研究所 研究報告 No.1 (2023)

Deterioration of Portland Cement-Based Materials
in Low-Temperature Seawater
低温海水がセメントの耐久性に及ぼす影響に関する研究
Mari Kobayashi, Keisuke Takahashi
This study is based on a paper published in Materials 2023, 16 (15).
本報告は Materials 16 (15) に掲載された論文に基づいたものである。

Cementitious materials have potential for infrastructure development in low-temperature marine
environments, including in seawater at high latitudes and in deep-sea environments (water depths of >1000 m) .
Although the marine deterioration of cementitious materials has been widely investigated, the influence of
seawater temperature has not been elucidated. In this study, to determine the effects of low-temperature seawateron the durability of cementitious materials, cement paste specimens were immersed in a seawater tank at room
temperature and 2 °C for 433 days. The specimen immersed in low-temperature seawater exhibited significant
deterioration with a partially collapsed surface, and Ca dissolution was more pronounced near the surface of thespecimen. Such significant deterioration can be attributed to the increased solubility of portlandite under lowtemperature conditions, which could promote Ca dissolution and subsequently lead to C– (A–) S–H
decalcification and the formation of M– (A–) S–H and thaumasite. These insights are expected to contribute to
the successful construction and maintenance of cementitious structures in low-temperature seawater.
Keywords: Deep sea, Seawater attack, Thaumasite,
Scanning Electron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS)
深海での海洋技術ブレークスルーを達成するため、将来的に深海でのインフラ構築にセメント系材料が利用される可能性が考えられる。セメント系材料の海水劣化は多く研究されているが、温度影響に焦点を当てた研究はなく、水深約 1000ｍ以上の低温海水環境におけるセメント系材料の耐久性を調査する必要がある。本研究では、
ポルトランドセメントを使用したペースト試験体を２℃の海水に 433 日間浸漬させ、その耐久性を評価した。その結果、低温海水に浸漬した試験体は表層が崩壊するなどの著しい劣化を示し、さらにその表層では Ca の溶脱が顕著であることが確認された。低温条件下では水酸化カルシウムの溶解度が上昇することで、Ca の溶脱が促進され、続いてカルシウムシリケート水和物の脱灰、マグネシウムシリケート水和物やソーマサイト (Thaumasite)が生成し、表層から崩壊が進行したと考えられた。本研究結果は、深海環境下のみならず、低温海水中でのコンクリート構造物の建設や維持管理に貢献するものと期待される。
キーワード：深海、海水劣化、ソーマサイト、走査型電子顕微鏡エネルギー分散型Ｘ線分析装置 (SEM-EDS)

1 Introduction

environment are shown in Fig. 1. Because sunlightcannot reach beyond the depths of 200–500 m,
photosynthesis is not possible. Furthermore, thehydraulic pressure increases at a rate of 0.1 MPa
every 10 m. The water temperature of deep-sea
environment is lower than that of the shallow-sea.

It is well-known that approximately 70% of theearth’s surface is covered by ocean. In terms ofvolume, approximately 95% of the total ocean
volume constitutes the “deep sea,” which is deeper
than 200 m. Characteristics of the deep-sea- 24 -

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appear to be more severely damaged, andsignificant Ca dissolution near the surface wasobserved (Fig. 2). This result suggests that
deterioration of PC specimen could be promoted
under deep-sea condition.
Low-temperature seawater is suspected as acause of promoted PC deterioration in deep-sea. In
this study, the effect of low temperatureenvironment on PC durability was investigated byusing PC paste specimen.

2 Experimental

Fig. 1 Deep-sea environment.

PC with no limestone added was used for the
paste specimens. The paste was mixed using amechanical mixer at 700 rpm for 2 minutes and castinto a 40 × 40 × 160 mm³ mold. The water-to-cement
ratio was 0.6. After 28 days of curing under sealedcondition at 20 °C, the specimens were immersed inroom- and low-temperature seawater chamber.
Room temperature seawater refers to seawater in alaboratory where room temperature was notprecisely controlled. The temperature of lowtemperature seawater was controlled by a thermoscontroller to be at 2 ± 1 °C. Fresh seawater wasalways supplied to each tank at a rate of 150 ml/min,
which replaced the seawater in the tankapproximately every 24 hours. (Fig. 3). The
specimens were immersed in each tank for 433 days.
After the immersion test, each specimen was
sliced into two pieces with a thickness of 10 mm
using a table bench saw with liquid paraffin, as

