Gopalakrishnan K., Birgisson B., Taylor P., Attoh-Okine N.O. (Eds.) Nanotechnology in Civil Infrastructure: A Paradigm Shift

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springerlink.com

Optimization of Clay Addition for the

Enhancement of Pozzolanic Reaction in

Nano-modified Cement Paste

Bjorn Birgisson and Mahir Dham

Abstract. This chapter presents a process for obtaining nano-size silicate platelets

for use in cement paste from exfoliating Cloisite Na+ clay particles. The resulting

silicate platelets require all surface electrical charges to be neutralized. The results

showed that this is achievable through the use of organic ammonium chloride sur-

factant. The exfoliation process was evaluated through the use of X-ray diffraction

and the optimum stable concentration of organic ammonium chloride was deter-

mined with the help of electrokinetic studies and turbidity measurements which

determine the stability of the clay surfactant solution. The ultimate effects on hy-

drated cement paste are evaluated using compressive strength measurements for

the subsequent replicates of samples prepared.

1 Introduction

In terms of volume used, concrete is the most successful material in the world. It

is made up of aggregates bound together with hydrated cement. In general it con-

tains between 10 and 15 wt. % cement. Globally 2.6 billion ton of cement is pro-

duced annually, giving more than 3 m

of concrete per person. Due to the size of

the cement production, it consumes large amount of energy and is one of the larg-

est contributor to carbon dioxide (CO

) release. Cement production accounts for

some about 4-5% of man-made CO

emissions. Thus the amount of cement used

in concrete production must be minimized. The challenge is to get more strength

out of the concrete with less cement.

One concept recently developed is to use nanoparticles made from exfoliated

clays, carbon tubes and/or precipitated silica particles (nano silica). Results from

Bjorn Birgisson

Professor and Division Head, Department of Civil and Architectural Engineering,

Brinellvägen 34, KTH Royal Institute of Technology, 10044 Stockholm, Sweden

e-mail: bjornbir@kth.se

Mahir Dham

Development Chemist, Mapei Corporation, Deerfield Beach, Florida 33442

e-mail: mahirdham@gmail.com

226 B. Birgisson and M. Dham

experiments show that by modifying the cement paste structure, it is possible to

modify both the hydration path and the structure of the binder. As an example, na-

no-SiO

additions with fly ash or with fly ash and silica fume combined have

shown more pozzolanic activity than traditional silica fume alone [Collapardi, et

al. 2004; Li 2004; Li et al. 2005]. This is due to the fact that their size allows them

to react more readily with the calcium hydroxide, thereby increasing C-S-H con-

version [Ji 2005].

In recent years, the addition of nano-particles into cement paste is gaining at-

tention due to their exceptional properties due to their high surface area and there-

fore high reactivity. Recent experiments have shown that nano silica do not only

change the structure of the C-S-H but it will also change the path of and mode of

hydration [e.g. Lagerblad and Fjällberg 2008], which in turn will influence the

structure of the paste. Nano-silica (nano-SiO2) is an industrial product that is more

fine-grained and thus reacts more readily with calcium hydroxide (CH) [Jo et al.

2007].

Nano-modification of the cement inherently affects the hydration process,

which in turn affects the structure and the physical properties. The addition of the

nano-clay particles also simulates a process similar to the pozzolanic reaction

where silicates and water react with calcium hydroxide to form C-S-H. It has been

shown that the rate of this reaction is proportional to the amount of surface area

available for the reaction [Jo et al. 2007]. Basic pozzolanic reactions produced

from the composite cements have been known to effect the production of CH and

C-S-H, the two main hydration products that have been attributed to the strength

of the cement [Escalante-Garcia and Sharp 2004; Taylor 1997; Shayan and Xu

2006]. Generally speaking, the pozzolanic reaction works by reducing the amount

of CH produced during the hydration either by consumption of CH itself of its hy-

dration components [Saeki and Monteiro 2005, Taylor 1997].

