ALKALINE ACTIVATION OF FLY ASHES. MANUFACTURE OF CONCRETES NOT CONTAINING PORTLAND CEMENT

Transcription

1 ALKALINE ACTIVATION OF FLY ASHES. MANUFACTURE OF CONCRETES NOT CONTAINING PORTLAND CEMENT A. Palomo (1) and A. Fernandez-Jiménez (1) (1) Institute Eduardo Torroja (CSIC) Madrid. Spain Abstract ID Number: 201 Author contacts A. Fernández-Jiménez Authors Fax Postal Address A. Palomo +34 (1) A. Fernández-Jiménez +34 (1) Contact person for the paper: A. Fernandez Jiménez Presenter of the paper during the Conference: A. Fernández Jiménez Total number of pages of the paper (this one excluded): 8

2 ALKALINE ACTIVATION OF FLY ASHES. MANUFACTURE OF CONCRETE NOT CONTAINING PORTLAND CEMENT A.Fernández-Jiménez (1) and A. Palomo (1) (1) Institute Eduardo Torroja (CSIC) Madrid. Spain Keywords: alkaline activation, fly ash, concrete, sleepers Abstract The alkaline activation of industrial sub products (specially those residues coming from the mining activity and from the energy sector) is a chemical process that allows obtaining new cement materials. It is spreading a lot among the groups of investigation around the world, since it offers the possibility of making more inexpensive conglomerates, which are more ecological and have properties that in many cases allow to overcome and to improve the well known advantages of portland cement concrete. In this document there is a rough description about the mechanisms through which the alkaline activation of the fly ashes passes. Nonetheless, it is given a special importance to the technological applications of the material due to the fact that some of its properties improve considerably when it is compared with Portland cement concrete. Therefore, from a technological point of view, it is necessary to emphasize that the mortars and concretes elaborated with activated fly ash, without Portland cement, develop high mechanical strength in short periods of time. Also, they seem to have a very low level of shrinkage and an excellent capacity for adherence to the aggregate and to the reinforcement steel. In the present investigation there is a specific case of a possible industrial application of these new concrete in the sector of precast industry. 1. INTRODUCTION Power stations, using coal like fuel are worldwide energy sources and consequently high quantities of fly ashes are nowadays generated. Simultaneously only a small part of these ashes is used (20-30%). The alkali activation of fly ashes (AAFA) is a singular procedure by which the grey powder proceeding from the coal combustion (FA) is mixed with certain alkaline activators (alkaline dissolutions), and the resultant paste is cured under mild temperature producing hardened materials (1-9). In previous studies (10,11) we have found that the main reaction product of such systems is a short-range order amorphous aluminosilicate gel: a 3- dimensional structure where the Si occurs in a variety of environments, with a predominance of Q 4 (3Al) and Q 4 (2Al). Additionally small amounts of certain zeolites such as Hydroxysodalite or Herschelite are often detected in these systems (2, 10-12). This material can consequently be considered to be a zeolite precursor. 1

3 The mechanisms of activation of fly ash can be divided in two main stages: Dissolution: Initially the vitreous component of the fly ash (aluminosilicate glass) in contact with the alkali solution is dissolved, forming a series of complex ionic species; Polymerisation: These small molecules present in the dissolution can join each other and form large molecules that precipitate in the form of gel, in which some degree of short range structural order can be identified (10). The composition and structure of this alkali aluminosicilate gel depends essentially on the size, structure and concentration of the ionic species present in the medium, as well as on the synthesis temperature on the curing time, and the ph of the mixture (10-12). The hypothetical evolution of this gel would be to form some sort of zeolite crystal. However, due to the very low alkaline solution / fly ash ratio used in the synthesis of this type of materials (or prevailing under experimental conditions), this evolution is extremely slow. The studies done up to today concerning the manufacturing of mortars and/or concretes of activated fly ash following the alkaline process (without Portland cement) turned out into very promising results. The above mentioned studies have demonstrated for example that the properties of the concrete of alkali activated fly ash are influenced, like those of the conventional concrete, by a set of factors related to the dosing of the mixture and to the conditions of the curing process (13-15). Nevertheless, these new concrete can manage to develop very high mechanical strength within a few hours. These strengths continue to grow more slowly with time. Also, the mentioned studies have manifested the high potential of these materials that could be used in the near future in the construction industry, especially in the precast industry. This is the reason why the goal of this work is focused on the establishment of some engineering properties for this type of materials (mechanical strength development, matrix-steel adherence and shrinkage). Finally, there is a specific case of an industrial application of these new concrete for the manufacturing of pre-stressed mono-block sleepers on railroads. 2. EXPERIMENTAL In this investigation a Spanish type F fly ash was used. The chemical composition and specific surface of the ash are presented in Table 1. Table 1. Chemical composition and specific surface of fly ash 1 L.o.I 2 Specific IR SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 K 2 O Na 2 O Surface FA (%) (m 2 /Kg) 1 L.o.I. = Loss on ignition; 2 IR. = Insoluble Residue; 2.1 Elaboration of test specimens For the production of the alkaline concretes of fly ash (without Portland Cement) it was used an alkaline dissolution containing NaOH + sodium silicate solution (Na 2 O=32 % SiO 2 =5 %, and H 2 O = 63 %). The concretes were prepared using the following proportions: siliceous aggregate (6-12 mm) / washing sand (0-5 mm) = 1.26; aggregate+sand / FA =4/1; solution / FA = All the concretes were made which a constant ash content = 465Kg/m 3. 2

