This video documents the construction of a circular brick dome using primitive techniques, starting from raw materials. The process begins with laying the foundation, followed by on-site production of bricks using a custom-built updraft kiln. A key focus is the creation of a strong, water-insoluble mortar. The mortar's binder is produced by calcining sieved wood ash into calcium oxide, while the aggregate is made from crushed, fired brick (grog). The video details the 3:1 mixing ratio of grog to ash, the chemical reactions involved (formation of calcium carbonate and an aluminosilicate geopolymer), and a successful water-resistance test of the set mortar. Finally, the video demonstrates the bricklaying process, including staggering joints and employing a specialized measuring stick to guide the construction of a pointed, gothic-arch dome profile. The project progresses until the supply of calcined ash is depleted, prompting plans for a more efficient furnace.
Building a Brick Dome: From Clay and Ash to a Finished Structure
00:00:00 Foundation and Layout: The project begins by marking out a 1.25m radius circle and digging a 25 cm deep by 12.5 cm wide foundation trench, which is then compacted.
00:02:00 On-Site Brick and Kiln Production: A small updraft kiln is constructed from existing bricks. New bricks are formed from clay using a water-soaked, porous terracotta mold and are then fired in the kiln to make them hard and waterproof.
00:03:33 Creating the Binder: Calcined Ash: Wood ash from the kiln is sieved to remove charcoal and formed into pellets. These pellets are fired (calcined) in the kiln at a dull red heat, converting calcium carbonate into calcium oxide, the primary binding agent for the mortar.
00:07:47 Preparing the Aggregate: Crushed Brick (Grog): Instead of sand, crushed fired brick (grog) is used as the aggregate. This method is chosen for its convenience and as an effective way to reuse broken bricks. The grog is sifted to ensure a consistent particle size.
00:08:53 Mortar Mixing and Chemistry: The mortar is mixed using a 3:1 ratio of crushed terracotta (grog) to calcined ash by volume. The mixture forms both insoluble calcium carbonate and a geopolymer (aluminosilicate chains), resulting in a strong bond. Due to the porosity of the grog, this recipe requires more water than a sand-based mortar.
00:09:42 Bricklaying Technique: Before laying, bricks are soaked in water to prevent them from drawing moisture out of the mortar too quickly. Mortar is applied to the bricks, with joints angled to accommodate the structure's circular wall.
00:11:13 Mortar Durability Test: After 9 days of curing, a sample block of the mortar is tested and proven to be completely insoluble in water. The creator notes it feels stronger than previous sand-based mortars.
00:14:31 Dome Construction Method: A measuring stick equal to the dome's diameter is used to create a pointed, gothic arch profile. By pivoting the stick from the base of the opposite wall, the precise position and angle for each brick in the rising dome can be determined.
00:16:57 Progress and Future Work: Construction halts after the supply of calcined ash runs out. The creator concludes by mentioning plans to build a specialized furnace for more efficient ash production to continue the project.