Imagine the entire ocean becomes extremely corrosive due to ocean acidification over a couple of decades, all nations have agreed to erect a permanent barrier along all shorelines to seperate land and acid which is gradually eating away the beach. The acidity of the ocean on average is below 1 on the ph scale and the cause remains unknown and leads to many speculations and this is a major mass extinction event on a planetary scale. My question is how can I cordon the ocean effectively? I believe the wall should be made of a particular kind of ceramic or other corrosion resistance material casts into huge blocks before lowering into place but that will take too many centuries (compare to Great Wall of China even with modern day technology) My ideal duration for the Poseidon's Towel Project should not be more than a decade.
Without going into the total implausiblity of such a situation (we'd already be extinct), you don't need big blocks.
If the coast is made from some kind of rock that is subject to the acid (e.g. limestone), it will use up the potentcy at the site of the reaction, and can't continue until the water circulates.
A paper-thin membrane is enough to keep the coastal water fully reacted and not mix with the rest of the ocean. The membrane needs to resist the acid on one side.
Plastic is good. Normal acids don't affect hydrocarbons, and the substance can be chosen based on that specific acid.
Back to plausibility: how about making it an interim state in a terraforming project of some alien world?
First turning the Ocean into acid would kill all life in the ocean, that is estimated to be between %50-80 of all life on earth. Next, it will kill everything in the shorelines and tidal estuaries.
%70 of the oxygen on the planet is produced by plants in the ocean. Most of our rain comes from ocean evaporation. this would cause an Acid rain like never before. Blocking your shorelines would be fairly pointless. Most of the world would die within a couple decades of the start of the process.
Since the ocean no longer support anything of any value that requires human interaction, you can simply withdraw inland.
To begin with the stone and sand of the abandoned coastline will be fairly inert. This is because the atmosphere contains a highly corrosive chemical element, oxygen. Water is also fairly corrosive, especially when combined with oxygen as it is along the coastlines. Also unless the result of the acid eating your coast is water soluble it will almost instantly create a protective barrier superior to artificial protection. I do not think it is a realistic worry to hypothesize an acid that creates water soluble products when reacting with all minerals. And even if we did do so, such a super-acid would certainly kill us all.
So if you simply abandon the coasts the acid should stop eventually. Atolls and small islands would be gone though. It might make sense to bolster some low areas by shipping dirt to add mass to them. The acid should make simply dumping waste in the ocean or the coast safer and waste waste would help create a mass barrier along the coast.
Note that I am assuming the positive water pressure will stop the acid from permeating thru the water table. This might not happen in areas arid enough. But I do not think people would care about losing Atacama or Kalahari, if a disaster of this kind was threatening to make us extinct.
In any case the key point to worry about is to get rid of the acid. Everyone would assume that we will otherwise go extinct. There would be little incentive for building artificial barriers with resources that could be used to fight the actual problem.
Especially since the erosion of the coastlines would help neutralize huge amounts of the acid. Eventually whatever generates it would have to run out of materials.