https://doi.org/10.14311/APP.2022.33.0295 Acta Polytechnica CTU Proceedings 33:295–299, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague OPTIMIZING THE ACID RESISTANCE OF CONCRETE WITH GRANULATED BLAST-FURNACE SLAG Luca-Alexander Kempfa,∗, Rolf Breitenbüchera, Christian Gertenb, Andreas Ehrenbergb a Ruhr University Bochum, Department of Civil and Environmental Engineering, Institute for Building Materials, Universitätsstrasse 150, 44801 Bochum, Germany b FEhS - Building Materials Institute, Bliersheimer Straße 62, 47229 Duisburg, Germany ∗ corresponding author: Luca-Alexander.Kempf@rub.de Abstract. Concrete structures exposed to high levels of chemical attacks are assigned to exposure class XA3, which recommends separate concrete protection or a special expert solution to ensure durability. Due to the partial substitution of Portland cement by blast-furnace slag, an increased resistance to acid attacks could be achieved within the framework of a research project. The technical and ecological advantages of cements containing granulated blast-furnace slag were exploited through chemical, granulometric and concrete technological optimizations. Despite extensive parameters, a statistical test design (DoE) was able to limit the experimental effort, thus defining principles for the conception of binder systems with increased chemical resistance.Mortar prisms indicated that the use of (ultrafine) blast-furnace slags (up to 13,000 cm2/g according to Blaine) with a broad particle size distribution can have a positive effect both on the capillary/gel pore ratio and on the calcium hydroxide content in the cement stone. Furthermore, the chemical composition of the blast-furnace slag as well as the water-binder ratio are decisive influencing factors for the acid-resistance, which was confirmed in accelerated acid resistance tests on concretes (pH-stat method). After 13 weeks of storing concrete specimens in sulfuric acid (H2SO4, pH 3.5), reduced damage depths and lower weight losses were observed compared to conventional binder compositions. The results serve as a basis for the development of highly acid-resistant concretes using blast-furnace slag-containing binder systems. Currently, the acid resistance of those concretes is being investigated in a long-term study by outsourcing representative test specimens into the Emscher sewer. Keywords: Blast-furnace slag, concrete, durability, sulfuric acid. 1. Introduction Concrete structures in agriculture as well as in in- dustrial and wastewater engineering plants may be exposed to various types of chemical attacks. Even though concrete is known for its high durability, the material is susceptible to acid attacks [1, 2]. Acidic media can diffuse through the porous binder matrix and dissolve it. To increase the acid resistance of the binder, priority is given to reducing the porosity. This is done on the one hand by the choice of a low w/b- ratio and on the other hand using an optimized grain size distribution up to the finest aggregates. For a complete hydration of cement, a w/b-ratio of about 0.40 is required. The larger amount of water is chem- ically bound in the calcium silicate hydrates (CSH), a minor part remains in the gel pores < 30 nm. With increasing w/b-ratio, more and more capillary pores are formed, causing a connected network of capillary pores at a w/b-ratio of approx. 0.60 [2, 3]. Espe- cially the usage of ordinary Portland cement (OPC) lowers the resistance of concrete towards acidic attack because it releases readily soluble calcium hydroxide (Ca(OH)2) and large portlandite crystals while hy- dration [4]. The Ca(OH)2 preferably develops where water is located as well as on the surface of the aggre- gates [5], which results in a three-dimensional inter- connected structure of Ca(OH)2 [6]. The substitution of OPC by granulated blast-furnace slag (GGBFS) reduces those effects due to a reaction of the GGBFS with the surplus Ca(OH)2 forming further CSH. In addition, these CSH are less soluble because of an in- creased silicate amount and the porosity of the binder matrix decreases which thus, obstruct harmful media from penetration into the material [5, 6]. The main objective of the research project is to optimize the acid resistance of commonly used concretes while at the same time maintaining good workability with the greatest possible avoidance of costly additives. The potential of GGBFS to strengthen the acid resistance is being evaluated by means of a comprehensive re- search programme involving a wide range of parame- ters. 2. Research Program 2.1. Binder optimization Due The research program was divided into two parts. In a first step, the binder was optimized on a mortar scale before a series of binder com- 295 https://doi.org/10.14311/APP.2022.33.0295 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en L. Kempf, R. Breitenbücher, Ch. Gerten, A. Ehrenberg Acta Polytechnica CTU Proceedings Figure 1. Testing area of the binder compositions. Figure 2. Schematic sketch of the acid test rig. positions, based on the results of the mortar tests, got examined with coarse grain on a concrete scale. To evaluate a wide range of GGBFS, three gran- ulated blast-furnace slags with different reactivities were considered. The reactivity is defined as the ra- tio of calcium and magnesium to silicon ((C+M)/S)). Before the blast-furnace slag was used to produce concrete, it has been grounded into four degrees of fineness, resulting in two GGBFS (4,200 cm2/g and 7,000 cm2/g) and two fine GGBFS (10,000 cm2/g and 13,000 cm2/g). To investigate the characteristic val- ues for the acid resistance, the w/b-ratio was variated between 0.30 and 0.44 and the percentages of OPC and GGBFS got amended. Due to the huge quantity of parameters, a statis- tical Design of Experiments (DoE) with 45 different binder compositions was able to limit the experimen- tal effort, thus defining principles for the conception of binder systems with increased chemical resistance. The testing area of the DoE is shown in Figure 1. 