UDK 691:620.1 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 46(6)573(2012) INFLUENCE OF DELAYED ETTRINGITE FORMATION ON THE MECHANICAL PROPERTIES OF AERATED CONCRETE VPLIV ZAKASNELE TVORBE ETRINGITA NA MEHANSKE LASTNOSTI AERIRANIH BETONOV Samo Lubej1, Andrej Ivanic1, Rebeka Rudolf2, Ivan Anžel2 fakulteta za gradbeništvo, Smetanova 17, 2000 Maribor, Slovenia 2Fakulteta za strojništvo, Smetanova 17, 2000 Maribor, Slovenia samo.lubej@uni-mb.si Prejem rokopisa — received: 2012-03-23; sprejem za objavo - accepted for publication: 2012-06-11 Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on the mechanical properties of a hydrated concrete. Ettringite crystals can cause cracks and their widening due to the pressure on the crack walls caused by a positive volume difference in the reaction. Concrete may show improvements in strength at early stages, but a further growth of the cracks causes widening and spreading of these cracks through the concrete structure. In a hydrated concrete, crystals of ettringite can also cause a disintegration of the concrete. In this paper we investigate the potential to utilise a positive volume difference in DEF in order to improve the mechanical properties of a hydrated fine-grained concrete. Finely dispersed crystallization nuclei, achieved by adding an air-entraining agent (AEA) and a short vibration of specimens, is presented as the main prerequisite for such improvements. The control of the expansion and mechanical properties were performed on the concrete samples with and without AEA by inducing DEF. For a microstructure examination of the fine-grained concrete an optical microscope and a scanning electron microscope were used. We found that controlled DEF, which is guaranteed by adding AEA and with the formation of the uniformly dispersed air bubbles, which are the crystallization sites for ettringite crystals, improves the mechanical properties. The specimens with induced DEF were measured and found to have a 6 % increase in the compressive strength. Keywords: delayed ettringite formation (DEF), concrete, aerated concrete, microstructure, mechanical properties Zakasnela tvorba etringita (DEF) je kemična reakcija z dokazano škodljivim učinkom na mehanske lastnosti hidratiziranega betona. Kristali etringita lahko zaradi pozitivne prostorninske razlike med reaktanti in produkti povzročijo nastanek razpok, rast kristalov etringita na stenah razpok pa povzroči njihovo širjenje. Pri mladih betonih rast kristalov etringita poveča trdnost, v hidratiziranem betonu pa nastanek in napredovanje razpok, kar lahko povzroči tudi razpad betona. V tem članku predstavljamo idejo, kako bi lahko pozitivne volumenske spremembe, ki so posledica DEF v hidratiziranem, drobnozrnatem betonu izkoristili za izboljšanje mehanskih lastnosti. Pogoj za izboljšanje le-teh je zagotovljen z uporabo dodatka aeranta (AEA), s katerim v hidratiziranem betonu zagotovimo enakomerno razpršene zračne mehurčke, ki so nukleacijska jedra. Zgostitev betona smo izvedli z vibriranjem, ki lahko traja le kratek čas. V raziskavi smo prikazali način in postopek izdelave betona z enakomerno razpršenimi nukleacijskimi jedri iz zračnih mehurčkov. Kontrolo nabrekanja in mehanskih lastnosti smo opravili pri vzorcih betona z dodanim aerantom in brez njega z induciranjem zakasnele tvorbe etringita. Za raziskave mikrostrukture vzorcev drobnozrnatega betona smo uporabili optični in elektronski vrstični mikroskop. Ugotovili smo, da lahko z nadzorovano zakasnelo tvorbo etringita, ki je zagotovljena z dodanim aerantom in nastankom enakomerno razpršenih zračnih mehurčkov, ki so nukleacijska mesta za kristalizacijo etringita, izboljšamo mehanske lastnosti. Pri vzorcih z inducirano zakasnelo tvorbo etringita smo izmerili 6-odstotno večjo tlačno trnost. Ključne besede: zakasnela tvorba etringita (DEF), beton, aerirani beton, mikrostruktura, mehanske lastnosti 1 INTRODUCTION Delayed ettringite formation (DEF) in cementitious materials is considered as a harmful chemical reaction leading to a variety of damages.1-3 The volume of the formed DEF crystals in a hardened concrete is larger than the volume of reactants and the main results are the forces arising from the growing crystals acting upon the walls of the crack. As a consequence, DEF