Water stability mechanism of silicification grouted loess

ORIGINAL PAPER

Water stability mechanism of silicification grouted loess

Qingfeng Lv • Shengxin Wang • Dekai Wang •

Zhumin Wu

Received: 27 January 2013 / Accepted: 30 June 2014 / Published online: 18 July 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Silicification is one of the chemical stabilisa-

tion methods used in the treatment of collapsible loess

soils. The water stability therein is a key parameter in the

silicification of grouted loess. Based on slaking tests, per-

meability measurement, X-ray diffraction spectra, X-ray

energy dispersive spectroscopy, and scanning electron

microscopy, the water stability mechanism inherent in the

CO2-silicification grouted loess was investigated. Samples

of original, compacted, and CO2-silicification grouted loess

in 30 days curing were tested. To assess the long-term

water stability, CO2-silicification grouted loess samples in

13, 19, and 24 years of curing were analysed. The study

showed that the CO2-silicification grouted loess had good

water erosion resistance, no disintegration, and good water

stability over time. The water stability of CO2-silicification

grouted loess depended on the strong bond strength of the

grains and a low permeability. The complex physico-

chemical reactions among CO2, water, alkali earth metal

salts, clay minerals, and organic matter in loess produced

hydrate calcium (and magnesium) silicate gels, which were

mainly coated on the surface of the soil skeleton grains and

original cements. A few filled in the trellis pores. The gels

coated on the soil skeleton limit the hydrophilicity of clay

minerals and organic matter and improve water resistance,

and if coated on original cements reinforce bond strength,

consequently, the water stability of CO2-silicification gro-

uted loess was improved.

Keywords Mechanism � Water stability � CO2-

silicification grouted loess � Disintegration � Permeability �Microstructure

Introduction

In China, there are about 35,280 km2 of loess, which

account for 6.6 % of the total land area of China and 4.9 %

of the world’s loess by outcropping area (Xu et al. 2007;

Qian et al. 1985). Almost all loess is sensitive to water (Li

et al. 2007), that is, the characteristic of large, rapid, uneven

settlement after ponding and softening, which can cause

construction foundation failures or geological hazards, is

always a risk (Kruse et al. 2007; Dijkstra et al. 1995).

Therefore, reliable methods of treatment of loess soils

should be researched to assist in the construction of indus-

trial and civil structures thereon (Yu et al. 2007; Xie 2001).

The most common treatment methods for collapsible

loess can be classified into two types, which are physical

methods and chemical methods (Jefferson et al. 2003;

Rogers et al. 1994). Physical methods are used to form

compacted loess mainly by means of a soil cushion method,

dynamic compaction, soil pile compaction methods, impact

compaction methods, etc., all of which are widely used in

groundworks for new buildings (Evstatiev 1988; Weng

2007; Wang et al. 2006). Silicification, a typical chemical

method, entails a water glass grout being injected into or

permeated through loess soils to form silicification-loess

mainly through a physicochemical processes (Arshakuni

and Golubkov1979; Sokolovich 1965, 1984). The resultant

Q. Lv � Z. Wu

Key Laboratory of Mechanics on Disaster and Environment in

Western China, The Ministry of Education of China, School of

Civil Engineering and Mechanics, Lanzhou University, Tianshui

Road, 222, Lanzhou 730000, Gansu, People’s Republic of China

S. Wang (&) � D. Wang

Geological Hazards Research and Prevention Institute,

Gansu Academy of Sciences, Dingxi Road, 229,

Lanzhou 730000, Gansu, People’s Republic of China

e-mail: [email protected]

