CHAPTER 6. MICROSTRUCTURE ANALYSIS FOR THE PRODUCTION OF THE COLD-
6.5. Reaction Products in the Concrete Railroad Ties Samples
Figure 57 shows the powder XRD patterns of coarse aggregates, which were prepared by taking from the PSC ties. The numbers in parentheses are the ICSD and ICDD reference numbers for the identified mineral phases. The identified minerals in the coarse aggregates from XRD were listed in Table 11.
It is worth noting that no aggregates contained a hydrous form of silica (SiO2) (e.g., opal, SiO2·nH2O) or an amorphous form of silica (e.g., obsidian or silica glass, SiO2), which are well-known reactive and deleterious minerals causing ASR [97].
98
Figure 57. XRD results of coarse aggregates taken from the PSC ties.
O
Abbreviation – Mineral
NOTE indicates that two or more peaks overlapped.
N
N
N N
Q
Q
Map-cracked Non-cracked
Longitudinal-cracked Brand-new
10 20 30 40 50 60
2 Theta degree (Cu-Ka1)
Q Q Q Q
Q
Q
Q
Q
Q
Q Q
Q Q Q
Q
Q Q
Al
Al
Al
Al Al
Al
Al
Al
Al
Al
Al Al
B B
B
S S S O
O
O
O
O O O
O
O O
O
O O O Mi
Mi
Mi Mi
Mi Mi Mi
Mi
Mi
Mi
Mi Mi
Mi
Mi
Mi N
N
N
N N
N
N
N N
N N
An An
Mu
Mu Mu
Mu
Mu
Mu Mu
Mu Mu
Mu
Q
Q
Q
Q Q Q
Q Q
Al
O O O
Coarse aggregates
Al
O O O
Mi Mi
O O
O O
Mi B
O Mi S
S
Albite (NaAlSi3O8) (00-004-2465) Quartz (SiO2) (01-083-2465)
Orthoclase (KAlSi3O8) (98-003-4783)
Sanidine (KAlSi3O8) (00-010-0353) Nimite (Ni,Mg,Al)6((Si,Al)4O10)(OH)8 (00-022-0712)
Microcline (KAlSi3O8) (00-019-0932)
Andesine (Na,Ca)[Al(Si, Al)Si2O8] (98-006-6127)
Biotite (K(Mg,Fe)3(AlSi3O10)(F,OH)2) (98-016-1230) Muscovite (KAl2(Si3AlO10)(OH)2) (00-007-0042) Reference patterns of identified phases
Feldspar group
Mica group
Alkali-bearing phases
An
Al O
Al
O Mi Mu
Fluorphlogopite (KMg3(Si3Al)O10F2) (00-004-2465)
Q- Quartz Al - Albite B- Biotite Mu- Muscovite
N- Nimite An- Andesine
O- Orthoclase Mi- Microcline
S- Sanidine
F- Fluorphlogopite
F
Mu
99
Table 11. Summary of minerals in the aggregates, identified from XRD Brand-new Non-
cracked
Longitudinal- cracked
Map-
cracked Note
Quartz ○○ ○○ ○○ ○○
Nimite ○○ ○○ ○○ -
Albite ○○ ○○ ○○ ○○ Plagioclase
feldspar
Andesine - ○○ - - Plagioclase
feldspar
Orthoclase ○○ - ○ ○○ Alkali feldspar
Microcline ○○ - ○ ○○ Alkali feldspar
Sanidine - - ○○ ○○ Alkali feldspar
Biotite - - ○○ - Mica
Muscovite ○○ - ○ ○○ Mica
Fluorphlogopite - ○○ - - Mica
Note – ○○: present, ○: possibly present, and -: not detected
In all coarse aggregates, quartz (SiO2) and albite (NaAlSi3O8) were commonly identified as shown in Figure 57. Quartz is a mineral that generally constitutes concrete aggregates and is chemically and mechanically stable unless it is micro-crystalline or highly strained [97, 179].
All coarse aggregates contained feldspar group minerals, which were albite (NaAlSi3O8), microcline (KAlSi3O8), sanidine (KAlSi3O8), orthoclase (KAlSi3O8), or andesine ((Na, Ca)Al(Si, Al)Si2O8). The feldspar group minerals are silicate minerals that contain aluminum (Al) and form a ternary system in terms of K-feldspar (KAlSi3O8), albite (NaAlSi3O8), and anorthite (CaAl2Si2O8). Most natural feldspars can be classified into two series: (1) solid solutions between K-feldspar and albite (i.e., alkali-feldspars (KAlSi3O8–NaAlSi3O8)) and (2) solid solutions between albite and anorthite (i.e., plagioclase feldspars (NaAlSi3O8–CaAl2Si2O8)) [180, 181].
