When restoring historic stone monuments, markers, and multi-piece cemetery structures, selecting the correct mortar is just as critical as the cleaning technique. Historic preservation relies on a foundational rule: matching the physical characteristics of the original materials. For decades, however, well-intentioned repairs have caused catastrophic damage because modern, rigid materials were used on historic stone.
A Brief History of Hydraulic Mortars
Long before the invention of modern Portland cement, builders relied on lime-based binders. Ancient Romans famously perfected hydraulic lime by mixing slaked lime with volcanic ash (pozzolans), allowing the mortar to set even in damp conditions or underwater.
By the mid-19th century—the era of many historic American cemeteries—French and British engineers rediscovered natural hydraulic lime (NHL). Produced by burning limestone that naturally contained clay impurities, NHL set through a chemical reaction with water rather than just air-curing. This gave historic masons a strong, durable, yet flexible mortar that could support stone monuments while accommodating natural thermal expansion, shifting, and moisture movement.
The Pros of Using Natural Hydraulic Lime (NHL) in Cemetery Restoration
Natural Hydraulic Lime has become the gold standard for historic masonry and cemetery restoration for several key reasons:
- Vapor Permeability (Breathability): Historic stones need to “breathe.” NHL mortars allow moisture absorbed by the stone to evaporate harmlessly through the mortar joints rather than getting trapped inside the stone.
- Flexibility and Movement: Ground freeze, thaw cycles, and soil movement cause historic monuments to shift. NHL remains slightly flexible, absorbing these micro-movements without cracking.
- Sacrificial Nature: Mortar should always be softer and weaker than the stone it holds together. NHL acts as a sacrificial layer; if stress occurs, the mortar joint gives way rather than fracturing the irreplaceable historic stone.
- Compatibility: Because historic foundations and stones were originally bedded in lime-rich environments, NHL bonds harmoniously with ancient substrates without introducing harmful chemical reactions.
The Dangers of Portland Cement-Based Mortars
In stark contrast to natural hydraulic lime, modern Portland cement-based mortars have no place in historic cemetery restoration. Using Portland cement on historic stone is often destructive, leading to permanent damage.
- Extreme Rigidity: Portland cement cures into a hard, brittle, inflexible sheet of concrete. When the ground freezes or monuments settle, the mortar refuses to flex. Instead, the stress transfers directly into the historic stone, causing the stone faces to spall, crack, or break apart.
- Trapped Moisture: Portland cement is dense and virtually impermeable. Moisture entering the monument cannot escape through the hard cement joints. Instead, moisture is forced to migrate through the stone itself. As trapped water freezes or evaporates, it pushes salts to the surface, causing the stone to erode and crumble from the inside out (spalling and granular disaggregation).
- Irreversibility: Portland cement forms a bond that is often stronger than the historic stone. If a repair fails or needs to be undone, chipping away hard cement mortar typically destroys the edges of the historic stones it was meant to protect.
Summary
In cemetery conservation, preservation means respecting the age and fragility of the stone. By utilizing Natural Hydraulic Lime, conservators ensure that monuments remain breathable, flexible, and protected for generations to come, avoiding the destructive pitfalls of modern Portland cement.