4 Fly ash and aggregate were mixed in a pan mixer for 3 minutes; then the liquid component of the material was added to the solid components and mixed for other extra 5 minutes. The mixture obtained was cast in different type of mould (as function of the test to be carried out), in three layers. Each layer was vibrated for seconds with a vibrating needle. Immediately after casting, the samples were kept in an oven and cured for 20h at 85ºC. At the end of the thermal curing period, the specimens were demoulded and left at room temperature (in the laboratory) until testing. In the case of the production of mortars (also without Portland cement) it was used a ratio of "Sand/FA = 2/1 and dissolution alkaline/fa =0.4. Siliceous standardised sand with 99.9 quartz content was always used. The curing process conditions were, as with the concretes, 20h to 85ºC and 99 % of r. h. After the thermal curing process, the materials were kept at the laboratory at 21ºC and 50 % of relative humidity. 3. RESULTS The mortars and concretes, which were prepared following the procedure indicated in the previous paragraph, were studied in order to know their mechanical properties, adherents and shrinkage. Finally, an industrial test was performed at a railroad sleeper s factory. 3.1 Mechanical strength In Fig. 1 (a) appear the values of flexural strength on prismatic prisms of 15x10x70 cm, and in Fig. 1 (b) the values of compressive strength on cubic samples of 15x15x15 cm. developed by the activated ash concrete. The obtained results indicate that this type of concrete reaches very high mechanical strength (both flexural and compressive) after a few hours of its preparation; thus, after 20 hours we obtain values of around 6MPa to flexion and 50Mpa to compression (values which are very higher than those obtained with a conventional concrete). Another remarkable fact is that the mechanical strengths continue to increase progressively, after the thermal curing, although at a slower rate. Flexural Strength (MPa) (a) h. 7d. 28d. 90d. Time Compressive strength (MPa) (b) h. 8h. 12h. 16h. 20h. 2d. 7d. 28d. 90d. Time Fig.1: Mechanical strength in alkaline concrete of fly ash (a) flexural strength (b) compressive strength. 3.2 Adherence steel-matrix: With the purpose studying the adherence between concrete and steel, some "Pull-out tests were done, following the regulated procedures of RILEM/CEB/FIP. The adherence is the phenomenon that describes the tension transference between steel and concrete. This characteristic makes it possible to combine the good

5 behaviour of the concrete when submitted to compressive loads and the high strength of the steel to traction in the structures of reinforced concrete. In this "Pull-out test, which was done for this investigation, the adherent length of the bar (which depends on its the diameter, and must fulfill the requirement of l > 5Φ), was placed in the center of the cubic samples of 20x20x20 cm. In the test 16mm diameter bars were used. Thus, the zone of longitudinal adherence was 16cm. To avoid the adherence in the unwanted parts of the iron bar, plastic hoses at both ends were placed. (See Fig. 2. (A)). The B-500-SD textured bars had a total length of 70 cm. The purpose was to measure the displacement of the bar in the opposite end to which the load is applied, passive end. The bar, in the active end is not adhered to the concrete. The load is applied on the longer end (see Fig. 2 (b)) with a hydraulic jack of 156 KN of capacity, at 72 N/seg of speed, up to maximum load. During the test, the relative displacement of the bar is registered with respect to the concrete in the opposite side from where the load is applied, by means of three displacement sensors. The cube lays over a rubber surface of 5 mm of thickness and at the same time, this surface lays over another surface made out of steel of 10 mm. (a) φ=16mm (b) 70 cm (c) (d) Fig. 2. (a) Bar of 16 mm.; (b)"pull-out test (c) and (d) samples after being submitted to the above mentioned test The tension of local adherence (τ) in Mpa is calculated by means of the equation [1]. Where Q is the applied load (N), Φ the nominal diameter of the bar (mm), and l b the length adherent (mm). Q Q = = A πφ τ [1] I b The results obtained on having applied Pull-out " tests on three samples of concrete of activated fly ash appear on Table 2. These results show that the failure of the material is produced by "Splitting to a maximum tension about 12 N/mm 2, highly overcoming the value of 9.70 N/mm 2 minimum demanded by the Spanish procedure (EHE) 4