123

Bull Eng Geol Environ (2014) 73:1025–1035

DOI 10.1007/s10064-014-0646-0

silicification-loess is noncallapsible, low in permeability,

and high in strength (Sheinin 1989). The silicification

method has already developed from double liquid silicifi-

cation and single liquid silicification in the early stages to

CO2-silicification (Rzanitsyn et al. 1969). In double liquid

silicification, sodium silicate and calcium chloride solutions

are injected into the soil in turn. In single liquid silicifica-

tion, only sodium silicate solution is injected. In the CO2-

silicification method, gaseous CO2 is firstly injected into the

soil, then the sodium silicate solution is grouted, and CO2

gas is injected into soil again as a final step. Compared with

dynamic consolidation methods and other physical meth-

ods, CO2-silicification is not widely used. However, it is an

effective method because it enables wet and saturated loess

soils under buildings and structures to be stabilised with

sufficient reliability and without interrupting their normal

operation (Wang 2005). Silicification is sometimes the only

option, such as treatments for the existing buildings or

construction loess foundation accidents. The stabilisation

effect is evaluated more on the basis of collapsibility, shear

strength, compressibility, bearing capacity, microstructure,

and dynamic characteristics than on water stability (Yin

et al. 2008; Wang 2005). The goal of soil stabilisation is the

improvement not only of a soil’s mechanical strength but

also its water stability. Therefore, it is necessary to further

research the water stability of silicification-loess.

Based on several different foundation treatment projects

in Lanzhou, China this research was focussed on the

mechanism of the CO2-silicification grouted loess water

stability by carrying out slaking tests, permeability mea-

surement, X-ray diffraction spectra (XRD), X-ray energy

dispersive spectroscopy (EDS), and scanning electron

microscopy (SEM).

Sample preparation and experimental methods

Engineering geology and ground treatment

All samples were collected from a construction site. The

construction, including an office building and a test

building, is located in Level 2 Terrace on the right bank of

the Yellow River in Lanzhou, Gansu Province, China. Its

foundation soil consisted of yellow or tan loessial silt clay

or silty soil (Q4al-pl) with horizontal bedding, slightly wet to

wet, and was plastic to soft plastic. There were worm holes

and plant root holes, with white calcium powder hole walls

present. The mineral composition of layer soils includes

detrital minerals, carbonate minerals, and clay minerals.

Detrital minerals are composed of quartz, feldspar, and

mica, and account for 80–90 % and carbonate minerals

account for 20–30 %. Clay minerals are composed of illite,

montmorillonite, and kaolinite. They account for 10–20 %.

This layer was the main bearing stratum and was

8.40–9.70 m in thickness. The foundation soil was a mod-

erate to highly compressible soil, with a loess collapse set-

tlement Ds of 16.2–27.90 mm, and self-weight collapsible

settlement Dzs between 7.80 and 12.75 mm. It was a non-

self-weight collapsible site, with an estimated collapsibility

of Grade I. The thickness of the collapsible soil layer was

4.50–5.50 m. The depth to the water table was 9 m.

The treatment method for the office building foundation

was a 500 mm thick lime-soil cushion. Since 1980, three

foundation settlement accidents occurred in different parts

of this loess deposit, and the CO2-silicification method was

successfully used to reinforce those foundations. The

reinforcement depth was 5.50–6.00 m, and the normal use

of the building was unaffected during the reinforcement.

The construction process for the CO2-silicification

method included, firstly, injecting CO2 gas and, secondly,

injecting water glass solution, and then injecting CO2 gas

again. The injection pressure was 0.15 MPa. The water

glass is sodium water glass, and the Baume degree is 20�Be

The mode is 3.

Sample preparation

Natural loess samples were collected at 2.0 m depth beside

the construction site by using a cutting ring from the bot-

tom of an inspection pit which was 2.0 m long and 2.0 m

deep (see Fig. 1, Nos. 1, 2, and 3). The average values of

physical, mechanical, and chemical indices of these natural

loess samples are presented in Tables 1 and 2.

CO2-silicification grouted loess samples in different

curing ages (30 days, 13, 19, and 24 years) were taken by

using a cutting ring from the bottom of the inspection pit of

the reinforced foundation of the office building. The sam-

pling site is shown in Fig. 1 (Nos. 4, 5, 6, and 7). The

samples that had undergone 24 years of curing came from

No. 4, 19 years from No. 5, 13 years from No. 6, and

30 days from No. 7. The average values of physical,

mechanical, and chemical indices of these CO2-silicifica-

tion grouted loess samples are presented in Tables 1 and 2.

