Concrete can make a durable and long lasting garden pond, although the effects of freeze thaw action should be taken into consideration. There are three main types of concrete pond.
For a free form pond:
• Dig hole, ram down soil, slope sides at 45o
• Line hole with heavy gauge polythene sheet
Then for a concrete liner:
• Cover base and sides with a 100mm layer of concrete
• Press galvanised wire mesh into the fresh moist concrete
• Cover wire netting with further 50mm layer of concrete and smooth with trowel.
Or for a ferrocement liner:
• Line the pool with 2 or 3 layers of galvanised wire mesh (approx 20mm opening size)
• Work a layer of mortar (1 part cement to 1.5 to 2 parts sharp sand) into the mesh
• The finished lining should be 20mm thick with 10mm of mortar cover to the mesh
For a more formal pond use an in situ concrete slab for the base with rendered masonry units for the sides.
Whichever type of pond is built it is important to cure the concrete, keeping it covered with a polythene sheet (or similar) for the first few days to prevent it drying out too rapidly. Once cured, allow the concrete to dry and then seal with a sealer from a pond shop to prevent the alkaline concrete harming the fish. Alternatively keep the pond filled with water for at least a month before adding fish or plants; change the water three or four times during this period and scrub down the concrete while it is empty.
While handling concrete water proof glooves should be worn. See H&S entry on Dermatitis