Non hydraulic Lime vs Natural Hydraulic Lime - Guidance for Older & Listed Buildings with Bespoke Aggregates, Pigments and Mortar Analysis

What they are

• Non‑hydraulic lime (lime putty): Pure calcium‑hydroxide that sets by carbonation (reacts with CO₂). Very workable, flexible and slow to gain strength — excellent for fine pointing, sheltered replasters and delicate historic substrates.
• Natural hydraulic lime (NHL): Contains natural clay/mineral impurities that produce a hydraulic set (reacts with water). Available in grades (NHL 2, 3.5, 5) with increasing strength and faster set; suitable for exposed, damp or structural situations when the lowest compatible grade is chosen.

Setting & curing

• Non‑hydraulic (lime putty): Sets by carbonation (reacts with CO₂). Slow cure, needs air contact.
• Natural hydraulic lime (NHL): Sets hydraulically (reacts with water). Faster cure, gains strength in damp conditions.

Strength & stiffness

• Non‑hydraulic: Low strength, very flexible and soft.
• NHL: Higher strength; available in grades (NHL 2, 3.5, 5) — increasing stiffness and load capacity.

Workability & finish

• Non‑hydraulic: Extremely workable and forgiving; excellent for fine tooling, renders and limewash substrates.
• NHL: Good workability but firmer; better for bedding, exposed external work and structural tasks.

Breathability & compatibility

Both are vapour‑permeable; non‑hydraulic is generally more flexible and kinder to very soft historic masonry. NHL is breathable but less accommodating to movement.

Durability & exposure

• Non‑hydraulic: Best for sheltered/internal locations and fine finishes; not suited to prolonged immersion or heavy structural loads.
• NHL: More durable in exposed, damp or load‑bearing situations; select the lowest adequate grade to match substrate softness.

Typical uses

• Non‑hydraulic: Internal replasters, fine pointing, limewash substrates, delicate historic finishes.
• NHL: External repointing, bedding, structural repairs and exposed elevations (choose grade per substrate).

Selection guidance for older/listed buildings

• Use non‑hydraulic where maximum flexibility, softness and breathability are required (very soft stone/brick, delicate tooling).
• Use NHL (choose appropriate lower grade) where faster set, greater early strength or wet‑condition performance is necessary.
• Avoid cement or overly strong mortars on traditional fabric.

Cement on lime mortar substrate

Cement on lime‑mortar buildings - it’s physically and chemically incompatible and will cause damage.

Key reasons

• Too stiff/hard: cement joints concentrate stress on softer historic bricks/stone → cracking, spalling.
• Impermeable: cement blocks vapour flow, traps moisture and causes damp and salt damage.
• Salt/migration issues: cement changes salt paths and locks salts into masonry, causing flaking.
• Differential movement: cement won’t accommodate thermal/hygroscopic movement, forcing damage into the fabric.
• Hidden decay: hard cement skins can mask ongoing internal deterioration and delaminate.
• Aesthetic & heritage loss: alters colour, texture and tooling; destroys historic evidence.
• Conservation compliance: often unacceptable to conservation officers and may breach listed‑building conditions.

Aggregates, pigments & texture control

• Aggregates: Select sand source, grading and particle shape to reproduce original joint profile and surface texture — coarse, angular sands for pronounced texture; fine, rounded sands for smooth finishes. Local or quarry‑matched sands are preferred to match mineralogy and weathering.
• Pigments & reclaimed fines: Mineral pigments (iron oxides, ochres) and small proportions of crushed reclaimed brick/stone dust can be used to achieve the correct hue and subtle patina without compromising mortar performance. Pigment dosages are kept low and mineral‑stable.
• Texture & tooling: Final appearance is controlled by mix grading, binder ratio, tooling technique, joint profile and optional sacrificial shelter coats or limewash.

Mortar analysis & performance matching

• Laboratory testing: Petrographic and chemical analysis, grain‑size distribution and salt testing identify original binder type, binder:aggregate ratio, porosity and residual strength.
• Performance targets: Mixes are designed to replicate carbonation/hydraulic behaviour, compressive strength range, water absorption and vapour permeability so repairs behave like existing fabric.
• Trial panels & validation: On‑site sample panels validate colour, texture, workability and curing; laboratory or in‑situ tests confirm absorption and strength where required.

Why this matters for older & listed buildings

• Compatibility: Matching softness, porosity and movement accommodation prevents stress transfer that can accelerate decay of soft bricks and stones.
• Breathability: Replicating vapour transmission and salt transport characteristics controls moisture and salt‑related deterioration.
• Authenticity & reversibility: Bespoke mixes, matched aggregates and reclaimed fines restore the original appearance while allowing future reworking.
• Durability: Tested, site‑proven mixes with correct detailing and curing deliver long‑term, low‑maintenance repairs.

Selection guidance (summary)

• Very soft, sheltered or fine‑finish work: Prefer non‑hydraulic lime with fine, matched sands and subtle pigments.
• Moderately exposed or structural repairs: Use NHL (selecting the lowest adequate grade, e.g. NHL 2 or 3.5) with graded aggregate and colour matching.
• Highly exposed/damp/load‑bearing: Higher‑grade NHL may be necessary, but avoid over‑stiff mortars on soft substrates.
• Always avoid cement‑rich or inappropriate high‑strength mortars on traditional fabric.

Typical workflow

• Survey & sampling — site inspection, photographic record and mortar/substrate sampling.
• Laboratory analysis — petrography, binder identification, grain‑size and salt testing; report with recommended mix range.
• Mix design & trials — produce trial mixes with selected aggregates and pigments; create sample panels for approval.
• Approvals — supply trial documentation and panels for conservation officer sign‑off.
• Application — hand‑applied by conservation masons using approved mixes, tooling and curing regimes.
• Validation & handover — provide test data, mix recipes, maintenance guidance and monitoring recommendations.

Practical note

Specification should state binder type/ageing, binder:aggregate ratio, aggregate source and grading, pigment type/dosage, application method and curing/aftercare. Trials and skilled application are essential. If you supply photos, substrate details and access for sampling, we will prepare a bespoke mortar analysis, pigment/aggregate options and trial‑panel programme to match strength, breathability and restore your building to how it was originally.