How to darken grout after drying

Joint renovation in brick masonry

A masonry joint has essential static and structural functions that require different properties of masonry and grout for different masonry constructions and bricks. These include compressive strength, adhesive shear strength, longitudinal and transverse expansion modulus, as well as defined thermal and hygric expansion characteristics and certain properties for moisture transport or water repellency.

The type and amount of binder used have the greatest influence on these parameters. An increasing proportion of hydraulic binders (e.g. cement, highly hydraulic lime or plaster and wall binders) increases the compressive strength of the set mortar and causes a higher modulus of elasticity, the compressive strength reaches up to about 30 N / mm2. Completely or partially carbonated mortars with binding agents such as air lime or water lime harden more slowly and achieve significantly lower compressive strengths of 1-2 N / mm2.

There is no such thing as the universal joint mortar that always gives the best result in combination with all bricks. The inventor of dry mortar, Sakret, therefore offers a range of masonry and joint mortars with very different properties tailored to specific masonry blocks and load cases. The assortment ranges from cement-based joint mortars with relatively high hardness and high weather resistance to natural stone-suitable lime-trass mortars with pronounced capillary and diffusion properties. The wide range of colors that can be achieved with these grouts is also important for the client.


Damage mechanisms in joint and stone

A mortar that is too soft or insufficiently bound will lose its structural strength over time due to plastic load deformation. This process is intensified by weathering and penetrating water - the consequences are sanding and washing of the joint.

A grout that is too hard, on the other hand, often shows a flawlessly hard and smooth surface, but cracks in the joint cross-section, as well as large areas of detachment from the stone flanks. The cracks form a capillary network that can transport rainwater deep into the masonry. Unfavorable load peaks can cause the stone heads to be sheared off if the grout is too hard.

Joint or joint pattern can also be impaired by damage mechanisms that lie in the stone or an unfavorable subsequent treatment of the masonry. Too soft stones weather strongly and expose the mortar to increased erosion, hard mortar often remains like a scaffold. Hydrophobizing the joint or an entire masonry will only bring the desired result under ideal conditions; often water penetrates in some places and the hydrophobization inwards. The “trapped” moisture then bursts off joint and stone surfaces in the event of frost.

Refurbishment step one: remove the old joint

Damaged joints must be removed before re-grouting. As a rule of thumb, an installation depth of around 2.5 to 3 cm applies to bricks. Chisels, flex and joint milling cutters are popular, but have the disadvantage that the stone flanks are easily damaged because the tools work unintentionally effectively and only allow limited precision. For the craftsman, it is worth looking at alternative solutions such as the plunge-cutting hand saw. Low-dust work with suitable extraction systems on the device is mandatory, not only because of the pollution, but also for health protection.


Refurbishment step two: new grouting

A well-cleaned joint is crucial for the flank adhesion of the new grout. Pre-wetting takes place according to product-specific recommendations. This also reduces efflorescence, because the properly pre-wetted stone hardly sucks water and its ingredients out of the fresh grout. When using carbonated joint mortar (air lime), pretreatment with acetic alumina causes better adhesion to the stone.

The new joint mortar is applied in one or two layers with a suitable joint iron made of stainless steel, depending on the depth of the joint. The consistency of the grout during processing is product-specific, usually roughly earth-moist.

Remediation step three: protection and follow-up treatment

Both hydraulically and carbonatically bonded grout achieve optimal properties when the ambient conditions - temperature, humidity and air circulation - are within the range specified by the manufacturer when the material in question sets. Controlled rewetting of hydraulic mortars and protection against drying out too quickly can have a positive effect on the result.

After the mortar has set, the joint is formed. In addition to the smooth joint, other joint geometries are also permissible, which guarantee good water drainage. Joint irons, a piece of hose or, in the case of lime mortar in historical natural stone masonry, possibly even a brush with coarse, stiff bristles are used.

The mortar is compacted a little as a result of the shaping process. This closes some of the small shrinkage cracks and removes any thin sintered layers on the surface that can change the color of the joint and form an undesirable membrane. Excessive rubbing of the joint leads to unfavorable accumulation of binder on the surface.

By Thomas Adolph

There is no such thing as the universal joint mortar that produces the best result with all bricks

On our website you will also find an infographic on the correct formation of masonry joints. To do this, please enter the web code in the search bar.

Grouting correctly

Here you will find an infographic on the correct formation of masonry joints.