CHAPTER 4. MICROSTRUCTURES AND MECHANICAL PROPERTIES OF BARIUM
4.4. Hydration Products of Barium Hydroxide-Activated Slag Cement
4.4.1. X-ray diffraction investigation of barium hydroxide-activated slag cement
The XRD patterns of hardened pastes are presented in Figure 4- 4 with reference patterns.
The analyses for the XRD reflections were conducted with the ICDD PDF-2 database [127] using X’Pert HighScore Plus software. For identifying the presence of C-S-H, the experimental pattern of 23-year-old hydrated β-C2S, measured by Mohan and Taylor [62], was used in this study after removing the reflections of calcium hydroxide as mentioned in chapter 2.
The hydration products of BHAS pastes were mainly C-S-H, strätlingite (Ca2Al2SiO7·8H2O), barite (BaSO4), hydrotalcite-like phase [or hydrotalcite, Mg6Al2CO3(OH)16·4(H2O)] [4, 124], portlandite [Ca(OH)2], and witherite (BaCO3), as shown in Figure 4- 4(a). C-S-H is generally a main hydration product of cementitious binders (e.g., portland cement, activated GGBFSs).
Strätlingite is a layered calcium aluminosilicate product, frequently identified in slag- blended OPC, calcium aluminate cement (CAC), or CaO-activated slag systems [5, 36, 54, 55, 75]. In the BHAS at 3 days, strätlingite was not identified, but it was recognized after 7 days [see Figure 4- 4(a)]. Most reflections of strätlingite were strong after 7 days except the basal peaks at ~ 7°.
Barite and witherite are Ba-bearing products, which often form when barium coexists with sulfate and carbonate sources [120, 121]. In particular, barite can be formed from the reaction between barium hydroxide and calcium sulfate. The formed witherite might be a result of barium hydroxide reacting with calcite, which was originally present in the raw GGBFS [see Figure 4- 4(a)].
Hydrotalcite is a magnesium-bearing hydration product with a layered double hydroxide structure that has been frequently identified in various cementitious systems such as mature slag- blended OPC [99], AAS [38, 40, 55, 102], and Ba(OH)2-Na2SO4-GGBFS systems [120, 121]. In this study, the reference reflections for hydrotalcite did not precisely match those of the experimental pattern. Similar poor matches were also reported in previous studies [4, 5]. Note that hydrotalcite may show slightly different patterns when it forms in different synthesizing conditions. Hydrotalcite often has modified structures because some of its Mg2+ ions could be replaced with other types of trivalent cations (typically Al3+ and Fe3+), and its CO32- anions could also be exchanged with other types of anions [54]. In particular, Mobasher et al. [120, 121] suggested the possibility that Ba2+ ions might substitute for Mg2+ ions. These substitutions could slightly change the original structure of hydrotalcite.
(a) (b)
Position [r2Theta] (Copper (Cu))
10 20 30 40 50 60
Witherite (98-015-8390) Barite (98-003-3730) Strätlinigite (98-006-9413)
Hydrotalcite (98-000-6296) Portlandite (00-044-1481)
BHAS at 28days
BHAS at 14days
BHAS at 7days
BHAS at 3days
23-year old C-S-H pattern of fully hydrated β-C2S paste
30 25
20 35 40 45 50 55 60
Position [2θ, r] (CuKα)
Position [r2Theta] (Copper (Cu))
10 20 30 40 50 60
Monocarboaluminte (00-036-0129) Ettringite (98-002-7039) Portlandite (00-044-1481) CHAS-WR at 28days
CHAS-WR at 14days
CHAS-WR at 7days
CHAS-WR at 3days
23-year old C-S-H pattern of fully hydrated β-C2S paste
30 25
20 35 40 45 50 55 60
Position [2θ, r] (CuKα)
(c)
Figure 4- 4: Results of XRD analysis of activated pastes with reference diffraction patterns: (a) BHAS, (b) CHAS-WR, and (c) CHAS-MR samples. ▼ indicates hydrotalcite-like phases.
Despite no inclusion of Ca(OH)2 in the mixture proportion of BHAS, the reflections of portlandite were identified in the XRD patterns of hardened BHAS samples [see Figure 4- 4(a)]. The presence of calcium hydroxide was likely due to the calcium dissolution and precipitation from raw GGBFS, but it also could be yielded from the possible reaction between barium hydroxide and calcite which was originally contained in raw GGBFS. Mobasher et al. [121] suggested that barium hydroxide could react with calcium carbonate, producing calcium hydroxide by the following reaction:
Ba(OH)2 + CaCO3 → BaCO3 + Ca(OH)2.