Deep-sea exploration has been actively
conducted so far, and as a result, many marine
resources have been discovered. Recently in mining
development as trials to obtain seabed resources
have been launched(1)-(3). In addition, carbon
dioxide storage under the seafloor is being
investigated. Such kind of technologicalbreakthrough require the establishment of
infrastructures in deep-sea environment.
Cement can be a base material for infrastructure
in deep-sea environment. However, very few
studies have investigated the durability of
cementitious materials under deep sea conditions.
In our first trial, exposure test of mortar specimens
with Portland cement (PC) were conducted at a
depth of 1680 m and constant temperature of theseawater at this depth was 4 °C(4). The result
showed that the specimens exposed for 608 days

Fig. 2 Map of locations where exposure tests were conducted (left) , mortar specimens exposed in deep-sea
for 608 days (middle) (4), and SEM montage image of cross-section from the specimen (right) .

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研究所 研究報告 No.1 (2023)

shown in Fig. 4. For analysis by electron probe
micro analyzer (EPMA) (JXA-8200, JEOL, Ltd.),

Fig. 5 PC paste specimens immersed in roomFig. 3 Experimental

set-upoftemperature seawater (a) and in low-

seawatertemperature seawater (b) after 433 days.

circulation chamber. Fresh seawater wasalways supplied to each tank at a flow rateisopropanol to stop hydration and pulverized to lessthan 90 μm for X-ray diffraction (XRD) (D2
PHASERdiffractometer,
BrukerAXS)
measurement.

of 150 ml/min.

one sample was impregnated with epoxy resin,
polished, and coated with carbon. After EPMA,
the sample was cut into 20 × 20 × 10 mm3 pieces
and polished again for scanning electron
microscopy–energy-dispersive X-ray spectroscopy(SEM-EDS) (JSM-IT-300, JEOL Resonance)
analysis. The other sample was cut using a lowspeed cutter (IsoMet Cutter, Buhler) into 2 mm
slices from the surface to a depth of 10 mm, and theremaining sample was denoted as the bulk sample.
Mashy collapsed areas on the surface of thespecimen immersed in low-temperature seawater
were collected directly and denoted as the collapsed
surface sample. Each piece was treated with

3 Result
Figure 5 shows the paste specimens after
immersion in seawater in the laboratory for 433
days. The specimen immersed in room-temperatureseawater was not significantly damaged, although
the surface was covered with white precipitates ofbrucite. Conversely, the specimen immersed in
low-temperature seawater exhibited significant
surface deterioration. The specimen was partiallycollapsed, did not maintain its original shape, andhad a fragile and mashy structure.

Fig. 4 Preparation of paste specimens for analysis following seawater immersion. The dotted lines indicate thecutting planes.

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Fig. 6 EPMA elemental maps of the PC paste specimens immersed in (a) room-temperature seawater and (b)low-temperature seawater.

Fig. 7 XRD patterns and concentration profiles of element in the specimens obtained after immersion in (a)room-temperature and (b) low-temperature seawater. The profiles of elements are plotted from thesurface (0 mm) to a depth of 12 mm within the area enclosed by the red square in Fig. 6. Ett: ettringite,

F’s: Friedel’s salt, CH: portlandite, CC: calcite.
The results of elemental maps for each specimen,
measuring the diffusion of ions from seawater andthe associated dissolution of ions from cement
hydrate, are presented in Fig. 6. In the case of thespecimen immersed in room-temperature seawater,changes in chemical composition were observed
exclusively on the surface (Fig. 6 (a)). However, forthe specimen immersed in low-temperatureseawater, significant changes in chemicalcomposition were evident at greater depths (Fig. 6(b)). Notably, Ca dissolution and SO42− diffusion
fronts were observed within the low-temperature

specimen. Additionally, in the corner area where
the specimen experienced collapse, there weremarkedly lower Ca concentrations and higher Mg
concentrations. This heightened Mg concentrationsuggests the formation of a Mg phase, a topic thatwill be discussed in greater detail based on the
results from EDS.
Concentration profiles for each element wereanalyzed from the surface (0 mm) to the interior (20mm) of the area enclosed by the red square in the
cross-sectional photographs found in Figs. 6 (a) and(b). These profiles, extending from the surface to a

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研究所 研究報告 No.1 (2023)

Fig. 8 SEM montage images of the specimens after immersion in (a) room-temperature seawater and (b) lowtemperature seawater. The area surrounded by the red box was analyzed and calculated the porosity by
image processing.

