Issue 2 -  2001/02

 ISSN 1311-8978

Adhesion of Polymer Coatings for Fine-grained Slag Concrete

Panayot Angelov Panayotov – LTU, Blvd. Kl. Ohridski 10, 1756 Sofia

Violeta Jordanova Petkova, Central Laboratory of Physico-Chemical Mechanics,

Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl 1, 1113 Sofia,

e-mail: malahit@clphchm.bas.bg

Received 10.10.2002; Cited 22.11.2002

Abstract.

Investigations have been carried out on the adhesion of epoxy resin paint coatings at the surface of fine-grained slag concrete under conditions of two regimes of hardening. The influence of an impregnating ground coat on the adhesion is studied too. It has been established that the porous structure at the surface of the slag concrete is most important for the adhesion as well as its moisture content and strength parameters. The obtained results show the possibility of wide practical application of the coating in the construction of basins and different irrigation structures and equipment.

Key words: paint coatings, concrete, porous structure, impregnating ground, water content.

Introduction

The surfaces of conventional building materials (concrete, lime, gypsum, gypsum fibreboard, wood and other surfaces) need improvement. Protective-decorative polymer coatings are most often applied for this purpose. The composition of the coating components depends on the operation conditions (air and water environment, aggressive air environment, low and high temperatures). According to the chemical nature of the basic film-forming component the polymer coatings are classified as epoxy, acrylate, polyvinyl acetate, polyester, polyurethane, rubber, silicone, etc. The choice of the system to be applied is based on considering the net cost, ecological effect and durability of the coating and the substrate. In the present case the latter are differing in composition concretes.

The durability represents a complex of properties including adhesion, mechanical properties, porosity, water permeability, temperature resistance, etc. It is considered that adhesion is one of the important properties. It is the direct expression of the coating adherence to the substrate. The improvement of concrete surfaces of the walls and bottoms of swimming pools is most often performed by means of epoxy paints forming coatings with azure colour. The technical characteristics of this type of paints do not contain data about the adhesion of the coatings to different concrete surfaces and the main goal of this investigation is to determine the adhesion of the Italian epoxy paint Cloroplast to two types of concrete surfaces.

Materials and methods of investigation

The analysis of the adhesion of the polymer coating was made on the basis of 10 test specimens with sizes 5.0/5.0/1.5 cm for the two concrete series. The control samples were ordinary concrete (OC) specimens with natural aggregates. The second series were made of the fine-grained slag concrete (SC) compositions. The accepted technology for the sample preparation was in conformity with BDS EN 206-1. The applied Portland cement PC35D20 met the requirements of BDS EN 196-1. The formulation of the slag concrete included a new combined active additive (AA), consisting of two components – alkaline and pozzolanic ones, in a definite mass (weight) ratio (0.4:0.6). The alkaline ingredient represented a waste product from construction industry with high calcium ion content and the pozzolanic ingredient – a secondary waste product from the ceramic industry with high active kaolin anhydride (Al₂O₃.5SiO₂). The additive met all the requirements of BDS EN 196-5 and BDS EN 196-6 with respect to dispersity, chemical composition and activity. It can be used in different brands of cement with different mineral composition and quality. The artificial aggregates represented well-fractionated mixture of flotation sterile material used as sand and fine-grained granulated slag entirely replacing the gravel in the slag concrete composition. These are cheap and easily accessible secondary mineral raw materials from non-ferrous metallurgy.

A three-parameter assessment has been performed for the influence of both active additive components on the changes of the compressive strength using the methods of planned experiment and mathematical statistics [3]. Their optimal quantities and the total additive content had been determined. The effectiveness of the additive is expressed in reducing the Portland cement amount with about 7-11 % and reducing the quantity of water with about 15 to 20 % keeping the same consistency as that of the control series. The lower water-cement ratio and good workability of the new fine-grained slag concrete composition contribute to the increase of its mechanical parameters.

The kinetics of the Ca(ОН)₂ amount calculated as CaO was investigated when varying the active additive quantity in the slag concrete composition [2]. Empirical relationships were found for the approximate determination of the safe Ca(ОН)₂ content for different ages of curing. It is seen that the use of the new combined active additive intensifies the hydration processes and the structure formation in the slag cement stone at a very early age and contributes to the higher durability.

