Passive CO₂ capture and mineralization on existing mineral building surfaces: A falsifiable framework for Low-Operational-Energy carbon removal

Active direct air capture requires dedicated air contact, sorbent regeneration, and associated operational energy. This study theoretically examines a complementary concept in which existing mineral building surfaces act as distributed passive air contactors. The proposed Passive Carbon Skin architecture spatially separates reversible atmospheric CO₂ capture, climate-mediated carbon transfer, and mineral fixation within a Ca- or Mg-rich reservoir. Rather than claiming a validated material, the study defines a falsifiable system framework, closed carbon-balance boundary, theoretical capacity limits, climate-dependent operability variables, and an experimental validation pathway. For an illustrative Mg(OH)₂-based baseline containing 180 g Mg(OH)₂ m⁻², the stoichiometric storage ceiling is approximately 136 g CO₂ m⁻²; achievable retention remains unknown and depends on reaction kinetics, thermohygrometric cycling, transfer efficiency, passivation, weathering, and material lifetime. Four quantitative screening elements are introduced: an annualized capacity relation separating storage capacity from fill time; a mass-normalized carbon-intensity ceiling constraining embodied burden; a climate-operability state space based on experimentally validated thermal-humidity cycles; and timescale ratios requiring capture and transfer kinetics to fit within naturally available dry and wet periods. Five core hypotheses test collector performance, bounded climatic operation, interlayer carbon transfer, architectural advantage, and durable net removal. The framework requires environmental vetoes and predefined failure thresholds. It is intended not as a replacement for engineered direct air capture, but as a testable route toward complementary distributed carbon removal using existing surface area and naturally occurring environmental gradients.