Contribution to Resource-Saving Construction
Recycled Concrete to Replace Silica Sand in Tile Adhesives
Sand is the most heavily traded solid resource in the world, with human consumption of the material estimated at 50 billion metric tons per year – and that figure is rising. Sand can be found in concrete, bricks, glass, roads, insulation and solar installations, among other applications. Given how dependent humanity is on sand, the United Nations Environment Programme (UNEP) has called for rethinking its extraction and use  and for recognizing sand as a strategic resource.
According to a recent UNEP report , extracting sand from rivers and from coastal or marine ecosystems can lead to erosion, salination of aquifers, impacts on biodiversity and loss of protection from storm surges – this, in turn, threatens regional water supplies, food production, fisheries and tourism.
A Sustainability Boost
The construction industry is particularly reliant on sand as a raw material. The sector also generates an enormous quantity of construction waste. While over 90 percent of this is recycled in Germany according to the Federal Environment Agency , that figure includes low-quality uses – such as filler material in landscaping or landfill construction. The aims of a circular economy, however, would be better met by reusing materials such as concrete and building debris in a way that retains a high level of added value. One option here would be to use recycled concrete in dry-mix mortars, specifically in tile adhesives.
Tile adhesives are a type of dry-mix mortar consisting of at least three components: binders, additives and aggregates. Binders other than cement include vinyl acetate-ethylene (VAE) copolymers, which improve the bond between tiles and the substrate while making the tile adhesive more flexible.
The most important aggregate used in tile adhesives is quartz sand, which typically comprises 50 to 60 percent of the weight of the overall product. The results presented here show that up to half of that sand can be replaced with fine-grained, recycled concrete without compromising the properties of the tile adhesive – quite the opposite.
Analysis of the Recycled Concrete Used
Studies were performed on fine-grained fractions of recycled concrete in which particles ranged in size up to 1.25 mm. These fractions were produced using two different methods. The first method was to take recycled concrete obtained from Ettengruber GmbH Recycling & Verwertung and pass it through a screen at the WACKER site in Burghausen (Germany). The second method is more relevant from an economic standpoint, as it makes use of all of the recycled concrete: in this case, the Building Materials and Construction Chemistry Laboratories led by Professor Andrea Kustermann at the Munich University of Applied Sciences crushed the recycled concrete from the Ettengruber company until the particles were the required size. More in-depth studies demonstrated that the method of producing the fine-grained fraction did not impact the results within the limits of experimental error.
An X-ray diffractometer was used for determining the crystalline phases of the ground samples of the recycled concrete. Analysis showed the composition of the material to be calcite/quartz/dolomite in a 40/20/30 ratio by weight along with an amorphous component (10). It follows that the fine-grained fraction of the recycled concrete consists of gravel (limestone, dolomite/calcite, quartz), sand (quartz), carbonized portions of portlandite and amorphous calcium silicate hydrate phases from set cement. No appreciable amounts of unset cement or gypsum were found.
The particle density of the screened samples was determined in a capillary pycnometer as described in DIN 18124 , yielding values between 2.3 and 2.4 g/cm3. These are lower than the particle density values for a quartz sand reference (2.63 g/cm3) and are consistent with a larger number of fine pores having diameters between 10 nanometers and 5 micrometers. Confirmation was obtained from measurements taken on a mercury injection porosimeter.
Due to the large proportion of fine particles and fine pores in the samples of recycled concrete, tile adhesives based on this aggregate require more mixing water than traditional tile adhesives with the same consistency. This was taken into account in the sample formulations under investigation.
Properties of Fresh Tile Adhesive
The tile adhesive formulated as a reference for these studies fits the C2TE S1 classification defined in DIN EN 12004-1  (C = cementitious adhesive, 2 = high-strength adhesive, T = resists vertical slip, E = open time of at least 30 minutes, S1 = deformable adhesive, where deformation is greater than 2.5 and less than 5 mm). The polymer contained in the adhesive is a dispersible polymer powder based on VAE and vinyl chloride (VC) that, among other effects, improves the wet strength of the tile adhesive and makes it more flexible.
