Cement-Based Mortar Additive

Frequently Asked Questions

• Q: How do I select the right cellulose ether viscosity for a render mortar?
• A: For hand-applied renders, 20,000–60,000 mPa·s HPMC at 0.15–0.25% provides a good balance of workability, water retention, and sag resistance. For machine-applied (sprayed) renders, lower-viscosity grades (10,000–30,000 mPa·s) at similar dosages are preferred to avoid excessive pump pressure. Thermal or lightweight renders with expanded perlite or polystyrene aggregate typically require higher-viscosity grades to compensate for the increased water demand of the lightweight aggregate.

• Q: What is the effect of cellulose ether on compressive strength of mortar?
• A: At standard dosages (0.1–0.3%), cellulose ether has a neutral to slightly positive effect on compressive strength in most mortars. By improving water retention, it ensures more complete cement hydration, which increases strength relative to a water-retaining-deficient mortar. At very high dosages (>0.5%), the air-entraining effect of cellulose ether can reduce density and compressive strength. For structural mortar applications, verify compressive strength compliance through standardised testing at your intended dosage.

• Q: Can I use HPMC in lime-based mortars for historic building restoration?
• A: Yes. HPMC is compatible with hydraulic lime and natural hydraulic lime (NHL) mortars used in historic building conservation. It provides workability and water retention benefits similar to those in cement mortars. Dosage should be kept low (0.05–0.15%) to avoid over-retardation of the slower-setting lime system. For natural lime putty (non-hydraulic) mortars, HPMC can also be used to improve workability, but the very slow carbonation set means water retention is less critical than in hydraulic binder systems.

• Q: Why does mortar crack after application and how does cellulose ether help?
• A: Mortar cracking typically results from rapid drying (desiccation cracking), excessive water-to-cement ratio, or substrate movement. Cellulose ether addresses the first cause — by retaining mixing water, it allows the cement to hydrate gradually and evenly, reducing the drying shrinkage gradient that causes surface cracks. For substrates prone to movement (timber-framed walls, steel sheet), RDP in the formulation provides flexibility to accommodate movement without cracking.

• Q: How does cellulose ether affect the air content of mortar?
• A: Cellulose ethers are surface-active polymers that can introduce a small amount of air into mortar during mixing — typically 1–3% additional air at standard dosages. This mild air-entrainment improves workability and freeze-thaw resistance but slightly reduces compressive strength and density. If air content is critical (e.g., high-strength repair mortars), use a compatible defoamer at 0.01–0.05% to control air entrainment. Yisheng can recommend compatible defoamer combinations for your formulation.

• Q: Does Yisheng offer technical support for new mortar product development?
• A: Yes, our applications team provides comprehensive new product development support: grade selection consulting, starting formulation recommendations, laboratory-scale trial mixing, EN classification testing, and technical report preparation. This service is available free of charge for qualified customers placing commercial orders. For distributors entering new markets or launching new product lines, we can also provide market-specific formulation guidance based on local raw material availability and construction practice.