Microstructure and reactivity of calcined mud supported limestones

Microstructure and reactivity of calcined mud supported limestones A. M. Soltan*1, W.-A. Kahl2,3, M. Wendschuh4 and M. Hazem5 The petrographic characteristics of mud supported limestone samples from Egypt were examined before and after calcination at 950uC for 0?25, 0?5, 1 and 2 h. X-ray diffraction (XRD), X-ray fluorescence (XRF), transmitted light microscopy (TLM), scanning electron microscopy (SEM) and X-ray microcomputed tomography (m-CT) were used for technological samples characterisation. Both the free lime content and the hydration behaviour of the resulted quicklime after calcination were measured. The produced lime is unreactive at all applied firing conditions, except the Chalk lime at 1 and 2 h soaking, despite the high free lime content of the samples. The microstructure of the mud supported limestones is of significance on the resulted quicklime reactivity, where microfractures are formed along the cavities inside the microfossils. The mineralogical and the chemical compositions of the studied mud supported limestones are the main controlling variables on the degree of the quicklime crystallinity and consequently the reactivity. Samples enriched with P2O5 show higher crystallinity of the resulted lime crystallites associated with lowest reactivity. Keywords: Limestone, Microstructure, Quicklime, Reactivity

Introduction The mud supported limestones are composed of grains embedded in a muddy, mainly calcitic matrix (Flu¨gel, 2004). Quicklime results from the calcination of different types of limestones. The calcination is achieved in rotary or shaft kilns applying suitable firing schemes (Boynton, 1988; Oates, 1998; Stanmore and Gilot, 2005; Lech, 2006a, 2007). The limestone characteristics in terms of its microstructure, mineral content and physical properties affect the quicklime specifications (Kantiranis et al., 1999, 2003; Lech, 2006b, Lech et al., 2009a, b). The optimisation of the calcination process is therefore a function of the limestone characteristics (Boynton, 1988; Lech, 2006b, c; Abu-zeid et al., 2008; Serry et al., 2008a, b). The limestones of fine microstructure, mud supported, are more easily calcined compared to coarse grained ones (Soltan, 2008; Soltan 2009; Soltan and Hazem, 2009). An optimum temperature of 900uC for calcination has been suggested which is the temperature used in traditional kilns (Hartman et al., 1996). The specific

1

Geology Department, Faculty of Science, Ain Shams University, Cairo 11566, Egypt Institute of Geosciences, Christian-Albrechts-University, Kiel 24118, Germany 3 Now at: Department of Geosciences Bremen University, Bremen 28359, Germany 4 Department of Crystallography, Bremen University, Bremen 28359, Germany 5 Chemistry Department, Faculty of Science, Ain Shams University, Cairo 11566, Egypt 2

*Corresponding author, email [email protected]

ß 2012 Institute of Materials, Minerals and Mining and The AusIMM Published by Maney on behalf of the Institute and The AusIMM Received 16 January 2011; accepted 11 August 2011 DOI 10.1179/1743285511Y.0000000024

surface area and porosity increase in the quicklime when compared with the limestone before calcination (Paolo, 2002; Trikkel and Kuusik, 2003). Soltan et al. (2011) found that the amount of pores is not the main variable for increasing the lime reactivity, but it is the shape of pores. The lime enriched in the contraction fracture micropores is of higher reactivity when compared with the lime enriched with the preserved intraparticle pores. Both the surface area and rate of hydration temperature are directly proportional to the quicklime reactivity (Moropoulou et al., 2001). The firing conditions applied in the industrial lime kilns affect the quicklime reactivity. Soltan et al. (2011) found that highly reactive lime can result from different soaking times after firing at 950uC, based on the microstructure of the starting material. The crystalline phases, such as periclase, developed during the limestone calcination, retard the rate of hydration temperature (Gheevarhese et al., 2002; Potgieter et al., 2003). The main objective of this study is to consider the calcination effect on the microstructure of mud supported limestone and consequently, the characteristics of the quicklime in terms of its microfabric.