In this study, the aggregates in the Non-cracked samples only contained plagioclase feldspar minerals (i.e., andesine and albite) although the aggregates in the Longitudinal-cracked, Map-cracked, and Brand-new samples mostly contained several different types of alkali feldspar minerals (i.e., orthoclase, microcline, and sanidine) as well as albite.
It is worth noting that aggregates having alkalis might act as an alkali reservoir in concrete given that previous studies have reported the possibility of alkali leaching from feldspar minerals or other types of alkali-bearing aggregates into concrete pore solution, resulting in expansive ASR cracking even in low alkali cement concretes [180, 182]. The influences of the alkali content in aggregates on the damages of PSC ties are discussed more in Sections 3.6 to 3.7.
Biotite (K(Mg,Fe)3(AlSi3O10)(F,OH)2), muscovite (KAl2Si4O10(OH)2), and fluorphlogopite (KMg3(Si3Al)O10F2) belong to the mica group [183]. Previous studies [183, 184] reported that micas in concrete aggregates might cause concrete deterioration as micas have a fragile porous laminar structure
100
that may provide a path for mobility of oxygen and water, and micas might react with alkalis and form expansive hydration products. However, in this study, micas (i.e., biotite, muscovite, and fluorphlogopite) were also found in the undamaged PSC ties (Non-cracked) as well as in the damaged PSC ties (Longitudinal-cracked and Map-cracked). Thus, in this study, the presence of micas did not necessarily cause the deterioration of concrete; the additional factors should be considered such as the presence of reactive silica and content of alkalis. The influence of micas on deterioration of concrete will be more discussed in Section 3.5 and Section 3.7.
101 (a)
2 Theta degree (Cu-Ka1)
10 20 30 40 50 60
Map-cracked Non-cracked
Longitudinal-cracked Brand-new
* Ettringite (Ca6Al2(SO4)3(OH)226H2O) (98-001-6045)
* Portlandite (Ca(OH)2) (00-044-1481) Quartz (SiO2)(01-083-2465)
* Calcite (CaCO3) (01-086-2340) Albite (NaAlSi3O8) (00-004-2465)
Orthoclase (KAlSi3O8) (98-003-4783)
A B C
* C3S (3CaO·SiO2) (00-049-0442)
* C2S (2CaO·SiO2) (00-033-0302)
Impurity from fine aggregates
Cement hydration products
Unreacted
cement compounds D
Cement paste
Reference patterns of identified phases Measured patterns
Muscovite (KAl2(Si3AlO10)(OH)2) (00-007-0042)
102 (b)
Figure 58. XRD results of the cement pastes taken from the collected PSC ties: (a) full XRD results and identified phases and (b) the detailed magnified patterns for four 2θ ranges of the results (a). During the extraction of cement paste, as the fine aggregates were not completely removed, a small fraction of some minerals from fine aggregates still remained.
Figure 58 shows the XRD patterns of the paste, taken from each concrete sample.
In all the samples, quartz, albite, and orthoclase were found in common, which might be originated from the fine aggregates, as the fine aggregates might not be perfectly removed from the cement paste samples during sample preparation. However, due to the relatively strong diffraction intensity of crystalline phases of the aggregates, the identifications of cement hydration products and unhydrated cement compounds were somewhat difficult; thus, in Figure 58 (b), the XRD patterns were divided into four sections where hydration products and unhydrated phases were detected : (A) 8.5° - 9.5° 2, (B) 17.5° - 18.5° 2, (C) 29° - 30° 2, and (D) 32° - 33° 2 (also see broken line boxes with A, B, C, and D in Figure 58 (a)). Although, in 8.5° - 9.5° 2, the XRD peaks of muscovite and ettringite (Ca6Al2(SO4)3(OH)2 26H2O) were observed, but very close together, it is important to distinguish these phases as biotite originated from fine aggregates while ettringite was newly formed hydration product,
8.5 9.5
A B C
Reference patterns
Muscovite
9 17.5 18 18.5 29 29.5 30
2 Theta degree (Cu-Ka1)
Portlandite Calcite
8.5°−9.5°2 17.5°−18.5°2 29°−30°2
Ettringite
D
32 32.5 33
32°−33°2
C2S C3S
Map-cracked Non-cracked
Longitudinal- cracked Brand-new
Reference patterns Reference patterns Reference patterns Undamaged ties
Damaged ties
103
which is important evidence of DEF [159, 160]. It is worth noting that ettringite was only identified in the damaged PSC ties samples (i.e., Longitudinal-cracked and Map-cracked) while it was not identified in the Brand-new and Non-cracked samples (see the arrows in A in Figure 58 (b)).
