The ground oyster shell mixture will now be monitored to see if it could provide a long-term solution.
The first test was for filling quarry holes which cause road subsidence, but laboratory tests are under way to see whether oyster shell “sand” could also be used for housebuilding.
Officials in Gironde, where a number of roads are built over former quarry mines which need to be filled with mortar to prevent subsidence, decided to use oyster shells rather than sand for ecological reasons.
While the shells were more expensive in the trial – by about 20% in cash terms – the overall ecological benefits mean it could be widely adopted.
Alain Renard, vice-president of the Conseil Départemental of the Gironde, in charge of infrastructure and risk prevention, said: “Usually, we use a mortar similar to that used in building, with a mix of sand, cement and water. We have substituted sand for the experiment with oyster shells, which have been ground down to the ideal size for building sand, and used it along with cement and sand for a quarry hole.
“After a month, it is performing perfectly.”
This first test, in Prignac-et-Marcamps on the right bank of the Dordogne river, will be monitored at monthly intervals to see how the mortar ages compared to standard sand based material.
Around 65 tonnes of oyster shells were used for the 65m3 of mortar during the test.
A second site, which will require 1,000m3 of mortar, is being studied at a quarry in the Entre-deux-Mers region.
“There has been so much quarrying in the region over the years that we do not know where all the holes are,” said Mr Renard.
“When we discover holes which run under or next to the roads, we have to deal with the problem, and it has fitted in with our need to find another solution to getting rid of oyster shells other than burying them in landfills, which we do now.
“With the tax on landfill, putting shells in them is expensive and the tax will only go up.”
The cost for the experiment of collecting shells and grinding them ended up between 20% and 30% more than using bulk buys of builders’ sand.
That does not take into consideration the environmental benefits, such as less landfill, and less building sand dug out of functioning quarries.
The use of this new material for buildings is being tested. The length of the tests and the bureaucratic process mean approval is still several years down the line. As well as grinding the shells to sand size, there are also experiments in using them to replace gravel in concrete, which Mr Renard said were promising.
With most of the cost of the exercise coming from the collection of the shells in the first place, the Gironde department is working with oyster farmers in the Bassin d’Arcachon to gather the waste shells from their normal activities.
There are also initiatives by associations working to reduce waste in Bordeaux to collect oyster shells from restaurants, often in collaboration with the collection of food scraps headed for communal compost heaps.
In Charente-Maritime, one business collects oyster shells and crushes and transforms them either into blocks fed to laying hens to boost calcium levels to boost egg production or to be incorporated into saltlicks for cattle.
It also grinds them to make a fertiliser which reduces soil acidity levels, like lime.
Shells can also be placed around plants that are sensitive to slugs.
The crushed shells create a barrier and are said to be too difficult for the snails and slugs to traverse.