Effects Of Dolomite Aggregate On The Deterioration Of Concrete Floor

Transcription

1 Effects Of Dolomite Aggregate On The Deterioration Of Concrete Floor JC Rocha CA da Luz & M Cheriaf Summary: There are some aggregates that, when in contact to alkali cations in cements, are highly reactive, such as the dolomite aggregate. This way, even though it presents mechanical strength and low porosity, a material can show deterioration which can cause serious damages to the product over time. This paper presents one case study accomplished in one concrete floor with screed mortar (high strength) in an industry located in state of Santa Catarina/Brazil. This case study shows the formation of gel unlimited swelling around aggregate particles caused by effects of alkali aggregate reaction, showing up two years after having been cast. For the case analysis, eight cylindrical samples were extracted presenting the following dimensions: 5mm in diameter and a medium height of 12 mm where 15 mm represented the substratum and 15mm the cement mortar). Besides, scratching and superficial extractions of the cement mortar samples were also accomplished in those visibly deteriorated areas of the floor. The investigation of the case was accomplished using thermal differential analysis techniques (DTA), Scanning Electron Microscopy (SEM) and X-ray diffraction analyses (XRD), where in all of the samples, preparation and operation of the technique were made with extreme severity. Based on the presented results using the SEM determination, it can be concluded that the micrograph of mortar of the analyzed samples of the floor, as well as the samples of the scratching and extraction, presented chemical characteristics morphologies and compositions of the alkali-aggregate reaction. The results of the DTA of the XRD, accomplished in the aggregates used in the composition of the cement mortar of the floor indicated the presence of quartz grains, in the gravel (cheap aggregate) and the white aggregates consisted of calcium dolomite carbonate. Based on the manifestations and the results, the reactive characteristic of the white aggregate and the alkali effects was very noticeable. In the unlimited swelling, the appearance of the whitish structure is due to the white aggregate disintegration, with strong indication of a reaction belonging to the alkali-carbonate group. The results of analyses showed gel and of the crystallized products, around the white aggregate, presented calcium-silica-alkalis reactions, where the detected alkalis were potassium and sodium. Besides, it was possible to identify, ettringite, portlandite, Mg(OH) 2 and carbonate of calcium, which in relation to the gel, the reactive characteristic of the white aggregate was also confirmed. Keywords. dolomite aggregate, concrete floor, deterioration, alkali-aggregate reaction 1 INTRODUCTION The alkalis usually end up being included to the material (concrete, mortar...) through the cement, but they can also be due to other kinds of sources such as the mixing water where the product of the alkali-aggregate reaction can be manifested even by expansions or pop-outs. So, it is possible for a material, even if presenting mechanical resistance and low porosity, to manifest deterioration which can cause serious damages to the product over time. According to VEIA et al. (1999), the reaction alkali-aggregate is done between some mineralogical components and the alkaline hydroxides dissolved in the pores of the product. Based on his studies, the reaction alkali-aggregate that occurs among certain calcareous and the alkaline solutions contained in the pores results in an expansive hygroscopic gel. In relation to the aggregate types involved in such expansive phenomenon, LEA et al. (197) say that they are restricted to certain fine-grained soils, clay and dolomite aggregates. 9DBMC-22 Paper 53 Page 1

