Impact-Resistant Synthetic Concrete / Epoxy Resin Mortars (Mehmet Namık Kayaalp)

  Where paints and coatings prove insufficient for continuous protection of structures and substrates exposed to severe external conditions, synthetic resin mortars—alternatively termed epoxy resin binder-based mortars—are recommended. This is because they provide excellent mechanical properties and good chemical resistance. Two principal groups of synthetic resin mortars exist. 1. Trowel-applied mortars, 2. Self-levelling mortars. The difference between these two groups lies in the binder content of the composition and the particle sizes of the aggregates employed. Trowel mortars (highly filled—containing 85% or higher aggregate content) are typically applied at 5–10 mm thickness. Cured synthetic resin mortars are formulated to provide the following properties. • High mechanical strength (compressive strength), • Lateral and vertical impact resistance. Mortars with low thermal expansion coefficients remain spreadable even at low temperatures. These can only be applied with an appropriate tool such as a trowel. Some formulations can be levelled after application. Particularly modified formulations may be synthetic resin mortars suitable for application to vertical surfaces. Self-levelling mortars (85% or less filled) are typically applied at 3–6 mm thickness. Synthetic resin mortars produced for these formulations spread and apply rapidly. On horizontal surfaces, rough surface levelling may suffice. These mortars exhibit lower impact resistance compared to impact-resistant mortars. Unlike low-binder mortars, their surfaces are smooth, glossy, and impermeable to liquids. If any surface tackiness remains after curing, it can be removed with water. To prevent slipping, sand should be broadcast onto the coating before curing. Aggregates: Inorganic aggregates are among the factors that notably contribute to determining the strength properties of synthetic resin concrete. For this reason, the high strength of this type of concrete can only be achieved with strong aggregates. The effect of particle size and structure of the aggregate on the strength properties of synthetic resin concrete follows the same principles as with normal mortars. The Fuller curve can be recommended for assessment of particle size distribution. Here the function of the binder is seen in fine particles that will fill large and small voids on the surface. The most well-known suitable aggregates for epoxy resin mortars with specified particle sizes are given on the following page. When particle size composition differs from recommended values, particularly when the composition lacks quartz flour, the following conditions may result: • With the same binder-to-aggregate ratio, there is insufficient resin to fill all voids. A porous material with air gaps is formed. • When larger void volumes are filled with more binder—that is, when the sand-to-resin ratio changes—strength properties are compromised, particularly impact resistance is lower. Very high compressive strength can be achieved with high-hardness aggregates such as silicon carbide and corundum. The aggregates known for synthetic resin concretes have their own hardness less than that of the binder. When synthetic resin concrete and cement concrete are fractured simultaneously under strong pressure, cracking in normal cement concrete occurs in the binder, whereas in synthetic resin concrete it occurs in the sand particles. For this reason, using high-quality aggregate in mortars is more appropriate. When graphite, quartz, or silicon carbide is used together as filler, electrically conductive mortars with leakage resistance lower than 10⁶ Ohm can be formulated in accordance with guidelines. Addition of graphite does not impair the good mechanical strength of heavily filled mortars. In some cases, the dark color caused by graphite may be undesirable, but this can be mitigated by adding titanium dioxide. Epoxy resin mortars can also be modified with liquid diluents such as tar, Coumarone-Indene resins, and other plasticizers. Often these additives can provide an advantage in solvent-based and solvent-free paints, but in heavily filled coatings, disadvantages such as reduced reactivity and low strength may be observed. When Coumarone resin is added at approximately 20% by weight of the binder, curing is greatly delayed and mechanical strength will be approximately 40% lower compared to mortars that did not cure or provide mechanical strength. Pigments: Color development of sand-filled casting pastes depends on the color of the sand. With normal light-colored quartz sands, pigment addition at 0.5–1% of the total mixture will suffice. Inorganic pigments are the most suitable and best pigments for this purpose. Pigments are highly compatible with synthetic resin concrete and resistant to solvents and weathering. For special decorative effects, broken marble fragments, colored sands, and pigments can be mixed together. All aggregates must be dry and only sands containing no organic components should be used. Application: If the self-levelling mortar contains a high proportion of resin, after curing it has a thermal expansion coefficient 3–5 times greater than concrete or steel. This means this mortar should not be used on surfaces exposed to low temperatures (0°C or lower). When coating thickness exceeds 2 mm, stress conditions may develop that could cause coating cracking. These stresses have not been observed in coatings thinner than 2 mm. Producing