James Mount, Natural Cement, explains how a sustainable cement solution enabled complex underwater restoration at one of the UK’s most iconic heritage sites.

Leeds Castle in Kent is widely regarded as one of the most picturesque and historically significant castles in the United Kingdom. Its origins can be traced back to the 9th Century, and over the centuries it has served as a Norman stronghold, a royal residence, and a private home before becoming the major public attraction it is today. Surrounded by a large moat and extensive gardens, the castle’s architecture and layout reflect its layered past. But while it may appear timeless to the casual observer, centuries of exposure to the elements have taken their toll on its structural components – particularly those below the waterline.

In 2023, structural engineers raised concerns over voiding beneath the surface of the landward abutment of the moat bridge, one of the castle’s oldest and most visually recognisable features. With the potential for long-term damage to the bridge’s integrity – and the logistical and heritage challenges of undertaking repair work underwater – it was clear that a specialist solution would be needed. Natural Cement was approached to supply a material that could meet strict conservation criteria while offering rapid setting times and proven durability in saturated environments. The selected product, NATCEM® 35, allowed for a low-impact, high-performance repair process that protected both the structure and its surroundings.

Unlike conventional Portland cement, which requires temperatures of around 1100 – 1200°C to achieve partial fusion and form compounds such as alite, Natural Cement’s binder is produced through a significantly more sustainable process. The argillaceous limestone used in NATCEM products is calcined at a much lower temperature – typically between 600 and 700°C. This lower temperature burn not only conserves energy but also avoids the release of excessive CO

typically associated with the decarbonisation of fully calcined materials.

This gentle calcination retains much of the natural clay content in the limestone, allowing the binder to exhibit hydraulic properties without the need for synthetic additives or plasticisers. The result is a naturally high-performance material that sets quickly, gains strength rapidly, and resists aggressive environments – while also aligning with low-carbon construction goals.

NATCEM 35 is a rapid-setting, high-performance mortar designed for use in harsh, wet, and cold environments. It is particularly well suited to structural repairs where time constraints, water presence, or environmental sensitivity restrict the use of conventional materials. In addition to being entirely free from portland cement, it is chloride-free, low in alkali content, and has no need for resin-based admixtures. These characteristics make it ideal for conservation and heritage works, where compatibility with original materials is a key concern.

Furthermore, NATCEM 35 is approved for use in public water supplies subject to DWI Reg. 31. It can be applied in saturated environments, underwater, or even in freezing conditions down to 0°C – all without washout or loss of strength. It is resistant to chemical attack, sulfates, seawater, and aggressive ground conditions, and it demonstrates excellent long-term stability in freeze-thaw environments. All these factors made it a natural fit for the Leeds Castle project.

Leeds Castle is a Grade I listed structure, with much of its fabric protected under English Heritage conservation guidance. The surrounding moat, fed by the River Len, is both a historical feature and an important part of the site’s visual appeal. Any repair works had to be sympathetic to the existing masonry and completed without draining or altering the moat. These constraints ruled out many conventional repair strategies, such as cofferdam construction or full dewatering.

The problem was located within the landward abutment of the bridge, where voids had developed beneath and behind the stonework due to years of persistent water flow and erosion. Left untreated, these voids could have led to further settlement or collapse of masonry elements, endangering both the structure and its users.

To address these concerns, Paye Stonework and Restoration – an experienced conservation contractor – was appointed to carry out the works. The contractor proposed an in-situ repair using hand-applied mortar via diving access. Natural Cement was engaged early in the process to provide technical input and ensure the selected materials would be suitable for direct application underwater.

Before the main works began, a test panel was constructed underwater to verify the workability and performance of NATCEM 35 under real-world conditions. This pre-qualification phase allowed the contractors and divers to familiarise themselves with the handling characteristics of the mortar, including its working life, consistency, and set times. It also demonstrated NATCEM 35’s ability to adhere to damp masonry and resist washout, a crucial factor given the continuous flow of water through the moat.

Once the test panel was completed and approved, full-scale works commenced.

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