The CHASs produced the same types of hydration products: C-S-H, portlandite, ettringite [Ca6Al2(SO4)3(OH)12·26H2O], and monocarboaluminate [Ca8Al4O14(CO2)·26H2O], but their peak intensities were quite different. The presence of portlandite was induced from the activator and/or the
Position [r2Theta] (Copper (Cu))
10 20 30 40 50 60
Monocarboaluminate (00-036-0129) Ettringite (98-002-7039) Portlandite (00-044-1481) CHAS-MR at 28days
CHAS-MR at 14days
CHAS-MR at 7days
CHAS-MR at 3days
23-year old C-S-H pattern of fully hydrated β-C2S paste
30 25
20 35 40 45 50 55 60
Position [2θ, r] (CuKα)
dissolution of GGBFS. As represented in Figure 4- 4(b) and (c), the peak intensities of portlandite of CHAS-WR were larger than those of CHAS-MR mainly because the content of calcium hydroxide in CHAS-WR was larger than that of CHAS-MR. Ettringite is one of the commonly identified hydration products in cementitious systems such as OPC [75] and the Ca(OH)2-activated slag systems [124]. In CHASs, the formation of monocarboaluminate, which is one type of AFm phases, was more favorable than other types of AFm phases because (1) the contents of sulfate sources might be deficient for producing ettringite compared to those of calcium and aluminum dissolved from GGBFS and (2) the presence of calcite in GGBFS could supply sufficient CO32- ions into pastes.
In the BHAS, the presence of barium ions could inhibit the formation of ettringite because the reaction between barium and sulfate ions was more likely prevalent [67, 128]. Barium ions consumed sulfate ions to produce barite, and strätlingite and hydrotalcite were therefore alternatively produced due to the shortage of sulfate ions. The formation of monocarboaluminate also seemed to be prevented due to the formation of witherite, which consumed CO32- ions. Considering the above discussion, it could be suggested that in the BHAS, the formation of barium-bearing products may be more favorable than those of the AFm (e.g., monocarboaluminate) or AFt (e.g., ettringite) phases.
The no formation of ettringite was likely a major reason for the lowest strength of BHAS at 3 days despite the significantly higher Ksp of barium hydroxide than that of calcium hydroxide. Note that ettringite plays an important role in producing early strength in the cementitious system [75];
however, because ettringite was not produced only in the BHAS due to the presence of barium, the strength development of the BHAS at 3 days was much lower when compared to the other samples.
4.4.2. Thermogravimetric analyses of barium hydroxide-activated slag cement
The TGA results for hardened pastes at 3 and 28 days are presented in Figure 4- 5. Each phase was identified using the TGA data from earlier studies tabulated in Table 2- 3.
All the samples showed significant weight losses at 0–200 °C, which were mainly attributed to the dehydration of C-S-H, ettringite, or strätlingite. In this temperature range, the CHASs showed strong and sharp ettringite peaks, while the BHAS exhibited a relatively broad peak due to no ettringite formation. The shape peak at 180–210 °C and the broad hump around 350 °C in the BHAS were related to the strätlingite and hydrotalcite-like phase (or hydrotalcite), respectively. The small peaks around 150 °C in the CHASs corresponded to monocarboaluminate. The weight losses around 400 °C for all the samples were attributed to the decomposition of portlandite. This analysis of the TGA result is consistent with that of XRD (see Figure 4- 4). Barite and witherite were not seen in the TGA curves because their decomposition temperatures exceeded 1,000 °C, which was the instrumental limit of this study [67, 120].
In Figure 4- 5, the total weight loss in the BHAS at 28 days was the greatest, indicating that
the BHAS produced a greater mass of hydration products than the CHASs; in addition, the temporal change of the total weight loss of the BHAS between 3 and 28 days was notably greater than those of the CHASs. These results may explain the significantly greater increase in strength of the BHAS with curing time, compared to the CHASs.
(a)
(b)
0.00
0.02
0.04
0.06
0.08
0.10
0.12 82
84 86 88 90 92 94 96 98 100
0 100 200 300 400 500 600 700 800 900 1000
Deriv. Weight (%/Ș)
Weight (%)
Temperature (Ș)
BHAS at 3days BHAS at 28days C-S-H & Strätlingite
Strätlingite Hydrotalcite-like
phase Portlandite
0.00
0.02
0.04
0.06
0.08
0.10
0.12 82
84 86 88 90 92 94 96 98 100
0 100 200 300 400 500 600 700 800 900 1000
Deriv. Weight (%/Ș)
Weight (%)
Temperature (Ș)
CHAS-WR at 3days CHAS-WR at 28days C-S-H & ettringite
Monocarboaluminate
Portlandite
(c)
Figure 4- 5: TGA and DTG curve of barium hydroxide- and calcium hydroxide-activated slag pastes at 3 and 28 days: (a) BHAS, (b) CHAS-WR, and (c) CHAS-MR.
Similar to the XRD result, the portlandite content of the CHAS-WR was considerably greater than that of the CHAS-MR, which may be related to the continuously increasing strength of the CHAS-WR after 7 days, unlike the CHAS-MR, because Ca(OH)2 was a main activator.
4.5. Microstructures of Barium Hydroxide-Activated Slag Cement Comparing Those of Calcium