CHAPTER 5. MICROSTRUCTURAL CHARACTERISTICS OF CALCIUM OXIDE-ACTIVATED
5.3. Material Characterization for Collected Four GGBFSs
5.3.1. Physical properties of GGBFSs
The particle size distributions of the raw GGBFSs are shown in Figure 5- 2, which shows the averaged plots from three independent measurements for each GGBFS. The size distributions were not markedly different from each other because these GGBFSs should satisfy some standards such as Korean standard (KS) or ASTM for use in commercial purposes. Nonetheless, each GGBFS qualitatively had its own distinctive characteristics for size distribution as follows: (1) SK-2 possessed the finest size distribution; (2) SD had the coarsest size distribution; (3) The particle size distribution of SK-1 was between SK-2 and SD; and (4) SS simultaneously had large fractions of both very small and large particles but had the smallest percentage of middle sized particles ranging from 3 to 30 μm.
Figure 5- 2: Cumulative curves, particle size distributions and the fitting results (in-set table) of the RRB equation for the raw GGBFSs
0 10 20 30 40 50 60 70 80 90 100
0.1 1 10 100 1000
Cumulative distribution (%)
Particle size (μm)
SK-1 SK-2 SS SD
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1 1 10 100 1000
Density distribution
Particle size (μm)
SK-1 SK-2 SS SD
Raw GGBFS Fitting equation R2 n b x0
SK-1 0.989x-2.570 0.9961 0.99 2.57 13.43
SK-2 0.996x-2.501 0.9951 1.00 2.50 12.31
SS 0.939x-2.473 0.9941 0.94 2.47 13.92
SD 0.988x-2.691 0.9932 0.99 2.69 15.23
To quantitatively describe the characteristics of the particle size distributions, the distribution equation of Rosin-Rammler-Bennett (RRB) [139] was fitted to the particle size distributions of the GGBFS. The RRB equation is often used to characterize the particle size distributions of Portland cement [140] and GGBFS [135, 136]. The RRB equation is defined as follows:
ܴ ൌ ͳͲͲ݁ିቀ
௫
௫బቁ (6)
, where R is a fraction of the residual particles in the x-μm mesh in %; x0 is the mesh size at 36.8% of the particles retained; and n is the slope index. By defining Y = ln(100/R), X = ln(x), and b = nln(x), the RRB equation is linearized into Y = nX - b, which was used for the curve-fitting with first-order polynomial functions. The larger value of n represents the higher fraction of small sized particles in a sample of powder. On the other hand, the lager x0 value represents the higher portion of large particles in a sample. The curve-fitted results for four raw GGBFSs are presented in the in-set table of Figure 5- 2, which shows all the curves are well fitted with the RRB equation (all R2 values for each GGBFSs are larger than 0.99). The order of magnitude for the values of n and x0 from the fitted results was SK-2 > SK-1 ؆ SD > SS and SD > SS > SK-1 > SK-2, respectively. Although the n value for SS fails to show any considerable presence of very small particles, overall, the values of n and x0
reasonably describe the size distributions of the GGBFS particles, similarly to the qualitative description, above.
5.3.2. Chemical composition of raw GGBFSs
The oxide compositions of the GGBFSs from XRF spectroscopy are represented in Table 5- 2. Table 5- 2 is expressed except for elements less than 0.1%. The hydraulic reactivity of GGBFS, calculated from XRF results, have been often estimated using various types of hydraulic indexes or modulus [7, 19, 54, 123] and some examples of those indexes was already represented in Table 2- 1.
For instance, KS F 2563 [141] uses the term basicity defined as (CaO+MgO+Al2O3)/(SiO2), which is required to be larger than 1.60 and is presented as third index in Table 2- 1. The other hydraulic indexes containing some minor components of GGBFS are presented [7] such as (CaO+MgO+Al2O3)/(SiO2+MnO+TiO2) which is defined by Chinese standard and is already exhibited as 11th index in Table 2- 1. In Table 5- 2, all GGBFSs had few significant differences in their oxide compositions [50], basicity and chemical indexes; in particular, SK-1 and -2 are quite similar to each other except for the SO3 content because these two GGBFS was originally manufactured from same mill.
Table 5- 2: Oxide chemical compositions of four different raw GGBFS measured by XRF spectroscopy and calculated basicity and chemical index.