cracks continue to grow wider and spread through the concrete structure.4,5 A considerable recent research has led to a better understanding of the mechanisms of DEF.6 In general, it is acknowledged that DEF is a result of various factors and conditions including excessive temperatures of above 70 °C, the presence of sulphates, existing cracks, moist conditions and so on.7-10 Ekolu et al11 summarise various control measures that could be used for the prevention of DEF including the use of chemical additives. However, preventative measures and improvements with respect to durability require further attention. In practice, concrete and mortar mixes are normally based on Portland cement clinker, where the chemical process of hydration of the clinker minerals yields hydrates and hydroxides. However, because of the presence of gypsum, the chemical reaction between tricalcium aluminate (C3A), gypsum (CaSO4 ■ 2H2O) and water forms ettringite crystals (3CaO Al2O3 3CaOSO4 ■ 32H2O). The volume difference in this reaction is positive and ettringite crystals grow fast, growing quickly on the unhydrated cement particles, which can slow down the hydration12. The presence of ettringite in a liquid cementitious system is unproblematic but its formation or re-formation in an already hydrated concrete can lead to extensive damages.6 Due to the resulting volume difference, particularly in the presence of sulphates, an expansive force within the concrete can cause its disintegration (sulphate corrosion). It is well known that cements with a low C3A content are more resilient to sulphate corrosion, but that also depends on the form of C3A.13 For instance, crystalline C3A is more reactive than its amorphous version. A positive volume difference as a result of an early ettringite formation (EEF) in cementitious materials rich with C4A3S (calcium aluminate sulphate, expansive cement) can be used to compensate for the shrinkage during the drying.14 In this case, C4A3S hydrates within a few hours or days causing a uniform distribution of ettringite and a homogeneous expansion of the hardened concrete at early stages. However, it is less known that an ettringite formation and expansion in hardened cementitious materials can also be used for their controlled strengthening. The formation of a new phase characterized by a substantial volume expansion for the purpose of strength improvement is well known in the mainstream material science. Such strengthening is based upon the creation of an internal compressive stress on the contact between the existing matrix and the new-phase particles and depends on their shape, size and their overall dispersion. It is desirable that the newly formed particles are small, spherical and located sufficiently apart from each other to avoid overlapping the stress fields. In the case of metallic materials we know of the dispersion strengthening of the cooper matrix with the ZrO2 particles1516. A strength improvement of the A^O3 ceramics with finely dispersed ZrO2 particles is one example of this kind of internal stresses in an Al2O3 matrix created by applying an external-force-trigger polymorphic transformation of a tetragonal crystalline structure of ZrO2 into a monoclinic crystalline structure. An increased volume creates substantial compressive stresses in a matrix surrounding the transformed particles leading to a several-fold increase in the compressive strength as well as in the resistance to the spreading of the cracks. The studies that apply to this type of mechanism for a strength improvement of concrete are rare. 2 EXPERIMENTAL WORK 2.1 Description of the experiment Experimental work included a selection and research of concrete components, an investigation of a cement paste and testing of the samples of a fine-grained concrete. Experimental work was determined with an experimental design research with the aim of confirming that DEF in a hydrated concrete improved the strength properties and also that an addition of an air-entraining agent (AEA) creates, by volume, uniformly distributed nucleation sites for the formation of ettringite crystals and a target microstructure. 