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Sandstone is a popular building material, which was used by the ancient Romans. It is easy to process. However, not every mortar can be used to grout the sandstone wall.
In a nutshell
- Sandstone has been used as a building material for centuries
- very soft rock and easy to work with
- Sandstone consists of 50 percent sand
- Due to its properties, special grout is required for grouting
Trass mortar for grouting?
Many mighty structures were built from sandstone, such as the Brandenburg Gate. Very often trass was and is used in construction with natural stone. This is a kind of natural pozzolan, mainly composed of silicon and aluminum compounds. In addition to water, binding agents such as lime or cement are also added to this. In general, trass mortar produced in this way can be used to grout natural stone, but not sandstone
- Trass mortar absorbs moisture, i.e. water
- this can easily penetrate the sandstones
- the stones then weather faster
The choice of mortar is therefore particularly important when grouting a sandstone wall. It must correspond to the properties and peculiarities of the rock.
Tip: The joint filling should always be at least 2 mm below the stone surface.
properties of sandstone
Sandstone is a sedimentary rock with a visible layered structure. In other words, the rock is ground very finely over time and mostly deposited on the bottom of water bodies. Water and other sedimentations act on the existing sand deposits. This is pressed and at the end the sandstone is created and can be mined. Typical features of the sandstone are:
- usually soft, loose material
- sometimes bulky and solid
- at least 50 percent sand
- mainly quartz sand
- other parts of the mixture contained: ore minerals, calcite, mica minerals
- Binding agents in sandstone are calcite, pebbles, clay and lime
- partial inclusion of fossils such as vertebrates or plant parts
- Sandstone density between 2.6 and 2.72 g/cm³
- Grain size of the batch parts between 0.06 and 2 mm
The color of the sandstone depends on mineral admixtures:
- yellowish brown: high limonite content
- green: contain glauconite
- Red; high proportion of carbon and iron oxide
- black-blue: high bitumen content
Notice: The sandstone weathers over time. Light-colored stones acquire a dark tint due to oxidation of the manganese and iron-bearing minerals present, such as pyrite, goethite, and hematite.
Use soft grout
A rule of thumb states that the grout must always be softer than the sandstone used. Furthermore, he should not be in possession of strong binding properties. Also not to be forgotten is the size of the grain of the joint material. The following materials can be used
- NHL Mortar, a natural hydraulic lime
- Natural air-lime mortar in different grain sizes
- alternatively HL mortar made of cement and hydrated lime hydraulically produced binding agent
However, it must also be mentioned in this context that air-lime mortar is not as weather-resistant as hydraulic lime mortar. You should take this into account when using it on heavily weathered surfaces.
The grain size of the grout is also important. It can be selected depending on the joint width. It is also important whether an existing, weathered sandstone wall is being renovated or whether it has just been rebuilt.
Weathered old sandstone wall
- Mortar grain size up to 4 mm for deep grouting
- for surface grouting size of the grain 1mm
- Joints must be thoroughly scraped out and cleaned
Newly built sandstone wall
- Grain depends on the joint width
- should be at least 2 mm
To ensure that the grout adheres well, the joints between the stones of the sandstone wall should be sprayed well with a four percent clay solution before grouting. This strengthens the substrate and increases the adhesion of the joint mortar. You should also make sure that the consistency of the mortar is not too wet.
Tip: After grouting, you should immediately remove all mortar residue from the sandstone with a damp cloth and water.
frequently asked Questions
Can simple cement mortar be used for sandstone wall grouting?No, in no case is simple cement mortar suitable for the joints of sandstone. This one is just too hard and doesn't allow stretching. This can quickly lead to tearing and bursting of the joints.
Does a sandstone wall have to be grouted?No, not necessarily if it is a dry wall. Here, the sandstones are simply tilted into one another without any binding agent. The gaps or joints are then suitable for planting afterwards. In this way, a small valuable biotope can be created.
What are the differences between air lime mortar & hydraulic lime mortar?Hydraulic lime mortar is insoluble in water. No air contact is necessary for curing. It even hardens under water. Compared to hydraulic lime mortar, air lime mortar is much softer and more sensitive to pressure. It also relies on the carbon dioxide in the air to properly harden.