2 This paper is about a study regarding the reaction among dolomite aggregates and alkalis in a floor of high resistance where the consequences of this resulted in expansions and formations of vesicles with a whitish pigmentation. It refers to a case study in an industry located in Santa Catarina/Brasil, two years after having been cast. The study was totally based on in situ observations and using microstructural techniques of investigation. The differential thermal analysis techniques (DTA), Scanning Electron Microscopy (SEM) and X-ray diffraction analysis (XRD) were studied. The samples used were cylindrical cores, scratching fragments and superficial extractions of the mortar in visible places of the deteriorated floor, where the sample collection, preparation and operation of the technique were made with extreme severity in all of them. 2 MATERIALS AND METHODS For the investigation of the problems presented by the industrial floor, superficial fragments of mortar (Fig. 1) and 8 test cores (Fig.2) with the use of diamond drill were extracted with the following dimensions: Diameter = 5 mm; Medium height = 12 mm, representing the coating mortar (of approximately 15 mm) and substratum of 15 mm. Figure 1: Superficial extractions of the mortar Figure 2: Cylindrical Test Cores It was necessary to visit the place where the manifestations occurred in the floor in order to verify the incidences and any fortuitous conditions of moisture, collecting material for complementation of the analysis. Having test cores and samples in hand, they were tested using the differential thermal analysis techniques (DTA), Scanning Electron Microscopy (SEM) and X-ray diffraction analysis (XRD). 2.1 Differential thermal analysis techniques (DTA) The analysis were made under the following test conditions: constant speed of 1 o C/min; natural atmosphere environment; calcined caulinate as inert reference material; alumina sample carrier; Testing mass of 6mg (particles <15µm), K type thermocouple and automatic registration of the data during the test in HP s dataloger. The test was accomplished in the Núcleo de Pesquisa em Construção of the Civil Engineering Course - UFSC, using the equipment of differencial thermal analysis, endowed with Carbolite oven and manufactured by INSA_Lyon. 9DBMC-22 Paper 53 Page 2

3 Initially, the samples submitted to this test were obtained by the test cores P1 and P8. During the preparation it was noticed that some of the whitish particles in the analyzed material were easily taken to pieces. Because of this, they were divided into smaller samples so that they could be analyzed separately constituting a representative sampling of the manifestation the whitish grains with little mortar adhered to its surface. For each sample two analysis were made. In Figures 3 to 5, below, the results of the thermal analysis accomplished in the samples are represented. Variação de temperatura (ºC) P8 P1-6 Temperatura (ºC) Figure 3: Thermogram of the mortars of the Test cores P1 and P8. Variação da temperatura (ºC) Branco Temperatura (ºC) Branco Figure 4: Thermogram of the white grains with mortar adhered to the surface. Variação da temperatura ( o C) Branco 2 Branco Temperatura ( o C) Figure 5: Thermogram of the white grains contained in the aggregates- without mortar in the surface 9DBMC-22 Paper 53 Page 3

4 Of the thermal analyses, based on the peaks, the following components were identified: temperature C: loss of water; temperature 21 C: calcium monosulfate (CaSO 4 ); temperature 4 C: magnesium hydroxide Mg(OH) 2 ; temperature 57 C: quartz (); temperature 8-92 C: calcium carbonate CaCO3; It is important to highlight that the white aggregates showed the predominant composition of calcium carbonate. 2.2 X-Ray Diffraction Analysis (XRD) The preparation of the samples followed the same procedure used in the differential thermal analysis - DTA, that is, the samples were taken to pieces until the desired grain size was obtained. (passing through a 15mm sieve). The analysis were accomplished in the laboratory de Difratometria of the Physics Department of UFSC, using the X-ray diffraction equipment of the brand Rigaku, models Mini Flex, with grafite monocromatizer and radiation Cu Ka. The obtained results of the analyzed samples are presented in diffractograms shown in Figures 6 to 8. TheTable 1 assists the reader in interpreting the diffractograms CaO 35 Intensidade (cps) D CaO D 1 5 E P P P P theta (graus) Figure 6: XRD pattern of the mortar sample of the Test Core P Intensidade (cps) E P CaO D Ca O C P D theta (graus) P P Figure 7: XRD pattern of the mortar sample of the Test Core P8. 9DBMC-22 Paper 53 Page 4

