18th Jun 2024 -

The construction sector is undergoing a significant transformation towards sustainability. An often overlooked aspect is the practice of eco-friendly plastering. This article explores sustainable plastering materials and techniques that reduce carbon footprint and create healthier interior spaces.

What are the eco-friendly plastering benefits?

Clay and lime eco-friendly plasters offer substantial benefits. Here, we explore how eco-friendly plastering is reshaping the way we think about our built environment.

Mitigating environmental impact

A key benefit lies in its capacity to reduce the carbon footprint of construction projects. Conventional cement-based plasters are associated with high levels of embodied carbon. Cement production accounts for approximately 8% of global CO2 emissions

In contrast, materials like lime plaster mix offer an exceptional level of carbon sequestration during the curing process. This can offset up to 20% of the CO2 emitted during production.

Enhancing indoor environmental quality

Eco-friendly plasters contribute to the improvement of indoor air quality. Many traditional plasters contain volatile organic compounds (VOCs) that release pollutants over time. Natural plasters like clay or lime are capable of absorbing airborne toxins. 

For example, clay plasters absorb formaldehyde, an indoor pollutant originating from furniture and floor coverings. It shows resistance to mould and fungi because of high alkalinity or natural properties.

Optimising thermal performance

Green plaster materials offer exceptional thermal properties. Earth-based plasters possess high thermal mass, enabling them to absorb, store and gradually release thermal energy. This helps regulate indoor temperatures. 

Similarly, cellulose-based plasters made from recycled paper provide effective thermal insulation. The R-values are comparable to conventional insulation materials while offering the additional benefits of being non-toxic and biodegradable.

Regulating moisture and ensuring longevity

Many eco-friendly plasters exhibit hygroscopic properties (absorbing and releasing water molecules). This allows them to absorb excess moisture and release it when the air is dry. It prevents condensation and hinders the risk of moisture-related issues such as rot and mould growth. 

Lime plastering absorbs more moisture than gypsum plaster. This enhances comfort and extends the lifespan of structures. Also, materials such as lime and clay plasters are highly durable.

Conserving resources and minimising waste

Sustainable plastering prioritises renewable materials and many eco-plasters incorporate recycled materials. In lime renders, recycled glass replaces up to 40% of the sand content. 

Reclaimed ceramic waste can be used as an aggregate. This prevents the wide use of newer resources and diverts waste from landfills. These plasters may even utilise agricultural by-products such as rice husk ash or bagasse (sugarcane residue). 

Biophilic design and aesthetic versatility

Natural plasters contribute significantly to biophilic principles which introduce the natural environment into design. The earthy textures and tones of clay or lime plasters create a sense of warmth and environmental connection.

Biophilic elements can reduce stress levels. In addition, these plaster materials offer considerable aesthetic versatility. Clay plaster can be tinted with natural pigments to achieve different colours. Lime plaster finish can be burnished for a marble-like effect or textured for a rustic appearance.

How to tackle sustainable plaster issues

A metal plaster tool with a red handle held by a white gloved hand working with eco-friendly plastering material in a circular motion.

While natural and recycled materials offer various benefits, they also present unique performance issues. Below, we look at 6 common concerns related to eco-friendly plasters and provide strategies to overcome them. 

1. Moisture sensitivity

Some natural plasters like clay are highly sensitive to water. Prolonged exposure to moisture may lead to erosion, cracking or even structural failure. This vulnerability hinders their use in humid climates or areas prone to heavy rainfall.

Solution: The plaster mix itself can be modified. Adding natural hydraulic lime (NHL) to clay plasters increases waterproof capabilities without compromising breathable performance. A 10% NHL addition to the plaster mix may reduce water absorption by up to 50%. 

Also, surface treatments can provide additional protection. Other methods like applying linseed oil or beeswax can create a water-repellent layer while maintaining the plaster's vapour permeability.