depth of 12 mm, are depicted in Fig. 7, alongside
the XRD results. The profiles reveal that the Ca
dissolution fronts are situated at approximately 2
mm and 3 mm for the samples immersed in roomtemperature and low-temperature seawater,
respectively, as determined from the Caconcentration profiles. The XRD patterns of thesample immersed in room-temperature seawater at
a depth of 0–2 mm, and those of the sample
immersed in low-temperature seawater at depths of

Fig. 9 Porosity

(area

%)

of

the

0–2 mm and 2–4 mm (corresponding to the Caleaching area), exhibit very small portlandite peaks.
This observation suggests that portlandite
underwent dissolution, leaving smaller amounts ofportlandite in these regions. Therefore, it can beconcluded that portlandite dissolution was notably
increased by immersing the specimens in lowtemperature seawater. Thaumasite could also formin the collapsed surface, however, it is difficult toconfirm the formation of thaumasite using XRD
because of the similar structures of thaumasite andettringite, both of which exhibit characteristic peaks
at 2θ = 9°, 16°, and 23°. The obtained 29Si NMR
spectra confirmed the presence of the thaumasite
phase in collapsed surface (see section 3.6 in theoriginal paper).
The SEM montages in Fig. 8 reveal a grayscalechange from the surface to the interior of eachsample due to hydrate dissolution, indicating Ca
dissolution mainly. Each SEM montage image wasanalyzed in 1 mm sections from the surface to theinterior of the specimen in the area surrounded by
the red box, and the porosity (area %) was
calculated by image processing (Fig. 9). This
analysis reveals that more pores existed close to thesurface in both specimens. Compared with thespecimens

immersed in room- and low-temperatureseawater.
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Fig. 10 EDS analysis result in atomic ratio (a) Si/Ca vs. Al/Ca at 0 mm depth from specimens immersed roomand low- temperature seawater, and (b) Si/Mg vs. Al/Mg at 0 mm depth from specimen immersed lowtemperature seawater.

Fig. 11(a) Si/Ca vs. Al/Ca, (b) S/Ca vs. Al/Ca at different depths from specimen immersed low-temperatureseawater.

specimen immersed in room-temperature seawater,that immersed in low-temperature seawater tended
to have more pores and microcracks at the same
depth, indicating that hydrate dissolution was more
pronounced. Based on the EPMA and XRD results,
the dissolved hydrate is mainly portlandite (Fig. 6
and 7).
In Fig. 10 (a), the Si/Ca ratio is shown as afunction of the Al/Ca ratio near the specimen
surface (0 mm depth), determined by EDS point
analysis of the specimens immersed in room- andlow-temperature seawater. The circle represents the
typical C–(A–)S–H composition of ordinary PC, aswidely reported in the literature(5)-(7). For the
specimen immersed in room-temperature seawater,

the absence of data points near the origin suggests
the absence of portlandite due to portlandite
dissolution. The data points do not scatter within thecircled area corresponding to a typical C–(A–)S–H
composition, indicating that C–(A–)S–H has not yet
been decalcified. Additionally, the data points alignalong a trajectory toward the theoreticalcomposition of ettringite, indicating the presence of
ettringite formed by SO42− derived from seawater.
In contrast, for the specimen immersed in lowtemperature seawater, the data points are more
scattered and located outside the circle
corresponding to a typical C–(A–)S–H composition,suggesting either C–(A–)S–H decalcification or the
precipitation of new hydrates, such as Mg-based- 29 -

研究所 研究報告 No.1 (2023)

hydrates. Furthermore, in conjunction with the
XRD results (Fig. 7), the presence of some data
points near the origin indicates the precipitation of
calcite instead of portlandite.
Figure 10 (b) shows the relative Si and Al atomic
ratios concerning the Mg content at a depth of 0 mm.
The area enclosed by the red dotted line indicatesthe range of possible M–S–H and magnesium
aluminosilicate hydrate (M–A–S–H) compositions(8). Some data points are located in this area indicate
the formation of M–S–H or M–A–S–H.
Additionally, a cluster occurs outside this area, near
Si/Mg = 0.5, which indicates the intermixing of M–(A–)S–H and other Mg phases (likely brucite basedon the XRD results).
Figure 11 (a) shows plots of the Si/Ca ratio as afunction of the Al/Ca ratio at depths of 0, 1, and 3
mm for the specimen immersed in low-temperatureseawater. Portlandite is dissolved at both 1 and 3
mm considering no data points appear near theorigin. The decalcification of C–(A–)S–H wasobserved at a depth of 0 mm, many data points werelocated in the C–(A–)S–H circle at depths of 1 and
3 mm. This indicates that C–(A–)S–H
decalcification could not have occurred. Ettringite
is formed in the 1 mm depth and Friedel’s salt in the
3 mm depth (Fig. 11 (b)), indicating that phasechange of hydrates from the surface to the interior
are almost identical to those reported for marine
deterioration before(9)-(11).

