CHAPTER 3. CALCIUM HYDROXIDE-ACTIVATED SLAG CEMENT WITH AUXILIARY
3.5. Characteristics of Local Microstructures of Calcium Hydroxide-Activated Slag Cements with
3.5. Characteristics of Local Microstructures of Calcium Hydroxide-Activated Slag Cements
(a) (b)
(c) (d)
(e) (f)
Figure 3- 7: 29Si solid-state MAS-NMR spectra of calcium hydroxide-activated slag cement with or without auxiliary activators and spectra deconvolution results with mean chain length of C- S-H and Al-to-Si ratio in C-S-H: (a) 4CH, (b) 5CH, (c) 5CH+NH, (d) 5CH+NC, (e) 5CH+NS, and (f) 5CH+GY at 28-day curing.
In the activated samples, the Q0 peak represents the amount of the residual GGBFS remaining after activation; therefore, the smaller area of the Q0 peak indicates that the residual
ppm
-100 -90 -80 -70 -60 -50
No. Pos.(ppm) %Area Qn
1 -74.8 56.4 Q0/Q1 (slag) 2 -78.7 14.1 Q1in C-S-H (end) 3 -81.2 20.4 Q2(1Al) in C-S-H 4 -85.0 9.1 Q2(0Al) in C-S-H 4CH at 28 days
MCL = 7.6
Al/SI in C-S-H = 0.23 1
2 3
4
ppm
-100 -90 -80 -70 -60 -50
No. Pos.(ppm) %Area Qn
1 -74.7 58.4 Q0/Q1 (slag) 2 -78.6 11.7 Q1in C-S-H (end) 3 -81.1 22.3 Q2(1Al) in C-S-H 4 -84.7 7.6 Q2(0Al) in C-S-H 5CH at 28 days
1
2 3
4
MCL = 9.0
Al/SI in C-S-H = 0.27
ppm
-100 -90 -80 -70 -60 -50
No. Pos.(ppm) %Area Qn
1 -74.7 53.9 Q0/Q1 (slag) 2 -78.7 13.5 Q1in C-S-H (end) 3 -81.3 27.4 Q2(1Al) in C-S-H 4 -84.2 5.2 Q2(0Al) in C-S-H 5CH+NH, 28 days
MCL = 8.9
Al/SI in C-S-H = 0.30 1
2 3
4
ppm
-100 -90 -80 -70 -60 -50
No. Pos.(ppm) %Area Qn
1 -74.6 36.1 Q0/Q1 (slag) 2 -78.8 20.2 Q1in C-S-H (end) 3 -81.3 26.4 Q2(1Al) in C-S-H 4 -84.6 17.3 Q2(0Al) in C-S-H 5CH+NC, 28 days
MCL = 7.6
Al/SI in C-S-H = 0.21 1 23
4
ppm
-100 -90 -80 -70 -60 -50
No. Pos.(ppm) %Area Qn
1 -74.7 44.2 Q0/Q1 (slag) 2 -78.7 10.9 Q1in C-S-H (end) 3 -81.3 27.1 Q2(1Al) in C-S-H 4 -84.5 17.9 Q2(0Al) in C-S-H 5CH+NS, 28 days
MCL = 12.7 Al/SI in C-S-H = 0.24 1
2 3
4
ppm
-100 -90 -80 -70 -60 -50
No. Pos.(ppm) %Area Qn
1 -74.8 53.0 Q0/Q1 (slag) 2 -78.7 19.2 Q1in C-S-H (end) 3 -81.2 16.3 Q2(1Al) in C-S-H 4 -84.4 11.5 Q2(0Al) in C-S-H 5CH+GY, 28 days
MCL = 5.7
Al/SI in C-S-H = 0.17 1
2 34
GGBFS underwent further reactions during activation. Consequently, comparing the areas of the Q0
peaks between samples reveals that the raw GGBFS tended to react more when additional activators were used.