It is supposed that the adhesion of the polymer coatings to the concrete surfaces depends to a certain extent on the porous structure, the tensile strength and the water absorption. Experimental studies were carried out to obtain more details about the changes of these parameters as a function of age. The axial tensile strength was determined using prismatic test specimens (sizes 4/4/16 cm) according to BDS EN 206-1. They were cured under two stationary regimes – under water and in atmospheric conditions (temperature 20±2° С and relative air humidity j = 65±5 %). Three specimens were tested for each of the investigated ages – 1, 7, 14, 21 and 28 days.

The dynamics of the changes in pore structure (total pore volume and pore size distribution) were studied for both concrete series (OC and SC) by means of the modern version of the mercury porosimeter “Carlo Erba” (range of pore diameter from 50 to 7500 A).

The polymer coatings were formed by the Cloroplast paint – a product of the Italian company ATRIA s.r.e. – Cloroplast. It is composed of modified epoxy resins and non-saponificated plastifiers. It forms a semi-glossy light blue coating, which is resistant to continuous impact of chlorine containing water. The relative density of the paint is r = 1.35 g/ml, the inflammation point is 30° С according to DIN 53213, the covering capacity is from 250 to 350 g/m² depending on the impregnation capacity and relief of the surface and it can be diluted to 10 % with DSN 300 thinner (synthetic turpentine). The specimens were covered by three layers placed at minimal intervals of 24 h. The final hardening of the coating was after at least 7 days, after which the specimens (the site) were ready for operation. The coatings were placed for comparison on concrete surface with and without primer. The priming was performed by dipping in Murofix - a colourless solution of polyvinyl resins. It is also a product of the ATRIA company, Italy. It is intended to neutralise the alkalinity of the painted surface and to improve the adhesion of the coating (water soluble or diluted by water). The density of the primer is r = 0.90 g/ml, the inflammation point is 36° С, the viscosity is 15-20 s according to FORD-4, the dry mass content is 18 %, рН = 4 and it can be diluted to 50 % (1:1) with a thinner of the company DSN 300. The primer forms a film with a thickness of 15-20 mm for one spreading of the layer.

The adhesion of the coating was determined according to the method of pulling-out a metal stamp glued on it with the Moment Super Bond cyancrylate adhesive of the Henkel company, Germany. The adhesion is calculated according to the formula:

s_а = 0.032 F                                      [MPa],

where F is the destructive force (tensile force) in kg;

0.032 is an empirical coefficient.

A steel stamp with a diameter of 2 cm was used. The test specimens were treated in an air-dry state (А) (air temperature 20±2° C and relative humidity j = 65±5 %) or in a water-soaked state (W). The testing was carried in the course of time – 7, 14, 21 and 28 days. The two series were designated respectively with age indices – OC-A-7 and SC-W-7, OC-A-14 and SC-W-14, OC-A-21 and SC-W-21, OC-A-28 and SC-W-28. The non-impregnated specimens were designated by the indices N and the impregnated – by the indices I. For example, the designation SC-IPW-14 should be understood as: specimens of slag concrete impregnated (I), covered with paint (Р) and cured in water (W) in the course of 14 days.

The two test concrete series were investigated without impregnation for comparison. In this case it is possible to determine the adhesion between the film formed by the cyancrylate polymer and the concrete surface or the cohesion in the surface concrete layer. The fracture character is determined visually in percents. The symbol K_c refers to the cohesive destruction in the concrete surface, the symbol A_cp – to the adhesive destruction between the concrete surface and the polymer coating and the symbol A_pm – to the adhesive destruction between the polymer coating and the stamp metal surface.

Results and analyses

The obtained results are presented in six tables and three figures. Table 1 shows the data, characterising the polymer coating and the adhesion to the concrete surfaces.

Table 1. Mean arithmetic values for the adhesion of the epoxy polymer coating to different concrete surface.

   Table 2. Character of fracture of the bond concrete-polymer coating.