Methodology and experimental procedure Four lithostratigraphic sections were represented by serial samples collected from Abo-Roash (the Chalk Series, Campanian-Maastrichtian; Moustafa 1988), the White Desert (the Khoman Chalk, Maastrichtian; Issawi, 1972), Gabal El-Shaghab (Tarawan Formation, Paleocene; Awad and Ghobrial, 1965) and Gabal Tanka

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(Tanka Formation, Upper Eocene; Boukhary and Abdelmalik, 1983). Thin sections were prepared from the serial samples to be examined by transmitted light microscope (TLM), whereas whole grains were examined by scanning electron microscope (SEM). The serial samples were crushed to pass the 5 and 10 mm sieves for the preparation of four industrial technological samples named Chalk, Khoman, Tarawan and Tanka. The chemical composition of the technological samples was determined by the X-ray fluorescence (XRF) technique using a spectrometer, Model PW/1404 with Rh target and six analysing crystals, whereas their mineral content was characterised by the aid of X-ray diffraction analysis (XRD) using a Philips X-ray diffractometer, Model PW/ ˚ ). 1800, with a Ni-Filtered, Cu Ka radiation (l51?542 A All technological samples were investigated with Xray microcomputed tomography (m-CT) by using the SkySkan 1172 system (SkyScan, Belgium). This analysis was achieved at the Institut fu¨r Geowissenschaften, Christian-Albrechts-Universita¨t zu Kiel, Germany. Selected grains of each sample were scanned with a beam energy of 70 kV, a flux of 141 mA and a Al/Cu foil at a detector resolution of 2?4 mm per pixel. SkyScan software was used for reconstruction, segmentation and volume rendering of the raw images. Three-dimensional stereology image analysis was performed on cubic volumes of interest of 1000 voxel edge length. Three-dimensional models were achieved by adaptive rendering of condensed data sets (nine times fewer voxels than the original). The calcination was achieved by loading the samples in porcelain crucibles and firing at 950uC for 0?25, 0?5, 1 and 2 h soaking times in an electrical muffle furnace. The calcination products, quicklime, were examined for their phase contents (XRD), microfabric (SEM), reactivity and free lime content. The latter was quantitatively determined using the sugar method (ASTM C25-06, 2006), where a definite portion of the quicklime is dissolved in sugar solution and titrated against standardised HCl solution. The reactivity was determined according to the Deutsches Institut fu¨r Normung in terms of the Rdin values (Potgieter et al., 2002) and the rate of hydration temperature increase (Moropoulou et al., 2001). The quicklime lumps were crushed to the size range (1– 2 mm), mixed with water (1 lime : 4 distilled water) in a calorimeter. Then the increase of hydration temperature was measured each 15 s for a period of 20 min. The maximum hydration temperature (Tmax/uC), its time span (DTmax/s), the rate of temperature increase (RTI/ uC s21), the time for 60uC temperature increase (T60/s) and the Rdin values were all determined and calculated. Based on the Rdin values, the quicklime was classified as

unreactive (UR; Rdin,10), reactive (R; 10,Rdin,30) and highly reactive (HR; Rdin.30). The higher the RTI, the higher the quicklime reactivity (Moropoulou et al., 2001).

