  Like-charge attraction, driven by ionic correlations, challenges our
understanding of electrostatics both in soft and hard matter. For two charged
planar surfaces confining counterions and water, we prove that even at
relatively low correlation strength, the relevant physics is the ground-state
one, oblivious of fluctuations. Based on this, we derive a simple and accurate
interaction pressure, that fulfills known exact requirements and can be used as
an effective potential. We test this equation against implicit-solvent Monte
Carlo simulations and against explicit-solvent simulations of cement and
several types of clays. We argue that water destructuring under nanometric
confinement drastically reduces dielectric screening, enhancing ionic
correlations. Our equation of state at reduced permittivity therefore explains
the exotic attractive regime reported for these materials, even in absence of
multivalent counterions.
