CHAPTER 3. CALCIUM HYDROXIDE-ACTIVATED SLAG CEMENT WITH AUXILIARY
3.4. Crystalline Phases Formed in Calcium Hydroxide-Activated Slag Cements and Its
Overall, the strength testing results indicated that using additive activators favored early strength development regardless of whether the chemicals were added or substituted, but did not improve the 28-day strength above that of the control sample (i.e. 5CH). Previous studies have often reported similar behaviors when alkaline chemicals were added to Portland or GGBFS cement:
accelerated early strengths with reduced (or maintained at most) 28-day strengths. However, the fundamental mechanism behind this behavior has not been fully explained [97, 98]
3.4. Crystalline Phases Formed in Calcium Hydroxide-Activated Slag Cements and Its
Figure 3- 4 exhibited the diffraction patterns of calcium hydroxide-activated slag cements without auxiliary activators i.e. 4CH and 5CH samples at 28days and includes simulated reference patterns from the ICDD PDF-2 database for the identified phases. In the 4CH and 5CH samples, the main hydration products were C-S-H (PDF# 00-33-0306), monosulfate (kuzelite) [Ca4Al2(SO4)(OH)12·6H2O, AFm phase, PDF# 01-083-1289], calcium hydroxide [Ca(OH)2, PDF# 01- 081-2040], ettringite [Ca6Al2(SO4)3(OH)12·26H2O, AFt, PDF# 00-041-1451], and hydrotalcite [Mg6Al2CO3(OH)16·4H2O, PDF# 00-014-0191]. These reaction products are frequently identified- phases in hydrated OPC systems [75], except for hydrotalcite, which has been often observed in CaO- or Ca(OH)2-activated slag pastes [5] and mature Portland cement-GGBFS blends [99] as well as alkali-activated GGBFSs [36, 38, 40, 55, 100-102]. Kuzelite is a sulfate-rich AFm phase that shares a chemical formula and structure with the monosulfate phase [Ca4Al2(SO4)(OH)12·6H2O] of a hydrated cement [103]; in general, monosulfate is considered as poorly crystallized sulfate-rich AFm phase and kuzelite is regarded as a highly crystallized form of monosulfate phases; Thus, the diffraction pattern for monosulfate phase could not well identified in XRD [104]. Despite the large difference in strength between 4CH and 5CH, the diffraction patterns of these two samples contained only few differences, supporting the notion that the higher strength of 4CH was due to its physical factors (e.g., porosity reduction by the use of a smaller amount of water) instead of its chemical aspects (e.g., types of hydration products) [75].
3.4.2. Modification of crystalline phases induced by adding auxiliary activators
As represented in Figure 3- 5, adding sodium hydroxide, NaOH (i.e. 5CH+NH), generated U-phase [NaCa4Al2(SO4)1.5(OH)12·9H2O, AFm, PDF# 00-044-0272], as well as C-S-H, hydrotalcite and Ca(OH)2. After adding sodium carbonate, Na2CO3, U-phase and calcite [CaCO3; PDF# 01-086- 2340] were produced with C-S-H, ettringite and hydrotalcite in 5CH+NC; however, no Ca(OH)2 was found. Adding sulfate-bearing chemicals i.e. sodium sulfate (Na2SO4) and calcium sulfate dihydrate (gypsum) predominantly generated C-S-H and ettringite while removing (or almost removing) the other phases. In particular, the peak intensities of ettringite increased significantly compared to those of 5CH.
The list of reaction products formed in each sample are summarized in Table 3- 3 to compare the relative quantities of each crystalline phase; the relative quantities were approximately assessed by carefully overlaying the diffraction patterns of specific samples to compare the profile peak intensities by adjusting the background of each diffraction pattern to become identical. Table 3- 3 represented that (1) there was an inverse-relationship between the quantities of ettringite and U-phase when Na- based auxiliary activators were used [105]; and (2) no auxiliary activators increased the formation of C-S-H.
(a) (b)
(c) (d)
Figure 3- 5: Diffraction patterns of calcium hydroxide-activated slag cements, adding four auxiliary activators with reference patterns; (a) 5CH+NH, (b) 5CH+NC, (c) 5CH+NS, and 5CH+GY at 28 days.