   Table 3. Percent pore size distribution for different ages and regimes of hardening

Table 4 shows the trends in the tensile strength changes for the two types of concrete and for different ages. The regime of hardening – in air (А) or under water (W) exerts substantial influence on the strength value. For example, the series OCOW and SCOW cured under water yield higher tensile strength on the 1^st day as well as on the 28^th day. The strength of the slag concrete specimens are with 42.9 % and 37.0 % higher than the corresponding control compositions for the same age. The obtained results are plotted in Fig. 1. Analogous relationship is observed for the series OCOA and SCOA cured in air (Table 4, Fig. 2).

The alteration of the pore volume (V_p) in cm³/g is shown in Table 5 and Fig. 3 for the two test series of concrete as a function of the age and regime of hardening. It is seen that the pore volume decreases with increasing the age. For example, the ordinary concrete composition OCOA cured in air has a recorded value 0.48 cm³/g on the 1^st day and 0.039 cm³/g – on the 28^th day. In the case of curing under water the volume varies from 0.045 cm³/g to 0.032 cm³/g. The slag concrete series SCOA cured in air yields a more dense structure than OCOA. It is logical that the measured pore volume in it is lower – 0.036 cm³/g and 0.027 cm³/g for the same ages of testing. The pore volume of the SCOW series is 0.032 cm³/g and 0.020 cm³/g, while the pore volume of the OCOW composition cured under water yields higher pore volume with 28.9 % and 37.5 %.

The pore size distribution (Table 3) is indicative for the quality of concrete pore structure and its influence on the durability. It is seen that the micropores (r = 50-100 А) form 27 % and 24 % of the pore volume of the OCOW and OCOA respectively. However, the micropore participation in the volume of SCOW and SCOA is higher – 30 %.

The comparative analysis of the data in Table 5 and Table 6 shows that there is a direct dependence between pore volume and water sorption. The latter decreases with the diminution of pore volume. For example, the OCOW series at the age of 14 days exhibits water sorption of 5.42 % for a pore volume of 0.034 cm³/g. At the same time the SCOW series exhibits water content of 3.02 % for a pore volume of 0.023 cm³/g. A trend is observed for increasing of water sorption with age. For example, OCOW exhibits water sorption of 9.23 % at the age of 28 days, while it is 4.14 % for the age of 7 days. The pore volume has been decreased with 24.5 %, while the water sorption has been increased 2.23 times. In the case of the SCOW series the water sorption for the mentioned ages is 5.79 % and 2.64 % respectively. It is clear that for the two concrete compositions the water sorption depends on the qualities of the pore structure and the duration of water soaking.

Table 4. Kinetics of the tensile strength of ordinary concrete and fine-grained slag

 concrete for two regimes of hardening – in air (A) and under water (A).

Table 5. Changes in the pore volume (V_P) for ordinary concrete composition and fine-grained slag

concrete under hardening regime in air (A) and under water (W).

Table 6. Water sorption of concrete series subjected to tensile stresses according to the metal stamp method.

Fig. 1. Changes of the tensile strength for the regime of curing under water

 Fig. 2. Changes of the tensile strength of series subjected to the regime of curing in air

 Fig. 3. Changes of the pore volume for different ages and regimes of curing

For example, the ordinary concrete composition OCOW at the age of 7 days absorbs 4.14 % of water, while the slag concrete sample SCOW absorbs 2.64 % of water for the same time. The water sorption at the age of 28 days for the two specimens is 9.23 % and 5.79 % respectively.

The analysis of the results in Table 1, Table 2 and Table 4 proves that the adhesion of the coatings depends to a great extent on the structure of the two types of concrete (pore volume and percent pore size distribution) as well as on the water content in them. For example, the adhesion of the coating for the OCPW series with 4.12 % water content is 0.67 МРа at the age of 14 days and the adhesion of the coating for the SCPW series with 3.02 % water content is 0.95 МPa.

