Miner Petrol DOI 10.1007/s00710-014-0329-3
ORIGINAL PAPER
United Arab Emirates limestones: impact of petrography on thermal behavior Sulaiman Alaabed & Abdel Monem Soltan & Osman Abdelghany & Bahaa Eldin Mahmoud Amin & Mohamed El Tokhi & Abbas Khaleel & Abdullah Musalim
Received: 18 November 2013 / Accepted: 7 May 2014 # Springer-Verlag Wien 2014
Abstract The thermal behavior of selected limestones from representative localities of the United Arab Emirates is investigated for their suitability for soft-burnt lime production. The limestone samples were collected from the Ghalilah, Musandam, Shauiba, Muthaymimah, Dammam and Asmari formations. The samples were characterized for petrography, mineral and chemical composition, together with physicomechanical characteristics. Investigative methods included transmitted light microscopy (TLM), cathodoluminescence (CLM) and scanning electron microscopy (SEM), as well as X-ray micro-tomography (μ-CT), XRD, XRF and Archimedes method. The limestone samples were fired in an electrical muffle furnace for 0.25, 0.5, 1 and 2 hours at 800, 900, 1,000 and 1,100 °C. After firing the lime grains were tested to determine their hydration rate and microfabric. The Ghalilah and Musandam limes show the lowest and highest maximum hydration rates, respectively, due mainly to the impure nature of the former, and the smaller lime crystallites and dominance of post-calcination micro-cracks of the latter. The Dammam and Asmari limes preserve a “ghost” microfabric of the original limestone. Higher allochem contents impose lower activation energy requirements for calcination, which implies earlier calcination of the allochems. The Editorial handling: J. G. Raith S. Alaabed : O. Abdelghany : B. E. M. Amin : M. El Tokhi : A. Musalim Geology Department, College of Science, United Arab Emirates University, Al-Ain, P.O. Box 15551, Abu Dhabi, UAE A. M. Soltan (*) : O. Abdelghany Geology Department, Faculty of Science, Ain Shams University, Cairo, P.O. Box 11566, Egypt e-mail:
[email protected] A. Khaleel Chemistry Department, College of Science, United Arab Emirates University, Al-Ain, P.O. Box 15551, Abu Dhabi, UAE
Musandam, Shauiba and Muthaymimah limestones may be useful for the production of reactive soft-burnt lime under the applied firing conditions, however, the Dammam and Asmari limestones need more advanced calcination conditions than the applied ones. The Ghalilah limestone was found to be unsuitable for the production of lime.
Introduction The outstanding importance of limestone in industrial applications results from the fact that it is dominantly composed of the mineral calcite (Chatterjee 2009). One of the key industrial applications of limestone is calcination for lime production (Oates 1998). Based on the applied firing temperature during calcination, the resulting lime can be described as soft-burnt lime (also referred to as quicklime), or dead-burnt lime. The former is produced when the applied temperature is in the range (900–1,150 °C) whereas the latter results from temperatures >1,150 °C (Stanmore and Gilot 2005). Soft-burnt lime generally has higher surface area and consequently is more reactive when compared with the dead-burnt lime (Ar and Dogu 2001; Moropoluoua et al. 2001; Gheevarhese et al. 2002; Kantiranis et al. 2003; Cheng and Specht 2006; Li et al. 2008; Serry et al. 2008a, b). The industrial process of limestone calcination is carried out in rotary or vertical shaft kilns where the limestone is fed as lumps (6–25 and 25–50 mm) (Boynton 1988; Cheng and Specht 2006). The calcination requires about 3.2GJ/ton and begins at ~780 °C (Kirk-Othmer 1981; Moffat and Walmsley 2004, 2006). The shrinking core reaction model is the most popular model used to illustrate the limestone calcination process. It assumes that calcination of the limestone lumps proceeds from the exterior to the interior of the lumps, leaving the core intact (Moffat and Walmsley 2006).
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Limestone calcination proceeds in five steps: a) hot gases of the kiln are transferred to the limestone lump surface; b) the surface begins to calcine and the hot gases are then transmitted through the micro-porous surface lime layer to the interior of the lump core, which becomes the new reaction interface; c) continuous calcination of carbonates forms CaO and CO2; d) CO2 migrates to the lime lump surface, then e) CO2 migrates from the lump surface to the kiln (Lech 2006a). The petrography of the limestone (facies) and the developing lime microfabric influence the hot gas transfer within the limestone lump and the escape of CO2 during calcination (Lech 2006a; Lech 2007). The detailed petrography, mineralogy, chemistry and physicomechanical characteristics of the limestone lumps drastically affect the quality of the calcined lime as outlined in the following sections.
