Hello first off I used to post to this forum as Ztevan Whyte but have been unable to get back into my account so will be using this one instead.
The problem with using cement is that it is very costly and in time it cracks and crumbles. The higher quality cement costs a lot more and for such a large project as you aspire it would be a logistical nightmare and the cost would be insane.
I do not have a stake in this project and couldn’t afford to be a part of it but I love the concept and would like to help if I can so here is my suggestion.
Whether you are building a submerged sea wall or barrier or a floating platform I think it may be possible to use sand to make glass to do so. You could make the glass sponge like and filled with pockets of air so it may be able to float or you can make it slab like for submerged sea walls. This sounds impractical but let me explain more how to do this onsite:
Use Mobile and modular glass induction forges. If you have a way to generate electricity such as solar, wind etc then this is possible it’s just a matter of scaling the power to meet the needs. If you are going to have a sea island you would need this amount of power anyways for the residents to use.
With induction forges you can melt a lot of different metals and maybe quartz or silica as well. You could always add iron flakes to the sand mixture and that may help to get the rest of sand to heat up and melt if it doesn’t do so on its own.
In this way the limiting factors are just the amount of power you have. Speed of production is limited to power as well.
I hope this idea helps.