Because the XRD analysis verified that no silicon-containing phases other than C-S-H and the raw GGBFS were present in this study, the smaller Q0 peak also might be related to the formation of more C-S-H. Accordingly, the additional C-S-H formation might be ranked as follows based on the Q0 peak areas from the deconvolution results: 5CH+NC > 5CH+NS > 5CH+GY ≈ 5CH+NH > 4CH ≈ 5CH. However, the XRD results in Table 3- 3 countered this claim because no C-S-H peaks larger than those of 5CH were observed in any sample. Furthermore, the decreases in the Q0 peaks were not proportional to the strengths. Although the activating additives (e.g., Na2CO3) largely reduced the Q0
peaks, no significant differences in strength were apparent at 28 days after the additions. Most of the previous studies on alkali-activated slag concluded that newly formed Q1 and Q2 peaks were attributed to the silicate chains in C-S-H [38, 101, 102, 109, 113]; however, the situation during this study might be different because (1) this study used Ca(OH)2 as a major activator and (2) when the GGBFS was activated with Na-based activators (e.g., NaOH or sodium silicate solution), the degrees of reaction were considerably higher than those in this study. Therefore, a considerable portion of the formed Q1
and Q2 peaks in this study may not belong to C-S-H; instead, they may be attributed to an amorphous state for Q1 and Q2 silicon atoms, which have not been used for C-S-H formation. Previous studies [38, 101] of alkali-activated slag collected similar 29Si NMR spectra that produced large growths in the Q1
and Q2 peaks while greatly reducing the residual GGBFS peak (i.e. Q0), even at very early ages (i.e.
1~3 hours). Although the authors primarily assigned those Q1 and Q2 peaks to silicate chains on C-S- H, considerable amounts of C-S-H may not form within such short time periods (i.e. less than 3 hours).
Therefore, some portion of those peaks might not be attributed to C-S-H.
If most of the Q1 and Q2 peaks belonged to C-S-H, the calculated values using these Q1 and Q2 peak areas for MCL of silicate chains in C-S-H should reveal a relationship with compressive strengths because when the MCL increased, the Ca/Si tended to decrease in C-S-H [42]. The calculated values are obtained using equation (5) [112, 114] as follows: 7.6 (4CH), 9.0 (5CH), 8.9 (5CH+NH), 7.6 (5CH+NC), 12.7 (5CH+NS), and 5.7 (5CH+GY) as exhibited in Figure 3- 7.
However, Figure 3- 8 demonstrates that the calculated values did not show any relation with 28-day compressive strength, implying that considerable portion of Q1 and Q2 peaks did not belong to C-S-H, supporting the earlier discussion.
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Figure 3- 8: The relationship between 28-day compressive strength and calculated MCL for calcium hydroxide-activated slag cements with or without auxiliary activators at w/s = 0.5.
Adding sodium hydroxide, NaOH, may have induced high ionic competition between the Na and Ca cations, limiting the 28-day strength. Although sodium hydroxide could accelerate the initial dissolution of GGBFS [55] and increase the early strength, it may have suppressed the dissolution of Ca(OH)2 from raw GGBFS, which is a principal activator for producing C-S-H, and consequently lowered the 28-day strength. Figure 3- 5(a) and Table 3- 3 verify that a large quantity of Ca(OH)2
remained in 5CH+NH. Compared to 5CH, the stronger Ca(OH)2 reflections might be attributed to additional Ca cations precipitation after a considerably higher GGBFS dissolution due to the high alkaline condition. However, this scenario remains unlikely because Figure 3- 7(c) presents that the degree of reaction for the slag (see Q0 peak) decreased only slightly after adding sodium hydroxide.
Therefore, the strong Ca(OH)2 peaks in diffraction pattern were more likely generated by unreacted (or undissolved) Ca(OH)2 due to the high alkaline environment induced by adding NaOH. Therefore, adding NaOH may have inhibited the consumption of Ca(OH)2 during C-S-H formation, decreasing strength gain.