Results and discussion Calcite (CaCO3) is the main carbonate mineral recorded from the XRD analysis in all samples, whereas dolomite [CaMg(CO3)2] is present only in the thin section of Tarawan. The non-carbonate minerals are quartz (SiO2), fluorapatite (Ca5 (PO4)3F) and gypsum (CaSO4.2H2O). Quartz occurs in all samples except the Chalk, whereas fluorapatite and gypsum were recorded only in the Tanka sample. CaO is the main major oxide in the four samples which is related to the calcite and dolomite. The content varies between 51?11 wt-% (Tarawan) and 55?61 wt-% (Chalk, see Table 1). SiO2 contents are low with a maximum value 5?93% in the Tarawan sample due to the sparse presence of quartz. The P2O5 and F are only recorded in the Tanka sample (1?65 wt-% and 2435 ppm respectively) proving the presence of fluorapatite. MgO is related to dolomite in the Tarawan sample (0?99 wt-%), whereas SO3 appeared only in the Tanka sample (0?40 wt-%) due to the presence of gypsum. The description of the limestone samples was achieved by the aid of Bacelle and Bosellini (1965), Pilkey et al. (1967), Friedman (1965) and Choquette and Pray (1970) charts. These charts were adopted for the identification and semiquantification of carbonate components (alloand orthochems), allochem roundness and size in addition to porosity types respectively. The schemes of Folk (1959, 1962) and Dunham (1962) were used for the sample classification. The Chalk and Tarawan samples are petrographically similar (mudstone, fossiliferous micrite). Planktonic forams, peloids and coccoliths (Fig. 1a) were recorded as the main allochems (5 and 10% respectively) floating in micritic groundmass (90%). The grains are medium (Tarawan) to fine (Chalk) sand sized, rounded to very well rounded and closely packed but of no contact. Dolomite rhombs are occasionally scattered in the groundmass of the Tarawan sample. The Tanka and Khoman samples are wack- to packstones (sparse biomicrite). Both are dominated by planktons (Fig. 1b), piloids and coccoliths, the same as the Chalk and Tarawan. However, the allochems are of higher proportions (45%), rounded to very well rounded and of no contacts. The allochems are mainly fine sand sized; however, about 15% of the grains in the Tanka are medium grained. Although the thin sections are almost devoid of any pores, the reconstructed and the rendering images (Fig. 2) are showing dispersed intraparticle pores in all

Table 1 Concentrations of major oxides in mud supported limestone technological samples Major oxides/wt-% Sample

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

P2O5

MnO

Na2O

K2O

SO3

F/ppm

LOI*

Chalk Tarawan Tanka Khoman

… 5.93 2.73 1.24

0.09 0.37 0.49 0.61

0.09 0.39 0.20 0.34

55.61 51.11 53.01 54.06

0.31 0.99 0.67 0.27

0.01 0.03 0.03 0.04

0.01 0.19 1.65 0.11

… 0.02 … 0.02

0.11 0.12 0.24 0.12

0.02 0.06 0.14 0.15

… 0.06 0.40 0.02

… … 2435 …

43.55 40.56 39.91 42.82

…: below detection limit. *Loss on ignition at 1000uC.

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1 Photomicrographs showing microstructure of studied limestone samples: a coccoliths (white arrow) surrounded by micritic matrix in the Chalk and Tarawan samples (SEM image); b planktons (white arrow) ﬂoating in micrite in the Tanka and Khoman samples (X-Nicols)

samples (Supplementary Materials 1 http://dx.doi.org/ 10.1179/1743285511Y.0000000024.s1, 2 http://dx.doi.org/ 10.1179/1743285511Y.0000000024.s2, 3 http://dx.doi.org/ 10.1179/1743285511Y.0000000024.s3 and 4http://dx.doi. org/10.1179/1743285511Y.0000000024.s4 for the Chalk, Tarawan, Tanka and Khoman respectively). The porosity in 3D, as well as pore size distribution, orientation and visualisation of the internal structure of the specimen, can be provided by the m-CT method (de Graef et al., 2005; Cnudde et al., 2009). The Tanka is of the highest porosity (y1%) (Fig. 2a and b) which means that all the samples are structurally compact. The pore size distribution (Fig. 3) based on mCT shows that the most frequent pore size diameters exist between 15 and 45 mm in all samples. Figure 4 shows that the free lime content increases with increasing soaking time in all samples. The incomplete calcination of limestone grains at 0?25 and 0?50 h soaking (Boynton, 1988; Oates, 1998) is interpreted from the low content of free lime (maximum 65 and 72% respectively) at these conditions respectively

