Options for finishing cast stone products

Homeowners increasingly focus on a sophisticated look of their floor and wall coverings both in interiors and in outdoor spaces. Many of them favor concrete products for this purpose, including a variety of slabs. Compared to quarry stone or tile products, concrete products come at a relatively low cost, are easy to install, and are suitable for everyday use in both private and public spaces thanks to their dimensional stability and robustness.

Producers of upmarket concrete slabs and blocks increasingly provide their customers with top-class, premium-quality products that emphasize the sophisticated design of the property thanks to their visual appeal and texture in combination with a wide variety of laying patterns (Fig. 1). SR Schindler based in Regensburg, Germany, supplies a wide range of individual machines and production lines for manufacturing premium-quality products that stand out visibly and tangibly from high-volume, mass-produced items thanks to their custom concrete recipes and top-class surface quality. Using such equipment and machinery enables concrete companies to offer products for any design preference and area of application, thus expanding their business to new market segments.

When producing finished concrete slabs and blocks, we can generally distinguish between wet-side and dry-side surface finishing methods.

Wet-side surface finishing

On the wet side of the production line, products are treated immediately after the end of the manufacturing process, which means that they have not hardened yet. Methods in this category include washing as well as coloring and marbleizing, especially for slabs produced on hermetic presses.

Using various matrices or tamper heads during the production process is another appropriate option for turning a smooth-surface item into a textured slab or block, for instance in an authentic slate or wooden look (Fig. 2). Matrices are custom-made plates composed of steel and rubber inserted into the molds of hermetic presses in order to function as the negative profile of the envisaged surface texture (Fig. 3).

SR Schindler has been manufacturing these matrices for many years at its own production facility. The company’s portfolio currently includes over 1,500 designs of up to 1,200 mm × 1,200 mm in size.

Dry-side surface finishing

The dry side of the production line provides many more finishing options. This is where cured products are treated mechanically and/or chemically in order to create their final appearance.

Blocks and slabs are most commonly finished using one  or more of the following methods:

grinding/polishing,

blasting,

curling,

impregnation/coating.

Additional options for paving stones and masonry blocks include:

bush hammering/ageing,

splitting,

edge treatment.

Finishing by grinding/polishing

In the grinding/polishing step, concrete products are treated using special grinding or polishing tools, such as high-performance industrial diamonds, in a wet or dry process so as to create a specific ground or polished finish, reaching from a slightly ground to a highly polished, glossy surface. Depending on the intended look and feel four to ten of these stations are in use to create the individual products.

This process step lends a bright appearance to products manufactured with colored aggregates while polished grey products come across as particularly elegant and top-class. In addition, polished surfaces are less susceptible to soiling and easier to clean than untreated smooth surfaces. It is precisely for this reason that polished concrete products are also often used in public areas such as airports, hospitals and shopping malls (Fig. 4).

Finishing by blasting and curling

Blasting using steel or stainless-steel microspheres exposes the aggregate particles by partially removing the cement layer in which they are embedded, thus slightly roughening the product surface. This is why blasted products are frequently used for outdoor spaces because they provide greater slip resistance (Fig. 5). Blasting can be performed either after slight polishing or as an intermediate step prior to curling, or as the only finishing process. Depending on the specific application and modification of process parameters such as conveyor speed, rotational velocity of the turbines and sphere diameter, a wide variety of surfaces can be created to expand the product portfolio considerably.

Blasting can be followed by curling in order to create a silky smooth surface. This method uses rotating brushes (Fig. 6) for removing excess cement and exposing and slightly polishing the aggregate particles. Curling is recommended not just as a finishing step but also as a virtually mandatory process prior to coating and printing. The brushes and optional high-performance cleaning system remove residual dust from the product surface. The dust-free surface then enables the coating agent to penetrate into the face mix, which is an indispensable prerequisite for creating a durable surface protection and long-lasting prints.

Post-treatment by impregnation or coating

Impregnation or coating is the last step in the surface finishing sequence. It protects finished products from weathering and soiling while making the items easier to clean. Depending on the specific chemical components, a primer coat may first have to be applied, followed by spraying or rolling the coating agent onto the surface (Fig. 7, Fig. 8). Infrared or UV modules ensure pre-heating or drying of the products (Fig. 9).

Bush hammering, ageing, splitting, and edge treatment

Paving stones, masonry blocks and block steps can additionally be aged/bush-hammered or split, or their edges can be broken if the surrounding architecture calls for an antique and/or historic appearance.

In the bush hammering/ageing step, the product surface is hammered and/or aged using mechanically operated tools (Fig. 10). Bush hammers are used to treat the entire surface and to knock off the edges while ageing hammers are used exclusively for knocking off the edges. A plastic film running between the product surface and the tools prevents hammering traces from appearing on the surface during the ageing process. This method makes it possible to use just a single machine to manufacture products with four different features: bush-hammered, aged without plastic film, aged with plastic film, and finished with knocked-off edges (Fig. 11, Fig. 12).

Splitting is the most appropriate method for replicating quarry stone walls and naturally aged quarry stone paving blocks. Splitting blades mounted on the top and bottom of the machine in a tongs-like arrangement apply a hydraulic splitting force of up to 150 tons from the bottom to the top in order to perform the splitting process. A splitting line equipped with two consecutively arranged splitting machines and a 90 degree turning device between these two machines permits longitudinal and transverse splitting of the products (Fig. 13, Fig. 14, Fig. 15 ).

Split products have straight edges as a result of this process that may disturb the otherwise natural splitting pattern, particularly of masonry wall blocks. To make them resemble natural quarry stone, the splitting line can be complemented by a bypass edge treatment station. This is where the edges of the split products are worked using rotating chains. Such a unit can knock off up to 12 edges depending on its design and layout.

SR Schindler has earned a sound global reputation as an experienced, innovative partner and supplier of top-class equipment and machinery. Its portfolio provides concrete products businesses with a wide variety of surface finishing options for manufacturing visually appealing concrete products with attractive textures. The modular equipment solutions designed and engineered by SR Schindler as well as the sophisticated control systems including high-quality hardware components and the smart safety concepts offered by the company enable producers to respond to future market demand in a highly flexible manner.

Text: Dr. Frederic Scholl, SR Schindler