a synthetic resin mortar is quite straightforward. If the resin component mixing ratios are observed precisely, proper mixing is achieved, and the mortar is mixed thoroughly in an anhydrous and moisture-free environment, satisfactory results will be obtained. Standard values for a typical application are given below. These values may vary somewhat depending on surface conditions. For coating quality, correct determination and establishment of working conditions are crucial. Resin systems reach their optimum properties at the mixing ratios specified in the formula. When the two components are uniformly mixed completely, the mortar reacts throughout in a satisfactorily manner. For this reason, the need for sufficient and complete mixing should not be forgotten. Mechanical mixers with edge scrapers of 50–80 litre capacity are more often preferred. Gravity mixers provide insufficient mixing action. First, the appropriate amounts of epoxy resin and hardener are uniformly mixed in a mechanical mixer. Subsequently, aggregates are mixed and kneaded in the mixer until completely wetted. The mixing process takes approximately 5 minutes. To prevent excessive air entry during mortar mixing, silicone oil at 0.5% by weight of the binder can be added. Silicone oils are particularly recommended for self-levelling synthetic resin mortars. Because after the coating spreads, bubbling occurs on the surface due to air entry. Under normal conditions, the binder curing has a limited pot life varying between 20 minutes and 6 hours depending on hardener type. Pot life is also significantly affected by the quantity of the mixture and the ambient temperature at that moment. To avoid local heating generated after mixing that accelerates the reaction, large containers can be divided into small equal portions. The binder's good adhesion properties, depending on the formulation, can be a disadvantage during the spreading of synthetic resin mortar. After some time, the mortar adheres to the trowel, becoming quite difficult to clean. This difficulty can be overcome by using paraffin oil in the formulation. Silicone oil, used as an air release agent in the formulation, also reduces mortar adhesion to the trowel. If the resin content in the mortar is 10%, use of a primer or bonding agent is recommended. The resin/hardener mixture used in the mortar, without aggregates, can be thinly applied to the surface before spreading the mortar and used as a bonding agent. In practice, this application acts as an adhesive between the substrate and low-binder mortar. A filled bonding agent has application advantages. Quartz flour and fumed silica are suitable aggregates that prevent the bonding agent from acquiring a speckled, oily appearance on the coat surface. The bonding agent should be coated with synthetic resin mortar immediately after application. By contrast, when the mortar contains approximately 15% binder, a bonding agent is not necessary. Surface preparation before application of synthetic resin mortar or bonding agent should receive careful attention. Perfect surface adhesion can only be guaranteed by perfect surface preparation. If a weak concrete substrate must show better resistance to mechanical stress, the synthetic resin concrete must be sufficiently thick—that is, minimum thickness should be 15–20 mm. On a good, sound substrate, 5 mm thickness is normally sufficient and perfectly appropriate. Resin viscosity, pot life, and hardener depend on the ambient temperature. Resins and aggregates should not be cooler than 15°C to ensure complete mixing. This temperature is also the lower limit of the curing temperature for non-accelerated binder systems. When low curing temperatures are unavoidable, acceleration with catalyst accelerators should be employed. At normal temperature, synthetic resin concrete cures completely within 24–28 hours depending on binder reactivity. Under these conditions, the concrete will be strong enough to resist light mechanical stress after only 12 hours. Full mechanical loading will be possible after 3–7 days. Concrete should never be exposed to chemical influences in less than seven days. Surface preparation: Water-cemented or cement-dusted concrete surfaces have low surface tension and penetration remains quite poor. For this reason, such layers must be removed before application. Completely dried upper concrete surface layers must be removed in their entirety. The mechanical soundness of old concrete must be checked. Additionally, oil, chemicals, or dirt particles must be completely removed from the concrete surface. Cleaning surfaces with solvent often proves insufficient. The safest method is to scrape off and remove deteriorated areas and then clean the concrete surface with sandblasting that does not damage the concrete or by flame treatment. Newly laid substrate concrete must be at least 2 cm thick and have aged for at least 28 days. On highly moist surfaces, particularly wet concrete surfaces, adhesion difficulties arise because varnish cannot spread on water-filled pores. For this reason, the concrete surface must be dry and clean during application. On old epoxy-painted surfaces to be repainted, if the paint still adheres well to the surface and is undamaged, the surface can be lightly sanded and cleaned with solvent before applying new epoxy paint. Mehmet Namık Kayaalp / Chemical Engineer - Ecelak Boya Kimya Ltd. Co. References: 1. Schering Industrie-Chemikalien. 2. H. Lee, K. Neville Handbook of Epoxy Resins. 3. UPPC GmbH, Baltringen.