SBR reduces shrinkage compared with water, so hopefully it won't be a problem. However for my real build I will source expanding cement, or at least 'non-shrink' variety.

You can also get synthetic cement such Polycrete (Granite Resin Mortar) which is supposedly good at damping and really stiff.

https://www.vubaresinproducts.com/ep...ir-mortar.html