Marine Georesources & Geotechnology
ISSN: 1064-119X (Print) 1521-0618 (Online) Journal homepage: http://www.tandfonline.com/loi/umgt20
Cementation of Sand Due to Salt Precipitation in
Drying Process
Jia He & Jian Chu
To cite this article: Jia He & Jian Chu (2016): Cementation of Sand Due to Salt Precipitation in
Drying Process, Marine Georesources & Geotechnology, DOI: 10.1080/1064119X.2016.1168498
To link to this article: http://dx.doi.org/10.1080/1064119X.2016.1168498
Accepted author version posted online: 30
Mar 2016.
Submit your article to this journal
Article views: 4
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=umgt20
Download by: [Gazi University]
Date: 05 April 2016, At: 10:26
Cementation of Sand Due to Salt Precipitation in Drying
Process
Jia He
Lecturer, College of Civil and Transportation Engineering, Hohai University, Nanjing, China
Downloaded by [Gazi University] at 10:26 05 April 2016
Jian Chu*
Professor, School of Civil and Environmental Engineering, Nanyang Technological University,
Singapore
Abstract
Salt in soil can cause cementation effect and increase the shear strength and stiffness of soil during
the drying process. In this paper, an experimental study is presented to explore the strength,
stiffness, and particle level structure of a salt-cemented sand at the dry state. Unconfined
compression tests were carried out on sands with various amounts of precipitated dry salt.
Scanning electric microscopic and elemental analysis were also conducted. The study shows that
the strength and stiffness of sand can increase significantly with salt content. The strength versus
salt content curve displays a convex shape, instead of a concave shape as found in cemented or
biocemented sands. This implies that the effect of dry salt on soil strength is strong at low level of
salt content. The microscopic and elemental analysis evidence that salt tends to precipitate at
particle contacts and form bridges between particles at relatively low salt content. The results
presented in the paper may explain why even small amount of salt can lead to an overestimation
of soil strength.
1
Keywords: cementation, salt precipitation, sand
INTRODUCTION
Recently, there have been a lot of studies on the effect of biocementation on sand using the
microbial induced carbonate precipitation (MICP) approach (Chu et al. 2013; DeJong et al. 2013;
Li 2015). In the MICP method, calcium chloride salt is usually used as a regent. Past studies have
indicated that the presence of salt can affect the strength of soil. When the salt used for the MICP
Downloaded by [Gazi University] at 10:26 05 April 2016
process has not been fully consumed, the strength of the biocemented sand can be attributed to
both biocementation and cementation by dry salt. For example, Li (2015) reported that, in the use
of biocement for strengthening sands, residual calcium chloride in soils led to an overestimation
of soil strength in the dry state, see Figure 1. It is also reported that a very small amount of salt
(as low as 0.065%) can greatly change the small-strain stiffness of soil at dry state (Truong et al.
2012). Under these circumstances, it is of research interest to understand the effect of precipitated
salt on the strength of soils.
Another objective of this paper is related to the soil strength and slope stability in the
seashore areas. At the water fluctuation zones of seashore areas, soil may undergo drying and
wetting processes, which leads to the variations in the soil strength. This variations in the soil
strength could be ascribe to several mechanisms: the change in the matric suction and osmotic
suction, and the precipitation or dissolving of salt when soil dries or wets. The last mechanism has
not been fully realized in the past. It is reported in limited studies that precipitated salt could be
relevant to the weakening of loess and saline skbkha in a wetting process (Abduljauwad and AlAmoudi 1995; Rinaldi, Santamarina, and Redolfi 1998; Cho and Santamarina 2001). The overlook
2
of the effect of precipitated salt in soil may cause an overestimation of soil strength and slope
stability, especially in seashore areas.
Salt (either calcium chloride, sodium chloride or other salts) can affect the soil properties
in different ways. When salts are dissolved in pore water, they will give rise to osmotic suction,
which in turn alters the compression and swelling behavior, Atterberg limits, shear strength, and
maximum dry density of soil (Mitchell and Soga 2004; Ajalloeian et al. 2013; Xu et al. 2014).