The character of fracture is also taken into account when determining the adhesion of the polymer coating to the surface of the concrete specimens (Table 2). It is seen that the series without coating exhibit mainly destruction character of the cohesive type (Kc) on the concrete surface. The fracture depth is considerable for some of the series with and without coating – 2-3 mm. For example, the destruction of the OCOA and SCOW specimens at the age of 7, 14 and 21 days is of the cohesive type (Kc) on the concrete and varies from 70 to 90 %. With the growing age the adhesive destruction becomes more pronounced and is predominant in some of the series. The destruction of the SCPW series at the age of 21 and 28 days is 90 % of the adhesive type at the concrete-polymer coating boundary (Acp). The observed adhesion values for the same series at the age of 14 days are s_а=0.95 МРа, at the age of 21 days - 1.95 МРа and at the age of 28 days - s_а=0.85 МРа. This proves that for this series the adhesion values between the coating and the concrete are higher than the tensile strength of the surface concrete layer. The adhesion has been also determined for concrete without coating – OCOA, SCOA, OCOW and SCOW. For example, at the age of 14 days the adhesion of the OCOA series is 0.72 МРа and of the OCPA series - 1.04 МРа. The value for the SCOA series is 0.77 МРа with predominating cohesive destruction. The adhesion of the SCPA series with polymer coating is 1.71 МРа. The adhesive destruction is predominating. The analysis of the results presented in Table 1 proves that the adhesion between the coating and the ordinary concrete exhibits its maximum values on the 21^st day both for OCPA cured in air (1.12 МРа) and for OCPW cured under water (1.60 МРа). The maximum values for the adhesion between the coating and the slag concrete are observed on the 14^th day for both SCPA and SCPW - 1.71 МРа and 1.95 МРа respectively. These results confirm explicitly one more advantage of the new slag concrete composition compared to the ordinary one – the good adhesion.

The influence of the impregnating primer on the polymer coating adhesion can be observed in Table 1 (for series OCIPA and SCIPA). It is seen that with the age growth slight decrease in adhesion is observed for the regime with air curing. The adhesion of the polymer coating of OCIPA at the age of 14 days is 0.88 МРа and on the 28^th day - 0.69 МРа. The adhesion values of SCIPA are respectively 1.31 МРа and 0.91 МРа. The adhesion of the coating on the specimens with primer is lower than that of the specimens with no primer. Hence the recommended by the company vinyl impregnator is not the most suitable one for the two concrete compositions and this type of coating. This implies that the compatibility between the primer and the concrete and coating should be defined very precisely for each type of concrete and coating.

The observed diminution of the adhesion of the polymer coating for with concrete age can be explained by the decrease in pore size and volume (Table 3, Table 5 and Fig. 3). According to the mechanical theory (the disjoining theory), the polymer coating adhesion to the solid body is due to the permeation of the polymer macromolecules in its pores [1]. The degree of permeation of the polymer molecules in concrete is decreased with volume reduction. The character of fracture of the impregnated samples shows that for all variants it is mainly of the cohesive type on the concrete surface. This fact is the reason for continuing further investigations until the adhesion between the polymer and the concrete acquires lower values than the tensile strength of the surface concrete layer.

Conclusion

The following conclusions can be made on the basis of the obtained experimental results:

1.   The applied modified epoxy paint forms coatings with higher adhesion on the slag concrete specimens compared to the ordinary concrete ones.

2.   The age and regime of hardening exert substantial impact on the adhesion of the polymer coating to the concrete surfaces.

3.   The used impregnating Murofix primer does not improve the adhesion of the Cloroplast epoxy coating to both the ordinary and the slag concrete.

References

1.   Panayotov P. Adhesives and Materials for Protective-decorative Coatings, 2002, Publ. House of the Forest Engineering University, Sofia, 276 p. (in Bulgarian).

2. Petkova V. Pozzolanic Active of the Combined Additive for a New Silicate Composite, X Jubilee Symposium “ECOLOGY-2001”, 07-09.06.2001, Bourgas, 353-358.

2.   Petkova V, R. Krastev. On the Variation of the Compressive Strength of New Class Silicate Composites”-Journal of international Research Publications [serial online]. Bulgaria, Science Invest Ltd - Branch Bourgas, 2000/2001, Issue 1 ro8/2001. http://www.еjornalnet.com (Contents) Issue 1/8/8.200,htm. ISSN 13311-8978.