Compacted loess samples were made from natural loess

by the standard Proctor laboratory compaction tests Its

maximum dry density is 1.74 g/cm3. Its optimum gravi-

metric moisture content is 16 %. The shear strength is

71.8 kPa, and the friction angle is 27.88.

Experimental methods

1. Slaking tests: all samples were immersed in distilled

water and were then described in terms of crack

development and disintegration.

2. Permeability, atterberg limits, and particle size ana-

lysis tests: all samples were taken for a laboratory

1026 Q. Lv et al.

123

coefficient of permeability and Atterberg limit testing

according to the prevalent standard (Chinese Standard

for Soil Test Methods, GB/T 50123-1999, same as BS

1377-2-1990 and ASTM D4318-05). Particle size

analysis testing was carried out using a SediGraph5100

Automatic Particle Analyser.

3. XRD: EDS, and SEM were carried out on the said

samples. XRD is used to study chemical composition,

and EDS is used to determine cement elements in

specified microareas, and SEM to observe soil skeleton

particles’ morphology and pore structure.

4. Micropore observation: a porous medium surface area

analyzer is used to quantify micropore characteristics,

such as surface area, pore volume, pore size distribu-

tion, and average pore diameter.

Results and discussion

Characteristics of particle size

Particle size distribution analysis of the samples before and

after stabilisation are presented in Fig. 2. As shown in

Fig. 2, the percentage of natural loess with particle size less

than 5 lm was 10.1 %, and the percentage of silicification

grouted loess with particle size less than 5 lm was between

24.1 % and 31.4 %. The fine particle content (i.e., that with

an equivalent spherical diameter of less than 5 lm)

increased after silicification. These fines were a type of

colloidal particle produced by the reaction between the

sodium silicate solution and the soil itself (Wang 2005).

Table 3 indicates that the liquid limit and plastic limit

increased after silicification. The Atterberg limits depended

upon the colloidal particle content, and the liquid limit and

plastic limit increased with an increased colloidal particle

content, as also verified by Skempton (1953).

Slaking and permeability characteristics

The hydraulic characteristics of soil usually include per-

meability and disintegration. Slaking and permeability

characteristics of different samples are presented in

Table 3. The CO2-silicification grouted loess had the

characteristics of lower permeability and higher plasticity.

The CO2-silicification grouted loess at different curing

ages has good water erosion resistance, underwent no

Table 2 Full chemical analysis of samples (average value)

Chemical

components

SiO2

(%)

Al2O3

(%)

CaO

(%)

MgO

(%)

K2O

(%)

Na2O

(%)

Fe2O3

(%)

TiO2

(%)

MnO

(%)

P2O5

(%)

SO3

(%)

Calcination

loss (%)

Organic

matter

(%)

CO2

(%)

pH Acid

indissoluble

substance

(%)

Water

insoluble

substance

(%)

Nature loess 56.34 11.67 5.82 2.31 2.52 2.9 5.24 0.72 0.079 0.18 0.73 10.77 0.46 8.16 8.75 69.27 88.75

CO2-silicification

grouted loess

59.51 11.17 5.81 1.83 2.19 3.57 4.04 0.58 0.06 0.19 0.40 10.32 0.43 7.32 9.73 74.19 90.43

Table 1 Physical and mechanical indices (Average value)

Type Nature density,

q (g cm-3)

Nature water

content, w (%)

Void

ratio, e

Dry density,

qd (g cm-3)

Shear

strength,

C (kpa)

Friction

angle, u (�)

Coefficient of

compressibility,

a1–2 (MPa-1)

Nature loess 1.662 20 0.954 1.384 13.8 29.63 0.423

CO2-silicification grouted loess 1.665 18.1 0.926 1.405 109.6 38.1 0.045

NO.6

NO.5

NO.4

NO.2

NO.1

NO.3

1 m1000

Inspection pit

NO.7

Fig. 1 Schematic diagram of

sampling inspection pit sites

Water stability mechanism of silicification grouted loess 1027

123

disintegration, and exhibited good water stability. Com-

pared with the natural loess, the compacted loess had an

altered microstructure after compaction; therefore, its

permeability was reduced, and its shear strength improved.