2.2 Materials 2.2.1 Selection of fine-grained concrete components For the sample preparation we used the cement known as CEM 142.5 R. In order to ensure the formation of the AFm phase, we used fly ash, for which we defined the components and the specific surface area. The role of fly ash was twofold - chemical and physical; its chemical role was possible because fly ash is actively involved in the pozzolanic reaction, while its physical impact was in place because it works as a nucleation site and as filler. For the aggregate we used chemically neutral standard sand with its properties being in line with EN ISO 196-1 and ISO 697. The purpose of adding a chemically neutral aggregate was to prevent an alkali-silica reaction (ASR), which would result from the reactive minerals in the aggregate. The most important precaution was the use of the petrographic components of the aggregates, which will not react with alkali.17 The used additive - an air-entraining agent - was the anion-based abietic acid. The components were mixed with clean drinking water. 2.2.2 Characterization of the samples and the investigation procedure Concrete-mixture components were determined on the basis of the standard consistency of the cement paste. We prepared two different series of the samples. The compositions of the mixtures are shown in Table 1. The samples were mixed with a laboratory mixer to meet the requirements of EN 196-1. Fresh concrete was built in the standard moulds with the dimensions of 4/4/16 cm using a vibrating table with a vibration time of 5 s, a frequency of 50 Hz and amplitude of 0.75 mm. The samples were monitored for 28 d in a climatic chamber at a temperature of (20 ± 2) °C and a relative humidity of (98 ± 2) %. In all the samples we performed the measurements of density, compressive and flexural strengths in the intervals of (7, 14, 28 and 56) d. On the set of the samples that contained fly ash, after 28 d of treatment in the climatic chamber, we performed a Duggan test. This test is used to achieve an accelerated delayed ettringite formation. The test consists of soaking the samples in demineralised water at (20 ± 2) °C and drying them at a temperature of (81 ± 2) °C. The test cycles are shown in Figure 1. After the Duggan18 test the prisms were put through a laboratory conditioning in a desiccator for 48 h in the time between the above phases, Table 1 : Compositions of fine-grained concrete mixtures Tabela 1: Prikaz komponent drobnozrnatega betona Specimen Aggregate (g) Cement (g) Fly ash (g) Water (g) AEA (g) AI 1350 450 218.2 6.8 Figure 1: Cycles of Duggan test Slika 1: Prikaz ciklov Dugganovega testa and were then once again immersed in demineralised water for 24 h in order to fill the capillaries and voids with water. In these samples we controlled the delayed ettringite formation by measuring the expansion caused by ettringite-crystal growth. At the moment when the expansion was no longer observed, in micrometers, we stopped the measurements. We considered that the delayed ettringite formation had been completed. The measurements of density and strength were then performed on the samples. These measurements were repeated on the samples where the Duggan test was not carried out, so we could obtain the comparisons of these values. 3 RESULTS AND DISCUSSION 3.1 Results of the investigations of concrete components Cement, standard sand and an air-entraining agent were used as the benchmark for the standard-quality results. Therefore, declared properties are not included. Fly ash contains silicates, carbonates and phosphates of calcium, magnesium, iron and aluminum and other elements. Illite/kaolinite clays, apart from illite and kaolinite minerals, also contain a-quartz, Fe2O3 and CaCO3.19 The results of the laboratory analysis for fly ash are shown in Table 2. 