(Table 2). However, the lime is maximised (98 and 99%, the Chalk lime) at 1 and 2 h due to the almost complete calcination of the carbonate minerals. Belite (Ca2SiO4 : C2S) is the only phase associated with lime in all samples after calcination at 1 and 2 h, except the Chalk lime. At all firing conditions, the lime is unreactive (Table 2), except that of the Chalk at 1 and 2 h (Fig. 5). The unreactivity of the lime produced at 0?25 and 0?5 h soaking times is mostly attributed to its low content. At 1 h, the Tanka lime (66%) is of higher reactivity (0?18 RTI, 9 Rdin) than the Khoman (88%, 0?05 RTI, 5 Rdin). The same observation is recorded at 2 h, between the Tanka (77%, 0?12 RTI, 8 Rdin) and Tarawan lime (88%, 0?07 RTI, 5 Rdin). Therefore, the amount of free lime is not the only controlling variable affecting its reactivity, especially at the longer soaking times. This could be interpreted in terms of microfabric of the lime crystallites. The reactivity of both the Chalk and Tarawan lime can be compared because of their petrographic

Table 2 Hydration behaviour and free lime contents of calcined technological limestone samples at different ﬁring conditions* Sample 950uC, 2 h Chalk Tarawan Tanka Khoman 950uC, 1 h Chalk Tarawan Tanka Khoman 950uC, 0.5 h Chalk Tarawan Tanka Khoman 950uC, 0.25 h Chalk Tarawan Tanka Khoman

Free lime/%

Tmax/uC

DTmax/s

RTI/uC s21

T60/s

Rdin

99 88 77 92

100 73 69 114

270 1125 570 615

0.37 0.07 0.12 0.19

130 465 300 255

18 5 8 9

R2 UR3 UR UR

98 67 66 88

81 53 51 70

540 990 285 1305

0.15 0.05 0.18 0.05

180 NC1 NC 435

13 NC NC 5

R UR UR UR

72 50 41 64

71 42 42 72

585 585 285 1215

0.12 0.07 0.15 0.06

240 NC NC 420

10 NC NC 5

UR UR UR UR

65 36 32 61

63 40 39 73

420 480 285 765

0.15 0.08 0.14 0.10

285 NC NC 300

8 NC NC 8

UR UR UR UR

Reactivity

*NC: not calculated; R: reactive (10,Rdin,30); UR: unreactive (Rdin,10).


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2 Photomicrographs showing reconstructed (a, c, e, g) and rendering (b, d, f, h) micro-CT images of Chalk, Tarawan, Tanka and Khoman samples, respectively (online resources 1, 2, 3 and 4 are for 3D orientation of samples respectively)

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3 Normalised volume based pore size distribution of limestone samples

similarities in the precedent limestone; both resulted from the decomposition of micrite dominating limestone (mudstone, fossiliferous micrite). The Chalk lime (98 and 99%) is more reactive, with higher Rdin (Table 2), than the Tarawan lime (67 and 88%) at 1 and 2 h respectively, because of the higher proportions of lime from one side and the microfabric of lime crystallite from the other side. The Chalk lime microfabric is characterised by the dominance of pinhole micropores (PMs) in the micrite derived lime (MDL) at 1 and 2 h (Fig. 6a). These PMs, resulting from the crystallites grain growth, increase the lime crystallite surface area subjected to hydration, and consequently the reactivity. However, it is not possible to characterise the allochem derived lime (ADL), due to the disappearance of the ghost structure of the precedent fine grained allochems at 1 and 2 h.