6 Table 2. Results of " Pull-Out " test applied on bars of iron absorbed in concrete of activated ash following the alkaline process, without Portland cement. Concrete Identification Bar Diameter Maximum Load (Q = KN) Maximum Adherence Tension τ(n/mm 2 ) Gap Area 1 AAFA W mm Slides and spliting AAFA W mm Slides and spliting AAFA W mm Slides and spliting 1 = τ minimum as per EHE for 16 mm diameters bars: 9,70 N/mm² 3.3-Shrinkage The shrinkage to drying was determined on mortar specimens of activated fly ash (prisms of 2.5x2.5x23 cm) in agreement with the standard ASTM C The mortars were elaborated as it is specified in the experimental procedure. In the Fig. 3 are the values of shrinkage of the alkali activated fly ahs mortars together with the ones obtained in Portland cement mortars (the above mentioned addresses to the information published by the authors of this investigation (13)). These results show that the mortars of activated fly ash experience a very slight shrinkage when they are dried; clearly lower than that of the cement mortars: within 90 days, for example, the values of shrinkage are lower than %, whereas the Portland cement mortars, especially those cured at room temperature, present a shrinkage nearly 0.09 % within 70 days. These results give an idea of the great dimensional stability of these new conglomerates. 0,10 AAFA-W CEM 22ºC CEM 40ºC Shrinkage (%) 0,08 0,06 0,04 0,02 0, Time (days) Fig. 3. Shrinkage to drying in activated fly ash mortars and portland cement mortars (13). 3.4 Industrial application: Manufacture of railroad mono-block sleepers. To carry out an industrial test on the manufacture of some prestressed mono-block sleepers of activated fly ash concrete, a homogeneous concrete mixture, exempt of Portland cement, was prepared in a small concrete mixer (125 litres capacity). The dosage of the concrete (per cubic meter) was carried out as follows: Fine aggregate (Silica sand)=825 Kg/m 3 ; Coarse 5

7 aggregate (Siliceous aggregate)=1050 Kg/m 3 ; Fly ash = 465k/m 3 ; activating solution/fly ash ratio = 0.50; Abraham`s cone =4. The experimental manufacture of the sleepers involved the fitting of a curing chamber where heat, in the form of a water steam, could be supplied to the moulded concrete (a double mould for making a couple of sleepers at the same time was designed specifically for this test). High power vibrators were applied to consolidate the alkaline concrete after it was placed into the mould. When consolidation rendered the mixture perfectly uniform, the material was steam-cured in the chamber. Fig. 4 shows the system of water vapour distribution, the sleeper s mould (once assembled into the heating system), and the vibration operation. Finally, once the thermal curing was completed, the mould was left to cool; the sleeper was taken out of the mould and finally submitted to the corresponding mechanical test with a static load. The data related to the breaking test with the static load are given in Table 3. Table 3 Sleepers data related to the breaking test with static load Sleeper A Sleeper B Tones Loading Duration Remanent Cracking (mm) Tones Loading Duration Remanent Cracking (mm) min min min min min min min min min Breaking down min min. Breaking down One tendon was broken Two tendons were broken (a) (b) (c) (d) Fig. 4 (a) Sleepers mould and the heating system; (b) Vibration operation; (c) Static test of a sleeper of alkali activated fly ash concrete; (c) Sleepers broken 6