3.2 Characterization of the macrostructure With an optical microscope Olympus SZX we observed significant break areas of the hydrated concrete. The control of the dispersion bubbles of the air-entraining agent showed that they were mostly distributed uniformly. The bubbles had a diameter between 25 pm and 50 pm. The average measured distance between the bubbles was 0.1 mm to 1.2 mm. 3.3 SEM characterization With the electron scanning microscopes JEOL JSM 5610 and QUANTA 200 3D, we observed the characteristic fields of the air-entraining bubbles. In these bubbles we discovered ettringite crystals. This proved our assumption that the bubbles of the air-entraining agents are nucleation sites. The morphologies of individual ettringite crystals are very similar. Ettringite crystals grow in bunches, and they are all needle-like and thin. An examination of a large number of bubbles showed that sample A has fewer ettringite crystals, as shown in Figure 2. The samples that were labelled as BI and BI-DT showed many more bunches of ettringite crystals. These crystals grow mainly on the porous sites in the air bubbles (Figure 3). On the samples that were labelled as BI-DT, we noticed more microcracks in the bubbles than on the samples labelled as BI. In these microcracks we also observed the presence of ettrigite crystals that were very thin and needle-like, suggesting their rapid growth (Figure 4). A comparison of the microstructures of the specimens from the fine-grained concrete mixes AI, BI and BI-DT, presented in Figures 2, 3 and 4, clearly demonstrates that, as expected, similar AEA-induced nuclei exist in AI, BI and BI-DT specimens. Although ettringite crystals can be found in the concretes produced by using pure Portland cement, no visible ettringite crystals were detected in any of the numerous prisms from the AI mix. However, ettringite crystals did appear 20kU X9,000 ~ 2 Mm NTF OMM LJ Figure 2: Micro fractography of sample AI with ettringite crystals; SEM, SEI Slika 2: Mikrofraktografija vzorca z oznako AI s kristali etringita; SEM, SEI Table 2: Results of the laboratory analysis for fly ash Tabela 2: Rezultati laboratorijskih analiz elektrofiltrskega pepela v masnih deležih, w/% Component part Content, w/% Loss on ignition 0.41 Insoluble residue 16.67 SiO2 impure 13.08 SiO2 pure 47.62 SiO2 soluble 0.64 SiO2 total 48.26 SiO2 active 35.18 Figure 3: Micro fractography of sample BI with ettringite crystals; SEM, SEI Slika 3: Mikrofraktografija vzorca z oznako BI s kristali etringita; SEM, SEI in the specimens of all the other fine-grained concrete mixes. The microstructures of the specimens from the fine-grained AI concrete mix show very few ettringite crystals in the AEA nuclei themselves, but they were detected in the microcracks (Figure 2). In line with the findings established by Myneni et al.20 these crystals have a thin, needle-shaped morphology and the length of approximately 2 pm, revealing a rapid growth. The fly ash in the fine-grained BI concrete mix may well be a source of soluble calcium required for an ettringite formation, as reported by Solem and McCarthy21, Zhang and Reardon22 or Chrysochoou and Dermatas,23 because its crystals were found in large quantities in the microcracks and within the AEA-induced nuclei. Figures 2 and 3 show that ettringite crystals have a thin, needle-shaped morphology, while those found in the microcracks are only approximately 2 pm long as opposed to the 10-pm-long crystals found in the nuclei (Figures 3 and 4). The microcrack that appeared on the surface of a nucleus in Figure 4 can be associated with shrinkage of the matrix during the hydration24. The comparison between various different BI specimens shows that ettringite crystals start growing wherever there is enough space for the growth before further damaging the concrete, which enables further growth. The AEA-induced nuclei may, therefore, act as relief reservoirs enabling the growth of the substances like ettringite crystals in a hardened concrete with minimum or no damaging effects. Hime25 even recommends air entrainment as a way to prevent DEF and reports on only a single incident where an air-entrained concrete suffered from DEF. 3.4 TGA analysis A simultaneous thermal analysis (STA) and a thermal gravimetric analysis (TG) were performed on the cement-paste samples of concretes A and BI in a static air atmosphere. Measurements were performed with a device Netzsch STA 449C Jupiter. We measured the change in the mass of the samples at 25 °C. Each measurement lasted 60 h. The maximum change in the mass, which was 19.88 %, was found in the cement-paste sample of concrete AI after a period of 446.8 min (Figure 5). The change in the mass of the cement-paste sample of concrete BI was lower and amounted to 19.66 % after a period of 438.2 min (Figure 6). The TG measurements have