Hydrotalcite (00-014-0191)
22-year old C-S-H pattern of fully hydrated β-C2S paste
>&D2+SHDNVDUHUHPRYHG IURPWKHRULJLQDOSDWWHUQ@
30 25
20 35 40 45 50 55 60
Position [2θ, ] (CuKα)
C-S-H (00-033-0306)
Ca(OH)2(01-081-2040)
10 20 30 40 50 60
Position [2θ] [Cu-Kα]
5
U-Phase (monosulfate) (00-044-0272) Ettringite (00-041-1451)
22-year old C-S-H pattern of fully hydrated β-C2S paste
>&D2+SHDNVDUHUHPRYHG IURPWKHRULJLQDOSDWWHUQ@
30 25
20 35 40 45 50 55 60
Position [2θ, ] (CuKα) C-S-H (00-033-0306)
U-Phase (monosulfate) (00-044-0272)
Calcite (01-086-2340)
10 20 30 40 50 60
Position [2θ] [Cu-Kα]
5
Ettringite (00-041-1451)
Hydrotalcite (00-014-0191)
10 20 30 40 50 60
Position [2θ] [Cu-Kα]
5
22-year old C-S-H pattern of fully hydrated β-C2S paste
>&D2+SHDNVDUHUHPRYHG IURPWKHRULJLQDOSDWWHUQ@
30 25
20 35 40 45 50 55 60
Position [2θ, ] (CuKα)
C-S-H (00-033-0306)
Ettringite (00-041-1451)
Hydrotalcite (00-014-0191)
10 20 30 40 50 60
Position [2Ƌ] [Cu-KƄ?
5
Ettringite (00-041-1451) C-S-H (00-033-0306)
22-year old C-S-H pattern of fully hydrated β-C2S paste
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30 25
20 35 40 45 50 55 60
Position [2θ, ] (CuKα)
Table 3- 3: List of crystalline phases formed in each sample and relative increase or decrease in quantity compared to 5CH sample.
Sample
Reaction product C-S-H Monosulfate
(kuzelite) (AFm) Ca(OH)2 Ettringite
(AFt) Hydrotalcite U-phase (AFm)
5CH ○ ○ ○ ○ ○ N
5CH+NH ○ ≈ (or ↓) N ○ ↑↑ ○ ↓↓↓ ○ ≈ ○ ↑
5CH+NC ○ ≈ (or ↓) N N ○ ↓↓ ○ ≈ ○ ↑↑
5CH+NS ○ ≈ N N ○ ↑ ○ ≈ N
5CH+GY ○ ≈ N N ○ ↑↑ N N
4CH ○ ≈ ○ ≈ ○ ≈ ○ ≈ ○ ≈ N
*ɂ: product formation; N: no formation of product; Ȥ: slight increase; ȤȤ: intermediate increase;
ȤȤȤ: large increase; Ȧ: slight decrease; ȦȦ: intermediate decrease; ȦȦȦ: large decrease; ≈:
no change compared to 5CH.
Although U-phase often formed when a large amount of sodium sulfate, Na2SO4, was added to Portland cement systems in previous studies [106, 107], adding sodium sulfate, Na2SO4, did not yield any U-phase [see Figure 3- 5(c)] during this study; instead, strong ettringite reflections were produced in the diffraction pattern. The high sulfate content of the 5CH+NS sample might have favored the formation of ettringite in 5CH+NS sample because AFt phase i.e. ettringite in this study is preferable to AFm phases in high sulfate environments [75]. However, note that U-phase can only form at high NaOH concentrations ( > 1~2 M [106, 108]), which is the most important synthesis parameter; no U-phase formation was reported in ordinary Portland cement systems because the NaOH concentration was generally below 1 M [106, 108]. The pH values of solutions presented in Table 3- 2 indirectly provide a relative comparison of the concentrations of NaOH between the samples. Therefore, the lack of U-phase implies that the NaOH concentration in 5CH+NS did not reach 1~2 M until the 28-day, while those of 5CH+NH and 5CH+NC remained above 1~2 M for 28 days.
When the pH decreases with long curing time(e.g., due to carbonation), U-phase can cause an expansive degradation in cement-based systems through the following conversion process, according to Li et al. and Sánchez-Herrero et al. [106, 107]: U-phase → C4AH13 → C4AܵҧH12 → C6Aܵҧ3H32 (= ettringite). During this conversion, the solid volume is significantly increased. Therefore, using sodium hydroxide, NaOH, and sodium carbonate, Na2CO3, in calcium hydroxide-activated slag cement may cause expansive failures after longer aging because U-phase forms.
3.5. Characteristics of Local Microstructures of Calcium Hydroxide-Activated Slag Cements