Ceramic Adhesives

Ceramic Tile Adhesives Contributing to Resource Savings in Construction Processes: Use of Recycled Concrete Instead of Silica Sand in Ceramic Tile Adhesives Sand is one of the world's largest commercial raw material sources. According to estimates, humanity consumes 50 billion tonnes of sand annually, and this volume is continuously increasing. In addition to many different applications, sand is used in concrete, bricks, glass, roads, dams and solar energy facilities. Given humanity's dependence on sand, the United Nations Environment Programme UNEP is calling for a reassessment of sand extraction and use [1]. It has also been noted that sand should be regarded as a strategic resource. According to a recent UNEP report [2], sand extraction from rivers, coastal and marine ecosystems can lead to erosion, salinization of aquifers, loss of protection against sudden floods and impacts on biological diversity, potentially threatening water resources, food production, fishing and the tourism industry at regional scale.

A Plus from a Sustainability Perspective

Raw materials in the form of sand are particularly needed in the construction sector. On the other hand, this sector produces enormous quantities of construction waste. According to Federal Environmental Agency data, while more than 90% of this waste is being reused, it should be noted that this figure also includes low-value uses such as use as fill material in landscaping or landfill sites [3]. However, it can be said that the objectives of a circular economy would be better achieved by reusing materials such as concrete and building rubble in a manner that provides high added value. One way to achieve this is the use of recycled concrete in dry-mix mortars, particularly in ceramic tile adhesives. Ceramic tile adhesives are a type of dry-mix mortar composed of at least three components: binders, additives and aggregates. In addition to cement, vinyl acetate-ethylene (VAE) copolymers, which provide better adhesion of ceramics to the substrate and increase the flexibility of ceramic tile adhesives, can also be used as binders. The most important aggregate in ceramic tile adhesives is typically silica sand, which comprises 50 to 60% of the total product by weight. The results presented in this article show that when approximately half of the sand used in ceramic tile adhesive formulations is replaced with fine-grained recycled concrete, the properties of the ceramic tile adhesive improve rather than deteriorate.

Analysis of Recycled Concrete Used

Fractions of recycled concrete with grain sizes up to 1.25 mm were used. These fractions were prepared using two different methods: In the first method, the fractions were separated by sieving from recycled concrete obtained from Ettengruber GmbH Recycling & Verwertung, supplied to WACKER's plant in Burghausen (Germany). In the second method, which is economically more important in connection with utilization of all recycled concrete, recycled concrete supplied by Ettengruber was ground to the required fineness at the Building Materials and Building Chemistry Laboratory of the Munich University of Applied Sciences under the direction of Professor Andrea Kustermann. Figure 1 shows virtually no difference in grain size distribution between the sieved and ground samples. Here, the proportion of fine grains up to 0.1 millimetres was higher in both cases compared to the sand-limestone mixture. Further testing revealed that the production method of the fine-grained fraction did not affect the results within measurement accuracy. [caption id="attachment_147611" align="aligncenter"] Figure 1. Grain size distribution in a silica sand-limestone mixture (top), a sieved (middle) and a ground (bottom) recycled concrete fraction. (Source: WACKER)[/caption]   The ground recycled concrete samples were measured in an X-ray diffractometer for the purpose of determining crystal phases. The evaluation revealed a composition consisting of approximately 40:20:30 weight ratio of calcite-quartz-dolomite and an amorphous portion (10). The fine-grained fraction of recycled concrete consists of aggregate (limestone, dolomite/calcite), sand (quartz), carbonated portlandite portions and amorphous calcium silicate hydrate phases from set cement. Set cement or gypsum were not present in notable proportions. The grain density of the sieved samples was determined by capillary pycnometer in accordance with DIN 18124 [4]. The values obtained ranged between 2.3 and 2.4 g/cm³. In addition to this grain density, which is lower compared to a reference silica sand (2.63 g/cm³), there are numerous fine pores with diameters ranging from 10 nanometres to 5 micrometres. Measurements carried out using mercury porosimetry also confirm this situation. The high proportion of fine grains and fine pores in the recycled concrete samples means that ceramic tile adhesives based on them require more water for the same consistency compared to conventional ceramic tile adhesives. This condition was taken into account in the test sample formulations.