shown that, by modifying the formation of a concrete sample, the changes in Figure 5: Results of a TG analysis for sample AI Slika 5: Rezultati TG-analize za vzorec AI ESkVJ X7>500 2Mm m ^ m MTF OMM LJ Figure 4: Micro fractography of sample BI-DT with ettringite crystals; SEM, SEI Slika 4: Mikrofraktografija vzorca z oznako BI-DT s kristali etringita; SEM, SEI Figure 6: Results of a TG analysis for sample BI Slika 6: Rezultati TG-analize za vzorec BI the fly ash did not transgress the setting time of the concrete, which must be longer than 360 min and shorter than 540 min. 3.5 XRM analysis X-ray mapping (XRM) analyses were performed with the electron scanning microscope JEOL JSM 5610 on the samples that are the fracture sites of the hydrated fine-grained concrete labelled as BI-DT. We analysed a particular field with the crystals of ettringite as shown in Figure 7. Based on the results, it was suggested that, in the areas of air bubbles in the cement matrix, the sulphate attack had been completed. These results confirmed that the process of delayed ettringite formation had been completed too. The chemical elements - sulfur and aluminum, which are characterized by their chemical reactions, are present only in the sites with ettringite crystals. 3.6 pH measurement The fine-grained hydrated concrete was sampled by breaking off a piece. This piece of concrete was crushed and sifted through a sieve with an aperture diameter of 0.02 mm. 50 g of the sample was mixed with a laboratory stirrer with 10 ml of distilled water. The solution was filtered, before being tested, through a filter paper N°40. The measurement procedure was taken from the literature.26 The measurements were performed with a pH meter Mettler Toledo S20 on three samples of each type of concrete in the intervals of (1, 2, 3 and 4) min. For each sample we performed three measurements and calculated the mean value. The results of the mean values are presented in Table 3. Based on the measurements, we found that the pH of the hydrated concrete with fly ash was reduced due to the Duggan test, although this value did not change significantly. It is known that the pH of a concrete with fly ash is lower than in the cases of the concretes without it. This is due to the binding of Ca(OH)2 with the reactive silicates contained in fly ash.27 The results confirmed that, with an addition of fly ash, the concrete's pH was decreased below the critical value of 9. This means that the corrosion resistance of concrete is guaranteed. Table 3: Measurements results for the pH values Tabela 3: Rezultati meritev pH-vrednosti Sample pH (mean value) AI 12.33 BI 11.88 BI-DT 11.86 3.7 Expansion measurement After a required 28-day curing period 6 prisms of the BI mix were exposed to a Duggan test in order to achieve the accelerated ettringite formation. The prisms were then placed into a standard apparatus for determining the length change of the hardened cement paste, mortar and concrete, constructed according to ASTM C490-86. During these measurements the apparatus itself was placed in a climatic chamber with a constant temperature of (20 ± 2) °C and a relative humidity of (98 ± 2) % (Figure 8). Ettringite formation was then monitored by measuring the length change (expansion) with a Mahr's MarCator 1080/12.5/0.005 mm digital micrometer. The results were recorded with an analogue/digital converter connected to a workstation. The expansion development was regularly measured in 15-minute intervals with a measurement accuracy of 0.005 mm, although the intervals could well be longer considering the slow pace of DEF. Figure 9 shows the change in the length for six fine-grained concrete prisms from the BI mixes that were exposed to the Duggan test and the final 24-hour immersion in demineralised water. The change in the Figure 7: XRM analysis of an ettringite crystal in an air bubble of AEA: a) the arrow on the micro fractograpy SEM, SEI, shows the analyzed crystal, b) the chemical elements S and Al are present only in ettringite crystal Slika 7: XRM-analiza kristalov etringita v zračnem mehurčku aeranta: a) puščica na mikrofraktografiji SEM, SEI kaže analizirani kristal, b) kemijska elementa S in Al sta prisotna samo na kristalu etringita Figure 8: Apparatus for measuring an expansion of hardened concrete according to ASTM C490-86, placed in a climatic chamber Slika 8: Aparat za meritve nabrekanja otrdelega betona, izdelan skladno z ASTM C490-86, v klimatski komori ¥ iMffih 1 OBIS jj OfJJ-tt--1 0PB5.