High-Temperature Mortar Ingredient
Hydrated Lime (Type S) for High-Temperature Mortar
Hydrated lime is the ingredient that confuses the most builders. Here's what it is, what it does in firebrick mortar, which bag to buy and the look-alikes to leave on the shelf.
Quick Answers
- Hydrated lime is a bright white powder that looks and feels like powdered sugar.
- It makes high-temperature mortar creamy and easy to work, slows the set so you have more time, and helps the mortar shed water.
- Buy Type S hydrated lime (the most common). Type N works too.
- Don't use granulated garden lime, ground limestone, quicklime or hydraulic lime.
What Is Hydrated Lime?
Hydrated lime (calcium hydroxide) starts as limestone. The stone is heated in a kiln to make quicklime, and the quicklime is then slaked with water under controlled conditions. What comes out is a soft, fluffy white powder that is far safer to handle than quicklime and is a standard ingredient in masonry mortar.
It's sold in 50-lb bags under several names: Type S hydrated lime, Type N lime, mason's lime, builder's lime, contractor's lime and powdered or pulverized lime. Masonry limes are graded as Type N, NA, S and SA. Type S ("special") is the one you'll find at most masonry suppliers.
Why Lime Is in High-Temperature Mortar
- Workability. Lime makes the mortar fluffy and creamy, so it spreads and sticks to firebrick instead of sliding off.
- More working time. Lime slows the cure, which helps on your first oven (and your tenth).
- Water resistance. Lime helps the mortar shed water, which matters for an oven that lives outdoors.
- Flexibility. Lime is less rigid than Portland cement, so the joints tolerate a bit of movement as the oven heats and cools.
Some experienced masons leave the lime out. We don't recommend that, especially on a first build. Just don't add more than the recipe calls for.
How Hydrated Lime Is Used in High-Temperature Mortar
Hydrated lime is 1 part of every 6 in the mix. It makes the mortar creamy enough to stick to firebrick, slows the set so you have time to work, and helps the joints shed water.
The 3 : 1 : 1 : 1 Mix (by volume)
Measure by volume, not weight. Lime is fluffy and weighs far less per scoop than sand, so weighing the ingredients throws the recipe off.
The short version: dry-mix all four ingredients by volume until the color is even, then add water a little at a time until the mortar is firm and buttery, like thick peanut butter. Dampen your firebrick, lay thin, full joints, and let the mortar cure for about a week before your first small curing fire.
Our full guide covers every step, the tools you need, where to use the mortar (and where not to), and how it compares to factory-made refractory mortar.
Which Lime Should I Buy?
| Product | What it is | Use it? |
|---|---|---|
| Type S hydrated lime | The most common masonry lime | ✓ Yes |
| Type N, NA or SA hydrated lime | Other masonry grades of hydrated lime | ✓ Yes |
| Mason's, builder's or contractor's lime | Trade names for powdered hydrated lime | ✓ Yes |
| Granulated or pelletized garden lime | Little lime balls for lawns and gardens | ✗ No |
| Agricultural lime / ground limestone | Crushed limestone (calcium carbonate), not hydrated lime | ✗ No |
| Quicklime | Calcium oxide. Reacts with water and gives off heat, and is very caustic | ✗ No |
| Hydraulic lime (NHL) | Sets much faster, which cuts your working time | ✗ No |
How to tell you have the right bag
Open it. Hydrated lime is bright white and soft, like powdered sugar. If it's pellets, gray granules or gritty crushed stone, it's the wrong product for mortar.
Don't Use Portland Cement With Lime Already Added
- Some bags of masonry cement or "Portland-lime" cement already contain lime, but not in the ratio this recipe needs. Buy plain Portland cement and add the lime yourself.
Lime is caustic, wet or dry. Wear safety glasses and gloves, and if it gets in your eyes, flush with clean water for at least 15 minutes and get medical help.
Find These Materials in Your Area
Need it today? Check a store near you.
We ship every bag FREE to the contiguous United States. If you would rather pick it up today, all four ingredients are stocked at stores near you, so check these places first.
Call ahead and ask for each item by the names on this page. Counter staff sometimes suggest a premixed substitute that doesn't belong in a wood-fired oven.
Hydrated Lime FAQ
What's the difference between quicklime and hydrated lime?
Quicklime is calcium oxide, made by heating limestone. Hydrated lime is quicklime that has already been slaked with water, which turns it into calcium hydroxide. Hydrated lime is safer to handle and is the one to use in mortar. Quicklime reacts with water, gives off heat and is highly caustic.
Type S or Type N hydrated lime?
Either works in high-temperature mortar. Type S is the most common at masonry suppliers, so it's usually the easiest to find.
Can I use garden lime?
No. Granulated or pelletized garden lime and agricultural lime are not hydrated lime. Look for a powder that looks and feels like powdered sugar.
Can I leave the lime out?
We don't recommend it. Lime makes the mortar workable, gives you more working time and helps it shed water. Use 1 part lime, no more and no less.
Where can I buy hydrated lime?
Landscape & masonry supply stores usually stock Type S hydrated lime in 50-lb bags. Some home improvement stores carry it, and plant nurseries can often order it.
Can I use hydraulic lime?
No. Hydraulic lime sets much faster, so your working time is much shorter. Stick with hydrated lime.
Can I use Type S mortar mix instead of Type S lime?
No. Type S mortar mix is a finished mortar that already contains sand, cement and lime. Type S hydrated lime is pure lime. They are completely different products.
Please Order Carefully: Refractory Materials Are Non-Refundable Once Shipped
Refractory materials are heavy, costly to ship and can't be resold once they leave our warehouse. For that reason, refractory products can't be cancelled, returned or refunded once your order has shipped.
We're happy to help you get it right the first time. If you're not sure which product or how much you need, contact us before you order and we'll walk you through it.
Shipping
FREE shipping on every refractory product to all locations in the contiguous United States (the lower 48 states). Orders ship by ground carrier or commercial freight, depending on size, and freight carriers will call to schedule delivery. Please leave a phone number we can reach you at in the order notes.
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