Downloaded by [Gazi University] at 10:26 05 April 2016
Another way that ionic salts affect soil behavior is that, as soil is drying, ionic concentration in
pore water increases and eventually reaches the saturated state, leading to the precipitation and
crystallization of salts. These dry salts tend to accumulate at particle contacts where water menisci
dry out, bridge soil particles, and enhance greatly the shear strength and stiffness of soils. There
were only limited research studies related to this topic in the past (Abduljauwad and Al-Amoudi
1995; Lin et al. 2005; Truong et al. 2010; Truong et al. 2012; Li 2015).
In natural conditions especially arid regions, salts affect strongly the mechanical behavior
of soil and influence related geotechnical problems. Abduljauwad and Al-Amoudi (1995) studied
the behaviour of a salty soil under soaking and leaching conditions. The soil in the wetting process
showed that percolation of clean water through the soil led to destruction of natural cementation,
causing an increase in settlement and a reduction in bearing capacity of soil ground; percolation
of brine water through the soil did not have major influence on the soil behaviour. Lin et al. (2005)
reported that, sandstones that contain dissolved minerals such as chlorite can be weakened during
a wetting process, due to the loss of dissolved components in the sandstones. Collins and Sitar
(2009) showed that, weakly-cemented and moderately-cemented sands manifested considerable
reduction in strength during wetting. Nowadays, chemical and biochemical modifications of soil
are becoming widely used. Soluble salts used in the soil treatment processes may also affect the
3
soil behaviour (Li 2015). The abovementioned studies aimed at investigating the discrepancies of
soil behaviour between dry and wet states. However the formation process of salt precipitation and
the relationship between salt content and mechanical behaviour of soil is still not well understood.
Studies on the properties of salt-cemented soil are still rare. A few published papers include Truong
et al. (2012), who studied the small strain behaviour of salt-cemented sand using elastic wave
methods. They concluded that salt-induced cementation can increase the small strain stiffness of
Downloaded by [Gazi University] at 10:26 05 April 2016
sand even with a salt content as low as 0.065%.
It is also interesting to note that methods have been proposed to preferably form cementing
agents at particle contacts, and thereby cemented soil can be much stronger than those using
conventional cementation methods. Valdes and Cortes (2014) proposed a method of using polymer
to strengthen sand. In heating and cooling process, polymer tends to accumulate at sand particle
contacts. The strength of such specimens is much higher than that of sand treated with Portland
cement. In biocement methods, a common way to treat bulky soil is to percolate treatment liquid
through soil body and cementation can be formed along flow path. It is concluded by Cheng et al.
(2013) that, in the treatment process, if degree of saturation can be controlled at a low level, more
calcite accumulates at particle contacts. The soil can be much stronger than that treated at fully
saturated conditions. From these studies, we can see that the formation and the spatial distribution
of cementing agents (either Portland cement, biocement, or precipitated salts) play an important
role in the strength of cemented soils. So it is a practical need to study the behavior of cemented
soil with cementing agents preferably formed at particle contacts. It is also deserve pointing out
that, the type of cementing agent may affect the strength and stiffness of host soil, because of the
difference in the strengths and the morphologies of cementing crystals. However, it is not in the
scope of this study.
4
Although research evidences have shown that salt precipitation can play an important role
in the strength of dry soil, a direct relationship between salt content and strength of host soil was
not seen in the past. Salt is omnipresent in natural soils (but is usually overlook in geotechnical
laboratories). This paper may shed light on the effect of salt on the variation of soil strength in
natural conditions, especially oceanic environments. In this paper, an experimental study on the
strength, stiffness, and particle level structure of salt-cemented sand was carried out. A series of
cylindrical samples were prepared with different salt contents. Unconfined compression tests were
Downloaded by [Gazi University] at 10:26 05 April 2016
conducted. Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray Spectroscopy
(EDX) analysis was used to analyze particle level structure.
TESTING METHODS
River sand (W9 grade, Gs = 2.63, D50 = 0.25 mm, poorly graded, sub-rounded, quartz
content > 99%) was used. Calcium chloride was used as the cementing agent. It is one of the
chemicals used in MICP-based biocement. The specimens were prepared in cylindrical PVC tubes
with rubber stoppers. The size of the PVC tubes was 3.8 cm in diameter and 7.6 cm in length.