2.2. Acid resistance test setup The acid resistance of the concretes was tested in an acid test rig, which is based on the pH-Stat method according to the MPA-Berlin-Brandenburg testing Figure 3. Concrete specimen in a storage tank with sulfuric acid. procedure [7]. The test stand consists of a reser- voir, which is connected to multiple storage tanks via acid-resistant hoses. The reservoir features a mag- netic stirrer as well as a titrator and a pH-electrode, which continuously measures the oxonium ion con- centration of the test medium. Sulfuric acid (H2SO4) has been used as the test medium, which was con- stantly maintained at a pH value of 3.5 µm 0.05 (au- tomatic endpoint titration). When the ph-value of the sulfuric acid exceeded an equivalence point, con- centrated sulfuric acid (H2SO4, 2.0 molar) was au- tomatically titrated and distributed by the magnetic stirrer. In order to prevent a high concentration of dissolved substances in the storage tanks, the sulfu- ric acid was also completely replaced every 14 days. The sulfuric acid was transported via pumps with a flow rate of approx. 15 dm3/h into the storage tanks, which always contained at least 34 litres of acid. Via a built-in overflow, the test medium is returned to the reservoir. The overall structure of the acid test rig is shown schematically in Figure 2. The test specimens were stored for 91 days in the storage tanks of the acid test rig starting at a prelim- inary age of 28 days as shown in Figure 3. The mass of the specimens was determined every two weeks in the process of the acid exchange. Figure 4 shows two mortar specimen after 91 days of storage, one in wa- ter and the other one in sulfuric acid with a pH-value of 3.5. 2.3. Concrete scale After the binder optimization with mortar prisms, the acid resistance was tested on concrete specimens with optimized binder compositions in sulfuric acid. Again, use was made of the DoE with a statisti- cal evaluation. For an optimal grain size distribu- 296 vol. 33/2022 Concrete Acid Resistance Optimization Figure 4. Mortar specimen after 91 days of storage in water (left) and sulfuric acid (right). Figure 5. Preparation outline of the concrete speci- men for acid storage. tion, the fuller principle with a maximum aggregate size of 16 mm was applied to achieve a high pack- ing density [8]. The concrete specimens were cut out of cubes with 150 mm edge length as shown in Fig- ure 5. Figure 6 shows a cuboid-shaped concrete spec- imen (150 mm × 100 mm × 50 mm) with a high pack- ing density of the coarse grain after preparation for the acid resistance test. The experimental programme for the acid tests consisted of 14 binder compositions were multiple pa- rameters got changed to each other. The results were evaluated in comparison to reference samples with CEM I 42.5 R and CEM III/B 42.5 N-LH/SR/NA. To quantify the acid attack, the depth of damage to the specimens was determined three times (after 28, 56 and 91 days) under a stereo microscope during the 91-day acid storage period. To determine the depth of damage, two different specimens were split off. The fresh fracture surfaces were sprayed with an indicator (1% phenolphthalein solution) and evaluated micro- scopically. The area with a pH value of 0 to 8.2 and above a pH value of 13 remains colourless, while the area between 8.2 and 13 turns pink-violet. Figure 7 shows an example of a fresh fracture surface after spraying on the phenolphthalein solution. The depth of damage was determined at each frag- Figure 6. Concrete specimen with high packing den- sity of the coarse grain. Figure 7. Position of the microscopic pictures on the fracture surface of an acid-stored test specimen after spraying on the phenolphthalein solution. ment at four equally spaced measuring points, each with 4 individual measurements (see Figure 7). The average damage depth was then calculated from two different fragments (thus 32 individual values). Fig- ure 8 shows one of those microscopic images of the acid-exposed concrete edge zone of a test specimen after 91 days of acid storage. To establish a refer- ence line for the measurement, the acid-resistant ag- gregates lying on the concrete surface were used as reference point. 