--II poo ■ -OP0Š- -iisr^e iiÎSf --ireniHAerageB/Dtl s Jit' m X ? i v 0 7 14 2] 28 35--42 43 56 63 70 TI 84 Time ['I.*/] Figure 9: Results of the expansion measurements for the samples labelled as B-DT after the Duggan test Slika 9: Rezultati meritve nabrekanja za vzorce z oznako B-DT po Dugganovem testu Table 4: Densities and mechanical properties of the hardened finegrained concrete (mix AI) Tabela 4: Vrednosti gostote in mehanskih lastnosti za otrdeli drobno-zrnati beton (mešanica AI) Time interval t/d p/(kg/m3) /m/MPa /c/MPa 7 1806 2.5 18.0 14 1804 4.0 14.8 28 1883 5.2 17.3 56 1885 5.3 19.5 121 1887 5.4 20.5 length of the prisms was measured daily and stopped after 83 d when the measurements did not show any further expansions. The result of the measurements of the length change (expansion) of prismatic samples was evaluated as the average value which is 0.0125 %. All the samples were visually inspected and no cracks were found. 3.8 Mechanical properties The density of the hardened fine-grained concrete (o), its compressive (f) and flexural strengths (fm) were measured on 6 additional prisms for each of the three mixes after the standard periods of (7, 14 and 28) d, and, additionally, after 56 d and 121 d when compressive and flexural strengths should reach a plateau. The mechanical properties of the fine-grained concrete were examined with a material testing machine Zwick Roell and a method according to EN 196-1. Table 5: Densities and mechanical properties of the hardened fine-grained concrete (mix BI) Tabela 5: Vrednosti gostote in mehanskih lastnosti za otrdeli drobnozrnati beton (mešanica BI) Time interval t/d p/(kg/m3) /m/MPa /c/MPa 7 1801 2.7 11.6 14 1807 4.1 14.3 28 1803 5.2 17.7 56 1817 5.3 18.8 121 1818 5.6 21.0 Table 6: Density and mechanical properties of the hardened mortar prisms (BI-DT: mix BI with Duggan test) Tabela 6: Vrednosti gostote in mehanskih lastnosti za otrdeli drobno-zrnati beton (BI-DT: mešanica BI z Dugganovim testom) Time interval t/d p/(kg/m3) /m/MPa /c/MPa 121 1810 6.0 22.5 Tables 4, 5 and 6 show the measured densities (o), flexural (fm) and compressive strengths (fc) of the hardened fine-grained concrete prisms for the mixes AI, BI and BI-DT. The compressive strength of the BI prisms, after 121 d, is by 6.6 % lower than that of the BI-DT prisms. A comparison of the results showed that the concrete with fly ash has a slightly lower initial strength and an increased final strength.28 4 CONCLUSIONS Controlling DEF by using AEA as a nucleation agent results in a slight increase of the compressive strength of the fine-grained concrete. Small and thin crystals of ettringite, resulting from a series of chemical reactions that take place in the hydrated concrete, caused a swelling of the concrete. The local stress concentration at the nucleation sites, which are the air bubbles of an air-entraining agent where ettringite crystals grew, did not cause an extension of the microcracks that could lower the compressive strength of the concrete. The ettringite-crystal growth in the porous parts of the walls of the air bubbles caused a change in the microstructure of the concrete. This change represents a transformation of the existing porous microstructure in line with the tiny crystals condensed in the microstructure. The result of these changes in the microstructure of the nucleation sites is a reinforced cementitious matrix. The strength improvement is a result of hardening of the cementitious matrix, causing an increase in the compressive strength of the concrete. 