Experiential investigation on the effect of heavy fuel oil substitution by high sulfur petcoke on the physico-mechanical features and microstructure of white cement composites

Samples of PC were collected on the following plan, (1) As delivered to the plant and collected from the storing yard. (2) As pulverized coke, which was collected isokinetically from the pipes leading from the pulverizer to the pre-heater and kiln main burner and calciner after the milling process and passing the dynamic separator then pulverizer collected from a hopper at the base of the pulverizer as shown in figure 1. HFO sample are taken from the boiler exit at the plant. White cement (WC's) samples are collected from Sinai white Cement Co. ELArish plant, Sinai Governorate. Physical features of petcoke and pulverized petcoke are shown in table 1. Also, main chemical composition and physical properties of HFO is shown in table 2. Table 3 explains the XRF analysis of both cement types.

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Generally, the physico-chemical and mechanical features of both WC_HFO and WC_PC were examined. Cement patches were molded into steel molds of (2.5 cm*2.5 cm*2.5 cm), then cured in humidity chamber of 98 ± 1% relative humidity (RH) at room temperature. After one day of curing,, the hardened cubes were de-molded and immersed directly in the cabinet filled by tap water with 98 ± 1% RH for 1,14, 28 and 90 days of hydration to measure the compressive strength. The curing condition was chosen according to ASTM [47] which showed the positive role of (high RH) on the performance of the cement-pastes. Setting times of WC_HFO and WC_PC cement patches measured by Vicate apparatus based on ASTM C191 [48]. Compressive strength (CS) was applied three- hardened white cement cubes for each paste according to ASTM C109M [49]. Sodium equivalent content (Na₂O_Eq) was calculated according to following equation: Na₂O_Eq,% = [Na₂O - 0.659 × K₂O] [50]. Whiteness reflection performed using 'Elerpho' apparatus according to DIN 5033 specs [51]. After 90 days the hydration rate of WC_HFO and WC_PC pastes were stopped by immersing of white cement cubes broken in acetone: methanol mixture as [1:1] ratio for one day, then followed by drying process at 60 °C for 8 h, then a part of dried sample was kept for testing with scanning electron microscopy (SEM), detailed industrial diagram for WC's samples collection from the cement plant is plotted in figure 2.

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The chemical composition of The WC's were performed by using XRF-ARL instrument (XRF-9900). Determine phases of hydration by using x-ray diffraction (XRD), which carried out using Philips (PW3050/60) diffractometer using a scanning range from 5 to 50 (2Ø), with a scanning speed of 1 s/step and resolution of 0.05°/step. On other side, another part of dried sample was milled to pass through (90 μ° mesh) to determine the microstructures (SEM-photos) were obtained with Inspect S (FEI Company, Holland) equipped with an energy dispersive.

Figure 3 Illustrates SEM micrographs of WC_HFO and WC_PC pastes hydrated for 90 days. I have noticed that the morphology of WC_PC pastes completely differ than that of WC_HFO, which can be attributed to the follows:

• Formation of excess hydration products of fibers calcium silicate hydrate (β-CSH-gel like flocks) adhered to the cement/clinker surface. Taylor has reported that the phases in which sulfur can occur in Portland cement clinker are as follows: arcanite (K₂SO₄), aphthitalite (3K₂SO₄·Na₂SO₄), thenardite (Na₂SO₄), calcium langbeinite (K₂SO₄·2CaSO₄), anhydrite (CaSO₄) and as a substituent in the major clinker phases, principally in the alite and belite with little in the ferrite phase [52].
• Appearance of new phase (red cycles in SEM images of WC_PC ) of the hydrated product with excess sulfur content in the PC combustion material and the alkali Na⁺, K⁺ and Ca⁺² equivalent present in the kiln hot meal which gives hardness compact microstructure than other WC_HFO pastes due to the incorporation of C2S-phase with these alkalis form Na₂SO₄, K₂SO₄ and Ca₂SO₄ salts promoting the late compressive strength and cement durability [53, 54].
• Finally formation of calcium hydroxide (plant like crystals) as a result of reaction of free lime content with sulfur which gives hardness and compact structure inside the cement pastes structure. WC_HFO pastes still have un-combined free lime even after 90 days of hydration (red cycles in SEM image of WC_HFO ), which effect badly on the compressive strength behavior and consequently negative effect on the microstructure of calcium silicate hydrate (CSH).

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Table 4 illustrate the sulfur phases in weight (%) using XRD analysis and Taylor's equations [52], which gives a great indications about the identity and the concentrations of phases contained sulfur in weight (%) percent. The main 3 minerals were detected by XRD and the other 2 phases calculated by Taylor's equations. The weight of anhydrite and Aphthitalite are high and this may be an indication that part of them were insoluble. In addition to that clinker sulfur tend to be double sulphate salt neither calcium langbeinite nor single sulphate salt as potassium sulphate, and it can't be directly detected but theoretically by using Taylor's equations. Presence of calcium langbeinite in white cement promote the lumps formation of cement due to presences of high alkali content detected in cement analysis.