Brine water was prepared by dissolving calcium chloride in de-ionized water. The concentration
of calcium chloride in brine water ranges from 6.5 ~ 37.5%. The specimens were prepared by
depositing dry sand into brine water in the PVC tubes. The dry mass and volume of sand was
controlled to be the same for all the specimens. Extra brine water above the sand surface was
removed. The specimens prepared in this way had a void ratio around 0.69. Then the specimens
were moved to an oven at a temperature of 60°C for 5 days. At this stage, the specimens were not
completely dry, but cementation or capillary effects were formed and the specimens could stand
without support. The PVC tubes were taken off. The specimens were oven dried at the temperature
of 105°C until they were completely dried. The specimens were kept in a sealed container to avoid
5
moisture absorption. The determination of salt contents at different parts of one specimen showed
that salt content in the specimen was relatively even and the specimen can be taken as a uniform
piece.
Unconfined compression tests were conducted. The strain rate was 1 mm/min. The tested
specimens were then used to determine salt contents. Salt mass was determined by weighing the
specimen before and after rinsing it in distill water. Some small pieces of the specimens were used
Downloaded by [Gazi University] at 10:26 05 April 2016
for particle level structure and elemental analysis, using SEM and EDX apparatus.
RESULTS AND DISCUSSIONS
Strength
Data of unconfined compression strength (UCS) versus salt content are plotted in Figure
2. Another two trend lines are also drawn in Figure 2. One is sand treated with Portland cement
(Consoli et al. 2010), and another is sand treated with biocement using the MICP method (Li 2015).
The sand used in this study and the study by Li (2015) is the same. The sand used by Consoli et
al. (2010) is also a poorly-graded clean quartz sand. Its mean size (D50) is 0.16 mm, which is a
little smaller than the size of the sand used in this study. The effective cementing agent in
biocement is calcite, a mineral consisting of calcium carbonate. For salt-cemented specimens, the
UCS increases with the salt content. Within the range of 1.9 ~ 15% salt content, the UCS increases
from 0.05 to 9.1 MPa. The strength of cemented and biocemented sands also show an increasing
trend as the cement content increases, but the strength is much lower in the same tested range.
Within the range of cement content of 5 ~ 12%, the UCS of cemented and biocemented soils are
only 1/2 to 1/4 of that of the dry salt-cemented specimens. Another difference is that the trend line
of the UCS of the salt-cemented specimens manifests a convex shape, that is, adding salt is more
6
effective in the lower range of salt content in terms of strength improvement. The trend lines for
the cemented and biocemented treated sand are concave. This is related to the spatial distribution
of cementing agents in soils pores, which will be discussed later in the paper.
Stiffness
The stiffness of the specimens is evaluated using the Young’s modulus determined from
the unconfined compression tests. The Young’s moduli versus salt content for all the tested
Downloaded by [Gazi University] at 10:26 05 April 2016
specimens are plotted in Figure 4. As salt content increases from 1.9 to 15%, the Young’s modulus
shows an ascending trend from 17 to 1,600 MPa. For relatively weak specimens (UCS < 0.2 MPa),
the cementation effect in the specimens progressively fails during compression, which leads to
soil-like the stress-strain curves, as shown in Figure 3a. The modulus is determined as the
tangential gradient at the origin point. For relatively strong specimens (UCS > 0.8 MPa), the
stress-strain curves display a rock-like pattern, showing an increasing trend in gradient in the initial
part of the curves and becoming linear at the latter stage of compression, as shown in Figure 3b.
This stress-strain pattern could be due to the compression of small pores in the specimens in the
initial stage of compression (Hoek and Martin 2014). The modulus is determined using the gradient
of the linear part of the stress-strain curve. The boundary of the two types of stress-strain curves
is likely to be within 2.7–3.1% range of salt content.
Particle Level Structure
SEM and EDX images of some salt-cemented samples are presented in Figure 5. The SEM
image of a specimen with 6.5% salt content is shown in Figure 5a. It can be seen that two sand
particles are bridged by salt precipitation. The salt precipitation takes on the shape of a water
meniscus. This particle level structure is evidenced by the EDX elemental analysis (Figure 5b),
7
which shows that salt (Ca and Cl elements) lies between two sand particles (Si and O elements).
