How Much Heat Can Concrete Blocks Withstand?

Concrete blocks are a versatile product used for everything from retaining walls and fun DIY planters in your yard to huge commercial and retail buildings. Given that these blocks are also used for firebreak walls, which contain a fire or slow its progress, some wonder how much heat they can take. The simple answer is up to 1832 degrees Fahrenheit, and even then they won't burn. Technically, they could melt, but that won't happen until around 2900 degrees Fahrenheit.

There are more than a dozen different types of concrete block in solid, hollow, and cellular forms. So while all are considered non-combustible, their actual fire resistance does vary. For example, a typical 4-inch block wall that might be used internally in apartments could withstand the heat of a house fire for a couple of hours. Make the wall from 8-inch blocks, and that time is extended to around four hours.

So, concrete blocks are among the best building materials for fire protection. However, you probably wouldn't want to use them to build a fire pit or a pizza oven. Let's look at why not. 

A wood-fueled fire pit can easily exceed 1000 degrees Fahrenheit. A pizza oven runs to about 700 or 800 degrees Fahrenheit. Both are well below the heat concrete blocks can withstand, so what's the problem?

Concrete is made by mixing water, cement, and aggregates (typically sand and gravel). Most of that water evaporates as the concrete block hardens, but not all of it. A small amount is trapped inside. With prolonged exposure to temperatures above 570 degrees Fahrenheit, that water starts to boil and becomes steam. The energy created is powerful enough to crack the concrete block or, in extreme cases, cause it to shatter. Not what you want when you're toasting marshmallows or cooking a pie.

So while you could make some cool patio furniture from concrete blocks, they aren't the best material for any kind of outdoor cooking setup or fire pit. That said, if you really want that industrial aesthetic, you could buy heat-resistant (refractory) concrete blocks. Although considerably more expensive, their concrete has special ingredients allowing them to withstand high cooking temperatures without problems. If you're using heat-resistant concrete blocks for a fire pit, an inner firebrick layer shores up structural integrity. Additionally, checking manufacturer specifications clarifies whether heat-resistant concrete blocks are rated for direct flame contact.