Lime and lime/cement plaster

Lime and lime/cement plaster

Applications for system
• Exterior sheathing, as a rainscreen or directly applied to wall substrate
• Interior sheathing
• Ceiling sheathing
Basic materials
• Hydrated lime (for air-curing lime plaster)
• Hydraulic lime (for hydraulic-curing lime plaster)
• Portland cement (for lime/cement mixtures)
• Pozzolanic material (fired clay, gypsum, slag, fly ash; for hydraulic-curing lime plaster)
• Sand
• Fibers (if required)
• Lath and/or mesh
How the system works
All of the plasters covered in this section are similar, in that they take a processed mineral material, mix it with sand and water and apply it to a surface where it chemically cures to closely resemble the original rock from which it was sourced. In substrate preparation, application method, finished texture and appearance and durability, the plasters are close to identical. However, in processing, curing process and environmental impacts, they differ enough to be considered separately.
Hydrated Lime Plaster. Also known as “air lime,” this type of plaster is based on limestone that has been heated to 900-1100°C (1650-2000°F) to form “quicklime” The powdered quicklime is then combined with water (a process that is highly reactive and generates a substantial amount of heat) to form a stable hydrated lime. Many people are surprised to find that “hydrated” lime is most often sold as a dry powder, but the hydration refers to the chemical absorption of water and not to the state of the final product.
The hydrated lime is mixed on-site with water and sand aggregate (and fibers, if required for additional tensile strength) and applied to the substrate, usually by trowel. The plaster on the wall will dry, losing water content to the air and the substrate (and this must be mitigated by continually moistening the plaster during the curing process), but the actual setting of the plaster is a chemical reaction involving the lime ingredients, water and air, during which the hydrated lime carbonizes. The carbonization process is long and slow. The plaster will become hard to the touch within a few days or weeks (depending on conditions) but will continue to cure and harden for decades as airborne carbon reacts with the lime.
Hydrated lime plasters are best suited for application in thin coats, building up thickness slowly. If the plaster is too thick, the deep areas will not carbonize for a long time. These plasters require careful application and tending during the early stages of curing. They can mature into strong, durable and beautiful plasters.
Hydraulic Lime Plaster. This type of lime plaster contains fired limestone and also some amount of a pozzolanic material, which can be naturally occurring in the limestone (as with natural hydraulic lime or NHL) or added to the lime at the time of mixing (as with fired clay, gypsum, slag, fly ash or other poz- zolans). Regardless of the source of the pozzolan, its addition in the mix creates a chemical reactivity with water that provides some setting action in the plaster. In hydraulic lime plasters, the limestone continues to carbonize over the long term as with “air limes,” but there is a fast, initial setting that occurs due to the pozzolanic reaction. The amount of water reactivity can vary depending on the type of pozzolan in the mix and its reactivity. NHL is rated based on its reactivity; site-mixed hydraulic limes can be adjusted by adding more or less of the pozzolan material to the mix. Pozzolans may be as low in volume as 5 percent and as much as 50 percent of the binder content.
Hydraulic lime plasters are similar to hydrated lime plasters in regards to the care required during curing (well protected from sun, wind and continuously moistened) and the need for relatively thin, even coats to encourage carbonization. These plasters will harden to the touch within hours, rather than days.
Lime/Cement Plaster. In this type of plaster, the portland cement content acts as the pozzolan, allowing the plaster to cure quickly in a water reaction. The difference is that portland cement is a very powerful pozzolan and, depending on the proportions used in Lime plaster.
Decorative plaster the mix, the plaster will take on characteristics that are more cement-like than lime-like. Lime plasters are relatively soft and permeable, while cement plaster is harder, more brittle and less permeable. In the right combination (25%-50% cement), a lime/cement plaster can have fast set times (2-6 hours) and cure to be hard and durable yet still be sufficiently permeable to be part of a vapor flow-through wall system. Too high a cement content and the plaster may become too brittle and impermeable.
These lime/cement mixes are commonly available as preformulated mortar mixes for laying brick and block.
Lime/cement plasters have two advantages over straight lime plasters. Rapid set times allow for the application of successive coats within short periods of time (1-2 days) and the hydraulic set allows for the application of thicker (and therefore fewer) coats, reducing labor input.
Tips for successful installation
1. Substrate preparation is important for all lime- based plasters. The smoother the surface, the less likely the plaster will have varying thicknesses that can cause cracking or soft spots.
If the surface to be plastered is uneven, lime plasters can be built up in low spots over one or more applications, to create a level surface for the first full coat of plaster. Alternatively, the plaster can be applied at a consistent thickness to follow the contours of the wall.
2. Lime plasters require a substrate with adequate mechanical adherence. If plastering over smooth substrates, a mesh of some type will be required and must be firmly adhered to the surface.
3. Each successive coat of lime plaster requires the previous coat to be scratched or roughened to provide mechanical grip for the subsequent coat, to avoid potential delamination.
4. Lime plaster has good permeability qualities, but is not very resistant to water penetration. Its open pore structure allows water to soak into the plaster relatively easily. While this will not affect the plaster itself, it may be problematic for the wall surface behind if repeated wetting is experienced. Rainscreen applications will leave a ventilation space behind the plaster to assist with drying. If the lime plaster is adhered directly to the wall, consider coatings that will repel bulk water in vulnerable areas.
5. Lime plaster requires care during its curing process. In particular, it is important with all lime-based plasters to maintain an adequate level of moisture in the curing plaster. Direct exposure to sunlight and wind can rob the plaster of moisture very quickly, resulting in poor curing and cracking. The plaster will need regular misting for about a week in addition to protection from excessive drying.
