CC BY
20UDK 666.972
STUDY ON CRYSTALLIZATION PROCESS OF CEMENT PASTE FOR
PORTLAND POZZOLAN CEMENT
111 2 Erdenebat Tserenjav , Usukhbayar B , Nyamjargal M , Lygdenov B.D
1 - Laboratory for Chemistry and Technology of New Materials, School of Applied
Sciences and Engineering, National University of Mongolia (Ulaanbaatar, Mongolia)
2 - East Siberia State University of Technology and Management
(Ulan-Ude, Russia)
* - corresponding author
erdenebatt@num.edu.mn; lygdenov59@mail.ru
1. Introduction
Coal is the most abundant resource of fossil fuel at the present in our country. Coal mines gradually shifted toward the modern industrial production and open-pit coal mines were created in almost every aimag and thermal coal domestic demand was completely increased too. Therefore coal has a relatively more importance for the economic growth of the country and our GDP rates, which is driven by coal mining industry's contribution. From other hands the blended cement with high volume mineral additive (HVMA) meet low carbon economics in cement by using a low carbon concrete to increase product effciency to cement and an excellent durability in terms of hydraulic free or sulphate resistance and chloride mitigation for use fundamental structures of construction and highway [1].
Figure 1. Coal reserves in Mongolia
Figure 2. Coal production and IBMA in Mongolia
Recent days Mongolia has over 300 known coal deposits in 12 coal basins and 3 regions, which total reserve (Fig1) is estimated in 152 billion tons and 22.5 tons of
these reserves have been proven [2]. The Tawan Tolgoi is one of the world's largest untapped coking and thermal coal deposits in our country, located in Tsogt-Tsetsii soum, Umnugobi (province) aimag in Southern Mongolia, and divided into six sections: Tsankhi, Ukhaa Khudag, Bor tolgoi, Borteeg, and Southwest and Eastern coalfields. The Tsankhi coal field section is the largest and the most focused part in recently, and it divided into East and West Tsankhi. The Baga Nuur coal mining deposit is located in a municipal district of capital city of Ulaanbaatar (UB) in 620 km2 area at the border between the Tuv (Central province) and Khentii (one of the East province) aimags, actually a separate city. The BN city is the keypoint of central heating city of UB. Transportion of coal through the freight trains of UB-Baga Nuur-UB and 138 km of paved road are accessible to UB and other neighbouring towns.
3
The overburden (Fig.2) of the mining is removed by dragline (with 15-20 m bucket), shovel (with 5-8m bucket) and dump truck (with 40-80t) then stored in open air environment at every local mining area [3]. Open-pit mining is a developmental activity, which is taken us to the natural ecosystem damage by several mining activities. By open-pit mining, the soil is removed and the fragmented rock is heaped in the form of overburden dumps [4]. But the average stripping ratio from year to year (overburden to coal) for the last two decades was
1.97m3/t [5, 6, 7, 8]. The
selection of coal mining overburden (OB) area depends on various parameters as physical and chemical properties of overburden and cement clinker compositions. The physico chemical properties of overburden dump materials are specific and differ at site from one dump to another dump due to different geological deposit of rocks. The objective of the present study was to characterization of blending effect of cement mortar with various amounts of additives by cement weight with same fineness value, including precipitation speed or thermal activation of Industrial byproduct as overburden for HVMA as alternative material to clinker substitution purposes of blended cement.
2. Experimental procedure 2.1. Materials and Methods
The binary blended cements were prepared by using OPC 42.5 Portland cement and cement substituting mineral additive as the TawanTolgoi, at west Tsankhi as 43o37'30" N, 105o28'27"E, Umnugubi province, owned by TawanTolgoi LLC and from Baga Nuur as 47o75'77"N, 108o33'33"E, owned by BagaNuur JSC overburdens. Chemical and mineralogical compositions of overburdens are relatively different depending on each type geographic zone and the Baganuur overburden samples are divided into 2types as no clay and clayish. Their and TT OB's chemical, and mineralogical analyses results and reference Portland cement OPC 42.5 MNS-ASTM of Khutul cement are given in Table 1-3, which was done by chemical, mineralogical and differential thermo analyses of TG/DTA undertaken by DERIVATOGRAPH, Hungary, with using of a-Al2O3 reference material in 10oC/min heating rate.
