CEM 805 – Advanced Hydroswelling Mastic for Waterproofing Construction Joints

CEM 805 is a one-component, polyurethane-based mastic specifically formulated to create a long-lasting seal against water ingress in construction joints and pipe penetrations. This solvent-free, hydroswelling compound is activated upon contact with moisture, expanding to fill gaps and cavities in concrete structures. Engineered for both new builds and remedial applications, CEM 805 is ideal for use in tunnels, basements, water tanks, retaining walls, and infrastructure exposed to groundwater or fluctuating environmental conditions.

Moisture-Reactive Expansion for a Secure Watertight Seal

The defining feature of CEM 805 is its ability to swell in the presence of water. Upon application, the material cures over a 24 to 36-hour period depending on ambient humidity and temperature. Once cured, it begins to absorb moisture and expand volumetrically, up to twice its original size. This swelling action allows the mastic to seal even the most irregular voids or hairline cracks around joints, providing a robust barrier against both hydrostatic and non-hydrostatic water pressure.

Single-Component Application

CEM 805 is supplied as a ready-to-use single-component paste, eliminating the need for mixing or special activation processes. This makes it particularly convenient for job sites where time and equipment access may be limited. It can be applied directly from a cartridge gun onto dry or slightly damp substrates, adhering firmly to concrete, steel, HDPE, and PVC. Its simple, direct application method reduces labour time and minimises error, ensuring a reliable outcome even in complex sealing tasks.

High Bond Strength Across a Range of Substrates

In addition to its swelling capability, CEM 805 forms a strong adhesive bond with construction materials commonly used in structural foundations and substructures. This includes cast-in-situ and precast concrete, plastic or metal pipework, and waterproofing liners or membranes. Its excellent substrate adhesion helps prevent dislodgement during curing or later structural movement. Once fully cured and swollen, the mastic remains securely in place under water pressure, mechanical stress, and thermal variation.

Performance Under Pressure in Wet and Subterranean Conditions

CEM 805 was developed for use in below-ground and high-moisture environments, where water pressure and persistent dampness pose serious risks to structural integrity. It performs exceptionally in vertical and horizontal construction joints, slab-to-wall interfaces, pile caps, and through-wall pipe entries. For water-retaining structures such as swimming pools and reservoirs, or water-excluding applications such as basements and lift pits, CEM 805 ensures long-term sealing where conventional sealants or gaskets may fail.

Chemical and Environmental Resistance for Harsh Installations

Beyond moisture protection, CEM 805 offers strong resistance to a wide range of aggressive substances. It resists common site chemicals including dilute acids and alkalis, fuels, oils, and hydrocarbons (making it suitable for industrial or commercial developments with chemical exposure risk). It also resists biological degradation from algae or bacteria, helping maintain the integrity of the seal in contaminated or naturally challenging soil conditions. This versatility extends the product’s usefulness across many different site types across the UK and Northern Ireland.

Flexibility During Curing

The mastic is formulated to retain a degree of flexibility even after curing and swelling. This flexibility accommodates minor structural movement, settling, or vibration that typically occurs in concrete and masonry over time. It helps prevent cracking or tearing of the seal, particularly important at construction joints, where two substrates may move independently. As such, CEM 805 contributes to the durability of joint systems in both static and dynamic settings.

Timeframe for Curing and Swelling Based on Site Conditions

Curing times for CEM 805 typically range between 24 and 36 hours. However, the precise timeframe is affected by ambient temperature and relative humidity. Warmer, more humid conditions can accelerate curing, while cold, dry environments may extend the process. After curing, exposure to water initiates the swelling phase. The rate and degree of expansion vary based on moisture availability but typically achieve a swelling ratio of up to 200%. This reactive behaviour makes the product self-adjusting to conditions underground or behind walls.

Application Guidance for Joint and Penetration Sealing

To apply CEM 805, surfaces should be cleaned of loose particles, oil, or standing water. The product should be applied in continuous beads along construction joints or around service penetrations, ensuring full coverage of the contact area. In pipe sealing applications, it should be applied around the circumference of the pipe where it passes through the concrete or membrane. Following application, it is recommended to allow sufficient curing time before any exposure to full water contact, unless otherwise advised by the site engineer.

Trusted in Infrastructure and Civil Engineering Projects

CEM 805 is widely used by structural engineers, groundworks contractors, and civil engineering firms for its proven performance in concrete sealing. It is suitable for use in power plants, underground utilities, wastewater treatment plants, stormwater systems, car parks, bridges, and public infrastructure. In housing developments and commercial builds, it is especially valuable in waterproofing below-ground extensions and converting basements into habitable rooms.

Environmental and User Safety Compliance

This product is solvent-free, reducing its environmental impact and making it safer to apply in confined spaces such as crawl spaces, basements, or tank linings. Its low-emission formulation minimises exposure risk to applicators, especially when used in accordance with appropriate PPE and ventilation protocols. CEM 805 is classified as non-hazardous under current EU and UK regulations, with a stable shelf life of 12 months when stored in cool, dry conditions away from direct sunlight.

Compatibility with Other Waterproofing Systems

CEM 805 integrates well into multi-layered waterproofing strategies. It can be used in conjunction with cavity drain membranes, external waterproofing coatings, or bituminous tanking systems. For enhanced protection, it can be applied alongside swelling waterstops or compression gaskets to create dual-seal protection at critical structural interfaces. Its broad compatibility with other products makes it easy to specify in complex construction details or when adapting standard designs for site-specific needs.

Reliable Supply and Technical Support in the UK and Northern Ireland

Available from Toner Dampproofing Supplies Ltd (UK) across the UK and Ireland. Our teams provide technical support, including guidance on application in extreme weather conditions, compatibility with existing waterproofing systems, and calculation of material quantities based on joint profiles. We work closely with professionals on site to ensure successful installation and long-term performance.

A Next-Generation Sealant for Water-Exposed Construction

In a market saturated with generic sealants and joint fillers, CEM 805 stands out as a high-performance, task-specific solution. Its swelling action, bonding strength, chemical resistance, and environmental safety credentials position it as the ideal sealing agent for use below ground and in high-risk moisture zones. With growing demand for watertight construction methods across the UK and Northern Ireland, CEM 805 delivers professional results where conventional materials fall short, ensuring structures remain dry, stable, and protected for years to come.