Effect of limestone petrography According to Dunham (1962), limestones may be divided into two broad categories (grain-and mud-supported limestones) depending on the content of grains (allochems) and matrix (orthochems). Grain-supported limestones are characterized by a fabric with little or no lime mud and abundant framework grains that support each other, whereas the mud-supported limestones are composed of framework grains embedded in a muddy, mainly calcitic matrix (Flügel 2004). For the production of lime, the calcination kiln feed may be composed of lumps of grain-and/or mud-supported limestone. Each limestone facies has its own characteristic optimum calcination conditions, which must be determined experimentally (Boynton 1988; Lech 2006b, c; Soltan 2007; Abu-zeid et al. 2008; Soltan 2008; Serry et al. 2008a, b; Soltan 2009; Soltan and Hazem 2009). During limestone calcination, the petrographic changes that occur depend on the limestone facies and the applied calcination conditions. Limestones with higher skeletal grain contents and/or lower impurity oxides give the highest liberated free lime. Grain-supported limestone rich in Nummulite fossils may preserve the fossil microstructure after calcination at 1,000 °C-2 h, while the common pores in Nummulites facilitate heat transfer and lower the activation energy (E) required for the commencement of calcination (Soltan et al. 2011; Soltan and Serry 2011). In addition, the presence of euhedral and subhedral neomorphic calcite crystals in the sparitic groundmass enhances formation of triple junction micro-fractures, which also contribute in lowering the activation energy (E) (Soltan and Serry 2011). By contrast, there is no preservation or ghosting of the microfabric of mudsupported limestones following calcination at 1,000 °C-2 h in the cases where the allochems are fine-to very fine-grained (Soltan et al. 2012).
Effect of limestone mineralogy and chemistry The main source of liberated lime is calcite. However other carbonate minerals such as dolomite may be present, and result in the formation of periclase. The latter phase has a low hydration rate when compared with lime (Potgieter et al. 2003). In addition, interface fractures may exist among the lime and the periclase grains due to the change of the molar volumes of these phases (Lech 2006b). The limestone chemistry is a reflection of its mineral composition. The CaCO3 content of the limestone should be higher than 98.6 % while the SiO2 content should be less than 1 % in order to produce highly reactive lime after calcination (Kantiranis et al. 1999). Each 1 % of total impurity oxides (TIO) (summation of SiO2, Al2O3, Fe2O3) will consume ~3 % free lime for the formation of the cement phases belite (Ca2SiO4), tri-calcium aluminate (Ca3Al2O6) and calcium alumino-ferrite (Ca2(Al,Fe)2O5) (Boynton 1988). These phases retard the quantity of liberated lime and consequently the surface area exposed to hydration. Some oxides such as P2O5 can positively affect the grain growth of the small lime crystallites and lower the surface area (Soltan et al. 2012). Effect of limestone physico-mechanical properties The physico-mechanical characteristics, particularly the precalcination porosity of the limestone, affect the quality of the calcined lime. Increasing porosity not only provides for uniform distribution of hot gases inside the limestone lump, but also accelerates the escape of the CO2 (Cheng and Specht 2006). Macro-and micro-fracture pores in the lime lumps are due mainly to calcination and can develop along pre-existed pore cracks in the limestone (Soltan and Serry 2011). The calcination fracture pores are the pathways for the hot gases inside the lump and also increase the outward diffusion of CO2 (Lech 2006a). This lowers the partial pressure exerted by the CO2, lowers the temperature of the lumps and consequently decreases the rate of lime grain growth (Kantiranis et al. 2003; Stanmore and Gilot 2005; Lech et al. 2009a, b). However, the low rate of CO2 diffusion accelerates the growth of lime crystallite grains, i.e., it promotes sintering, and thereby reduces the surface area and hydration rate of the lime (Adanez et al. 2002; Paolo 2002; Feng and Lombardo 2002; Trikkel and Kuusik 2003; Stanmore and Gilot 2005; Lech et al. 2009a, b). The lime hydration rate is not only affected by the porosity but also by pore morphology. Lime that is rich in micro-fracture pores has a higher hydration rate than lime rich in intra-particle pores (Soltan et al. 2011). From the previous sections, it can be concluded that limestone calcination is a petrographic, mineralogical-chemical, physico-mechanical and temperature dependent process. As previously noted, this contribution aims to clarify the role played by the petrographic composition of approximately
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chemically homogeneous limestones of the UAE on their calcination and hydration behavior. In the United Arab Emirates, the quarrying of limestone is the main mining activity, with ~150 mt/year of limestone production. This quantity is consumed by the building materials industry where it is used for production of cement, concrete, lime and ceramics (UAE Ministry of Energy 2010). The main limestone quarries are located at Ras-Al-Khaimah and Al-Ain.