Table 1. Chemical compositions of BN overburden /%
Compositions SiO2 AI2O3 CaO Fe2O3 TiO2 MgO R2O H2O+ LOI* E
BN OB, no clay 64.96 13.17 3.10 2.17 0.31 0.3 3.83 2.20 9.90 99.94
BN OB, clayish 64.91 16.92 0.90 3.05 0.52 - 3.97 2.38 7.34 99.99
TT OB 50.18 28.13 3.93 3.32 0.05 < 0.3 3.23 3.12 7.23 99.49
The reference cement as PC 42.5_MNS_0974:2008 Portland cement was supplied by "Khutul" CLC (Cement and Lime Company).
Table 2. Mineralogical composition of the Hutul cement,%
C3S C2S C3A C4AF Free lime
56 21 8.2 13.7 0.61
As seen from Table 3, slowest weathering and most stable series of plagioclase feldspar as the sodium rich siliceous and aluminum constituents for BN and clay minerals are dominant in mineralogical compositions. Detection of hydration group and its dehydration and phase transformation processes were controlled too by IR spectra measurement with using of Csl pellet method by Shimadzu, Japan, FT/IR-4400 spectrometer.
Table 3. Mineralógica! compositions of overburdens, /%
№ Minerals BN_OB, % TT_OB, % Specific gravity, g/cm3
1. Quartz: SiO2 35.0 7.50 2.65
2. Microcline: K[AlSi3O8] 25.3 11.0 2.60
Feldspars Oligoclase: Na,Ca[AlSi3O8] 37.20 6.50 2.70
3. Muscovite : K2Al4(Si6Al2)O20(OH)2 0.30 0.80 2.75
4. Magnetite: FeO ■ Fe2O3 0.20 - 5.00
Nlminite: FeTiO3 0.05 - 4.80
Biotite (mica-monocline):
5. K (Mg,Fe)3[Si3Al010]- [OH, F]2 1.10 0.30 2.80
Clayish material-Zeolite:
6. Ca(Al2Si10O24)*7H2O 0.40 5.50 2.65
7 Nacrite: Al2Si2O5(OH)4 0.20 35.0 2.55
8 Kaolinite: Al2Si2O5(OH)4 0.30 33.40 2.55
The cement mortar specimens for compressive strength were aged for 3, 7, 28 and 90 days in a laboratory medium of 20±2oC temperature and 50±5% relative humidity. Demolding after 24h, the specimens were kept in water until they were tested for compressive strength. Six specimens with dimensions 20mm x 20mm x 20 mm, were tested under same laboratory conditions. These compressive strength tests were carried out by using hydraulic compressor of 20 000N as PSU-35 (Russia) and reported the average of six measurement of each specimen. The hydration products were identified by X-Ray Diffraction Analyses (XRDA) and Transmission Electron Microscope (TEM). The hydrated products were grinded in an agate mortar. XRD measurements, carried out by DIFFRACTOMETER D-500 (Germany), by using Ni-filtered Co Ka, with a 0.001o step size in the range of 5-55°0. In order to determine
the properties of 28days hydrated phases of silicate calcium was used a scanning electron microscope as JEOL ARM-200F.
3. Results and Discussion
3.1. Activity of Mineral Additives
3.1.1 HVMA of BN_OB
The BN overburden was suspended in tape water while kept them for 24 hours and the ratio of solid: liquid as 1:9 [5]. Then the suspended BN_OB was activated by alkali and alkali earth solution of potassium hydroxide; sodium hydroxide; and calcium hydroxide solution of 1%, 3%, 5%, 10%, 15%, 20% respectively and pH value, suspension activity in some inorganic solution, and their coagulation speed were determined. The precipitation speed kinetics study of activated BN_OB by alkali and alkali earth solution was shown in Table 4 depending on their coagulation speed in detail.