The setting time of WC_HFO and WC_PC are presented in figure 4 obviously, the setting times of the WC_HFO-patches were shorter than the WC_PC-patches. The changes of initial and final setting time were carefully detected as explained elsewhere [55]. It can be expressed according the following order: WC_PC > WC_HFO. The longer initial and final setting time of WC_PC-patches can be attributed to the excess of SO₃ content in cement during the hydration process and half of that sulfate is anhydrate which is not soluble in water and unable to convert calcium tri-aluminate (C3A) into ettringite as sown in following equation:

$$\begin{eqnarray*}&&{\rm{C}}3{\rm{A}}+3{{\rm{CSH}}}_{2}+26{\rm{H}}\to {{\rm{C}}}_{6}{{\rm{AS}}}_{3}{{\rm{H}}}_{32}\left({\rm{ettringite}}\right)\end{eqnarray*}$$

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C3A is known as the highest reactive phase among all cement phases, and have a strong influence on the early hydration as well as rheology of portland cement and concrete. Initially C3A forms the metastable hexagonal hydrates (C₄AH1₉ and C₂AH₈) in the presence of water, which are then slowly converted to stable cubic hydrates (C₃AH₆) [56]. The hydration of C3A can be represented as follows [57]:

$$\begin{eqnarray*}&&2{\rm{C}}3{\rm{A}}+27{\rm{H}}\to {{\rm{C}}}_{4}{{\rm{AH}}}_{19}+{{\rm{C}}}_{2}{{\rm{AH}}}_{8}\left({\rm{hexagonal}}\,{\rm{hydrates}}\right)\to 2{{\rm{C}}}_{3}{{\rm{AH}}}_{6}\left({\rm{cubic}}\,{\rm{hydrates}}\right)\end{eqnarray*}$$

It was observed during setting time test for WC_PC that the cement tendency to false set or flash set due to precipitation of gypsum during hydration in the first minutes, is characterized by the fact that the false set disappear by remixing. Meanwhile, the WC_HFO has been the shortest setting times due to the sulfate/alkali ratio is in acceptable limits as HFO had the lowest sulfur content compared to PC and the produce clinker is more granular than dust product respectively.

Compressive strength results of WC_HFO and WC_PC are depicted in figure 5 As a function of hydration ages. As alkali benefits WC_PC showed the highest compressive strength comparing to WC_HFO. The compressive strength of WC_PC shows sharp increases with curing ages for all hardened pastes especially at the late strength records as a result of high SO₃/alkali ratio and Na₂O_Eq prompting formation of excess amount of hydration products especially, calcium silicate hydrate (β-CSH) which responsible for capacity building as shown in the detailed graph(6). Excess SO₃/alkali (ratio > 1.2) and Na₂O_Eq affects clinker burnability and increases the degree of volatilization as they acts as fluxes at combustion temp ≥ 1050 °C [58, 59]. Early strengths of WC_PC is lowered comparing to WC_HFO due to the incorporation of alkali equivalent to the C2S phase (rather than K₂SO₄, Na₂SO₄ which dissolves rapidly and leads directly to higher late strengths), which reduce the soluble alkalis in cement so the early strength of WC_HFO is higher than WC_PC.

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Sodium equivalent of WC_HFO and WC_PC are depicted in figure 6. WC_PC has the higher Na₂O_Eq compared to WC_HFO. Due to high sulfur content in the PC combustion material, the WC producers use a complementary raw material which known as Potassium Feldspar (PF) [Sanidine (monoclinic) (K,Na)AlSi₃O₈] [60, 61]. Potassium Feldspar is rich in alkalis (potassium, sodium and calcium) act as a flux, lowering the melting temperature of the hot meal mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the alkalis to sulfur and other excess components of the system during combustion together forming sodium sulfate Na₂SO₄, potassium sulfate K₂SO₄ and anhydrite Ca₂SO₄. These alkaline salts are desirable in the combustion atmosphere inside the cement kiln system, which prevents cyclone blockage and any accumulation at the entrance to the kiln or inside it, in addition to that it enhances the cement/clinker late strength and durability. On the other side the presence of insoluble anhydrite, that makes cracks, endearments and yellowish color, This is considered the worst in the use of cheap petroleum coke in manufacturing of white cement, in addition to increasing the percent of sodium equivalent in the final product by >0.6 maximum permissible, will reduces the applications of WC's. The properties and effect of the sodium equivalent are shown in the flow chart (7), where it is clear that when the alkali equivalent exceeds the boundaries of the standers, it reflected in various effects either in the cement processing line nor in its features and applications are shown in the flow chart as explained in figure 7.

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WC_PC showed very pale white color compare to WC_HFO i.e.: Ry = 82.20 versus Ry = 87.10 respectively, its reflection degree under the Elerpho was low especially on the Rz axis which reflect the green contrast i.e.: Ha = −1.22 versus Ha = −4.79 for WC_HFO, as showed visually in figure 8, this evidence supports the decrease at the WC_PC whiteness degree (Ry). It can be expressed according the following order: Ry- WC_HFO > Ry- WC_PC conform to DIN 5033 (relative to 100% Rx, Ry and Rz) [51].

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Clinker/Cement reflection degree (Ry) effect negatively by using PC in the combustion process due to precipitation of unburned PC particles on the core of cement/clinker particles and due to its poor granulated (dust product) so it needs a huge amount of quenching water during cooling system in order to oxidize its color and become 80% of its size white and the reaming 20% becomes yellow and save its reflection degree (Ry) and its applications as shown in the follows figure 9:

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