Mortar for wood fired ovens
Mortar refers to a paste of cements, sand and possibly other ingredients used to fasten bricks or construction blocks, and to fill spaces left among them. Builders use mortar in the construction of buildings as it results in a rigid, stable and reliable structure. Mortar is required at almost every step in the building of a brick based pizza oven. There are two types of mortars and neither should be substituted for the other.
- Refractory Fire Clay Mortar
- General Purpose Building Mortar
Builder’s mortar or refractory mortar coupled with Portland cement based concrete or refractory concrete is used extensively to build high quality pizza ovens. A layer of mortar 1/4" (6mm) thick should be applied between bricks to create solid adhesion. Mortar helps in building the firing chamber, but is unsuitable for the hot hearth face inside. The hot surface should be constructed entirely out of fire bricks which are butted together without mortar in between. The big spaces among bricks should not be filled with refractory mortar. The refractory mortar is prepared by mixing sand, calcium aluminate cement, and fire clay in the ratio of 10:3:1.5. Portland cement with lime may be used in place of refractory cement if desired. The mixture will contain sand, fire clay, Portland cement, and lime in the ratio of 10:6:2:3. The lime contains calcium which allows the cement to endure extremely hot conditions. This mixture should be mixed thoroughly under dry conditions, and then room temperature water is added to make a paste the consistency of peanut butter. The paste is then applied between bricks where gaps exist usually in a V form. This results in the mortar not being exposed to the high heat of the internal oven. The area between wall bricks and floor does not require mortar. In the case of round domes, no space between bricks should exist.
Generally should be minimal gaps between bricks inside the oven dome as it creates exposure of mortar to the high internal temperatures. Additionally it creates an unnecessary and thick mortar quantity on the outer side of the oven dome. The angles of bricks should be suitably cut to prevent the occurrence of gaps inside the dome. These bricks should be so placed that no gap is created. A thin layer of the mortar is used inside the dome only to strengthen the structure, and not to fill gaps.
Refractory Fire Clay Mortar
When flat fire brick joints are difficult or impossible due to curves, refractory mortar should be used. Its maximum application should be no more than about 1/4" (6mm) as thicknesses above this are prone to crack and shrink over time. It shouldn't be used to fill holes or empty spaces in the oven. Any space larger than 1/4" (6mm) should be filled with a piece of fire brick. A thin slice may be cut with either a tile saw or hand held angle grinder with an appropriate blade attached. Due to the use of fire clay in the mixture of refractory mortar the heat tolerance of the medium is greater than that of regular building mortar. If building mortar is use in applications where the heats experienced are excessive, it is possible that the integrity of the mortar may fail and cracks may develop. Mortar should not be used in areas where flat joins are possible between the fire bricks such as in the hearth.
Refractory Mortar can be prepared by mixing Sand, Calcium Aluminate Cement and Fire clay in the ratio 10 : 3 : 1.5:
|Sand||Calcium Aluminate Cement||Fire clay|
Portland cement with lime may be used in place of refractory cement if desired. The mixture will contain sand, fire clay, Portland cement, and lime in the ratio of 10:6:2:3.
|Sand||Fire clay||Portland Cement||Lime|
Note: Use grey Portland cement, not white.
General Purpose Building Mortar
This is the mortar that you will use to lay the bricks of your pizza oven where excessive heat resistance is not required. The mortar is for use with general building bricks, not refractory fire bricks. It is very easy to prepare and you will require:
- Portland Cement
- Hydrated Lime
To make this building mortar, you will need the above components in this ratio 6 : 1 : 1.
Mortar can also be made with just sand and cement without the hydrated lime in the ratio 4 : 1. Lime allows for mortar to be easier to handle and increases the drying period giving you three times as much time to work with.
Mortar Mixing Instructions
To prepare the mortar first mix all the components properly to a uniform state without the use of water. The mixture may be made within a wheelbarrow or on a flat surface such as existing concrete or a piece of metal. Tools used for mixing might include a builder's shovel or trowel. Once fully mixed and the ingredients for a solid colour, add a small amount of water whilst mixing. Continue adding water at regular intervals until a paste as thick as crunchy peanut butter is formed. The mortar should reach a workable state such that it is uniformly wet and mixed. Too much water should be avoided such that the mortar maintains its shape when piled, and does not flow completely. A hand sized ball of the mortar should hold its shape.
Application and Drying
Application of mortar to bricks is usually done via the use of a brick laying trowel. This is a one handed hand tool with a handle and triangular flat shovel surface. As with the setting of concrete, the setting of mortar in the oven should be slowed such that it does not occur too fast. If water is drawn from the mortar quickly due to evaporation, the curing process will be hindered and the resulting mortar may be brittle. To slow the drying process a plastic sheet or wet hessian cloth may be placed over the mortared area. Additionally the area may be periodically watered over the following 24 hours. If the oven and mortared structure is in direct contact with the midday sun, or is placed in a hot and dry climate, this slowing of the drying process will be more important.
Before the oven is fully fired for the first time it is important that the concrete and mortar be completely cured and dry. If water remains in the mortar when heat is applied the water may turn to steam and create cracks as it attempts to escape. These cracks may be both structurally and aesthetically damaging. To minimise the potential for issues it is suggested that multiple small fires be built in the oven to gradually dry and increase the temperature over time. The drying time will likely be much longer in cold or wet environments and as such extra care may need to be taken.