2. Natural material variation

Natural plasters have variability in colour, texture and performance because of differences in raw materials. A batch of clay from one site may perform differently than another. This leads to inconsistencies that can cause problems for large-scale or aesthetically sensitive projects.

Solution: With materials like clay, testing is key. Simple field tests for sand/silt/clay ratios will guide the appropriate mix design. Creating a 'material library' with samples and performance data ensures consistency for your project. For colour consistency, using natural pigments from a single source and batch will help. 

3. Lower compressive strength

Some eco-friendly plasters like clay may have lower compressive strength compared to traditional plaster options. This is a concern in applications where the plaster might be subject to high impact or abrasion. 

Solution: Understanding the specific strength requirements of the project is important. In residential and low-traffic commercial applications, the strength of eco-plasters is more than adequate. For higher-demand areas, the mix design can be adjusted. Adding a small percentage (5-10%) of NHL to clay plasters may boost compressive strength.

Another approach is to use layered systems. A base coat of stronger material, like a thin layer of natural fibre-reinforced concrete, provides the necessary strength. Also, a finishing plaster coat offers the desired aesthetic and breathability. 

4. Prone to biodeterioration

Many eco-plasters have a natural resistance to mould because of their high pH or mineral content. However, high organic content like straw-reinforced earth or cellulose-rich plasters can be susceptible to biodeterioration. This includes mould growth, insect infestation or bacterial decay, particularly in high-humidity spaces.

Solution: In high-humidity areas like bathrooms or kitchens, it is advisable to use plasters with higher lime content. This is because lime's high alkalinity prevents most microbial growth. For organic-rich plasters, natural preservatives can be effective. Borax (sodium borate) offers both insect and fungal resistance. Other natural additives like tannins (from oak bark) and essential oils (tea tree) have shown antimicrobial properties.

5. Thermal crack damage

Rapid temperature changes may cause thermal cracking. This is particularly true for earth plasters or NHL plasters, which have lower tensile strength than traditional plasters. Thermal cracking affects appearance and also leads to water ingress and reduced insulation performance.

Solution: For walls that may receive direct sunlight, using lighter colours for eco-plasters can reduce heat absorption. Incorporating phase change materials (PCMs) like paraffin or plant-based waxes into the plaster mix will help. They absorb heat as they melt during the day and release it as they solidify at night, smoothing out temperature fluctuations.

Adding natural fibres like hemp shiv to the plaster mix can improve tensile strength and crack resistance. In some cases, using a slightly coarser aggregate or incorporating small amounts of fine sand also helps reduce shrinkage.

Application techniques also play a role. Applying thinner, multiple plaster layers rather than 1 thick layer allows them to adjust to temperature changes more easily. Proper curing is also critical. Drying under shade cloths or damp burlap prevents the rapid shrinkage that often leads to cracking.

How Long Does Eco-Friendly Plaster Take to Dry?

The drying and curing times of plasters are important factors in the overall plaster performance. Below, we look at the drying and curing times of eco-friendly plasters in comparison to traditional cement and gypsum plasters.

FactorGypsum/cement plastersEco-friendly plasters
Initial set timeGypsum: 20-30 minutes; cement: 1-2 hours.Non-hydraulic lime: No initial set; NHL: 1-4 hours; Clay: No chemical set.
Touch-dry timeGypsum: 2-4 hours; cement: 12-24 hours.Non-hydraulic lime: Several days; NHL: 24-48 hours; Clay: 24-72 hours.
Full cure/hardening timeGypsum: 14-21 days; cement: 28 days.Non-hydraulic lime: Months to years (carbonation); Clay: years (particle interlocking).
Strength developmentRapid initial strength gain. Level after 28 days.Lime: Continues to gain strength for up to 3 years; Clay: 15% increase in compressive strength over a decade.
Drying factorsHumidity, temperature. Less affected by thickness due to the chemical set.Humidity, ventilation, temperature, thickness. For example, 10mm lime plaster: 1 day; 25mm: 3-4 days.
Additive effectsSynthetic additives can speed up the set or drying.Natural additives (e.g. rice husk ash) can speed up lime setting by 30%.
Climate sensitivityLess sensitive. The chemical set proceeds in a wider temperature range.More sensitive. In damp/cold, lime/clay drying can double. May need dehumidifiers.
Self-healing propertyNone. Cracks remain.Hairline cracks may self-heal over 2-3 months because of ongoing carbonation.

