CHAPTER 5. MICROSTRUCTURAL CHARACTERISTICS OF CALCIUM OXIDE-ACTIVATED
6.2. Raw Material Properties of GGBFS and Calcium Carbonate Powder and Experimental Details
6.2.1. Properties of raw GGBFS and calcium carbonate powder
Commercial GGBFS and captured calcium carbonate powder from carbon sequestration process were obtained for this research. The materials were investigated by a laser diffraction particle size analyzer (Sympatec, GmbH HELOS/RODOS), and conventional powder XRD (Rigaku, D/MAX2500V/PC). The results of laser diffraction for the materials are represented in Figure 6- 1.
(a) (b)
Figure 6- 1: Particle size distributions of (a) raw GGBFS and (b) carbon-sequestrated calcium carbonate powder from laser diffraction analyses.
As presented in Figure 6- 1 the particle size distribution of GGBFS used in this study does not have significant differences previous literature [5, 66, 124]. The calcium carbonate powder has a finer particle size distribution than GGBFS although the powder does not undergo any grinding
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 10 20 30 40 50 60 70 80 90 100
0.1 1 10 100 1000
Density distribution
Cumulative distribution (%)
Particle diameter (μm)
cumulative density
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procedures. In addition, the distribution of calcium carbonate powder used in this research is also finer than those of previous studies [68, 158, 160, 162-164] where most calcium carbonate powder was produced by grinding limestone. Median values obtained from laser diffraction are 9.53±0.09 μm for GGBFS and 4.74±0.03 μm for calcium carbonate powder.
The diffraction patterns of raw materials are presented in Figure 6- 2. Raw GGBFS does not contain any crystalline phases i.e., raw GGBFS used in this study was 100% of amorphous phase. The pattern of calcium carbonate powder is well matched with the reference pattern of calcite (PDF # 98- 016-4935) and does not contain any other crystalline phases, which means that the purity of calcium carbonate powder used in this study is significantly high.
Figure 6- 2: Diffraction patterns obtained by conventional powder XRD for raw GGBFS and calcium carbonate powder with a calcite reference pattern in ICDD PDF-2 database.
The oxide chemical composition of raw GGBFS was investigated by XRF spectroscopy (Bruker, S8 Tiger XRF spectroscope) as presented in Table 6- 1. The chemical composition of raw GGBFS does not have significant difference with previous literature [5, 50, 66, 124, 163] but because the GGBFS does not contain any calcium sulfate (i.e. anhydrite and gypsum), SO3 content in Table 6- 1 is lower than the GGBFS containing calcium sulfate sources.
Position [2Theta] (Copper (Cu))
10 20 30 40 50 60
Raw GGBFS
Reference pattern for calcite (98-016-4935) Raw calcium carbonate powder
Table 6- 1: Oxide chemical compositions of raw GGBFS obtained by XRF spectroscopy.
Formula Oxide content (wt./wt.%)
CaO 45.2
SiO2 34.0
Al2O3 13.7
MgO 3.6
SO3 1.4
TiO2 0.6
K2O 0.5
Fe2O3 0.4
MnO 0.3
Na2O 0.3
SrO 0.1
ZrO2 0.0
Raw calcium carbonate powder was investigated by SEM (Hitachi, S-4800) as shown in Figure 6- 3. Most calcium carbonate particles have rough surface and the shape of the particles are irregular. The particle sizes identified in SEM is consistent with the results of laser diffraction.
(a) (b)
Figure 6- 3: SEM SE image of raw calcium carbonate powder used in this study; (a) 3000X magnification, and (b) 120,000X magnification.
The chemicals used as activator were obtained as reagent grade chemicals. Calcium oxide (CaO, ~ 97% purity) was acquired from Daejung Chemicals and Calcium sulfate (CaSO4, ~ 99%
purity) was obtained from Sigma-Aldrich.
6.2.2. Experimental details for investigating the effects of calcium carbonate powder in CaO- CaSO4-GGBFS composite cements
The content of calcium oxide and calcium sulfate was fixed as 5 wt.% in weight ratio and the amount of GGBFS was substituted by calcium carbonate powder from 0 wt.% to 50 wt.%. The water- to-solid ratio was determined to be 0.38. The detailed mixture proportion is exhibited in Table 6- 2.
The sample labels in Table 6- 2 are indicating the substitution ratio by calcium carbonate powder. For example, the “10C” sample is indicating 10% substitution of GGBFS by calcium carbonate powder.
Table 6- 2: Mixture proportions for investigating the properties of CaO-CaSO4-GGBFS composite cements with different levels of calcium carbonate powder in weight ratio.
Label
Binder
water GGBFS CaO CaSO4 Calcium carbonate powder
0C 90 5 5 0 38
5C 85 5 5 5 38
10C 80 5 5 10 38
20C 70 5 5 20 38
30C 60 5 5 30 38
40C 50 5 5 40 38
50C 40 5 5 50 38
All materials except water were dry-mixed for the homogeneity of the materials for 5 minutes before mixing with water. After that, the mixing procedure was guided by ASTM C305 [125].
The mixed pastes were cast into cylinder molds of ϕ 2.54 × 2.54 cm for preparing MIP and SEM samples and into cube molds of 5 × 5 × 5 cm for compressive strength testing. The air voids entrapped in the pastes were removed by an electrical vibration plate for 1 minute. The casted samples were cured in humidity chambers under condition of 23±2°C temperature and above 95% RH. After 24 hours, all samples were de-molded and re-cured under the same curing conditions.
Compressive strength tests were carried out at 3, 7, 14, and 28 days for all samples. The average value of triple testing was determined to be final compressive strength of each sample. After
finishing compressive strength tests, damaged sample fractions were collected and finely grinded for preparing the powder specimen for XRD and TGA. The cylinder samples were sawed for preparing MIP specimens of 5 × 5 × 5 mm cube and SEM specimens of 2 mm thickness plate by using a precision cutter (Buehler, IsoMet 1000). A solvent-exchange method using IPA was selected to stop the hydration of samples as mentioned in chapter 2. All samples were stored in vacuum desiccators under pressure of ~ 60 cmHg for 1 day before tested.
The XRD patterns of hardened pastes were collected by using 6D beamline at Pohang Accelerator Laboratory using a wavelength of λ = 0.66 Å. Two dimensional diffraction images were converted to one dimensional diffraction patterns by adjusting its energy to have same wavelength with copper k-alpha radiation (λ = 1.5406 Å) to directly compare with the diffraction patterns obtained from conventional XRD. Obtained diffraction patterns were analyzed by Match software with COD [61].
The TGA experiments were conducted by SDT Q600 (TA Instruments) for 3-day and 28-day curing samples. The heating rate was determined to be 10°C/min within the range of ambient temperature, to 1000°C under nitrogen gas environment.
The pore structures of hardened pastes were investigated by MicroActive Autopore V 9600 (Micomeritics Instrument Co.) under generating pressure of 0.2 to 61,000 psia. The microstructures of hardened pastes were also investigated by FE-SEM (Hitachi, S-4800) with EDS analysis in BSE mode.
For preparing SEM BSE samples, hardened pastes were sliced and polished with 9, 6, 3, and 0.5 μm diamond suspensions to obtain a thickness of ~30 μm. Osmium coating was carried out before SEM analysis.
6.3. Compressive Strength Development of CaO-CaSO4-GGBFS Composite Cements with