5 Intensidade (cps) E P CaO SC D 2theta (graus) C P D D D D C C Figure 8: XRD pattern of the white grains extracted from the Test cores. Table 1: Notation of the minerals Notation Mineral 2θ uartz 21,5 ; 26,75 ; 36,75 ; 39,6 ; 4,4 ; 42,6 ; 45,95 ; 5,25 ; 5,95 ; 55,. E Ettringite 19,2. D Dolomite 29,7 ; 39,65 ; 43,45 ; 49, e 49,75. C Calcite 3,4 ; 51,6 ; 55, P Portlandite 18,15 ; 34,15 ; 47,3 ; 5,25 e 5,9. ypsum 21,5 ; 31,5 ; 36,75 ; 42,5 e 56,75. SC calcium silicate 27, Scanning electron microscopy (SEM) The scanning electron microscopy (SEM) was used for observation and microstructural analysis of the solid phases (morphology and crystalline lens of the particles). With the same technique the qualitative chemical analysis was used through the microprobe EDX, with a dispersive energy of x-rays (EDX), allowing a located chemical analysis. For the microscopic analysis an scanning electronic microscope - MEV, models PHILIPS XL3, equipped with a micro probe was used. The microscope belongs to the Laboratory de Materiais of the Mechanical Engineering Department of UFSC. The analysis were accomplished in fracture surfaces obtained through the fragmentation of the mortar of the samples, in points of interest (white aggregates, mortar, pebble aggregate). The samples were prepared covering the surface with carbon coating, accomplished in the equipment BALTEC - EARLY 3 - Evaporad Carbon. Figures 9 to 24 present the micrographs and the spectra obtained through the analysis on the scanning electronic microscope. Figure 9: SEM image of the mortar sample of the Test core P1 (2X) 9DBMC-22 Paper 53 Page 5

6 Figure 1: EDX spectra of the highlighted area in Figure 9. In Figure 9 the morphology of the mortar extracted from the test core P1 is observed, with an increase of 2 times, highlighting the pebble aggregates (medium dimension of 3mm), composed essentially of silicon, according to the spectrum on Figure 9. The presence of this component is also identified in the difractograms of the mortars (Fig. 6 and Fig. 7) in a quartz form, as well as in the endothermic transformation to 57 C observed in the thermograms (Fig. 3 and Fig.4). Figure 11: SEM image of the sample mortar of the test core P1 (8X) Figure 12: EDX spectra of the highlighted area of Figure 11 Figure 11 presents the micrograph of the mortar of the test core P1, with an increase of 8 times, with its referring spectrum in Fig. 12. In the micrograph obtained in the mortar a massive composition it is observed with occurrence of deposits in the surface. The spectrum reveals the composition silicon-calcium-potassium - magnesium, with little occurrence of iron, sulfur and aluminum. The presence of sulfur can be attributed to the calcium monosulfate (plaster - CaSO4), which was evidenced not only in the endothermic reactions at 21 C, shown in the thermograms of Fig 3 and Fig. 4, but also in the difractograms (Fig.6, Fig. 7 and Fig. 8), with a standard peak located in the angles (2θ), presented in Table 1. On the other hand, the presence of magnesium was verified in the endothermic reaction at 4 C (Fig. 3, Fig. 4 and Fig. 5), attributed to the occurrence of dolomite found in the difractograms (Fig. 5, Fig. 6 and Fig.7). 9DBMC-22 Paper 53 Page 6

7 Figure 13: SEM image of the mortar sample of the test core P8 (2X) Figure 14: SEM image of the mortar sample of the test core P8 (32X) a Figure 15: EDX spectra of the highlighted area of Figure 14. Figures 14 and 15 present the micrograph of the mortar of the test core P8, with an increase of 2 to 32 times, respectively, representing the pebble aggregate inserted in the mortar of the test core. In the micrography of Fig. 14, the aggregate with deposits of crystallized products of the aggregate-alkali reaction in a flocculated structure on the surface is observed, and the spectrum obtained (Fig. 15) reveals silicon, calcium, sodium, potassium, and c magnesium components with an occurrence of aluminum and iron. b Figure 16: SEM image of the white grains with adhered mortar (4X) 9DBMC-22 Paper 53 Page 7

8 Figure 17: EDX spectra of the highlighted area of Figure 16 The micrograph of Fig. 16, with an increase of 4 times, presents the morphology of the white grain with adhered mortar, where the grain and the mortar are dispersed in a gel phase of massive aspect. The spectrum (Fig.17) reveals the silicon, potassium and calcium composition. The a, b and c zones of the analyzed sample, defined in the Fig. 16, had an increase of 16 times, allowing better identification of the gel formation, of which the micrographies are presented in Figures 18,2 and 21. Figure 18: SEM image of the white grains with adhered mortar (16X) zone a highlighted Figure 19: EDX spectra of the highlighted area of Figure 18. In the highlighted zone a of the micrograph of Fig.18, the development of the crystalline phase of morphology acicular is observed, composed by silica, calcium, potassium, sodium and magnesium. 9DBMC-22 Paper 53 Page 8

