The chemicals used in swimming pools can injure plants when the filter is backwashed and the water is dumped in the root zones of plants. The foliage will turn brown as the plants die from repeated doses of the chemicals.
The problem: you don’t have a mortar box or wheel-barrow to mix up those small batches of concrete. Instead, use the ring method on a slab or sheet of plywood. Put down coarse aggregate on the bottom, followed by layers of sand and cement. Reserve the center for a mixing well. Then add water to the center and pull the ingredients into it with a garden hoe.