4 Free lime versus soaking time at 950uC


5 RTI versus soaking time at 950uC

The Tarawan lime microfabric can be differentiated into ADL (Fig. 6b and c) and MDL at 1 h. The former is related to the ghost medium grained allochems and characterised by the fossil pinhole micropores (FPMs) and suture microfracture pores (SMPs). The FPM are formed on the original pores of the planktons, whereas SMP formed along the ghost suture line contacts of the plankton chambers. The MDL is dominated by PM, the same as the Chalk lime (Fig. 6a). These pores, FPM, SMP and PM, are not enough to raise the reactivity of the Tarawan. This is because of the low amount of free lime (67%) at 1 h and the crystallinity of the octahedral lime crystallite (OLC) (Fig. 6d) formed at 2 h due to the effect of Al2O3 and Fe2O3. The higher the lime crystallinity, the lower its reactivity (Boynton, 1988). Although not recorded in the XRD analysis, periclase (MgO) from the decomposition of dolomite is another reason to retard the Tarawan lime reactivity at 1 and 2 h. Periclase has lower rate of hydration when compared with lime (Potgieter et al., 2003). The Tanka (66%) and Khoman (88%) limes are unreactive at 1 h soaking (9 and 5 Rdin respectively); however, the former is of higher reactivity. This is attributed to the fact that the Tanka lime is preserving the ghost structure of the medium grained allochems (Fig. 7a and b). FPMs are developed in the ADL, which in turn increase the lime relative surface area subjected to hydration and consequently increase the reactivity. However, disappearance of the ghost structure in the Khoman may be due to the fine grained allochems, as in the case of the Chalk lime. At 2 h soaking condition, however, both limes are unreactive, and the reactivity situation is reversed. The Khoman lime (92%) is of higher reactivity than the Tanka (77%) (9 and 8 Rdin respectively). This could not be interpreted by means of the higher proportions of the free lime. Figure 4 shows that in all samples the content of free lime increases from 1 to 2 h soaking time to a varying extent: from 88% to 92% (Khoman) and from 66% to 77% (Tanka). After 1 h, the 66% free lime of the Tanka are related to a much lower RTI than the 88% of Khoman. After 2 h, 77% free lime of Tanka corresponds


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6 Photomicrographs showing lime microfabric: a MDL and PM in Chalk and Tarawan lime at 1 and 2 h; b ADL of Tarawan at 1 h; c squared area in Fig. 5b) showing FPM and SMP; d OLC of Tarawan lime at 2 h

7 Photomicrographs showing lime microfabric: a ADL of Tanka at 1 h; b squared area in Fig. 6a showing FPM of Tanka at 1 h; c OLC of Tanka lime at 2 h

to a higher RTI than the 92% free lime of Khoman. However, the higher reactivity could be due to a feature of the microfabric: the crystallinity of the OLC of the Tanka (Fig. 7c). The OLC in the Tanka lime can result from the dominance of P2O5 (1?65%), sulphur (0?40%) and fluorine (2435 ppm) (Table 1), which may promote the lime agglomeration and crystallinity (Boynton, 1988; Oates, 1998; Ioannis et al., 2011)

Conclusion It can be concluded that for the specific heating conditions tested: 1. The lime resulted from the mud supported limestones is mostly unreactive.

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2. There is no preservation for the mud supported limestone microstructure post-calcination at 2 h if the allochems are fine grained. 3. The mineralogical as well as the chemical compositions are the main controlling variable on the reactivity of the mud supported limestones post-calcination. The chemical composition of the rock affects the degree of crystallinity of the lime crystallites and in turn plays a significant role in determining its reactivity. 4. The increase of P2O5 content increases the degree of crystallinity and consequently decreases the reactivity of lime.

Acknowledgements The experiments and data analysis were achieved in Bremen University during a 3 month postdoctoral


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DAAD-GERSS research fellowship. The authors are of deep gratitude to Professor Dr M. Serry, Professor Dr H.-J. Kuss, Professor Dr M. Boukhary, Dr S. Hogewoning and C. Mehling for their critical discussion during the implementation of this work. The authors also appreciate the peer reviewers of this work.

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