Geologic setting and materials The United Arab Emirates (UAE) is located in the eastern part of the Arabian Peninsula between latitudes 22°40’ and 26° 00'N, a nd l ongitude s 51°00' and 56 °0 0'E . Geomorphologically, the UAE can be subdivided into two distinct zones. The first larger province is located in the west and northwest of the country. This area is covered by desert sand and coastal plains. The coastal areas are dominated by dry evaporitic flats and sabkhas. The second province is
restricted to the east and northeast of the country. This is the area of the Oman Mountains (Abd-Allah et al. 2013) which form a range parallel to the coast of the Gulf of Oman. There are offshore islands in the Arabian Gulf, such as Dalma and Sir Bani Yas. These islands are composed of much older rocks than those of the Oman Mountains. The Paleozoic to Tertiary succession of the UAE has been divided into three major rock units (Glennie et al. 1973; Searle and Malpas 1980; Lippard et al. 1986; Glennie et al. 2005). From older to younger they are (Fig. 1): a) autochthonous units of Middle Permian to Upper Cretaceous carbonate rocks resting unconformably on older continental crust, representing a part of the stable Arabian Platform; b) allochthonous units that are composed mainly of Mesozoic rocks, including the Sumeini Group (slope sediments), Hawasina Complex (basinal sediments), Haybi Complex (distal oceanic volcanics and limestone exotics) and the Semail Ophiolite nappe (massive slice of former oceanic crust and upper mantle), that were emplaced as a series of thrust nappes in the Late Cretaceous; and c) a neo-autochthonous sequence of Maastrichtian to Lower Tertiary sediments that include the Qahlah, Simsima,
Fig. 1 Lithostratigraphic chart of the Northern Emirates (modified from Abdelghany 2006)
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Muthaymimah, Rus, Dammam, Asmari, Lower Fars and Barzaman Formations. In the present work, limestone samples were collected from Jabal Hafit, Jabal Buhays and Wadi El Bih (Fig. 2) to represent the Ghalilah, Musandam 3, Thamama, Muthaymimah, Dammam and Asmari Formations. The Ghalilah Formation is a mixed carbonate/siliciclastic formation straddling the Triassic/Jurassic boundary. It was defined as a single formation by Glennie et al. (2005). Its base is characterized by a 2 m thick band of ferruginous quartz sand, followed by limestone, quartz sandstone, shale and dolomites. It can be divided into three parts: the lower consists of reddish quartz sandstone and marl ~105 m thick; the middle part is an ~80 m thick limestone interbedded with marl; and the upper part consists of ferruginous quartz sandstone alternating with buff and grey marl and calcareous shale ~65 m thick. The Ghalilah limestone samples were collected from Wadi Hagil (Fig. 2). The Musandam Group was defined by Hudson and Chatton (1959) who named it after the Musandam Peninsula (the physiographical region that extends north from Ras Al Khaymah to the Straits of Hormuz). It is a monotonous sequence lacking key biostratigraphical markers. It is subdivided into three formations: the Musandam 1, Musandam 2 and Musandam 3 Formations, totaling ~1,250 m thick. The studied samples were collected from Musandam 3 Formation (Upper Jurassic), which is ~250 m thick. This formation is dominated by locally cross-bedded, bioclastic and peloidal limestones, many of which contain corals and are characterized by discontinuous chert bands and nodules towards the top. The top is marked by an erosion surface which is overlain by fine-grained, pale coloured limestones of the Thamama Group. In the present work, the Musandam 3 Formation samples collected from Wadi Hagil, Shamal area (Fig. 2) will be referred to as the Musandam Formation. The Lower Cretaceous Thamama Group is about 200 m thick (Fig. 1) and unconformably overlies the Musandam Group. It consists of calcirudites at its base with upwards coarsening and thickening trends, and is a locally channelized limestone sequence. The studied samples were collected from the topmost unit at Wadi Hagil, Shamal area (Fig. 2) from the Shauiba Formation, which is mainly represented by grayish white, thick bedded and highly fossiliferous limestone containing rudistid bivalves, Orbitolina spp. of benthonic larger foraminifera, molluscan shell fragments and microbial algae. The Muthaymimah Formation is Middle Paleocene to Early Eocene in age (Jabal Buhays) and unconformably overlies the Upper Cretaceous Simsima Formation. It is up to 100 m in thickness and consists of interlayered clay-rich marl with gypsum veinlets, followed by fossiliferous and argillaceous limestone. It is topped by thinly bedded marl interlayered with nummulitic, alveolinid limestone and