However, there are also some Ca and Cl elements on the particle surfaces. This could be due to
the drying of absorbed water on sand particles. Figure 5c, d present two SEM images with 6.5%
and 12.9% salt contents, respectively. The difference between the two images is that: the specimen
with 6.5% salt content has a lot of pore voids; the specimen with 12.9% salt content manifests that
salt precipitates distribute continuously among sand grains (although some voids may not be seen
from the image). Such a phenomenon is related to the water content and the morphology of water
Downloaded by [Gazi University] at 10:26 05 April 2016
phase when saline water reach its solubility and salt starts to precipitate. In an unsaturated soil,
water will be accumulate at soil particle contacts and form water menisci at relatively low water
contents (namely pendular stage). But at relatively high water contents, water phase will be
continuous (namely funicular stage). It is also reported that salt precipitation tend to form at soil
particle contacts at pendular stage and contribute greatly to the small-strain stiffness (Cho and
Santamarina 2001).
The particle level analysis given above can explain some strength and stiffness features of
salt-cemented sand as displayed in Figures 2 and 4. As salt precipitation are preferably formed at
soil particle contacts, its contribution to the strength is larger than other cementation methods,
where cementing agents are more randomly distributed in soil pores. But when salt content
increases, meniscus-shaped salt bridges between sand particles cannot be formed. In this range of
salt content, the cementation effect of salt becomes weaker. This is why UCS-salt content
relationship shows a convex shape.
CONCLUSIONS
8
This paper presents an experimental study on the effect of salt precipitation on strength and
stiffness of sand at dry state. Within a range of 1.9% to 15% salt content, the dry specimens show
great increases in both strength and stiffness. The strength versus salt content curve displays a
convex shape, instead of a concave shape as found in sands treated with Portland cement or
biocement. The SEM images of salt-cemented sand reveal that, at a relatively low level of salt
content, salt tends to precipitate at particle contacts and bridge sand particles together, while at a
high salt content, salt precipitates tend to distribute continuously among sand particles. This
Downloaded by [Gazi University] at 10:26 05 April 2016
explains why salt precipitation contributes greatly to the strength of salt-cemented sand, but the
strength increasing trend becomes slower as salt content increases.
Acknowledgements
This work is supported by Jiangsu Science Foundation Project BK20150814, China
Postdoctor Science Foundation Project 2015M581714, and Jiangsu Postdoctor Research
Foundation Project 1501026C.
References
Abduljauwad, S. N., and O. S. B. Al-Amoudi. 1995. Geotechnical behaviour of saline sabkha soils.
Géotechnique 45 (3): 425–45. doi:10.1680/geot.1995.45.3.425
Ajalloeian, R., H. Mansouri, and A. H. Sadeghpour. 2013. Effect of saline water on geotechnical
properties of fine-grained soil. Electronic Journal of Geotechnical Engineering 18:1419–
35.
Cheng, L., R. Cord-Ruwisch, and M. A. Shahin. 2013. Cementation of sand soil by microbially
induced calcite precipitation at various degrees of saturation. Canadian Geotechnical
Journal 50 (1):81–90. doi:10.1139/cgj-2012-0023
Cho, G., and J. Santamarina. 2001. Unsaturated particulate materials—Particle-level studies.
Journal of Geotechnical and Geoenvironmental Engineering ASCE 127 (1):84–96.
doi:10.1061/(asce)1090-0241(2001)127:1(84)
Chu J., V. Ivanov, V. Stabnikov, and B. Li. 2013. Microbial method for construction of aquaculture
pond in sand. Geotechnique 63 (10):871–75. doi:10.1680/geot.sip13.p.007
9
Downloaded by [Gazi University] at 10:26 05 April 2016
Collins, B., and N. Sitar. 2009. Geotechnical properties of cemented sands in steep slopes. Journal
of Geotechnical and Geoenvironmental Engineering ASCE 135 (10):1359–66.
doi:10.1061/(asce)gt.1943-5606.0000094
Consoli, N., R. Cruz, M. Floss, and L. Festugato 2010. Parameters controlling tensile and
compressive strength of artificially cemented sand. Journal of Geotechnical and
Geoenvironmental Engineering ASCE 136 (5):759–63. doi:10.1061/(asce)gt.19435606.0000278
DeJong, J. T., K. Soga, E. Kavazanjian, S. Burns, L. A. Van Paassen, A. Al Qabany, A. Aydilek,
S. S. Bang, M. Burbank, L. F. Caslake, C. Y. Chen, X. Cheng, J. Chu, S. Ciurli, A. EsnaultFilet, S. Fauriel, N. Hamdan, T. Hata, Y. Inagaki, S. Jefferis, M. Kuo, L. Laloui, J.
