A pyrochlore charge ice consisting of two magnetic cation types gives rise to a Heisenberg magnet with an interesting type of long-range correlated bond disorder. If the antiferromagnetic exchange among cations of the same type dominates over the interspecies exchange, closed antiferromagnetic chains following the fully packed loops of like cations emerge as nonlocal effective low-temperature degrees of freedom. Monte Carlo simulations reveal a fluctuation driven first-order phase transition from an algebraically correlated spin liquid to a long-range ordered spin nematic, which mean-field theory captures with very high accuracy. The global O(3) Heisenberg symmetry is suddenly reduced to a Z2 Ising symmetry as the Néel vectors of the strongly antiferromagnetically correlated loops align with each other to optimize the entropy of their thermal fluctuations. Interestingly, the nematic transition is found to be sensitive to the size statistics of the cation loops and thus provides a direct thermodynamic probe of otherwise elusive structural properties. In turn this sensitivity offers a structural route to engineering the nematic phase stability.