3.1. Mineralogical characteristics of produced clinkers
When the meta-schist sample was examined mineralogically, quartz, calcite and albite minerals made combustion difficult. In addition, minerals such as muscovite, chlorite and montmorillonite, which facilitate firing and positively affect burning have been determined. When the clay sample is evaluated in terms of burnability as a result of mineralogical analysis, it is known that quartz, calcite, anorthite and albite detected in the structure of the clay sample make burning difficult. In addition, minerals such as muscovite, chlorite and montmorillonite, which facilitate firing and positively affect burning have been determined. (Fig. 1). When the mineralogical structure of both minerals is compared, it is seen that the minerals that make burning difficult in the claystone structure are more and the minerals that make burning easier are less (Table 4). When the results are evaluated with the Rietveld method, it is seen that the amount of quartz in the claystone is 13% higher than that of the meta-schist (Table 4). In line with the mineralogical results of the raw material samples, it is predicted that raw meal produced with meta-schist (RM/M) can burn more quickly than raw meals produced with claystone (RM/C). High quartz content in clay substantially impacts the grinding of raw mix, clinker quality, and final cement produced. The presence of quartz requires excellent grinding and a long sintering time to react significantly, all of which are very expensive. High quartz content in clay is not suitable to be used as a component of raw material in the cement industry because the high quartz content creates problems during the grinding and clinkering stages. This is due to the neat and open chain bonding of Si-O that produces a solid crystal structure, making quartz hard, brittle and stiff, thus affecting the process. Although it is crucial to optimise the grinding circuit when dealing with high quartz content in clay, cost should be considered as it consumes a large amount of electricity [15, 16, 17].
Table 4
Mineral% and comparison with Rietveld method.
Names
of Minerals
|
R/Clay
|
Meta-schist
|
R/Clay-Meta-schist
|
%
|
%
|
%
|
Quartz
|
44.90
|
31.70
|
+ 13.20
|
Calcite
|
7.40
|
2.10
|
+ 5.30
|
Chloride
|
12.30
|
25.30
|
-13.0
|
Muscovite
|
8.30
|
15.0
|
-6.70
|
Montmorillonite
|
3.40
|
6.90
|
-3.50
|
Albita
|
16.30
|
10.0
|
+ 6.30
|
Orthoclases
|
0
|
5.70
|
-5.70
|
Ankerite
|
0
|
3.30
|
-3.30
|
Anorthite
|
7.40
|
0
|
+ 7.40
|
3.2. Raw meal burnability
The reactivity of the raw mix samples was evaluated based on unreacted lime (FCaO) content were programmed temperatures of 1200-1300-1350-1400 and 1450°C after sintering. Free lime content according to the sintering temperature of the examined mixtures is given in Fig. 2. As can be seen, the addition of meta-schist to the raw mix resulted in a well-burned clinker with a free lime value of 1.60% at 1450°C. The corresponding value for the clay raw meal sample was found to be 1.87%. The modules and phase percentages calculated with the chemical properties of the clinker samples produced at 1450°C are given in Table 5.
The free lime value of the meta-schist clinker is 1.60, indicating that the prepared raw meal has an easy-burning character (% Free lime < 2.00) (Table 5). It is also evident from the free lime values that the clinker, which is burned with the meta-schist raw material, is more easily burned than the clinker burned with claystone raw material. (Table 5). The main reason for this is that the amount of quartz in the clay is higher than the meta-schist. Different quartz content and particle size distribution have a decisive influence on the combustion process and the quality of the final product [15, 18]. Improving flammability with a faster heating rate and shorter sintering zones leads to smaller silicate crystals in portland cement clinker. This enhances the clinker's grindability and the cement's strength development [19].