dolomitic limestone of Early Eocene age interrupted by a 2 m thick conglomeratic layer. The Dammam Formation (Middle to Late Eocene) is~ 600 m thick. Its lower part is composed of grayish, thickbedded, fractured and rarely cavernous limestone that varies locally to chalky or dolomitic limestone and is regularly interbedded with soft marl beds. The top of the Dammam Formation consists of thinly-bedded yellowish coloured nummulitic limestone with marl interbeds. The Asmari Formation (Early Oligocene) is ~140 m thick and is lithologically variable. The lower part is 30 to 40 m thick and consists of greenish, gypsiferous mudstone and brownish nummulitic marly limestone. The upper part is composed of thick-bedded chalky limestone with some layers of dolomitic limestone and marl.
Methods Six representative limestone samples, one each from the Ghalilah, Musandam, Shauiba, Muthaymimah, Dammam and Asmari formations, were prepared by rock crushing and sieving of the crushed material through 5-and 10-mm sieves to select the particle size range similar to that used in the construction industry (Boynton 1988; Cheng and Specht 2006). A detailed petrographic examination was performed using transmitted light microscopy (TLM), cathodoluminescence microscopy (CLM) and scanning electron microscopy (SEM). For TLM an Olympus BH-2 microscope was used, while the CLM was performed using a Technosyn8200MK11 cold cathode stage mounted on an Olympus BX41 microscope with a beam current of 250 μA and voltage of 15 kV, with an attached Olympus DP72 digital camera for imaging. The SEM instrument was a Quanta FEG 250 instrument with accelerating voltage of 200 V–30 kV and magnification of 610 up to 6,400,000. The frequency of each petrographic component was semi-quantitatively determined using the comparison charts of (Bacelle and Bosellini 1965a, b). The allochem grain size was determined in thin sections by measuring the maximum apparent grain diameter using an ocular with a millimetre scale (Friedman 1965), and applying the grain bulk concept of Dunham (1962). In addition, the roundness of the allochem grains was described according to the chart designed by Pilkey et al. (1967). The mineral composition of the samples was determined by X-ray diffraction analysis (XRD) using a Philips X-ray diffractometer (Model PW/1840) with a Ni-filtered, Cu-Kα radiation (λ=1.542A). The chemical composition was determined by X-ray fluorescence (XRF) using a Philips spectrometer (Model PW/1404) with Rh target and six analyzing crystals. Bulk density and apparent porosity of the representative samples were determined using the liquid displacement
United Arab Emirates limestones Fig. 2 Regional map of the Northern Oman mountains showing the locations of the collected samples (modified from Abdelghany 2006)
(Archimedes) method (ISO 5018: 1983), where the samples were soaked for 1 h under vacuum in kerosene (specific gravity (γ)=0.8).
The samples were examined with X-ray micro-computed tomography (micro-CT) using the SkySkan1172 system (SkySkan, Belgium) at the Institut für Geowissenschaften,
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Christian-Albrechts-Universität Kiel, Germany. Single grains were scanned with a beam energy of 70 kV, a flux of 141 μA and a Al/Cu foil at a detector resolution of 2.4 microns per pixel, using a 360° rotation with a step size of 0.6°. Reconstruction, segmentation and volume rendering were done using SkySkan software. Three-dimensional stereology image analysis was performed on cubic volumes of interest of 1,000 voxel edge length. Calcination of the limestone samples was conducted by loading in porcelain crucibles and firing for 0.25, 0.5, 1 and 2 h at 800, 900, 1,000 and 1,100 °C (a total of 16 firing trials for each sample) in a lab scale electrical muffle furnace. After each calcination run, the lime grains were immediately examined for their microfabric characteristics (via SEM and CLM), free lime content and hydration rate. The CLM has the benefit of distinguishing the different crystalline phases b ased on their cathodoluminescence behavior (Karakus and Moore 2002; Karakus 2005). For the CLM, the fired lime grains were directly mounted in Araldite resin, soaked under vacuum for 2 h, dried overnight in electric dryer at 100 °C, mounted on a glass slide and then polished using ethanol. The free lime content was quantitatively measured applying the Sugar method, where a definite weight of the lime is dissolved in sugary solution and then titrated against a standardized HCl solution (ASTM - C25-06 2006). The hydration rate was measured in terms of the rate of hydration temperature increase (Moropoluoua et al. 2001). The lime grains were crushed to a 1–2 mm size range and then mixed with water (1 lime : 4 distilled water) in a thermostatically isolated container. The rise of hydration temperature was measured each 15 s for a period of 20 min. The time of the maximum hydration temperature (ΔTmax sec) and its value (Tmax °C), the rate of temperature increase (RTI°C/s.), and the time for 60 °C temperature increase (T60 s.) were all measured or calculated.
