SOME CONTRIBUTIONS TO THE EXAMINATION OF COMPOSITES FROM POWER PLANT WASTES

This paper describes an investigation into the possibilities of the use of all three types of solid power plant wastes: fly ash (FA), flue gas desulphurization gypsum (FGD gypsum) and bottom ash (BA) as components of composites for road construction. Two mixtures were made: 1. power plant wastes Portland cement (PC) and 2. power plant wastes sand (S) Portland cement (PC). The mass ratio of components in these mixtures was: 1. FA FGD gypsum BA PC = 2 : 1.5 : 5 : 1.5 and 2. FA FGD gypsum BA S PC = 2 : 1.5 : 4 : 1 : 1.5. For both mixtures, the compressive strength, the mineralogical composition, the water absorption and the resistance to freeze-thaw treatment were determined 7 and 28 days after preparation of samples (mixing with water). The obtained results showed that both mixtures could have potential to be used for sub-base layers in road construction and the second mixture (with natural aggregate-sand) has advantage over the first mixture in terms of compressive strength, mineralogical composition, and resistance to freeze-thaw treatment.


INTRODUCTION
The thermal power plants produce three types of solid wastes (fly ash -FA, flue gas desulphurization gypsum -FGD gypsum and bottom ash -BA).The disposal of these wastes is an economic and an ecological problem (pollution of air, waters and soil).Consequently, their utilisation in bulk (as materials for road construction) is of great importance.
The application of wastes from power plants as materials for road construction is connected with their characteristics.FA with low CaO content (the class F according to ASTM 618) may exhibit pozzolanic properties and could be applied as a substitute for Portland cement (PC) in concrete (Manz, 1999;Benavidez et al. 2003;McCarthy and Dhir, 1999;Freidin and Motzafi-Haller, 1999;Asokan et al. 2005).
BA is a more or less inert material, i.e., bottom ash usually does not exhibit pozzolanic properties, but it could be a potential aggregate in concrete as a replacement for natural sand (its particles are coarse, sized between sand and gravel, fused and with a glassy texture) (Benavidez et al. 2003;Cheriaf et al. 1999;Bai and Basheer, 2003).
FGD gypsum could be applied as a chemical activator of FA in FA/cement systems, based on the ability of gypsum to react with aluminium oxide in the glass phase of fly ash to produce ettringite that contributes to the strength in the early ages (Poon et al. 2001).
The aim of this work was to broaden the knowledge concerning the applicability of power plant wastes as components in cement based composites for road construction FA and BA, applied in this work, originated from Serbian power plant "Nikola Tesla" and FGD gypsum from Bohemian Power Plant -Hvaletice (no Serbian plant has a FGD system installed yet).

Materials
The following materials were used:

Proportions and methods of examination of mixtures
Two kinds of the mixtures (pastes) were made: the first was composed of power plant wastes (FA, FGD gypsum, BA), Portland cement (PC) and water (W) and the second contained sand (S) in addition to the mentioned components in the first mixture.
To simulate the stabilization (necessary for use of the mixtures in road construction), cylindrical specimens (diameter 10.2 cm and height 11.7 cm), so-called Proctor specimens, were made by the standard Proctor compaction procedure (ASTM D698).The necessary quantity of water applied for preparation of these specimens presented the content of water at which the mixtures became most dense and achieve their maximum dry density (so called the optimum moisture contents -OMC).
The content of components in solid phase and the content of water (OMC) in the mixtures are given in Table 1.