Table 5
The chemical properties of produced clinker samples (Bogue Formula)
Chemical contents
%
|
Loi
|
SiO2
|
Al2O3
|
Fe2O3
|
CaO
|
MgO
|
SO3
|
K2O
|
Na2O
|
R-clinker
|
0.28
|
20.55
|
4.99
|
3.72
|
64.40
|
1.35
|
0.56
|
0.81
|
0.34
|
M-clinker
|
0.42
|
20.19
|
6.88
|
2.86
|
65.11
|
1.63
|
0.01
|
1.28
|
0.01
|
Modules
%
|
Free Lime
|
LSF
|
SM
|
AM
|
C3S
|
C2S
|
C3A
|
C4AF
|
Liquid Phase
|
R-clinker
|
1,.88
|
97.82
|
2.36
|
1.34
|
57.86
|
15.36
|
6.94
|
11.31
|
25.83
|
M-clinker
|
1.60
|
97.90
|
2.07
|
2.41
|
54.80
|
16.55
|
13.39
|
8.70
|
29.64
|
The microstructure of Portland cement clinker samples was examined in polished sections with an optical microscope. In the clinker sample of Portland cement obtained as a result of firing raw meal produced with meta-schist, it is seen that it does not affect the formation of microstructure and characteristic mineralogical phases. As in the reference clinker, meta-schist clinker consists of 4 main phase components (C2S, C3A, C4AF and free lime) and 2–5% of the total secondary compounds. Both clinker samples contain more or less euhedral alite and exhibit a distribution of alite crystals. Free lime is dispersed at lower rates than the reference clinker and among the other phases, especially in the meta -schist clinker. The meta-schist clinker was seen as well-formed brown alite crystals in the optical microscope, while the alite crystals were seen as bluish round, rich in lamellar. In both clinker samples, belite crystals mainly appear in nests and clusters, and the amount of belite was found to be less in the meta-schist clinker (Fig. 2 and Fig. 3). The point counting method and Bogue formula calculations confirmed this result. In (PC) R/M clinker, the small number of belite crystals and their even distribution concerning alite indicate that the clinkerization reaction proceeds mainly in the direction of quality and the raw mixture is much more homogeneous. Finally, in the (PC) R/M clinker, the liquid phase occurred as uniformly dispersed fine crystals, while in the (PC) Ref case, large C3A crystals were observed.
To produce good quality cement clinker, it is necessary to control the growth and enlarge alite crystals while burning the clinker and providing a large amount of alite. Thus, for a particular raw material mixture minimising the crystal size and optimises the kiln regime to ensure proper composition formation [20, 21, 22]. The phase amounts of the produced clinker samples obtained by the dot counting method under an optical microscope are shown in Table 6.
When the crystal sizes of clay-clinker clusters and clusters are examined, it is seen that they have a dense structure in the alite phase. Belite and free lime are in clusters. This indicates that the raw mixture is coarse-grained. Coarse-grained minerals can cause difficulty in burning [23]. Fine-grained raw meal should be produced to prevent and facilitate burning difficulties. When the crystal sizes of the clay-clinker granules are examined, it is seen that the alite crystals are of variable size (5–40 microns) and the belite crystals are of average size (25–40 microns). However, large sized alite and belite crystals were encountered in some granules. Therefore, it was concluded that improvements should be made in firing properties to obtain smaller and more stable alite standart crystals.
When the crystal sizes of meta-schist-clinker clusters are examined, it is seen that alite and belite crystals are generally of average size, but variable in size (5–40 microns), while belite crystals are of normal size (25–40 microns). Variable-sized alite crystals result from insufficient homogeneity of the raw mix. However, some granules in claystone clinker were observed in large alite and belite crystals. For this reason, the combustion capability can be increased with the improvements to be made in the process [24, 25].
Table 6
Optical microscope phase determination results of claystone and meta-schist clinkers.
Content
|
Clay-clinker Optik
|
%
|
Alite C3S
|
59.97
|
Belite C2S
|
19.09
|
(C3A + C4AF + Free CaO + Alkali Sulfate)
|
25.85
|
Content
|
Meta-clinker Optik
|
|
%
|
|
Alite C3S
|
54.68
|
|
Belite C2S
|
17.31
|
|
(C3A + C4AF + Free CaO + alkali sulfate)
|
28.01
|
|
The typical size of six-sided angular crystals of alite in an ordinary clinker is 25 µm–50 µm long, while the average size of belite-rounded crystals is 25 µm–40 µm long. it can be see that the belite (dicalcium silicate) from the high quartz clay was larger (belite nest) than those of low and normal quartz clay. This was due to the coarse particles produced from the raw mix caused by the high quartz. The smaller the crystals of alite, the higher the strength of the cement produced from this clinker. The size of the alite crystals affects the grinding costs (much energy is required to reduce large crystals) and the cement strength. It is known that the strength of the smaller alite crystal size of the two cements of the same chemical composition is higher because it provides a larger surface for hydration [26, 27].
