CHAPTER 5. MICROSTRUCTURAL CHARACTERISTICS OF CALCIUM OXIDE-ACTIVATED
5.6. Microstructures of Calcium Oxide-Activated Slag Cements Produced from Different GGBFSs
5.6.1. Local silicon- and aluminum- structures based on the investigation of 29 Si and 27 Al MAS-
Figure 5- 6: Calcium carbonate contents calculated from the TGA results for the raw GGBFSs (SS and SD) and activated pastes at 28 days (CA-SS and CA-SD).
Although the TGA curves of CA-SK-1, -2 and -SD did not have significant difference, where the weight loss of these samples in Figure 5- 5 was similar, the strengths of these samples were significantly different at 28 days; thus, in this study, the quantity of the reaction products was not a dominant factor for strength development.
5.6. Microstructures of Calcium Oxide-Activated Slag Cements Produced from Different
(a) (b)
(c) (d)
Figure 5- 7: 29Si MAS-NMR spectra and deconvolution results of raw GGBFSs and activated pastes at 28 days: (a) SK1 and CA-SK1, (b) SK2 and CA-SK2, (c) SS and CA-SS, and (d) SD and CA-SD; Gaussian function was used for raw GGBFSs and deconvoluted component of 1;
Lorentzian function was used for the other deconvoluted components of 2-4.
Based on the deconvolution results, the DOD of silicon atoms in each GGBFS sample was calculated as ‘100 – area of Q0’; the MCL of C-S-H and Al/Si atomic ratio in C-S-H were also calculated using equations (5) and (7) [112, 114, 155]. Although being already mentioned in chapter 3, equation (5) is presented below, again.
ܯܥܮ ൌ ʹ ቄܳଵ ܳଶሺͲܣ݈ሻ ͵
ʹ ܳଶሺͳܣ݈ሻቅ
ܳଵ
(5)
ܣ݈Ȁܵ݅ ܽݐ݉݅ܿ ݎܽݐ݅ ൌ
ͳ
ʹ ܳଶሺͳܣ݈ሻ
ܳଵ ܳଶሺͲܣ݈ሻ ܳଶሺͳܣ݈ሻ
(7)
-100 -90
-80 -70
-60 -50
Chemical shift (ppm)
%#5- raw
activated simulated 1
2 3
4
-100 -90
-80 -70
-60 -50
Chemical shift (ppm)
%#5- raw
activated simulated 1
2 3
4
-100 -90
-80 -70
-60 -50
Chemical shift (ppm)
%#55 raw
activated simulated 1
2
3 4
-100 -90
-80 -70
-60 -50
Chemical shift (ppm)
%#5& raw activated simulated 1
2 3 4
Table 5- 4: Deconvolution results of 29Si MAS-NMR data for calcium oxide-activated slag cements at 28 days
Label Deconvoluted
peak number Q assignment Deconvoluted peak position (ppm)
Relative area of deconvoluted peak
CA-SK-1
1 Q0 (raw GGBFS) -75.7 61.2
2 Q1 -80.0 23.3
3 Q2(1Al) -81.5 2.6
4 Q2 -84.4 12.9
CA-SK-2
1 Q0 (raw GGBFS) -75.6 53.5
2 Q1 -80.2 25.4
3 Q2(1Al) -83.0 4.8
4 Q2 -84.8 16.4
CA-SS
1 Q0 (raw GGBFS) -75.5 54.1
2 Q1 -80.3 24.8
3 Q2(1Al) -82.6 6.1
4 Q2 -84.6 15.0
CA-SD
1 Q0 (raw GGBFS) -75.9 61.6
2 Q1 -80.3 28.5
3 Q2(1Al) -82.5 1.6
4 Q2 -84.7 8.3
The plots of the MCL and 28-day strength in terms of the calculated DOD are presented in Figure 5- 8. The MCL and strength approximately showed a linear relationship with the DOD;
however, the values of the Al/Si ratio in this study were significantly smaller than those from other types of activation such as Ca(OH)2- [39, 124] or alkali-activations for GGBFS [39, 146, 156].
