The choice between Lime and Ordinary Portland Cement (OPC) boils down to the following:
While OPC offers high strength, easily understood and predictable behaviour, and fast setting times, lime offers several meaningful advantages, particularly in terms of ecological sustainability and general performance.
- Lower Embodied Energy: The production of lime requires significantly less energy compared to the high temperatures needed for OPC manufacturing.
- Reduced CO2 Emissions: Lime production emits substantially less carbon dioxide (up to 80% less) pound-for-pound compared to OPC.
- Carbon Sequestration: As lime mortars cure, they re-absorb carbon dioxide from the atmosphere (a process called carbonation), partially offsetting the CO2 released during their manufacture. In the case of pure lime the offsetting is significant, in the region on 80% - 90%.
- Breathability (Vapour Permeability): Lime mortar allows walls to "breathe" by permitting moisture vapour to pass through and evaporate. OPC is denser, less moisture permeable, and more brittle, which can allow moisture in through cracks, then trapping it within the wall structure, leading to dampness, decay, and frost damage to the masonry.
- Flexibility and Movement: Lime has a lower compressive strength and a higher modulus of elasticity than OPC, making it more flexible. This allows it to accommodate slight structural movement, thermal expansion, and settlement (common in older buildings) without cracking the mortar or the adjacent masonry.
- Protection of Masonry: Lime mortar is softer and less rigid than OPC. This is crucial for use with soft masonry (like traditional bricks or stone), as the mortar acts as a sacrificial element, wearing away over time instead of causing the hard, rigid cement to spall (crumble) the face of the softer stone or brick.
- Self-Healing (Autogenous Healing): Lime mortars often contain "free lime" that can go into solution when minor cracks occur. As the water evaporates, this lime is re-deposited, forming calcite crystals that can effectively heal the small cracks over time.
- Salt Resistance: High-quality lime mortar is generally immune to sulphate attack, a corrosive reaction that can occur in OPC in the presence of certain salts.
- Aesthetics: Lime typically provides a softer, more traditional appearance that complements historic masonry materials.
- Reusability of Materials: The lower bond strength of lime mortar makes it easier to separate, clean, and reuse bricks and stone after demolition. OPC mortar often bonds too strongly, leading to damage when materials are separated. Tools, including mechanical mixers and wheel barrows are more easily cleaned. The waste produced at a mixing station is substantially less toxic and harmful than that of OPC.
- Improved Workability: Lime-based mixes are often reported to have superior plasticity and workability compared to OPC mixes.