Is Ready Mix Plaster Sustainable?

A plasterer wearing a white hard hat, grey t-shirt and white trousers holding plastering tools to mix eco-friendly plastering materials against a damaged wall structure and in front of a ladder.

Known for its convenience and consistency, the environmental impact of ready mixed plaster is based on various factors. The below looks at the key aspects that determine whether these pre-blended mixtures should be considered eco-friendly. 

Composition matters

Many ready-mix plasters are gypsum-based. Gypsum manufacturing requires temperatures of 150-180°C, significantly lower than 1450°C for cement. However, the increasing use of synthetic gypsum, a byproduct of coal-fired power plants, creates a negative impact.

While using this waste product diverts material from landfills, it also supports the coal industry, a contributor to climate change. Also, the inclusion of Portland cement in some traditional mixes for added strength increases their carbon footprint.

Water usage

Water usage in ready-mix plasters is a concern, especially in water-stressed regions. Both manufacturing and on-site mixing require substantial amounts of water. However, innovations are emerging in this area. Some plants use rainwater harvesting or recycle process water. Also, plaster manufacturers like Knauf have set goals by 2032 to reduce their water withdrawals by a fifth. 

End-of-life management

The end-of-life phase of ready-mix plasters is critical. Gypsum plasters are theoretically recyclable and can be reprocessed as a soil amendment. However, contamination with other building materials often prevents recycling. Also, cement-containing plasters will often end up in landfills. 

Innovative eco-ingredients

Boosting the eco-credentials of ready mix plasters is the incorporation of natural, recycled or industrial byproduct materials. Another innovation is the use of recycled gypsum from discarded plasterboards. 

Natural gypsum could be replaced with recycled gypsum without compromising performance. Even agricultural waste is finding its way into these mixes. For example, plaster incorporating 10% rice husk ash may increase compressive strength. 

How Much Does Eco-Friendly Plaster Affect Bonding?

Plaster bonding directly influences the durability, appearance and performance of ceiling or wall plaster finishes. The shift towards eco-friendly plastering raises important questions about how these materials compare to conventional plasters in terms of bonding. Here, we compare eco-friendly bonding plaster vs traditional materials.

AspectGypsum/cement plastersEco-friendly plasters
Primary bonding mechanismChemical bonding through hydration forms interlocking crystals. Quick setting and early strength.Lime also bonds chemically through carbonation over time.
Bonding strengthHigh initial strength because of the rapid chemical set.Lime plasters on brick surfaces match cement plaster strength after 90 days.
Substrate compatibilityGood on rigid substrates like concrete. Can be problematic on softer, flexible materials due to rigid nature.Excellent on a wide range of substrates, especially natural materials. Flexibility reduces cracking on movable substrates.
Moisture compatibilityLow breathability can trap moisture in natural substrates, causing damage.High breathability. Lime plasters allow more moisture transmission than cement.
Surface preparationRequires dampening to prevent rapid water suction. May need bonding agents on smooth surfaces.Lime benefits from dampening but can be applied dry. Clay often needs no prep on natural substrates.
Bonding additivesSynthetic polymers for workability and bond strength. Gypsum and cement plaster additives can off-gas VOCs.Pozzolans (metakaolin, ash) for lime (up to 50% strength increase). Natural fibres (sisal, jute) for clay (up to 25% strength increase).
Repair easePatching often requires large-area removal which generates waste.Seamless patching for surfaces like clay and lime plaster walls. New lime chemically bonds with old. Clay can be rewet and integrated.

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