6. There are many excellent resources to aid with the successful mixing and application of lime plaster. Be sure to research thoroughly before application.
Pros and cons
ENVIRONMENTAL IMPACTS
Harvesting — Moderate. Limestone is a non-renewable but abundantly available material. It is mechanically extracted from quarries. Impacts can include habitat destruction, surface and ground water interference and contamination. Manufacturing — High. Limestone is mechanically crushed and heated (900-1100°C / 1650-2000°F for lime and 1400-1600°C / 2500-2900°F for cement). This is an energy-intensive process during which large amounts of fossil fuels are burned, contributing to habitat destruction and air and water pollution and high carbon emissions. During the kilning process, large amounts of CO2 are driven out of the rock (approximately 1 kg of CO2 for every 1 kg of lime or cement). In the case of cement, this CO2 stays in the atmosphere. In the case of lime, it is slowly recombined with the lime through the carbonization process. In theory, the carbon uptake for lime can be close to 100 percent but in practice it is less because not all of the lime will have adequate exposure to the atmosphere.
Sand is mechanically extracted from quarries and mechanically crushed. Impacts can include habitat destruction and surface and ground water contamination.
Transportation — Moderate to High. Sample building uses 9,787 kg of plaster for exterior: 14.7 MJ per km by 15 ton truck 9.2 MJ per km by 35 ton truck 1.56 MJ per km by ocean freight
Limestone is available in many regions, but is not necessarily harvested and processed in all regions. Impacts for this heavy material will vary depending on distance from the site. Natural hydraulic lime (NHL) most commonly comes from France or Portugal, carrying high transportation impacts for use in North America.
Sand is locally harvested in nearly every region, and should have minimal transportation impacts. Installation — Negligible.
WASTE: Low
Compostable — Lime plaster can be left in the environment or crushed to make aggregate. Hydrated lime plaster can be kept wet indefinitely and doesn’t need to be disposed of. Quantities should be low, as it is mixed in small batches.
Transportation: Mortar mix transportation by 35 ton truck would equate to 9.2 MJ per kilometer of travel to the building site. Typically from engineeringtoolbox.com
Sheathing and cladding materials 205
Lime and lime/cement plaster
ENERGY EFFICIENCY
Lime plaster can be the primary air control layer on the exterior and/or interior of walls and the interior of ceilings. Properly applied and detailed, the plaster can be airtight and contribute to a high level of energy efficiency. Poorly applied plaster can allow leakage through cracks and shrinkage gaps around edges and at intersections with other materials, greatly reducing efficiency.
Lime plaster adds no thermal resistance to the enclosure.
MATERIAL COSTS: MODERATE
Source and quality of ingredients can influence costs. Standard lime/cement mortar mixes are widely produced and available and are relatively low cost. Specialty lime products can be more expensive, and are often imported from Europe.
LABOR INPUT: HIGH
Lime plaster requires significant labor input. Substrate preparation, mixing and application are all labor-intensive processes. The need for multiple thin coats (3-4 for lime, 2-3 for lime/cement) stretches labor time.
Health Warnings — Lime, cement and sand are high in silica content and are dangerous to inhale. Lime and cement are caustic when wet and can cause irritation of the skin that can range from mildly uncomfortable to painful chemical burns.
SKILL LEVEL REQUIRED FOR THE HOMEOWNER Preparation of substrate — Moderate to Difficult.
Lime plasters are only as good as the substrate preparation, making attention to detail very important. Extensive mesh and plaster stops may be required, as is the masking of all intersecting materials and the protection of floors.
Installation of sheathing — Moderate to Difficult.
The basics of trowel application of wet plaster can be
learned quickly, and an inexperienced homeowner can apply a functional plaster. Instructions for the curing of the plaster must be followed carefully to obtain a good result. Quality of finish will improve dramatically with experience.
Finishing of sheathing — Easy to Difficult. The final coat of plaster can be the final finish, and as long as it is functionally sound no further treatment may be necessary. Particular textures and degrees of finish will vary with skill and experience. In some applications, a final surface treatment may be brushed, rolled or troweled onto the plaster surface.
SOURCING/AVAILABILITY: EASY TO DIFFICULT
Lime/cement mixes are widely available as type-N mortar from building supply outlets and masonry supply shops. Hydrated lime is fairly widely available from masonry supply shops. Agricultural lime, available through farm supply shops, is not suitable for lime plastering. Hydraulic lime is a specialty product that will likely need to be special ordered from a regional distributor.
DURABILITY: HIGH
All forms of lime plaster can have a long life span of at least a hundred years, and potentially much longer. Proper maintenance will have a lot of impact on durability. Repair of cracks and reapplication of protective coatings (where required) will help to maximize life span.
CODE COMPLIANCE
Acceptable solution in all codes. ASTM and other standards exist for lime and lime/cement plaster and are referenced by codes.
INDOOR AIR QUALITY: HIGH
Used as an interior sheathing or cladding, lime plaster should have no negative impact on IAQ. The antiseptic nature of lime can discourage mold growth, helping to maintain good IAQ in damp areas. Lime plasters have excellent moisture-handling qualities and will absorb excess interior moisture rather than having condensation form on the plaster surface.
The dust from mixing lime plaster and from cleanup after plastering is high in silica content and can be pervasive. Be sure to protect air ducts and other vulnerable areas during plastering and clean up to avoid contamination.
FUTURE DEVELOPMENT
The practice of harvesting and manufacturing lime plaster is thousands of years old. Modern practices are efficient and the results very consistent compared to historical practices. It is unlikely that developments in the production of lime will change much. Greater efficiency in kilns and use of waste heat may reduce embodied energy as fuel costs rise.
Lime plastering may grow in popularity as the understanding of vapor-permeable wall systems increases, but it is unlikely that lime plaster will return to its once dominant place as a sheathing/cladding material.

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