More recently exfoliated nano-size exfoliated silica sheets are obtained from

montmorillonite smectite clays [Dham 2007]. Research by Dham (2007) has led to

a process where the researchers were able to exfoliate clays with weak platelet

bonds, resulting in exfoliated nano-sized montmorillonite clay (Cloisite Na+). Im-

portantly, the cost of the nanosilicates is pound for pound less than for silica

fumes, since the basic material is derived from natural (“as dug”) clays. The proc-

ess has also been used on phosphate waste clays in Florida with good results,

which are currently being land-filled [Boyd et al. 2007].

In this chapter, the exfoliation process is described, along with the methods

used to verify the exfoliation of Cloisite Na+ into nano-size silicate platelets that

are suitable for use in concrete. An important component of this system is the neu-

tralization of the surface charges of the resulting silicate platelets through the in-

troduction of a suitable surfactant.

2 The Exfoliation Process

Montmorillonite clay itself is a sheet-like structure (Fig. 1) composed of tetrahedra

SiO

and octrahedra AlO

at a 2:1 ratio. Each of these sheets sustains a charge on

their surface and edges. Clays are characterized by their ion exchange properties

Optimization of Clay Addition for the Enhancement of Pozzolanic Reaction 227

Silicate Platelet

Gallery

Fig. 1 Idealized layered silicate structure

that result in a high affinity to water, which makes the naturally incompatible for

use in concrete in their unmodified form.

The advantage of the sheet-like configuration over other small particles is that the

multi-layer silicate structure can be penetrated between layers by small molecules

(see Fig. 2), forcing the silicate platelets apart. This process is called intercalation.

Silicate Platelet

Gallery Expanded by Reactive Species

Fig. 2 Partial separation of silicate platelets (intercalation)

If the penetrating molecules reactive enough, this process may result in complete

separation of the silicate layers (i.e. exfoliation), as shown schematically in Fig. 3.

Polymer Chain

Fig. 3 Total separation of silicate platelets (exfoliation)

228 B. Birgisson and M. Dham

As a result of exfoliation, a small mass of montmorillonite clay can result in

numerous small, thin (e.g. 20-nm) platelets, with very large surface area, that are

fully separated. These nano-size silicate platelets may e.g. be used directly as

pozzolans in concrete or bonded with selected species of polymers which tend to

bond to all of these surfaces, creating a linkage effect among the silicate platelets.

This bonding can also be considered as a flocculated material since one polymer

chain can link several clay particles together. The resulting composite can exhibit

vastly improved properties.

3 Materials and Methods

In this study, Cloisite Na

clay, which is also known as sodium montmorillonite, is

added to the cement paste mixture along with a surfactant to ensure thorough dis-

persion of the clay. The surfactant used in this research was an organic ammonium

chloride called 2-(Dimethylamino) ethyl acrylate, methyl chloride [Sigma Aldrich

2006; BMT Inc. 2008; Cementec Inc. 2008; Southern Clay Inc. 2008].

For complete exfoliation of clay, it is advantageous if synthesized charged

organic/inorganic nanoparticles are used instead of conventional intercalating

agents. For this reason, organic ammonium chloride was perceived to be most

suited.

The first obstacle to overcome was determining the exact amount of surfactant

necessary to produce a stable clay/surfactant solution. Initially, specific propor-

tions of surfactant, with respect to clay, were investigated. Evaluation of the sta-

bility of the system became a necessity and thus electrokinetic phenomena (zeta

potential) and system turbidity techniques were used to ascertain the optimal

amount of surfactant required.

3.1 Electrokinetic Studies for Zeta Potential Measurements

The equipment used for evaluating the procedure included a Brookhaven Zeta

Plus zeta potential meter. Zeta potential is the potential of the surface at the plane

of shear between the particle and the surrounding media as the particle and media

move with respect to each other. In the presence of an applied electric field, the

charged surface (and the attached material) tends to move in the appropriate direc-

tion, while the counter ions in the mobile part of the double layer would have a net

migration in the opposite direction. On the other hand, an electric field would be

created if the charged surface and the diffuse part of the double layer were made

to move relative to each other. The zeta potential facilitates easy quantification of

the surface charge [Singh 2002].