Results Limestone petrography Siliceous mudstone, wackestone-packstone, packstonegrainstone and grainstone facies are all represented in the limestones of the Ghalilah, Musandam, Shauiba, Muthaymimah, Dammam and Asmari formations. The Ghalilah limestone (Late Triassic) is dominated by micrite (80 %) (Table 1), fine-to medium-grained carbonate clasts (20 %) and rare disseminated detrital quartz grains (Fig. 3a). This limestone is characterized by sparite veins crosscutting the compact, massive and nano-porous micrite groundmass (Fig. 3a). This limestone sample belongs to the siliceous micrite (intramicrite) facies.
Table 1 Average petrographic contents and physical properties of the limestone samples Sample
Allochems, (%)
Orthochems, (%)
Bulk density, g/cm3
Apparent porosity, (%)
Ghalilah Musandam Shauiba Muthaymimah Dammam Asmari
20.00 40.00 55.00 50.00 80.00 70.00
80.00 60.00 45.00 50.00 20.00 30.00
2.60 2.80 2.84 2.60 2.90 2.85
2.20 1.60 2.20 2.00 1.20 1.40
The Musandam limestone (Late Jurassic) is classified as wackestone-packstone facies (pelbiosparite). The allochems (40 %) (Table 1) are represented mainly by medium-and coarse-grained pelloids together with lower concentrations of smaller foraminifera, pelecypods (Fig. 3b) and perforated echinoidal fragments (Fig. 3c). The groundmass (60 %) is mainly neomorphic microsparite (Fig. 3b) with characteristic micro-cracks crosscutting the sparite crystals in places (Fig. 3d). Micrite is also recorded in many micro-areas and contains occasional sparite veinlets (Fig. 3e). The Shauiba limestone (late Early Cretaceous) is a packstone-grainstone (biosparite) dominated by echinoids, bryozoans and algal fragments as the main allochems (55 %) (Fig. 3f). These grains range in size from medium-to very coarse-grained and show mostly grain-supported fabrics. The latter skeletal grains are held in a blocky neomorphic pseudosparitic groundmass (45 %) of sub-hedral and anhedral shape (Fig. 3f). The Muthaymimah limestone also belongs to the packstone-grainstone facies (biosparite) (Fig. 4a). The recrystallized small foraminifera, bryozoan and algal fragments form about (50 %) of the carbonate content. These allochems are medium-grained and dispersed in a massive, granular, neomorphic pseudo-sparitic groundmass (50 %) (Fig. 4b). In places, the allochems are in point contact and the neomorphic sparite is micro-cracked (Fig. 4b). The Dammam limestone (Middle-Late Eocene) is characterized dominantly by gravel-sized (>80 %), well-rounded nummulite bioclasts sharing point and sutured grain contacts (Fig. 4c). In places, the nummulites may exceed 5 mm in diameter and the chambers are totally filled with orthosparite. Together with the nummulites, echinoids, bryozoans, miliolids and algal fragments (Fig. 4d) represent the predominant allochems (80 %). All grains are incorporated in a micritic groundmass (20 %) (Fig. 4c), however, neomorphic sparite is also common in the groundmass (Fig. 4d). This limestone belongs to the grainstone facies (biomicrite).
