The Principle
Lime mortar sets not by evaporation, but by carbonation: Ca(OH)2 + CO2 → CaCO3 + H2O. This reaction requires free water to transport ions, but halts if the pores saturate. It accelerates with temperature until dehydration begins at 180°C.
This solver estimates the critical hour when the compressive strength exceeds the shear force of a fine-grit abrasive. Derived from field observations of Hoosic lime in the North Adams mill house.
t_min (hrs) = K × (RH_adj) ÷ ((T_K − 273.15)2 × (CO2_ppm / 400))
Where:
• K = 1.8 × 10⁶ (empirical constant for Type N lime)
• RH_adj = 1.0 if 40–70%, else penalty factor
• T_K = Absolute temperature (Kelvin)
Where:
• K = 1.8 × 10⁶ (empirical constant for Type N lime)
• RH_adj = 1.0 if 40–70%, else penalty factor
• T_K = Absolute temperature (Kelvin)
Field Notes
In the winter of 2025, a batch applied at 18°C and 65% RH reached sandability in 42 hours. At 24°C and the same humidity, the window closed at 28 hours. The variable is not the calendar, but the energy state of the matrix.
Data source: Solvent Chemistry Notes, Lime Chemistry, Constants Schema.