8 4. DISCUSSION AND CONCLUSSIONS In general terms, the activation process of fly ashes allows getting a material with similar cementing features as the Ordinary Portland Cement. Besides, this process is the origin of important economical and environmental benefits when compared with the traditional fabrication of OPC. The results of the present work have shown that the material obtained by activation of fly ashes though the alkaline process develops excellent mechanical strength in a relatively short period of time (35.4 MPa to 12h and 50MPa to 20h, see Fig. 1)). In this respect, it must be emphasized that the temperature of curing plays a very important role, just like with the Portland cement concrete. Nevertheless, it differs from the new cementing material in that it does not suffer from the effect of temperature or other alterations that remarkably affect its durability. This fact can give the activated ash concretes a great added value, which means it could be used for prefabrication and the production rate could notably increase. Regarding the interface concrete-steel, these concretes prepared with activated ash and without Portland cement, are characterized by having a very good adherence. This is so high, that with compressive strength of around 50 Mpa (in cubes of 15x15x15), values of breakage higher than 50 tons are obtained in railroad sleepers, (see Table 3); and sometimes even breaking the steel during the testing. A good mono-block sleeper made with Portland cement concrete also starts to apply tons, but to reach this value it is necessary that the cubic specimens develop compressive strength of around 80 Mpa. Also, it is important to emphasize the excellent dimensional stability of this new conglomerate. The shrinkage is one of the most unwanted properties of the concrete. The consequence of this shrinkage is the breakage. It also reduces the load capacity and contributes negatively to its permanence, since it makes it vulnerable, among other things, to the negative effects of external agents. In Fig.3 it can be seen how the cement mortars, especially those cured at room temperature, experience a high shrinkage mainly due to losses of free water for drying. Nonetheless, the mortars of activated fly ash rarely shown shrinkage for drying. Finally the good results obtained in the specific application of these materials in the manufacture of mono-block railroad sleepers, justify a great potential for construction, especially in the precast industry of concrete. Beside the good technological features of these materials, it is very important to emphasize how easily this type of material can be adapted to the existing facilities in the current industry. Thus, for example the manufacture of railroad sleepers has taken place without having to significantly modify the current process. Above all, it has been done observing how the new material incorporates significant improvements in the essential features that are expected from elements such as sleepers. ACKNOWLEDGEMENTS The Directorate General of Scientific Research has funded the present study (project COO-1999-AX-038), and the Regional Government of Madrid awarded a post-doctoral grant associated with this research. The support provided by J.L. Garcia and A. Gil in the mechanical testing is also much appreciated. Authors wish to express their thanks to ALVISTRANVI personnel for their inestimable co-operation. 7

9 REFERENCES 1. Palomo, A., Grutzeck, M-W., Blanco, M.T. "Alkali-activated fly ashes a cement for the future" Cement and Concrete Research, Vol 29, 1999, pp Fernández-Jiménez A. and Palomo A. Alkali-activated fly ashes: properties and characteristics. 11 th International Congress on the Chemistry of Cement (Durban, South Africa) Vol 3, 2003, pp Fernández-Jiménez A. and Palomo A. Characterisation of fly ashes. Potential reactivity as alkaline cements. FUEL, 82, 2003, pp ,. 4. Krivenko, P.V., Alkaline cements, in: P.V. Krivenko (Ed.). Alkaline cements and concretes. 1. Vipol Stock Company. Kiev. 1994, pp Palomo, A., Fernández-Jiménez, A., and Criado M,. Geopolymers: one only chemical basis, some different microstructures Mater Construcc, (2004, in press). 6: Criado M., Palomo A., Fernández-Jiménez A., Alkali activation of fly ashes. Effect of curing conditions on the nature of the reaction products FUEL (submitted for publication 2004) 7. Puertas F. and Fernández-Jiménez, A. Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes. Cem. and Concr. Comp. No. 25, 2003, pp Fernández-Jiménez A., Palomo A. "Microstructural development of alkali-activated fly ash cement. A descriptive model Cement and Concrete Research (submitted for publication 2004) 9. Van Jaarsveld J.G.S., Van Deventer J.S.J., and Lukey G.C. The effect of composition and temperature on the properties of fly ash and kaolinite-based geopolymers Chemical Engineering Journal, Vol 89, 2002, pp Palomo, A., Alonso, S., Fernández-Jiménez A., Sobrados I. and Sanz J. Alkaline activation of fly ashes. A 29 Si NMR study of the reaction products J. Am. Ceramic. Soc. (2004, in press). 11. Fernández-Jiménez A and Palomo A. Alkali Activated Fly Ashes Structural Studies Trough Mid-Infrared Spectroscopy Microporous and mesoporous Materials (submitted for publication 2004). 12. Fernández-Jiménez A. and Palomo A. Microstructure of alkali activated fly ash mortars: effect of the activator Cem. Concr. Res. (submitted for publication, 2003). 13. Fernández-Jiménez, A., Palomo, A. "Hormigones alcalinos exentos de cemento portland Revista Ingeniería de Construcción, Vol. 18, No. 3, 2003, pp Hardjito, D., Wallah, S.E., and Rangan, B.V. Research into Engineering Properties of Geopolymer Concrete International Conference 'Geopolymer tur potential into profit', Melbourne, Australia, October 29, Fernández-Jiménez A. and. Palomo A, Some Factors Affecting Early Compressive Strength Of Alkali Activated Fly Ash Concrete (Opc Free) ACI Materials J. (submitted for publication 2004). 8