3. Results 3.1. Binder optimization A statistical evaluation of the test results was used to determine the influencing parameters that signifi- cantly affect the respective target values. The spec- imens were tested for capillary porosity, compressive strength after 7 and 28 days and for their Ca(OH)2 content. As shown in Figure 9, not only the w/b ratio but also the fineness of the binder (specific surface) decreases the capillary porosity of the specimens. Regarding this and the results of the other target values, 15 binder compositions with a high potential 297 L. Kempf, R. Breitenbücher, Ch. Gerten, A. Ehrenberg Acta Polytechnica CTU Proceedings Figure 8. Microscopic image of a concrete specimen after 91 days of storage in sulfuric acid. Figure 9. Impact of w/b ratio and specific surface on the capillary porosity. of an increased acid resistance were chosen for acid- tests in order to evaluate the correlation between the binder granulometry and the acid resistance of the mortars. Figure 10 shows the location of the opti- mized binder compositions in the defined testing area. 3.2. Concrete tests After the acid resistance tests, mathematical models have been created which allow the target values to be forecast within the selected system boundaries. The models were subsequently verified by separate experi- ments that were not part of the original experimental design. These models provided the basis to create further optimized binder compositions for the tests with con- crete specimen. Given the variation of multiple pa- rameters of the test series, the evaluation is carried out by a statistical test assessment. Figure 11 shows the output of the analysis by means of a contour chart. The chart consists of the calculated estimated damage depth after 91 days of storage in sulfuric acid, regarding the w/b ratio, the reactivity of the GGBFS and the parameter "n", which represents the RRSB slope according to DIN 66145 [9] to embody the par- ticle size distribution (PSD) of the finest particles. The contour chart shows that all three parameters Figure 10. Optimized binder compositions for acid resistance tests. have a massive impact on the acid resistance. In order to design an acid-resistant concrete, high reactivity should already be taken into account when choosing the blast-furnace slag. Comparing the two diagrams, damage depth under 500 µm can only be achieved with a high (C+M)/S ratio of 1.45. The PSD in the finest particle range is also of great importance. The modification from a narrow PSD (n = 1.25) to a broad PSD (n = 0.95) is about as effective as adjusting the w/b ratio from 0.4 to 0.3. 4. Conclusion The main objective of the research project was to achieve the highest possible resistance of mortars or concretes against sulfuric acid attack in combina- tion with sufficient strength development and suitable workability by using (ultra-fine) granulated blast fur- nace slag in the binder. It has been shown that a high (ultra-fine) granulated blast furnace slag content and a broad PSD down to the finest particle size has a favourable effect both on the capillary/gel pore ratio and on the calcium hydroxide content in the hardened cement paste. 298 vol. 33/2022 Concrete Acid Resistance Optimization Figure 11. Influencing factors to the damage depth of the concrete specimen after 91 days of storage in sulfuric acid. The results presented serve as a basis for the de- sign of highly acid-resistant concretes. The optimized concrete compositions developed in this project were able to undercut the damage depths of concretes cur- rently regarded as extremely acid-resistant by more than 50%. It is shown that the choice of material and the preparation of the granulated blast furnace slag play a decisive role when it comes to the acid resistance of concrete. Acknowledgements The IGF-Project 18949 N / 2 of the Steel Institute VDEh research association was funded by the Federal Ministry for Economic Affairs and Energy on the basis of a deci- sion by the German Bundestag as part of the program to promote Industrial Collective Research (IGF). References [1] R. Grün.Chemische Widerstandsfähigkeit von Beton, Berlin, Springer, 1924. [2] J. Stark, B. Wicht. Dauerhaftigkeit von Beton von Beton vol 2, Berlin, Springer, 2013. [3] Verein Deutscher Zementwerke e. V. Zement-Taschenbuch, Düsseldorf: Verlag Bau+Technik, 2002. https://www.vdz-online.de/fil eadmin/wissensportal/publikationen/zementindus trie/Zement-Taschenbuch_2002.pdf. [4] J. Kiekbusch. Säureangriff auf zementgebundene Materialien. Berichte aus dem Bauwesen, 2007. [5] R. D. Hooton. Canadian use of ground granulated blast-furnace slag as a supplementary cementing material for enhanced performance of concrete. Canadian Journal of Civil Engineering 27(4):754-60, 2000. https://doi.org/10.1139/l00-014. [6] C. Rieck, R. Hüttl. Beton mit erhöhtem Säurewiderstand für Rohre und Schächte-Prüung, Entwicklung, Anwendung. BetonW Intern (BWI) 3:140-6, 2011. [7] J. Gerlach, L. Lohaus. Sachstandsbericht Verfahren zur Prüfung des Säurewiderstandes von Beton vol 620, Berlin: Beuth, 2016. [8] M. Jalal, A. R. Pouladkhan, A. A. Ramezanianpour, et al. Effects of silica nano powder and silica fume on rheology and strength of high strength self compacting concrete. Journal of American Science 8(4):270-7, 2012. [9] DIN 66145-4. Darstellung von Korn-(Teilchen-)größenverteilungen RRSB-Netz, 1976. 299 https://www.vdz-online.de/fileadmin/wissensportal/publikationen/zementindustrie/Zement-Taschenbuch_2002.pdf https://doi.org/10.1139/l00-014