The papercrete in the grid of such small cross section will be prone to breaking. You might experiment with either wire reinforcement or maybe steel microfibers as an admix.
Might I suggest something really off the wall. How strong does the grid have to be? You might be able to use a sodium silicate C02 injection technique that is used in core and mold making. See http://www.svsu.edu/~rtuttle/FRG/HOWTOs/Sodium%20Silicate%20Coremaking.pdf Now this material will still have a brittle nature but if you framed the grid for strength you might get by. But it would be stronger than papercrete in the cross sections you are aiming for.__._,_.___