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Fig. 3 a A photomicrograph showing the Ghalilah limestone which is dominated by clasts (white arrow) in a micritic groundmass, sparite veins (red arrow) and detrital quartz grains (blue arrow) (PPL). b A photomicrograph of Musandam limestone which is composed mainly of structureless pelloids (red arrow) in neomorphic sparitic groundmass (blue arrow) (crossed polars). c SEM image showing echinoid fragment (black arrow) containing numerous pores (white arrows) of the Musandam limestone. d SEM image showing intra-sparite pores (white arrows)
and micro-cracks (black arrows) crosscutting the neomorphic sparite groundmass of the Musandam limestone. e Cathodoluminescence microphoto showing sparite veinlets (white arrows), small foraminifera (blue arrow) and an echinoid spine (green arrow) of the Musandam limestone. f Microphoto showing recrystallized algal allochem (red arrow) and echinoid plate (blue arrow) cemented together with other allochems by sparitic groundmass in the Shauiba limestone (crossed polars)
The Asmari limestone (Early Oligocene) (Fig. 4e and f) is petrographically similar to the Dammam (grainstone— biosparite). It differs only in having lower allochem (70 %) and higher orthochem (30 %) contents, compared with the Dammam Formation.
Limestone mineralogy, chemistry and physical characteristics XRD analyses (Fig. 5) independently confirm the mineralogical composition of the limestone samples determined by petrographic analysis. Calcite is the major mineral in all
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Fig. 4 a A photomicrograph of Muthaymimah limestone which is dominated by recrystallized foraminifera (red arrow) and pelecypod fragments in a sparitic groundmass (crossed polars). b SEM photomicrograph showing micro-crack of average 7 μm width (black arrow) crossing the neomorphic sparite in the Muthaymimah limestone. c A photomicrograph of Dammam limestone which is dominated by nummulites showing tangential and sutured-contacts (red arrows) The nummulite bioclasts (white arrow) are filled with orthosparite (blue arrow) and
embedded in a micritic groundmass (PPL). d Cathodoluminescence image of Dammam limestone showing bryozoa (white arrow) and echinoid plates (green arrow) cemented by sparitic groundmass. e A photomicrograph showing Operculina grains (white arrow) filled with orthosparite (red arrow) in a micritic groundmass of the Asmari limestone (PPL). f A photomicrograph of nummulite bioclasts (white arrow) and echinoid plates (red arrow) cemented by micritic groundmass, Asmari limestone (crossed polars)
samples with quartz as a significant mineral in the Ghalilah limestone. Dolomite occurs in minor amounts in the Asmari limestone. The chemical analyses (Table 2) are consistent with the petrography and mineralogy of the samples (Figs. 3, 4 and 5). CaO (48.15–55.53 %) is the major oxide in all samples due to
the predominance of calcite. The lowest content of CaO (48.15 %) in the Ghalilah limestone is linked with the highest content of SiO2 (12.85 %). The high SiO2 values are due mainly to the existence of the disseminated detrital quartz grains in the micritic groundmass of the Ghalilah limestone (Fig. 3a). Also the total impurity oxides (TIO) (summation of
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Fig. 5 XRD patterns of the studied limestone samples
SiO2, Al2O3, Fe2O3, TiO2 and MnO) is ~13 % in the latter sample. It is noticeable that with the exception of the Ghalilah limestone, the studied limestone samples form a rather homogeneous group in regard to their chemical composition. The physical characteristics, in terms of bulk density and apparent porosity, have been determined for the studied limestone samples. The bulk density ranged between 2.60 and 2.90 g/cm3 while the apparent porosity is bracketed between 1.20 and 2.20 %. Both characteristics show correlation with the contents of allochems and orthochems of the limestone samples (Fig. 6). Lime hydration, microstructure and calcination activation energy (E) Figure 7 shows the hydration behavior of the samples after firing for 0.25, 0.50, 1 and 2 h at 800, 900, 1,000 and 1,100 °C. The Ghalilah, Musandam, Shauiba and Muthaymimah samples have their lowest hydration rates at 1,100 °C-2 h firing condition (0.12, 0.25, 0.25 and 0.21 °C/s, respectively)
(Fig. 7a–d) (Table 2). The maximum hydration rates for these four samples are 0.80, 2.53, 2.49 and 2.18 °C/s, respectively, occurring at different firing temperatures and soaking times. The Dammam and Asmari show maximum hydration rates at 1,100 °C-2 h (0.65 and 0.70 °C/s, respectively) (Fig. 7e and f). The microstructure of the calcined lime is shown in Figs. 8 and 9. Lime is the main phase in all samples, however, Casilicate (C2S) and Ca-alumino-ferrite (C4AF) are recorded in the Ghalilah lime (Fig. 8a and b). The lime crystallites are minute (