However, as with the natural loess, compacted loess still

had only a weak resistance to water erosion. Its water

stability was poorer, and it disintegrated after ponding

within 4–5 min.

The results also showed that the coefficient of perme-

ability of the natural loess was the highest whereas that of

the CO2-silicification grouted loess was the lowest. The

permeability of the CO2-silicification grouted loess was

inversely proportional to the curing time.

Analysis of X-ray diffraction spectra

Figure 3 shows an XRD of nature loess, and Figs. 4, 5, and

6 show XRD of CO2-silicification grouted loess in different

curing ages. Figure 3 shows that the main mineral com-

ponents of loess were: quartz, feldspar, mica, calcite,

dolomite, and other clay minerals. Figures 4, 5, and 6Fig. 2 Particle size distribution of the tested samples

Table 3 Comparison of hydrogeological properties test results of different specimens

Sample Properties Disintegrating phenomenon Coefficient of

permeability,

k 9 10-6 (cm s-1)

Liquid

limit, WL

(%)

Plastic

limit, WP

(%)

Plasticity

index, IP

(%)

Compaction

loess

Compaction

coefficient

kc/0.95

Bubbles appearing, crack opening with minor

spalling in 4–5 min, complete disintegration

7.56 24.8 15.8 9.0

CO2-

silicification

grouted loess

Curing age

30 days Bubbles appearing and disappearing 3 min later.

No visible change ponding after 90 days, no

disintegrating

6.72 33.3 19.6 13.7

13 years 5.16 33.9 19.5 14.4

19 years 4.99 33.5 19.7 13.8

24 years 4.53 33.3 19.7 13.6

Natural loess Undisturbed

structure

Lots of bubbles, crack opening with major

spalling in 4–5 min, complete disintegration

27.71 24.8 15.8 9.0

Fig. 3 X-ray diffraction spectra of nature loess

1028 Q. Lv et al.

123

indicate that the XRD of CO2-silicification grouted loess

basically matched that of the original loess, that is, the

main mineral component of CO2-silicification grouted

loess had undergone no change. However, the diffraction

intensity of some minerals had decreased, and there were

no new X-ray diffraction peaks, but low and dense amor-

phous substance peaks appeared.

Analysis by scanning electron microscopy

Typical SEM micrographs of the natural loess, the com-

pacted loess, and the CO2-silicification grouted loess are

shown in Figs. 7, 8, 9, 10, and 11.

It can be observed from Fig. 7 that the soil skeleton of

natural loess was mainly a single grain with a surface that

was clean or adherent to less cement The pores were

mainly trellis pores indicative of a typical granular, trellis,

and point contact structure. Figure 8 shows that the pores

in the compacted loess were mainly intergranular pores

with secondly trellis pores, which was a typical granular,

mosaic, and surface contact structure.

Figures 9, 10, and 11 show that the pores in the CO2-

silicification grouted loess were mainly trellis pores, some

of which were filled with a little gel as shown in the black

box in Fig. 9. There was adsorptive cement on the grain

surfaces, which had gel-formed during the solidification

Fig. 4 XRD of CO2-silicification grouted loess at 13 years curing age



123

process. Gel was coated on the particle surface in the form

of a film, and point contacts became surface contacts. It

was indicative of a typical granular, trellis, and surface

contact structure.

Analysis by X-ray energy dispersive spectroscopy

The binding force between the particles was difficult to

measure directly, but EDS can be used to measure the

presence and extent of those chemical elements likely to be

able to cement individual grains. EDS spectra of the natural

loess and the CO2-silicification grouted loess are shown in

Figs. 12, 13, 14, and 15, and the surface elemental com-

positions of natural loess and CO2-silicification grouted

loess in EDS experiments are shown in Table 4 from where

the elements: C, O, Al, Si, K, Ca, Ti, and Fe can be tested

for, and found, in natural loess except for the expected

presence of Mg.