A smaller fraction of residual Si in raw GGBFS (i.e., smaller Q0 site) generally indicates a higher degree of GGBFS dissolution (i.e., DOD). If all dissolved phases of GGBFS are supposed to be used for producing reaction products, considering the similar TGA curves between CA-SK-1, -2 and -SD, the fractions of dissolved GGBFSs are also expected to be similar in Figure 5- 8; however, CA-SK-2 showed a much larger degree of dissolution than that of CA-SK-1 and -SD, even slightly larger than CA-SS. This might suggest that although SK-2 was highly dissolved, a large portion of
dissolved GGBFS could not participate in the reactions that produced the major reaction products.
Thus, the dissolution degree of Si in GGBFS is not necessarily a direct measure of the reaction degree of CaO-activation.
(a) (b)
Figure 5- 8: Relationship of the degree of dissolution of silicon atoms with (a) the mean chain length of C-S-H and the Al/Si ratio and (b) the compressive strength of the activated samples at 28 days; ○: activated GGBFS with calcium sulfate, ●: activated GGBFS with no calcium sulfate.
(a) (b)
(c) (d)
2 2.5 3 3.5 4 4.5 5
20 30 40 50 60 70
Mean chain length (unit)
Degree of dissolution of raw GGBFS (%) CA-SS
(Al/Si = 0.07)
CA-SD
(Al/Si = 0.02)
CA-SK-2
(Al/Si = 0.05)
CA-SK-1
(Al/Si = 0.03)
0 10 20 30 40 50 60 70
20 30 40 50 60 70
Compressive strength (MPa)
Degree of dissolution of raw GGBFS (%) CA-SS
CA-SD
CA-SK-2 CA-SK-1
-30 -10 10 30 50 70 90 110
Chemical shift (ppm)
CA-SK-1 SK-1
-30 -10 10 30 50 70 90 110
Chemical shift (ppm)
CA-SK-2 SK-2
-30 -10 10 30 50 70 90 110
Chemical shift (ppm)
CA-SS SS
-30 -10 10 30 50 70 90 110
Chemical shift (ppm)
CA-SD SD
(e)
Figure 5- 9: Solid state 27Al MAS-NMR spectra; (a) to (d) represent each raw GGBFS and activated sample at 28 days and (e) exhibits all 27Al MAS-NMR of activated samples at 28 days.
The MAS-NMR results of 27Al are shown in Figure 5- 9. Because GGBFS was the only source of aluminum in this study, the total number of Al atoms must be the same in each sample after activation. Thus, to directly observe the transition of the 27Al spectra after activation, the intensities of the spectra for the raw GGBFS and 28-day paste were normalized to have the same areas for all samples. The spectrum of each hardened sample consisted of three spectral components: 4-fold coordinated Al [i.e. Al(IV)] near 40 to 80 ppm, 5-fold Al [i.e. Al(V)] around 15 to 40 ppm, and 6-fold Al [i.e. Al(VI)] at about -20 to 15 ppm [73].
In Figure 5- 9(e), the peaks around 13 ppm were probably due to ettringite formation [117, 118], and the peaks near 10 ppm could be attributed to various AFm phases such as kuzelite (monosulfate), monocarboaluminate, and other possible AFm phases [117, 118, 132, 157]. Ettringite was identified in the TGA results of CA-SK-1, -2, and -SS; AFm phases were also found in all samples consistent with the XRD and TGA results.
In this study, the reduction of area of Al(IV) [or Al(IV) + Al(V)] after activation is almost equivalent to the DOD of aluminum atoms for the raw GGBFS because a very small quantity of Al(IV) was present in the reaction product (see the very low values of the Al/Si ratios in C-S-H in Figure 5- 8). Comparing Figure 5- 8 with Figure 5- 9(e), except for SK-2, GGBFS with the higher DOD of silicon atroms roughly exhibited a higher DOD of aluminum atoms; however, despite the highest DOD of silicon atoms in SK-2, SK-2 showed the lowest DOD of aluminum atoms, which explains the
-30 -10 10 30 50 70 90 110
Chemical shift (ppm) CA-SK-1
CA-SK-2 CA-SS CA-SD
Al(IV)
Al(VI) AFt AFm
0 2 4 6 8 10 12 14 16 18 20
Chemical shift (ppm) CA-SK-1
CA-SK-2
CA-SS
CA-SD
Al(V)
smallest growths of peaks of the AFt and AFm phases in CA-SK-2. This observation might suggest that the DOD of aluminum atoms could be different from that of silicon atoms for the raw GGBFS as an irregular case.
5.6.2. Comparison of the pore structure of each calcium oxide-activated slag cements