Content of water (OMC) [%]
[%] Twelve Proctor specimens of both mixtures were stored for 7days and the another twelve of both mixtures were stored for 28 days in a humid room (20°C ± 2°C, 85% humidity).After storage, the following properties of the specimens were determined: compressive strength, mineralogical composition, water content, absorption of water and resistance to freeze-thaw cycling.
The compressive strength was determined according to the Serbian Standard SRPS U.B1.030 (which is in accordance with the European Standard EN 14227-4).The values of compressive strength presented herein are average value of the compressive strength of three specimens of both hardening mixtures after 7 and 28 days.For an assessment of the validity of the results of compressive strength, they were compared with the criteria for compressive strength of sub-base layers proposed by Public Enterprise-Roads of Serbia.
The mineralogical compositions of pulverized and crushed specimens (during the determination of their compressive strength) were determined by the powder XRD technique using the Philips PW 1729 X-ray generator and the Philips PW 1710 diffractometer.The obtained XRD-diffractograms of samples were interpreted by means of JCPDS cards.Also, XRD-diffractograms were used to estimate the development of obtained products (phases) of the hydration reactions during period from 7 to 28 days.Namely, the peak intensities (I in cps) of products after 28 days (I 28 ) and after 7 days (I 7 ) were read from X-ray difractograms and than the ratios I 28 / I 7 were calculated.The ratio I 28 / I 7 < 1 signified a diminution and ratio I 28 / I 7 > 1 signified augment of content of phases (products).
Water content and absorption of water of monolithic (non-crushed) cylindrical (Proctor) specimens were determined as the average value of three specimens after 7 and 28 days, according to SRPS U.B1.012 and SRPS B.B8.010, respectively.
The resistance to freeze-thaw cycling of monolithic (non-crushed) cylindrical (Proctor) specimens was determined using the durability test procedure specified in SRBS U.B1.050 (as an average value of three specimens after 7 and 28 days).This freeze-thaw procedure is correlated to the mass loss and volume change after 14 freezethaw cycles.

Compressive strength
The results of the compressive strength determinations of the specimen S 1 (made from mixture M 1 in Table 1) and the specimen S 2 (made from mixture M 2 in Table 1) after 7 and 28 days are presented in Table 2.The estimated standard deviations for the compressive strength are also given in Table 2.The specimen S 2 (in which 20 mass % of bottom ash was substituted with sand) had greater compressive strength values than the specimen S 1 (in which sand was not present), both after 7 and 28 days of hardening (by 11.57% and 7.42%, respectively).These results are probably connected with the higher porosity and inferior physical properties (density, bulk density, maximum dry density) of bottom ash compare to those of sand.

Compressive strength
[MPa] 7 days 28 days Sub-base for motor ways and very heavy traffic load 2 -5.5 3.0 -6 Sub-base for heavy and medium traffic load 1.5 -5.5 2.5 -6 The criteria for compressive strength of subbase layers proposed by the Public Enterprise-Roads of Serbia are presented in Table 3.
Comparing the obtained results (Table 2) with the criteria in Table 3, it could be concluded that both mixtures prepared in this work are suitable as the sub-base for heavy and medium traffic load.

XRD analysis
Based on the results of the XRD analysis (interpreted by JCPDS cards) it could be stated that both specimens (S 1 and S 2 ) contained the same crystalline phases: quartz, mullite, ettringite, gypsum and calcite, as well as the amorphous phase of C-S-H.
Quartz and mullite detected in the specimens, were inherent to fly ash and bottom ash.
Ettringite and C-S-H were the products of the hydration reactions that occurred.Ettringite (3CaO•Al 2 O 3 •3CaSO 4 •32H 2 O) was formed by the reaction between calcium aluminate, from Portland cement and FA, and gypsum from FGD gypsum and Portland cement (Holly et al. 2006): C-S-H (hydrated calcium silicate) was the product of the reactions between tricalcium silicate (C 3 S) and dicalcium silicate (C 2 S), from Portland cement, and water, according to: (2) Because Ca(OH) 2 was not present in the specimens (stated by X-ray diffraction analysis) it could be concluded that the liberated Ca(OH) 2 (2) reacted with active silicates from fly ash (present in the amorphous phase of FA) to form also calcium silicate hydrate (C-S-H).
The presence of gypsum and calcite in both specimens (S 1 and S 2 ) could be interpreted and explained by means of the ratios of the intensities of the XRD peaks (I 28 / I 7 ) of these phases.The ratios of the intensities (in cps) of the XRD peaks of ettringite, gypsum and calcite for the specimens after 28 and 7 days (calculated on the basis of their values read from XRD diffractrograms of the specimens) are presented in Table 4. 1.70 0.68 1.05 * ratio < 1 signifies a diminution and ratio > 1 signifies augment of content of phases ** these peaks were chosen because they did not overlap with those of other phases Based on the results presented in Table 4, the following development of the phases during the hydration reactions (from 7 to 28 days) were stated: - The diminishing of the content of ettringite (I 28 / I 7 = 0.58) and the augment of the contents of gypsum (I 28 / I 7 = 1.32) and calcite (I 28 / I 7 = 1.52) in the specimen S 1 ; - The augment of the content of ettringite (I 28 / I 7 = 1.70), the diminishing of the content of gypsum (I 28 / I 7 = 0.68) and practically the same content of calcite (I 28 / I 7 = 1.05) in the specimen S 2 .
The diminishing of the content of ettringite as well as the augment of the contents of gypsum and calcite could be related to the carbonation phenomenon which occurred in the specimen S 1 , via reactions with atmospheric CO 2 , according to the equations (Garcia-Gonzales et al. 2006;Chen et al. 2007;Fernández Bertos et al. 2004): CxSHy zCO Cx zHy zCaCO     (4) Because the contents of gypsum and calcite were not increased while the content of ettringite was in the specimen S 2 , it could be concluded that the carbonation phenomenon is not occurred in this specimen.
The obtained results could be consequence of the fact that the diffusivity of CO 2 (and therefore the carbonation) depends of the porosity of a solid -low porosity inhibits the diffusion of CO 2 into the solid (Fernández Bertos et al. 2004).The substitution of 20 mass % of porous BA with non-porous sand in S 2 probably diminished the diffusion of CO 2 , which was a possible reason that carbonation did not occur in this specimen.