Although the reactions of Na2CO3 and Na2SO4 with Ca(OH)2 could also produce NaOH, these reactions may have produced much lower NaOH concentrations compared to the direct NaOH addition (consult pH values of solution for 5CH+NC and 5CH+NS in Table 3- 2). Therefore, the effects of ion competition should have been weaker than those for 5CH+NH. However, note that adding Na2CO3 created a stronger alkaline environment than Na2SO4, and this situation may have also occurred when Ca(OH)2 was partially replaced by Na2CO3 and Na2SO4. Consequently, the lower strength displayed by 5CHNC (= 11.3 MPa) relative to 5CHNS (= 15.4 MPa) may be explained by the higher ionic competition due to the higher alkaline generation of Na2CO3.
0 5 10 15 20 25 30 35 40
4 6 8 10 12 14
28-day compressive strength (MPa)
Calculated MCL 5CH+GY
5CH+NC
5CH+NH 5CH
5CH+NS
3.5.2. Distribution of aluminum atoms in calcium hydroxide-activated slag cements with or without auxiliary activators
Figure 3- 9 contains all of the 27Al MAS-NMR spectra. Because no other aluminum source was added to GGBFS, the integrated areas of the 27Al MAS-NMR spectra for all samples should be identical. For direct comparison, all of the integrated areas under the NMR spectra were normalized to share the same area.
Figure 3- 9: 27Al MAS-NMR spectra of raw GGBFS and calcium hydroxide-activated slag cement with or without auxiliary activators at 28 days.
The 27Al MAS-NMR spectrum for the raw GGBFS contained a broad tetrahedral peak [i.e.
4-fold coordinated aluminum atom, Al(IV)] near 60 ppm, which was also reported in previous studies [39, 40, 102, 115]. The deconvolution of the 27Al MAS-NMR spectra was not illustrated; the Gaussian curve fit may not be accurate due to the asymmetrical quadrupolar nucleus of 27Al. After activation, the tetrahedral peaks were noticeably reduced in all samples, indicating a considerable release of aluminum atoms from the raw GGBFS. Because the Al(IV) peaks also included the aluminum atoms substituted for the silicon in silicate chains in C-S-H, slight shifts in the centerlines of the Al(IV) peaks were observed after activation [39, 113-116]. New octahedral peaks [i.e. 6-fold coordinated aluminum, Al(VI)] distinctly grew at approximately 13 ppm and 10 ppm, implying the formation of new reaction products. The first peaks at ~ 13 ppm were assigned to the AFt phase (ettringite) and the
-20 0
20 40 60 80 100
ppm 4CH
5CH 5CH+NS 5CH+NH 5CH+NC 5CH+GY GGBFS
Tetrahedral Al(IV)
Octahedral Al(VI)
GGBFS 4CH 5CH 5CH+NS 5CH+NH 5CH+NC 5CH+GY GGBFS
0 2 4 6 8 10 12 14 16 18 20
ppm
5CH 5CH + NC 5CH + NH 5CH + GY
5CH + NS
NCC NHH
AFt AFm TAH
4CH Assigned Al(VI) peaks
m ned
Hydrotalcite
second peaks at ~ 10 ppm represented a combination of the AFm phase (monosulfate, or U-Phase) and hydrotalcite [105, 117-119]. The third peaks at the lowest frequency (near 3 - 5 ppm) have been reported in previous white cement hydration studies [117, 118] , which denote them as TAH (third aluminate hydrate) phase.
The 27Al MAS-NMR results agreed with the XRD results. The AFt (ettringite) peaks were clearly observed in 4CH, 5CH, 5CH+NS and 5CH+GY. A relative comparison of peak intensities for the ettringite provides the magnitudes of ettringite formation from the largest to the smallest:
5CH+GY > 5CH+NS > 5CH = 4CH. In particular, 5CH+GY and 5CH+NS generated very strong ettringite peaks at ~ 13 ppm with little to no intensity for the other peaks. The samples of 5CH+NH and 5CH+NC exhibited strong U-phase (AFm) and hydrotalcite peaks at ~ 10.6 peaks and ~ 10 ppm, respectively, in their 27Al MAS-NMR profiles.