9 Figure 2: SEM image of the white grains with adhered mortar (16X),zone b highlighted. Figure 21: SEM image of the white grains with adhered mortar (16X), zone c highlighted. The highlighted zone b shows a gel phase containing particles in its surface. In the highlighted zone c (Fig. 21), the internal structure of the gel presents pores. It is still verified the presence of Portlandite, registered in the difractograms of the mortars. In the samples extracted in situ, obtained by the manifestations and observing the mortar adhered to the seal coating, the presence of quartz pebble is verified, indicated by the presence of silicon in the spectrum of Fig. 23. From the analysis of the manifestation morphology, the agglomeration of the product in the surface of the mortar is observed (Fig. 22). Figure 22: SEM image in the sample extracted in situ (3X) Figure 23: EDX spectra of the highlighted area of Figure 22 (quartz grain). 9DBMC-22 Paper 53 Page 9

10 Figure 24: SEM image of the mortar of the sample extracted in situ of Korodur (1X) 3 ANALYSIS OF THE RESULTS AND CONCLUSIONS The visit in situ, as well as the test cores analyzed showed that the signs of deterioration of the floor were brought by the manifestations observed, marked by the formation of vesicles with a whitish pigmentation and a gel exudation in the surface, indicating alkali-aggregate reaction, which was confirmed by the analysis accomplished in the representative samples. According to the results presented using the microscope, it is possible to conclude that the analyzed micrographies of the Korodur mortar of test cores of the floor, as well as for the scratching and extraction of the samples in situ, presented typical morphologies and chemical compositions of the reaction alkali-aggregate. The results of the X-Ray difraction thermal analysis, accomplished in the aggregates used in the mortar composition of the floor, indicated the presence of quartz grains in the pebbles and white aggregates constituting dolomite calcium carbonate. According to the manifestations and based on the results, the white aggregates has reagent characteristics to alkalis. In the formation of the vesicles, the appearance of a whitish structure is due to the composition of the white aggregate, with strong indication of a alkali-carbonate reaction. From a morphological point of view, the gel presented a massive structure in a fine texture with cracking. The crystallized products presented acicular forms, and like the massive gel, in the mortar/aggregate interface. Punctual chemical analysis of the gel and of the crystallized products, done by EDX, showed calcium-silicon-alkaline compositions, where the detected alkalis were potassium and sodium. Additionally, it was possible to identify, by the scanning electronic microscope (SEM), ettringite in isolated crystals. Well crystallized portlandite was also rarely recognized and identified in the difractograms. In the ones referred to the mortar, the presence of Mg(OH) 2 and calcium carbonate was verified, which related to the gel, confirm the white aggregate reativity. Therefore, in relation to the evidence of the verification of the phenomenon, what should be understood and considered is the alkali-aggregate reaction s risk in the selection of materials for the concrete or any other product that involves dolomite aggregates and alkalis in excessive amounts. This does not mean that the same aggregates cannot be used but that the limitations for use should be evaluated. Besides, it is important to mention that microstructural analysis is always fundamental to determine and to understand the harmful manifestations that usually appear in residential and industrial constructions. 4 REFERENCES 1. Kropp, J., Relations between transport characteristics and durability. Performance Criteria for concrete durability, pp Lea, F. M., 197, The chemistry of cement and concrete, Basic Books, reat Britain. 3. Veiga, F. N et al., 1997, Reação agregado: a utilização da técnica de microscopia eletrônica de varredura na identificação de seus produtos, Proc. Simpósio sobre reatividade álcali-agregado em estruturas de concreto, Curitiba, Brasil, November 1997, vol. 1, pp DBMC-22 Paper 53 Page 1