The higher the ratio of di-calcium silicate (C2S), the more difficult the clinker is to grind. Cement produced from this type of clinker has a low heat of hydration and therefore low initial strength. Clinker is therefore caused by the presence of free CaO in the cement, improper selection of raw materials, not well mixed, not milled as desired, or that the mixture is not properly burned [28].
3.3. Hydraulic and mechanical properties of the produced cement samples
Portland cement samples were produced from clinkers produced using meta-schist and claystone. The clinker and cement sample produced with claystone was qualified as OPC/R (Ordinary Portland Cement/Reference) and cement sample produced with meta-schist was qualified as OPC/M (Ordinary Portland Cement/Meta-schist). Setting time and final setting times for Portland cements produced in Turkish standards are minimum of 1 hour and a maximum of 10 hours, respectively. According to ASTM Standards, the setting initial time is a minimum of 45 minutes and the set final time is specified as 375 minutes. Cement samples seem to satisfy both types of cement standards. The hydraulic and physical properties of cement samples are shown in Table 7.
Table 7
Hydraulic and physical properties of cement samples
Cement Types
|
setting time (minute)
|
Water percentage
|
(soudness)
|
sieve
|
spesific gravity
|
spesific surface area)
|
|
Initial
|
Final
|
%
|
mm
|
45 micron
|
90 micron
|
g/cm3
|
g/cm2
|
OPC/M
|
140
|
228
|
28,0
|
1
|
4,4
|
0,4
|
3,10
|
3204
|
OPC/R
|
152
|
232
|
28,1
|
1
|
4,2
|
0,3
|
3,10
|
3202
|
The mechanical properties foreseen for TS EN 197-1 cement (Cem I 42.5 R) are ≥ 20 MPa for the two-day strength class; it must be ≥ 42.5 MPa for the twenty-eight-day strength class. It is seen that the compressive strength results of both cement samples produced are above the Turkish Standard values (Fig. 4).
The changes observed in the phase and microstructure of the cured cement pastes were investigated using scanning electron microscopy (SEM) techniques and chemical analyses in micro domains (EDS analysis). A few hours after cement pastes are mixed, they form a solid structure that loses its plasticity. As time progresses, the soft plastic cement becomes less plastic and solidifies and hardens. Together with a calcium hydroxide (portlandite) and the hydration products of calcium aluminate, namely ettringite, hydrated silicates of calcium are formed, the so-called C-S-H-phase. Chemical reactions between the water and cement in the cement paste continue as long as the appropriate temperature and humidity environment is present. The main component of the cement paste, as well as the most critical factor affecting the strength of the hardening mass, is the phase indicated by the symbol C-S-H. As time progresses, the strength gained also increases with the increase of the C-S-H phase. The hydration and hardening mechanisms of cement are based on the density of the C-S-H gel with the addition of water and the crystallization of ettringite and calcium hydroxide intergrowths in the hardening cement paste and filling the pores in the hardening cement paste [29, 30]. Microstructure images of cement samples are shown in Fig. 5 and EDX images are shown in Fig. 6. The microstructure and morphology of the meta-schist cement mortar, characterized using scanning electron microscopy (SEM), showed a compact microstructure similar to OPC, with few microcracks, but with small cracks. The formation of some microcracks is more evident in OPC cement mortar.
C-S-H gels and very little portlandite were observed in both cement mortar samples. Depending on the C-S-H gels, compressive and splitting strengths gave high values. Depending on time, the reaction continues as long as C3S and water are present. As the volume (amount) of the formed products increases, the concrete gains strength and the permeability decreases [31].