In the sample formulations, either 25 or 50 percent of the sand in the reference tile adhesive was replaced with recycled concrete, whereby the water needed to be increased to 290 mL and 320 mL, relative to 270 mL for the reference mortar. Virtually no differences were found between the reference and the other samples in terms of sag resistance, viscosity or density – properties that are significant for fresh tile adhesive.
A study comparing wetting capability was carried out as described in DIN EN 1347 : the tile adhesive was applied onto a concrete slab and a tile was laid 10, 20, 30 and 40 minutes later; a force of 50 Newtons was subsequently applied to the tile for a period of 30 seconds. The contact surface between the tile adhesive and the tile was then determined. Formulations based on recycled concrete were found to be superior to the reference: the more recycled concrete used, the better the wetting capability (Fig. 2), as blends containing recycled concrete take on more water.
A higher proportion of recycled concrete increased open time, paralleling the improvement in wetting capability (Fig. 3). Open time was determined as directed in DIN EN 12004-2  by placing tiles in the adhesive after 5, 20 and 30 min. and then measuring the tensile adhesive strength. A relatively long open time is highly desirable for tile installers, as this extends the time available for laying tiles in the prepared mortar bed.
Improved wetting capability and longer open times for formulations based on recycled concrete have no negative impact either on the initial tensile adhesive strength or on tensile adhesive strength following storage in water, at elevated temperatures or under freeze-thaw conditions (Fig. 4). One possible explanation here is that the fine-pored particles in the recycled concrete act as something of a reservoir that gradually releases water over time. That is an advantage when it comes to cement hydration. A team at the Technical University of Lisbon studying the use of recycled concrete in mortar production proposed this hypothesis in 2013 .
The values for tensile adhesive strength following storage at elevated temperatures are evidence of the considerable flexibility of all of the formulations studied here. This is because storage at higher temperatures generates mechanical stresses due to differences in the coefficients of expansion for the tiles and substrate – the tile adhesive must be able to withstand those stresses. The exceptional flexibility demonstrated here can be traced to the VAE/VC polymer in the formulations. Deformation was determined for the formulations according to EN 12004-2 , whereby the values for all of the blends containing recycled concrete were the same as those for the reference, within experimental error.
Important to Note
Whether the use of recycled concrete as an aggregate material in tile adhesives is beneficial for the environment and cost-effective for dry-mix mortar manufacturers is largely dependent on three factors. First: Sufficient quantities of recycled concrete must be available near the dry-mix mortar production sites, as the environmental benefit of recycled concrete must not be compromised by long transport distances . Second: The quality of the recycled concrete used must not fluctuate excessively. Third: Facilities must be on hand for breaking down the recycled concrete (crushing, grinding).
Recycled concrete can replace quartz sand in tile adhesives, thus helping conserve resources. Because it consists of fine grains and fine pores, it increases the water demand of the adhesive. Nevertheless, formulations based on recycled concrete yield tensile adhesive strength values similar to those of traditional tile adhesives, while at the same time wetting tiles more effectively and extending open time.
Authors: Dr. Klas Sorger and Nick Bucksch, Wacker Chemie AG, Burghausen
 DIN 18124: Soil, investigation and testing – Determination of density of solid particles – capillary pycnometer
 EN 12004-1:2017, Adhesives for ceramic tiles – Part 1: Requirements, assessment and verification of constancy of performance, classification and marking
 EN 1347:2007, Adhesives for tiles – Determination of wetting capability
 EN 12004-2:2017, Adhesives for ceramic tiles – Part 2: Test methods.
 C. Neno, J. de Brito, R. Veiga, Using Fine Recycled Concrete Aggregate for Mortar Production, Materials Research. 2014; 17(1): 168-177