Portlandite (Ca(OH)2) peaks in 17.5° - 18.5° and calcite (CaCO3) peaks in 29° - 30° were found in all the samples in B and C in Figure 58 (b). Interestingly, the Map-cracked sample contained the fewest content of portlandite among the samples, indicating that consumption of Ca(OH)2 was more progressed in the Map-cracked PSC ties. This observation will be further discussed with ICP-OES results showing dissolution of aggregates by alkaline solution.
Note that unhydrated cement compounds (i.e., C2S (2CaO·SiO2) and C3S (3CaO·SiO2)) were present in the Brand-new and the Non-cracked samples while it mostly disappeared in the Longitudinal- cracked and the Map-cracked samples (see white arrows in D in Figure 58 (b)). It is because, in the damaged PSC ties, further hydration might have progressed due to the presence of cracks, resulting in water ingress from the outside for a long service time. However, all the samples did not contain C3A (3CaO·Al2O3) and C4AF (4CaO·Al2O3·Fe2O3) which are the minor phases of cement component.
Compared to the C3S and C2S phases, its hydration rates are much faster and its contents are much fewer [97, 115]; thus, they might be fully hydrated in all the samples.
Figure 59. TG and DTG curves for the cement paste, taken from each PSC ties.
70%
75%
80%
85%
90%
95%
100%
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C)
Weight (%)
Derivative weight (%/°C)
Ettringite
CaCO3 Ca(OH)2
ASR gel
Brand-new Non-cracked
Longitudinal-cracked Map-cracked
DTG curves TG curves
Ettringite
& C-S-H
Cement paste
104
The TG result and its differential curves (DTG) for the cement pastes, which were taken from each PSC ties, were shown in Figure 59. Unlike the XRD result, the TG result did not confirm the presence of albite, quartz, orthoclase, and muscovite as these minerals were not thermally decomposed or undergo phase change in the TG temperature range of this study [185]. In all samples, the large DTG peaks under 150 °C were due to dehydration of ettringite and C-S-H [115]. Although it was difficult to precisely distinguish DTG peak of C-S-H and ettringite from the overlapped DTG peaks [186, 187], the DTG peaks below 150 °C of the Brand-new and the Non-cracked samples were likely attributable to dehydration of C-S-H as ettringite was only detected in the Longitudinal-cracked and the Map-cracked samples in the XRD result (see Figure 58).
The 15-year used PSC ties (i.e., Non-cracked, Longitudinal-cracked, and Map-cracked) showed greatly larger C-S-H DTG peaks than that of the Brand-new PSC tie, possibly due to the longer hydration ages (i.e., ~17 years vs. ~1 year) causing more formation of C-S-H. Among the 15-year used PSC ties, the DTG peaks below 200 °C of the damaged PSC ties (i.e., Longitudinal-cracked and Map- cracked) were slightly larger than that of the Non-cracked sample (red-shaded area), probably due to the presence of ettringite and more hydration of cement compounds, which is consistent with the XRD result.
It is worth noting that the little DTG peaks around 350 °C were only found in the damaged PSC ties samples (i.e., Longitudinal-cracked and Map-cracked) (see dotted box in Figure 59). The previous study reported that these DTG peaks were resulted from the decomposition of ASR gel [188]. Hence, from the XRD and TG results, the deteriorations of the damaged PSC ties (i.e., Longitudinal-cracked and Map-cracked) might be strongly linked with both ASR and DEF. The large peaks around 410 °C owing to dehydration of Ca(OH)2 were found in all the samples [187]. Map-cracked sample had the fewest content (green-shaded area) of portlandite between all the samples, consistent with the XRD results. The other DTG peaks in 600-700 °C were attributed to the decomposition of calcite [70].