CHAPTER 4. MICROSTRUCTURES AND MECHANICAL PROPERTIES OF BARIUM
4.2. Raw Material Characterization and Experimental Methods for Verification of the Properties of
4.2.1. Materials used for preparing barium hydroxide- and calcium hydroxide-activated slag cements
† The contents of this chapter have been submitted for publication in journal paper.
Commercial GGBFS was obtained for this study. The material was examined using XRF spectroscopy (S8 Tiger; Bruker, Germany), a laser diffraction particle size analyzer (HELOS/RODOS;
Sympatec GmbH, Germany), and high-powered powder conventional XRD (D/MAX 2500V/PC;
Rigaku, Japan) for internal standard semi-quantitative analysis. The oxide chemical compositions from XRF are exhibited in Table 4- 1. The results shown in Table 4- 1 did not demonstrate a large difference from general GGBFSs [50].
Table 4- 1: Oxide chemical composition of raw GGBFS obtained by XRF spectroscopy.
Formula Oxide content (wt. %)
CaO 45.8
SiO2 31.1
Al2O3 13.0
MgO 5.1
SO3 3.1
TiO2 0.6
K2O 0.4
Fe2O3 0.4
MnO 0.2
Na2O 0.2
BaO 0.1 SrO 0.1
The 10 wt.% of crystalline corundum was added into 90 wt.% of raw GGBFS powder as an internal standard. The XRD pattern was taken with Cu-Kα beam (λ = 1.5418 Å) and 5°–60° scanning range at 2θ and is shown in Figure 4- 1. The results of semi-quantitative analysis using the reference intensity ratio (RIR) method with the X’pert HighScore Plus program are included with identified phases in Figure 4- 1; raw GGBFS contained about 3.6% of anhydrite (CaSO4), 0.4% of gypsum (CaSO4·2H2O), 0.4% of calcite (CaCO3), and 95.6% of the amorphous phase. Calcium sulfate sources (i.e., anhydrite and gypsum) are commonly added to GGBFS during the manufacturing process for improving grinding efficiency and meeting the required standard for chemical composition in Korea where the chemical composition of commercial GGBFS should be required to maintain the basicity defined as “b = (CaO + MgO + Al2O3) / SiO2” over 1.6. Calcite was also one of the phases, which have been often found in the GGBFS [123]. There is no singularity in raw GGBFS used in this chapter, indicating that obtained GGBFS have the representativeness of commercially available GGBFS.
Figure 4- 1: The diffraction pattern of raw GGBFS with reference diffraction patterns and results of semi-quantitative analysis. Numbers attached in reference patterns indicate the numbers of ICDD PDF-2 for each phase.
The results of laser diffraction particle size analysis are presented in Figure 4- 2. The characteristics of particle size distribution did not show any significant difference from those of previous studies [5, 66, 123, 124].
Figure 4- 2: Particle size distribution and cumulative curve for obtained GGBFS.
Position [rr2Theta] (Copper (Cu))
10 20 30 40 50 60
Calcite (00-005-0586) Gypsum (98-016-1626) Anhydrite (98-001-5876) Corundum (01-081-2267) Anhydrite: ~3.6 (wt.%) Gypsum: ~0.4 (wt.%) Calcite: ~0.4 (wt.%) Amorphous: ~95.6 (wt.%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 10 20 30 40 50 60 70 80 90 100
0.1 1 10 100 1000
Frequency
Percentage (%)
Particle size (μm) Cumulative
Distribution
4.2.2. Detailed experimental methods for the comparison of barium hydroxide- and calcium hydroxide-activated slag cements
Two types of activators were used for GGBFS: barium hydroxide of ~ 95% purity (Sigma- Aldrich, St. Louis, MO, USA) and calcium hydroxide of ~ 95% purity (Junsei Chemical, Tokyo, Japan). The mixture proportions are shown in Table 4- 2, where BH and CH indicate Ba(OH)2 and Ca(OH)2, respectively; the AS is an acronym for activated slag; WR and MR designate weight ratio and molar ratio, respectively.
Table 4- 2: Mixture proportions for investigating barium hydroxide-activated slag cement.
Sample label GGBFS
(wt.%)
Activator (wt.%) Water (wt.%)
Water/GGBFS (wt. ratio) Ba(OH)2 Ca(OH)2
BHAS 100 10.0 - 40 0.4
CHAS CHAS-WR 100 - 10.0 40 0.4
CHAS-MR 100 - 4.32 40 0.4
The quantity of Ba(OH)2 in the BHAS was fixed as 10 wt.% of GGBFS, which corresponds to ~ 0.0584 mole of Ba(OH)2 per 100 g of GGBFS. Compared to the content of Ba(OH)2 of the BHAS, the CHAS-WR and the CHAS-MR were prepared to have the same weight fraction (i.e., 10 wt.% of GGBFS) and the same molar fraction (i.e., ~ 0.0584 mole per 100 g of GGBFS) of Ca(OH)2 (i.e. 4.32 wt.% of GGBFS), respectively. The water-to-GGBFS weight ratio was constantly determined as 0.4 for all pastes.
Powder of barium hydroxide or calcium hydroxide was dry-mixed with raw GGBFS powder by hand for 5 minutes. De-ionized water was placed in a mixing bowl, and the pre-mixed powder was added to the bowl. The mixing process for paste samples was done according to ASTM C305 [125].
The mixed pastes were cast into cubic brass molds of 5 × 5 × 5 cm for compressive strength tests and into cylindrical molds with a 2.54 cm diameter and a 2.54 cm height for producing the samples for SEM and MIP tests. The casted samples were manually compacted enough to remove air voids trapped in the pastes. All samples were cured at 23 ± 2 °C and above 95% relative humidity for 24 h. After 24 h, the samples were de-molded and then cured in the same conditions until testing.
Compressive strength tests were conducted at 3, 7, 14, and 28 days of the curing process.
The strength for each sample was determined as an average value of three identical specimens. After the strength testing, the fractured pieces were collected and ground to prepare powder specimens for XRD, NMR, and TGA. The cylindrical pastes were sliced into cubic pieces of 5 × 5 × 5 mm to prepare MIP specimens and 2 mm-thick pieces for SEM specimens using a precision cutter. The solvent-exchange method with isopropyl alcohol (IPA) was used to stop further hydration as
mentioned in 2.2.6.
The XRD patterns for hardened samples were taken with Cu-Kα beam (λ = 1.5418 Å) and a 5°–60° scanning range at 2θ. The TGA experiments for 3-day and 28-day samples were carried out using a thermal analyzer (SDT Q600; TA Instruments, New Castle, DE, USA) with alumina pans. The range of the heating temperature was set from an ambient temperature to 1,000 °C with a heating rate 10 °C/min in a nitrogen gas environment. The pore size distributions of the hardened pastes were examined with a mercury porosimeter (Autopore IV 9500; Micomeritics, Norcross, GA, USA) under a pressure range from sub-ambient to 414 MPa (60,000 psi) at 3 and 28 days. Solid-state MAS-NMR experiments were carried out for the 28-day pastes with a 400MHz Avance II + Bruker Solid-state NMR instrument. The microstructures of 28-day samples were examined with a FE-SEM (Hitachi S- 4800; Hitachi, Tokyo, Japan) with EDS spot analysis in BSE mode. All samples were thin-sectioned with a thickness of ~30 μm and polished with 9, 6, 3, and 0.5 μm diamond suspensions. The samples were coated with osmium before being tested.