4 Discussion
The specimen immersed in low-temperatureseawater showed more significant degradation thanthat immersed in room-temperature seawater. The
temperature affects the durability of cementitious
materials; for example, SO42− attack is accelerated
at low temperatures owing to the increased
solubility of portlandite at low temperatures and theweakening of the surface layer owing to theformation of thaumasite(12). The solubility of
portlandite in cement increases at lower
temperatures(13). The saturation concentration of
Ca2+ in portlandite was 1.08 times higher at 2 °C
than at 20 °C(14), and more portlandite can dissolve

in low-temperature environments. A steeper Ca2+
concentration gradient between seawater and poresolution could further drive Ca dissolution. As the
portlandite dissolves, the porosity increases, as
shown in Fig. 9, promoting the ion diffusion inseawater into the specimen and increase portlandite
dissolution. As a result, the specimen immersed in
low-temperature seawater exhibits significant
deterioration than the specimen immersed in roomtemperature seawater.
While portlandite dissolution increases theporosity of the specimen, as illustrated in Fig. 9, it
does not result in a mushy structure or surface
collapse. In the specimen exposed to roomtemperature seawater, no other damage wasobserved despite the dissolution of portlandite in
the surface layer. However, portlandite dissolution
does increase seawater permeability, thereby
facilitating C–(A–)S–H decalcification and,ultimately, the formation of M–(A–)S–H andthaumasite. The formation of these hydrates is
expected to cause the surface layer to collapse.
Furthermore, the specimen near the surface canbe damaged due the formation of ettringite.
Ettringite does not always lead to serious SO42−
attack in concrete, with no cracking or spallingobserved in sulfur-rich zones(9),(15). However, in thespecimen immersed in low-temperature seawater,
sulfur-rich zone owing to the formation of ettringite
was observed near the surface at a depth of 1 mm,
and fine parallel cracks appear (Fig. 11 (b)).
Ettringite precipitation in smaller (typically < 20nm) and supersaturated with respect to ettringite
can cause specimen expansion(15). The solubility of
ettringite decreases at lower temperatures(16), i.e.,
precipitation would be more pronounced. Although
the dissolution of portlandite has caused an increasein porosity, C–(A–)S–H is present in the area where
ettringite is formed, and additional ettringite
formation can occur in the microporosity derived
from C–(A–)S–H, resulting in the collapse of thespecimen surface layer.

5 Conclusion
PC paste specimens were immersed in room- and- 30 -

ＵＢＥ三菱セメント株式会社

low-temperature seawater for 433 days. The specimen
immersed in low-temperature seawater underwent
more pronounced deterioration than that immersed inroom-temperature seawater. The deteriorated
specimen immersed in low-temperature seawater
exhibited the following characteristics:•
The Ca dissolution front penetrated deeper within
the sample, and portlandite dissolution was moresignificant compared to the specimen immersedin room-temperature seawater.
•
The porosity of the specimen increased as
portlandite dissolved.
•
On the collapsed mushy surface, C–(A–)S–H
was decalcified, and Mg-based hydrates (e.g.,
brucite and M–(A–)S–H) and thaumasite formed.
The increased solubility of portlandite at low
temperatures led to substantial Ca dissolution andsubsequent deterioration through C–(A–)S–H
decalcification, resulting in the formation of M–(A–)S–H and thaumasite, ultimately resulting in theweakening and collapse of the surface. Therefore, lowtemperature seawater could mainly contribute to PC
deterioration under deep-sea condition. As alternative
binders to PC, one with less or no portlandite would bebetter. Suitable candidates would be calcium
aluminate cement and magnesium binders, as well asbinders with inorganic additives, such as fly ash andsilica fume, which consume portlandite in thehydration process and create denser structure.

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pp. 1–18 (2008)小林真理・こばやし まり
研究所 セメント研究室
セメントグループ 研究員
高橋恵輔・たかはし けいすけ
研究所 セメント研究室
セメントグループ 主幹- 32 -