The elements C, Si, O, Ca, and Mg were analysed in

CO2-silicification grouted loess, and the C, O, and Si

contents were significantly higher than the corresponding

values in natural loess. It was noteworthy that Mg was

assayed after CO2-silicification, which showed that Mg

dissolved from the original rock salt after alkali activation.

The mass ratio of Si (located at the black box in Fig. 9)

reached its maximum of 37.74 %. Hereby, the cement

between the soil skeleton’s largest grains was manifested in

the compounds containing elemental C, Si, O, Ca, Mg, etc.


Fig. 7 SEM micrograph of natural loess specimen (9500)

Fig. 8 SEM micrograph of compacted loess specimen (9500)

1030 Q. Lv et al.

123

The compounds were predominantly calcium (or magne-

sium) silicate hydrate, and hydrated silica gel.

Analysis of micropore characteristics

Comparison of micropore characteristics of natural loess

and CO2-silicification grouted loess is shown in Table 5.

As can be seen from the table, the pore size distribution

remained basically unchanged after silicification in the

range 17–1500 A. The pore volume and average pore

diameter are little changed. The surface area substantially

increased by 56.48 %, which indicates that the fine parti-

cles are increased after silicification, particularly nanoscale

particles. The fine particles should be predominantly

calcium (or magnesium) silicate hydrate, and hydrated

silica gel. The larger the surface area, the stronger the

connection strength.

Mechanism of water stability

The plasticity index of loess increased after silicification,

which showed that the range of plasticity increased, and the

clay particles (various silicide gels formed in the solidifi-

cation process) increased in number. Meanwhile, the

coefficient of permeability of the natural loess, the com-

pacted loess, and the CO2-silicification loess decreased in

turn.

Immersed water can infiltrate the loess trellis pores

after the natural loess and the compaction loess suffered

ponding in water and in the penetration process, and the

water interacted with the non-water-resistant cement

materials comprising the loessian microstructure, such as

clay minerals and organic matter. Clay minerals have

strong expansibility after coming into contact with water,

and the trellis pores provided the space to allow such

expansion. On the other hand, there were uncompensated

oxygen atoms and hydroxyl groups on clay mineral sur-

faces, and the interactions between uncompensated oxy-

gen atoms and structural units of water formed the initial

adsorption layer, which became gradually thicker as the

infiltration progressed. Therefore, the binding force

between the soil skeleton’s particles decreased, and,

consequently, the loess microstructure became unstable

and slaking occurred. It can be said that the physical and

mechanical processes only changed the strength of the

loess but not its water stability.

The main pores were trellis pores in the CO2-silicifica-

tion grouted loess as was the case for the natural loess.

Fig. 9 SEM micrograph of CO2-silicification loess for curing

13 years (9500)


19 years (9500)


24 years (9500)


123

Water-resistant hydrated silicate calcium (magnesium) and

hydrated silica gel were generated in a series of complex

physicochemical processes, which involved the interaction

between CO2, sodium silicate, rock salts, clay minerals,

and organic matter in the loess. Because a little gel filled

some trellis pores, the interconnectivity of the pores

decreased and the permeability decreased. Therefore, the

amount of water that infiltrated the soil during saturation

decreased. Gel coated the surface of the soil skeleton’s

particles and the original cementation surface to inhibit the

activity of clay minerals and organic matter. This new

cementation bonded the soil skeleton and formed a whole

network, which can enhance the bond forces and improve

the water-resistance of the soil and retain its cementation.

The previously hydrophilic connections became hydro-

phobic and the water stability improved to such an extent

that slaking no longer occurred. Analysis indicated that the

enhanced water stability of the CO2-silicification grouted

Fig. 12 EDS spectra of natural loess

Fig. 13 EDS spectra of CO2-silicification grouted loess at 13 years curing age

1032 Q. Lv et al.

123

loess originated from the largest soil skeleton particles’

connection strengths and low permeabilities.