Table 4. Coagulation rate kinetics for alkali activated specimen of BN OB
(by time, min)
Coagulants Samples 1%, 3%, 5%, 10%, 20%,
Clayish 4,01 3,50 3,50 4,00 7,01
KON No clayish 4,00 4,00 4,00 7,00 7,00
Clayish 4,30 4,30 4,20 4,30 6,30
NaOH No clayish 3,22 3,22 3,22 4,23 5,23
Clayish 3,20 3,00 2,20 4,40 5,00
Ca(OH)2 No clayish 3,20 3,00 2,30 4,40 5,00
The physics-chemical characteristics as grain size distribution, hydrogen ion saturation factor, pH (see Table 5), and mineral and chemical some characteristics, suspension activity in most suitable coagulant solution were determined in detail for each BN_OB samples.
Table 5. Hydrogen ion saturation factor and pH value of BNOB
Overburden № suspension* рН value
1 2
1 Unactivated BN_OB 6,04 6,07 6,06
2 Activated BN_OB 7,15 7,15 7,15
*(S : L = 1 :9)
3.1.2 HVMA of TT_OB
The nacrite and kaolinite was changed by various amount into metakaolin respectively (eq.1) due to dehydration during heating, the other minerals such as feldspars, cericite, biotite and quartz in overburdens remained no change in thermal activated TT Overburden.
Al2Si2O5(OH)4 ^ Al2Si2O7 + 2 H2O (eq.1) The TT_Overburden activated by thermal method in 550, 650,750oC from 1, 2, 3 hour respectively (from physics-mechanical study preferred 2h) and ground by a ball mill to a fineness of 0.01% residue on a 65 ^m sieve. Physical properties of the OBs as fineness, particle size distribution were investigated by Blaine method and results shown in the Table 6.
Table 6. Physical properties of the TT OBs
Specimens
OPC HVMA_BN HVMA_TT
42.5
Blaine specific surface area, 346 423 460
m2/kg
3.2 Blended Cement properties 3.2.1 Cements' compressive strengths
The cement strength is the main point to evaluate the cement quality. Besides cement strength, cement setting time is an important property in terms of the workability of the cement. In practically gypsum may be interground with the clinker and the HVMA in order to regulate the setting time of the cement. Physics-mechanical properties as compressive strengths of TT or BN blended cements were determined under comparison control of OPC42.5 without and with 5, 10, 15 and 20% amounts of HVMA by cement weight while after strengthened them in water condition for 3, 7, 28, 90 days.
The results of the physics-mechanical tests for the binary blended cement mortars were described by Figure 3 and 4. In Table 7 and 8 are shown the range of the BN_OB and TT_OB blended cement setting times and their performance of compressive strength.
Table 7. Study results for the BNOB blended cements' strength kinetics
Blended Setting time, min Compressive strength, kg/cm2
cement, Initial Final 3 days 7 days 28 days 90 days
0 175 500 410 420 430 450
10 170 490 280 440 530 530
15 160 425 330 510 540 540
20 150 380 360 640 710 730
25 140 445 330 550 600 660
The final setting time of BN_OB blended cements was slightly accelerated and compressive strength increased by 300kg/cm2 about at a dose of 20% replacement of
PC.
HVMA BN OBBlended cement strength test
□ PC-42 5R MNS 0974:2005
i| И 10% HVMA of BNOB
□ 15 % HVMAof BNOB Я 20% HVMA ofBNOB Ш 25% HVMA ofBN OB
time, day
Figure 3. Physics-mechanical properties of HVMA BN_OB blended cements mortar for 3, 7, 28, 90 days
In the Figure 3, a case of 20% of HVMA of BN_OB specimen showed the highest strength development at the age of 90 days even in early age it has lower effect than referred material of PC 42.5 MNS 0974:2008.
Table 8. Study results for the TTOBblended cements' strength kinetics
Blended Setting time, min Compressive strength, kg/cm2
cement, Initial Final 3 days 7 days 28 days 90 days
0 175 500 410 420 430 450
10 164 445 360 400 460 460
15 150 435 380 460 580 580
20 130 410 400 440 560 560
The final setting time of TT_OB blended cements at a dose of 20% replacement of PC was slightly accelerated and compressive strength increased by 100kg/cm2 over at a dose of 15% replacement of PC.