Water content and absorption of water
The water content and the absorption of water of monolithic (non-crushed) cylindrical (Proctor) specimens after 7 and 28 days are presented in Table 5.The increase of water content and the decrease of absorption of water in the Specimen 1 from 7 to 28 days (Table 1) could be related to the carbonation phenomenon which took place during mentioned period in the specimen.It seems that ettringite decomposition with the release of a lot of water (3) caused the increase of water content and that calcite formation (4) caused the decrease of absorption of water, because of calcite which was deposited in pore volume of material during carbonation (Valls and Vàzquez, 2001;Johannesson and Utgenannt, 2001).

T a b l e 5 -Water content and absorption of water of the specimens
The obtained results (presented in Table 4), which suggested that practically no differences in water content and absorption of water between 7 and 28 days for S 2 specimen, are very probably connected with the fact that the carbonation phenomenon did not occur in the Specimen 2.

Freeze-thaw resistance
The results of the freeze-thaw resistance are given in Table 6.The criteria of SRPS U.B1.050 standard are: the maximum of 14% of mass loss and the maximum of 2% of volume change after 14 freeze-thaw cycles.On this way both specimens (S 1 and S 2 ) had satisfactory freeze-thaw resistance, after 7 days and 28 days as well (Table 6).
Moreover, it is evident from Table 6 that the specimen S 2 (composed of FA, PC, FGD gypsum, BA and S) after 28 days exhibited the highest freeze-thaw resistance.

CONCLUSION
Based on aforementioned it could be concluded the following: 1.Both mixtures: FA -FGD gypsum -BA -PC -W (2 : 1.5 : 5 : 1.5, MOC = 36.533%)and FA -FGD gypsum -BA -S -PC -W (2 : 1.5 : 4 : 1 : 1.5, MOC = 35.211%),satisfied the criteria for sub-base layers, regarding the compressive strength values (proposed by the Public Enterprise-Roads of Serbia) and the freeze-thaw resistance (SRPS U.B1.050).2. The examinations performed in this work indicated that carbonation reactions occurred in the mixture without sand (the first mixture).The products of these reactions could have both positive role (formed CaCO 3 can improve structural properties of material), and negative role (formed gypsum may cause delayed ettringite formation and eventually cracking of material).Consequently, longterm examination (longer than 28 days) should be included in future investigations.

T a b l e 4 -
The ratios of the intensities of the XRD peaks of the specimens after 28 and 7 This paper was realized as a part of the project "Study of Possibilities for Valorization of the remaining Coal Reserves to Provide Stability of the Energy Sector of Republic of Serbia" (TR 33029) financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia within the framework of Programme of research in the field of technological development for the period 2011-2015.

Table 2 -
Compressive strength results for hardened specimens

Table 6 -
The mass loss and volume change of the specimens