrete slab requires careful planning to ensure durability, safety, and longevity. A 6- to 8-inch-thick slab with proper reinforcement (such as rebar and wire mesh) provides a strong foundation.

    To prevent heat damage, consider fire-resistant materials, such as fire bricks and heat-resistant mortar. Additionally, the slab should extend 12 to 24 inches beyond the fireplace footprint for structural stability and safety.

    By following these guidelines and using high-quality materials, you can create a long-lasting outdoor fireplace that withstands heat exposure and enhances your outdoor space. If you're unsure about the best mix design or installation method, consulting a professional can help ensure a successful build.

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