HVA1A TT OB_ blended cement strength test
700
J 7 28 90
Time, days
Figure 4. Physics-mechanical properties of HVMA TT_OB blended cements mortar for 3, 7, 28, 90 days
In the case of HVMA of TT_OB specimen the 15% of cement weight amount was more effective strength development at the age of 90 days and it clear described in Fig.4, even in early age it has lower effect than referred material of PC 42.5 MNS 0974:2008 too.
3.2.2 TEM and XRDA
Blended cement pastes of PC 42.5_MNS_0974:2008 with and without HVMA mortars hardened in water conditions for 3 days respectively were investigated by using XRDA and TEM analyses.
Cement Hotol
Figure 5. X-Ray diffraction pattern and TEM micrograph of C-S-H phases in
hydrated cement referring material of PC 42.5_MNS_0974:2008 for 28 days
Figure 6. X-Ray diffraction pattern and TEM
micrograph of C-S-H phases formed in hardened HVMA_BN_OB_Hydrated Blended Cement for 28 days
From hydrated referring material of 3days hydration age PC 42.5_MNS_0974:2008 TEM showed that unhydrated cement grains were showed needle shaped fibers of ettringite in the middle of C-S-H crystals gel which is surrounded and widely resembled that XRD pattern of C-S-H were dispersed through the cement paste (Fig.5).
The hydrated blended cement of 3days hydration age HVMA_BN_OB (Fig.6) C-S-H gel calcium silica hydrates gel pores and unhydrated cement belite grains were being in middle of C-S-H gel. The obtained C-S-H is surrounded by resembled calcium silica hydrates gel, which was widely dispersed through the cement paste in TEM and XRD pattern of C-S-H (I and II) phases as hydraulic free phases in cement colloidal system[6, 10].
Figure 7. X-Ray diffraction pattern and TEM micrograph of C-S-H phases formed in hardened HVMA_TT_OB_Hydrated Blended Cement for 28 days
From the TEM analysis results of 3 days hydration age hydrated blended cement of HVMA_TT_OB can be suggested that calcium silica hydrates more stable crystal structures were formed on the result of hydration reactions. The calcium silica hydrates gel containing capillary pores and hexagonal crystals of calcium hydroxide (portlandite) conglomerations were observed (Fig.7) especially in the case of HVMA_TT_OB. This shows that HVMA_TT_OB intensifies the formation of sheetlike C-S-H stable crystal phases [12, 15, 16, 18].
Conclusions
The following conclusions can be drawn from the study: Both TT and BN overburden used in this study had up to 20% of major cement weigth, conforming with the chemical requirements of the ASTM and Mongolian
Standards. They fulfilled physics-mechanical requirements concerning compressive strength.
The colloidal solution precipitation speed was more effective when use 5% Ca(OH)2 for alkali activation of BN overburden. The fineness of the HVMA blended cement analysis results show that the final product of hydration C-S-H sheet like gel structure forms dominantly in early age of 3 of them. TEM analysis results show that compared to PC42.5R ASTM: MNS and with addition of TT or BN HVMA crystallizes with forming of stable crystals with round shaped or hexagonal geometric forms of CSH. This supports the results of XRDA. X-ray analysis results show that the content of Ca(OH)2 phase decreases and the formation of hydrated calcium silica phase increases due to the addition of both TT or BN overburden than to "PC cement" system at the early age of hydration.
The addition of BN_HVMA leads to content of Ca(OH)2 phase decreases and the formation of hydrated calcium silica phase increases due to the addition of both TT or BN overburden than to "PC cement" system at the early age of hydration. HVMA cement contributes to the reduction of carbon dioxide and other emissions at source. Due to its better eco-compatibility, the market share of HVMA cements will increase in the future.
Acknowledgements
The authors wish to acknowledge financial contribution made with Project as
N°.SSA_010/14 which is supported by the Ministry of Education, Culture and Science of Mongolia and Mongolian Foundation of Science and Technology.
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