Larrahondo, D. A. C. Manning, B. Martinez, B. M. Montoya, D. C. Nelson, A. Palomino,
P. Renforth, J. C. Santamarina, E. A. Seagren, B. Tanyu, M. Tsesarsky, and T. Weaver
(2013). Biogeochemical processes and geotechnical applications: Progress, opportunities
and challenges. Geotechnique 63 (4):287–301. doi:10.1680/geot.sip13.p.017
Hoek, E., and C. D. Martin. 2014. Fracture initiation and propagation in intact rock – A review.
Journal of Rock Mechanics and Geotechnical Engineering 6 (4):287–300.
doi:10.1016/j.jrmge.2014.06.001
Li, B. (2015). Geotechnical properties of biocement treated soils. Doctoral thesis, Nanyang
Technological University, Singapore.
Lin, M. L., F. S. Jeng, L. S. Tsai, and T. H. Huang. 2005. Wetting weakening of tertiary
sandstones—Microscopic mechanism. Environmental Geology 48 (2):265–75.
doi:10.1007/s00254-005-1318-y
Mitchell, J. K., and K. Soga. 2004. Fundamentals of soil behavior, 3rd ed. Hoboken, NJ: John
Wiley & Sons.
Rinaldi, V. A., J. C. Santamarina, and E. R. Redolfi. 1998. Characterization of collapsible soils
with combined geophysical and penetration testing. In Proceedings of 1st international
conference on site characterization, ISC’98, ed. P. K. Robertson and P. W. Mayne, 581–
88. Brookfield: Balkema.
Truong, Q. H., Y. H. Eom, Y. H. Byun, and J. S. Lee. 2010. Characteristics of elastic waves
according to cementation of dissolved salt. Vadose Zone Journal Soil Science Society of
America 9 (3):662–69. doi:10.2136/vzj2009.0131
Truong, Q. H., C. Lee, Y. U. Kim, and J. S. Lee. 2012. Small strain stiffness of salt-cemented
granular media under low confinement. Géotechnique 62 (10):949–53.
doi:10.1680/geot.10.t.004
Xu, Y., G. Xiang, H. Jiang, T. Chen, and F. Chu. 2014. Role of osmotic suction in volume change
of clays in salt solution. Applied Clay Science 101:354–61. doi:10.1016/j.clay.2014.09.006
Valdes, J., and D. Cortes. 2014. Heat-induced bonding of sands. In Proceedings of Geo-Congress
2014 Technical Papers, Atlanta, Georgia, USA, 3721–33.
10
Downloaded by [Gazi University] at 10:26 05 April 2016
Figure 1. UCS versus calcite content for biocemented sands at wet and dry conditions (data from
Li 2015).
11
Downloaded by [Gazi University] at 10:26 05 April 2016
Figure 2. UCS versus content of cementing agents.
12
Downloaded by [Gazi University] at 10:26 05 April 2016
Figure 3. Unconfined compression curve of salt-cemented sand specimens with (a) 2.3% and (b)
3.7% salt content.
13
Downloaded by [Gazi University] at 10:26 05 April 2016
Figure 4. Stiffness of salt-cemented sand.
14
Downloaded by [Gazi University] at 10:26 05 April 2016
Figure 5. SEM and EDX analysis of salt-cemented sand (a) A SEM image of the specimen with
6.5% salt content (×800 magnification); (b) EDX analysis of Si, O, Ca and Cl elements on the
specimen in (a); (c) A SEM image of the specimen with 6.5% salt content (×50 magnification);
(d) A SEM image of the specimen with 12.9% salt content (×50 magnification).
15