Properties of Fresh Ceramic Tile Adhesive

A ceramic tile adhesive was formulated as reference that can be classified as C2TE S1 according to DIN EN 12004-1 [5] (C = cement-based adhesive, 2 = adhesive with high adhesive properties, T = high resistance to vertical slip, E = open time of at least 30 minutes, S1 = flexible adhesive with deformation of at least 2.5 mm and less than 5 mm). The polymer in the adhesive is, among other properties, VINNAPAS ® 8620 E, a VAE and vinyl chloride (VC) based dispersible powder polymer that increases the water aging resistance of the ceramic tile adhesive and makes the adhesive more flexible. In the sample formulations, 25% and 50% of the sand in the reference ceramic tile adhesive was replaced with recycled concrete (Table 1). Accordingly, the water requirement also increased to 290 and 320 ml compared to 270 ml for the reference mortar. No significant difference was observed between the reference and other samples in terms of properties important for fresh mortar characteristics such as sagging resistance, viscosity and density. [caption id="attachment_147612" align="aligncenter"] Table 1. Overview of tile adhesive formulations tested
(Source: WACKER)[/caption] Comparative tests of wetting capability were carried out in accordance with DIN EN 1347 [6]: A ceramic tile is placed on a concrete slab to which ceramic tile adhesive has been applied, 10, 20, 30 and 40 minutes after application, and a load of 50 Newtons is applied for 30 seconds. Subsequently, the contact surface between the ceramic tile adhesive and the ceramic is determined. Looking at the results, formulations based on recycled concrete were superior to the reference: As the proportion of recycled concrete increased, wetting capability also improved (Figure 2). This is due to the increased water requirement in mixtures made with recycled concrete. [caption id="attachment_147613" align="aligncenter"] Comparison of wetting test results (left to right: Formulation 1 = Reference,
Formulations 2 and 3 = Partially substituted with silica sand obtained from recycled concrete, 25% and 50%). (Source: WACKER)[/caption] Parallel to improved wetting capability, open time also increases as the proportion of recycled concrete increases (Figure 3). This value was determined in accordance with DIN EN 12004-2 [7] by placing ceramics in the adhesive after 5, 20 and 30 minutes, followed by measurement of adhesive strength. The extension of open time is also a desired property for ceramic installation specialists, as it provides a longer period for applying the ceramics to the prepared mortar bed. [caption id="attachment_147614" align="aligncenter"] Figure 3. Determination of open time by measurement of adhesive strength. After 20 and 30 minutes, adhesive strengths for recycled concrete based formulations 2 (middle) and 3 (right) are superior to the reference. Accordingly, open times are also longer. (Source: WACKER)[/caption] The improved wetting capability and longer open time of recycled concrete based formulations do not result in a loss in terms of initial adhesive strength or adhesive strength following water aging, thermal aging and freeze-thaw cycles (Figure 4). This situation can be reasonably explained by the fact that the fine-porous grains in the recycled concrete act as a type of water reservoir that gradually releases water. This provides an advantage for cement hydration. This hypothesis was proposed in 2013 by a team from the Technical University of Lisbon investigating the use of recycled concrete in mortar production [8].   [caption id="attachment_147615" align="aligncenter"] Figure 4. Testing of initial adhesive strength (grey) and adhesive strength following water aging (light blue), thermal aging (red) and freeze-thaw cycles (purple). There is no significant difference between recycled concrete based formulations 2 and 3 (middle and right) and reference formulation 1. (Source: WACKER)[/caption]   High adhesive strength values following thermal aging demonstrate that all of the formulations examined here have high flexibility. This is because the thermal aging process creates mechanical stresses that must be withstood by the ceramic tile adhesive, arising from the different expansion coefficients of the ceramics and substrate. The high flexibility of the formulations stems from the VAE/VC polymer they contain. Determination of the deformability of the formulations in accordance with EN 12004-2 [7] gave the same values as the reference within measurement accuracy for all mixtures containing recycled concrete. The tests were subsequently repeated with ceramic tile adhesive formulations from which fine-grained fractions were sieved from a new batch of recycled concrete produced by Ettengruber six months after the initial batch used in this work. It is gratifying that the same test results from before were reproduced with these ceramic tile adhesives. This indicates that both recycled concrete series were of very similar quality.

What Should Be Considered?

Whether the use of recycled concrete as an aggregate in ceramic tile adhesives is ecologically advantageous and economically viable for dry-mix mortar producers depends on three main factors. First, recycled concrete must be available in sufficient quantities near the dry-mix mortar production facility, as the ecological advantage of recycled concrete should not be diminished by long-distance transport [9]. Second, the quality of the recycled concrete used should not show high variability. Third, facilities must be available for crushing and grinding the recycled concrete.

Conclusion

Recycled concrete can be used as a substitute for silica sand in ceramic tile adhesives and thus contribute to resource conservation. Due to its fine-grained and fine-porous nature, its use increases the water requirement of the adhesive. However, recycled concrete based formulations achieve similar adhesive strengths to conventional ceramic tile adhesives. At the same time, they provide better wetting of ceramics and have a longer open time.

References

[1] https://www.unep.org/news-and-stories/press-release/our-use-sand-brings-us-againstwall-says-unep-report [2] https://www.unep.org/resources/report/sand-and-sustainability-10 strategicrecommendations-avert-crisis [3] https://www.umweltbundesamt.de/daten/ressourcen-abfall/verwertung-entsorgung-ausgewaehlter-abfallarten/bauabfaelle#verwertung-von-bau-und-abbruchabfallen [4] DIN 18124: Soil, investigation and testing - Determination of density of solid particles - capillary pycnometer [5] EN 12004-1:2017, Adhesives for ceramic tiles - Part 1: Requirements, assessment and verification of constancy of performance, classification and marking [6] EN 1347:2007, Adhesives for tiles - Determination of wetting capability [7] EN 12004-2:2017, Adhesives for ceramic tiles - Part 2: Test methods. [8] C. Neno, J. de Brito, R. Veiga, Using Fine Recycled Concrete Aggregate for Mortar Production, Materials Research. 2014; 17(1): 168-177 [9] https://www.schwenk.de/recyclingbeton-baustoff-der-zukunft/ Dr. Klas Sorger European Technical Services Manager Construction Polymers Wacker Chemie AG Nikolaus Bucksch Business Development Manager Construction Polymers Wacker Chemie AG