Based on the microstructure of the natural loess, the

compacted loess, and the CO2-silicification grouted loess,

the mechanism of loess collapsibility can be discussed. The

natural loess had a granular, trellis, and point contact

structure. The bond was mainly a hydrophilic connection,

and the interparticle bond strength was low. Thereby, the

natural loess exhibited characteristic collapsibility and

slaking. The compacted loess had a granular, mosaic and

surface contact structure. Because there were no macrop-

ores in this compacted loess, it was characteristically non-

collapsible and non-slaking. The CO2-silicification grouted

loess has a granular, trellis, and surface contact structure.

The interparticle bonds were mainly hydrophobic and the

bond strength between particles was high; therefore, the

CO2-silicification grouted loess was characteristically non-

collapsible and non-slaking. Obviously, changing the trellis




123

pores into intergranular pores or keeping trellis pores

unchanged and improving the cement strength can elimi-

nate collapsibility. Accordingly, the internal cause of loess

collapsibility was its weak soil skeleton particles’ inter-

connection strength, and trellis pores were not the key

factor: the external cause was water and superimposed load

or self-weight stress.

Conclusions

This study presented an analysis of the water stability of

CO2-silicification grouted loess samples after 30 days, and

13, 19, and 24 years of curing based on slaking tests and

permeability tests. The soil behaviour was examined

through its XRD, EDS tests, and SEM.

The CO2-silicification grouted loess with different cur-

ing ages had good water erosion resistance, did not disin-

tegrate, and exhibited good water stability. The coefficient

of permeability of the natural loess was the highest and that

of the CO2-silicification grouted loess was the lowest. The

permeability was inversely proportional to the curing time.

The water stability of CO2-silicification grouted loess

depended on the strong bond strength of its soil skeleton

grains and its low permeability. The complex physico-

chemical reactions among CO2, water glass, metal salts,

clay minerals, and organic matter in the loess produced

hydrated calcium (or magnesium) silicate gels, which

mainly coated the surface of the soil skeleton’s grains and

their original cementing agents. A few filled the trellis

pores. The activities of clay minerals and the organic

matter were inhibited by these hydrates, which caused a

reinforced bond strength between the soil skeleton’s grains

and the water-resistance of the cementing agents: conse-

quently, the water stability of CO2-silicification grouted

loess was improved.

Acknowledgments The work was supported by Gansu province

science and technology support program (No. 1011FKCA093) and

Project of The National Natural Science Foundation of China (No.

51178290).

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Table 4 Comparison of surface elements of natural loess and CO2-silicification grouted loess

Sample Index C O Mg Al Si K Ca Ti Fe

Natural loess Weight % 35.61 12.24 – 5.18 23.21 2.53 9.47 0.39 11.32

K-ratio 0.0495 0.0153 – 0.0317 0.1588 0.0210 0.0811 0.0031 0.0928

Cnts/s 5.18 15.84 – 68.44 340.39 32.45 115.82 3.57 65.37

Atom % 55.93 14.69 – 3.67 15.89 1.24 4.56 0.16 3.88

CO2-silicification grouted loess Weight % 36.99 15.506 0.97 5.82 29.22 2.7167 7.91 0.57

K-ratio 0.0443 0.018 0.006 0.0408 0.2156 0.0219 0.0713 0.004

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Atom % 54.72 17.213 0.725 3.8533 18.5 1.24 3.7967 0.213

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and CO2-silicification grouted loess

Type BET

surface

area

(m2/g)

Pore

volume

(cm3/g)

Average

pore

diameter (A)

Pore size

distribution

(A)

Nature loess 17.8104 0.0378 98.3464 17–1,500

CO2-

silicification

grouted loess

27.8698 0.0614 99.7968 17–1,450

1034 Q. Lv et al.

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Water stability mechanism of silicification grouted loess