5 REFERENCES 1 S. Diamond, Delayed ettringite formation - Processes and problems, Cement and Concrete Composites, 18 (1996) 3, 205-215 2 M. D. A. Thomas, Delayed ettringite formation: recent developments and future directions, in: S. Mindess, J. Skalny (Eds.), Materials Science of Concrete VI, American Ceramics Society, Westerville, OH 2001, 435-482 3R. Barbarulo, H. Peycelon, S. Prene, J. Marchand, Delayed ettringite formation symptoms on mortars induced by high temperature due to cement heat of hydration or late thermal cycle, Cement and Concrete Research, 35 (2005) 1, 125-131 4S. Sahu, N. Thaulow, Delayed ettringite formation in Swedish concrete railroad ties, Cement and Concrete Research, 34 (2004) 9, 1675-1681 5M. Thomas, K. Folliard, T. Drimalas, T. Ramlochan, Diagnosing delayed ettringite formation in concrete structures, Cement and Concrete Research, 38 (2008), 841-847 6 M. Collepardi, A state-of-the-art review on delayed ettringite attack on concrete, Cement and Concrete Composites, 25 (2003) 4-5, 401-407 7H. F. W. Taylor, Cement Chemistry, Academic Press, London 1990 8 C. D. Lawrence, Mortar expansions due to delayed ettringite formation. Effects of curing period and temperature, Cement and Concrete Research, 25 (1995) 4, 903-914 9C. D. Lawrence, Delayed ettringite formation: an issue?, Materials Science of Concrete, 4 (1995), 113-154 10 S. O. Ekolu, M. D. A. Thomas, R. D. Hooton, Dual effectiveness of lithium salt in controlling both delayed ettringite formation and ASR in concretes, Cement and Concrete Research, 37 (2007) 6, 942-947 11 S. O. Ekolu, M. D. A. Thomas, R. D. Hooton, Implications of pre-formed microcracking in relation to the theories of DEF mechanism, Cement and Concrete Research, 37 (2007) 2, 161-165 12 S. Swaddiwudhipong, D. Chen, M. H. Zhang, Simulation of the exothermic hydration process of Portland cement, Advances in Cement Research, 14 (2002) 2, 61-69 13 B. Mather, Field and Laboratory Studies of the Sulphate Resistance of Concrete, Performance of Concrete-Resistance of Concrete to Sulphate and Other Environmental Conditions, Thorvaldson Symposium Proceedings, University of Toronto Press, Toronto, 1968, 66-76 14 M. Collepardi, Ettringite formation and sulfate attack on concrete, Proceedings of the 5th CANMET/ACI International Conference on Recent Advances in Concrete Technology, Singapore, 2001, 27-39 151. Anžel, L. Kosec, A. Križman, Disperzijsko utrjanje hitro strjene zlitine Cu - Zr, Kovine, zlitine, tehnologije, 30 (1996) 3, 225-229 161. Anžel, A. C. Kneissl, A. Križman, R. Rudolf, Dispersion strengthening of copper by internal oxidation of rapidly solidified Cu-RE alloys. Part 1, Z. Metallkd., 94 (2003) 2, 127-133 17 A. Mladenovič, N. Vižintin, S. Drnovšek, Alkalno-reaktivne petro-grafske komponente v agregatu za beton iz porečij Drave in Mure, Mater. Tehnol., 34 (2000) 1-2, 103-105 18 E. Grabowski, B. Czarnecki, J. E. Gillott, C. R. Duggan, J. F. Scott, Rapid test of concrete expansivity due to internal sulfate attack, ACI Mater. J., 89 (1992) 5, 469-480 19 M. Krgovic, M. Kneževic, M. Ivanovic, I. Boškovic, M. Vukčevic, R. Zeljak, B. Zlatičanin, S. Durkovic, Lastnosti sintranega produkta iz elektrofiltrskega pepela, Mater. Tehnol., 43 (2009) 6, 327-331 20 S. C. B. Myneni, S. J. Traina, T. J. Logan, G. A. Waychunas, Oxy-anion behavior in alkaline environments: Sorption and desorption of arsenate in ettringite, Environ. Sci. Tech., 31 (1997), 1761-1768 21 J. K. Solem, G. J. McCarthy, Hydration reactions and ettringite formation in selected cementitious coal conversion byproducts, Materials Research Society Symposium Proceedings, 71 (1992) 9, 245 22 M. Zhang, E. J. Reardon, Removal of B, Cr, Mo, and Se from wastewater by incorporation into hydrocalumite and ettringite, Environmental Science & Technology, 37 (2003) 15, 2947-2952 23 M. Chrysochoou, D. Dermatas, Evaluation of ettringite and hydro-calumite formation for heavy metal immobilization: Literature review and experimental study, Journal of Hazardous Materials, 136 (2006) 1, 20-33 24 H. Stang, Significance of shrinkage - induced clamping pressure in fiber matrix bonding in cementitious materials, Advanced Cement Based Materials, 4 (1996) 3, 106-115 25 W. G. Hime, Delayed ettringite formation - A concern for precast concrete?, PCI Journal, (1996), 26-30 26 J. Grubb, H. Limaye, A. Kakade, Testing pH of Concrete, Concrete International - ACI publication, 2007 27 N. Koukouzas, I. Papayianni, E. Tsikardani, D. Papanikolaou, C. Ketikidis, Greek fly ash a cement replacement in the production of paving blocks, World of Coal Ash, Covington, US 2007 28 A. Zajc, Kemijski in mineralni dodatki v tehnologiji betona, 14. slovenski kolokvij o betonih; Posebne lastnosti betonov z dodatki, Zbornik gradiv in referatov, Ljubljana, 2007