Chemical elements SK-1 SK-2 SS SD
CaO 44.9 45.4 45.5 45.7
SiO2 32.8 33.2 30.7 32.0
Al2O3 12.8 13.2 12.7 13.7
MgO 3.8 3.5 4.6 4.9
SO3 3.7 2.5 4.5 1.7
TiO2 0.6 0.6 0.6 0.6
K2O 0.6 0.5 0.4 0.4
Fe2O3 0.4 0.4 0.3 0.5
MnO 0.2 0.3 0.2 0.2
Na2O 0.2 0.2 0.2 0.2
BaO 0.1 0.1 0.1 0.1
SrO 0.1 0.1 0.1 0.1
ZrO2 0.0 0.0 0.1 0.0
Basicity of GGBFS
(KS F 2563) 1.87 1.87 2.04 2.01
Chemical index
[7] 1.83 1.82 1.99 1.96
5.3.2. Crystalline phases and microstructures of raw GGBFSs
Table 5- 3: Phase contents contained in four different GGBFS in wt.% from the semi- quantitative analysis with conventional XRD. Note: the calcite contents were also measured with TGA: 1.9% for SS, and 2.8% for SD.
Raw GGBFS CaSO4
(Anhydrite)
CaSO4·2H2O (Gypsum)
CaCO3
(Calcite) Glass phases
SK-1 5.8 0.2 - 94.0
SK-2 5.1 0.1 - 94.8
SS 4.7 3.2 2.6 89.5
SD - - 2.4 97.6
Using the RIR method [142, 143] with an internal standard, the phase contents in each GGBFS were approximately calculated (presented in Table 5- 3) from the measured XRD patterns for
the raw GGBFSs, which are included in Figure 5- 4 in the later section. To improve the accuracy of the RIR analysis, the best well-matched pattern among PDF databases was selected with consideration of the score provided by X’pert HighScore Plus software. Although the accuracy of the RIR method is generally lower than that of the Rietveld method [57, 144], the accuracy of the RIR in this study is likely to be comparable because the resolutions of the measured patterns were not sufficiently high for the Rietveld method.
The calcite contents were estimated for SS and SD. Note that the strong peak of gypsum at ~ 29° was closely located with the strongest reflection of calcite, and accordingly, the RIR method is likely to overestimate the calcite content in SS. Thus, the calcite contents were also measured with TGA for better accuracy shown in Figure 5- 6. It is not clear whether calcite was formed through carbonation of the calcium in the GGBFS, or whether it was intentionally added during the milling process. In general, the presence of calcite could affect cement hydration in several ways: (1) it can modify the AFm phases by converting monosulfate to hemi-carboaluminate or mono-carboaluminate, and also, it could generate an additional AFt phase resulting in improved mechanical strength; (2) it could increase the strength by acting as fillers; (3) however, it could reduce the strength because of un-reacted residues when it is added in large amounts [104, 145]. Further discussion will be made on the influence of calcite in the CaO-activated GGBFS system in a later section of the paper.
Calcium sulfates (i.e., anhydrite and gypsum) were present in the raw samples of GGBFS.
These were probably added during the milling process to improve the grinding efficiency or to meet the standard requirements for the chemical composition of commercial GGBFS powders (e.g., KS basicity or other chemical indexes). The basicity and chemical indexes can be modified through the addition of anhydrite or gypsum due to the increase of the CaO content. The addition of these sulfate sources also increases the SO3 content in the oxide composition shown in Table 5- 2.
When anhydrite and gypsum are excluded in the calculation for the glass content, the inherent contents of the glass phase could be 100% for SK-1 and -2, 97.6% for SD, and 97.2% for SS.
If we also regard the calcite as external inclusions, the original glass contents of all the raw GGBFSs could be 100%.
The 29Si and 27Al MAS-NMR spectra were taken for the raw GGBFSs but are also included in the later section in Figure 5- 7 and Figure 5- 9, respectively, with those of the hardened pastes for discussion purposes; the intensities of all spectra were rescaled to have visually the same area so that they could be directly compared between GGBFSs. The peak locations of the 29Si MAS-NMR spectra for the raw GGBFSs ranged from -75 ~ -76 ppm, agreeing with previous studies; Schilling et al. [38]
and Bernal et al. [28] proposed -74.0 ppm and -76.0 ppm, respectively. In the 27Al MAS-NMR results, most spectra for the raw GGBFSs showed a 4-fold coordinated aluminum similar to earlier studies [28, 38, 102, 124, 146].
5.4. Compressive Strength Development of Calcium Oxide-Activated Slag Cements Prepared