For our replicates of various samples it was found that when the absolute value

of zeta potential is around 50 mV the dispersions are very stable due to mutual

electrostatic repulsion and when the zeta potential is below this value, coagulation

(formation of larger assemblies of particles) occurs very quickly, causing rapid

sedimentation. Even when the surface charge density is very high but the zeta po-

tential is low, the colloids are unstable. Also, the velocity of heterocoagulation

Optimization of Clay Addition for the Enhancement of Pozzolanic Reaction 229

(coagulation of different particles) depends on the zeta potentials of each type of

particle. Therefore, zeta potential is an important parameter characterizing colloi-

dal dispersion. The instrument used to measure the zeta potential is called a zeta

potential meter.

3.2 Turbidity Measurements

Turbidity in water can be caused by suspended and colloidal matter such as clay,

silt, finely divided organic and inorganic matter, and plankton and other micro-

scopic organisms. Turbidity is an expression of the optical property that causes

light to be scattered and absorbed rather than transmitted with no change in direc-

tion or flux level through the sample. The Turbidity Meter can measure this tur-

bidity, in units of nephelometric turbidity units or NTU. This term originated be-

cause turbidimeters with scattered-light detectors located at 90° to the incident

beam are called nephelometers and so the units ascribed to their Turbidity meas-

urements is NTU [EPA 1999].

The principle of this method is based on the intensity of light scattered by the

sample under defined conditions. The higher the intensity of scattered light, the

higher is the turbidity. For the replicate clay samples mixed with OAC, it was pos-

sible to determine the optimum amount of the surfactant required by the system to

ensure a stable turbidity measurement.

The turbidimeter used in this research was a Hach Model 2100AN Laboratory

turbidimeter capable of measuring Turbidity values from 0 to 10000 NTU. The opti-

cal system was comprised of a tungsten filament lamp, lenses and apertures to focus

the light, a 90° detector to monitor scattered light, a forward scatter light detector, a

transmitted light detector, and a back scatter light detector [Lab Manual 2006].

3.3 X-Ray Diffraction: Exfoliation of Clay Platelets

The X-Ray Diffraction (XRD) technique is used to measure atomic spacing be-

tween lattice layers in a crystal. This can be calculated from the X-Ray Diffraction

test results using the Braggs Law equation, is as follows: n = 2d sin , where,  =

wavelength of the incidental X-Ray beam; d = the atomic spacing between the

layers;  = angle of incidence; n is taken as unity.

Three samples were examined with the X-Ray diffraction technique, using a

Philips X'Pert system [MAIC 2009]. These samples were:

• Sample 1 consisted of Cloisite Na

alone.

• Sample 2 contained 9.5% by weight of Cloisite Na

and 9.5% by weight

OAC, mixed in water and then dried.

• Sample 3 contained 5.6% by weight of Cloisite Na

, 5.6 % by weight

OAC, and 5.6% by weight of poly-vinal alcohol (PVA) polymer, mixed in

water and then dried.

230 B. Birgisson and M. Dham

4 Results

Initial laboratory observations indicated that very low dosages of OAC surfactant

were required to induce stability. An experimental matrix was formulated to evalu-

ate a range of surfactant contents to be used per sample. This matrix is tabulated

in Table 1.

Since the amount of surfactant to be used had a very low weight to be meas-

ured, a buffer solution of the surfactant was used where 1 g of surfactant was

added to 1000 g of water.

Table 1 Experimental matrix

Sample

Number

Clay

(g)

Water

(g)

Surfactant

(mg/ g of

clay)

Buffer

(g)

1 1 98 1 1

2 1 97 2 2

3 1 96 3 3

4 1 95 4 4

5 1 94 5 5

6 1 93 6 6

7 1 92 7 7

8 1 91 8 8

Fig. 4 The effect of OAC concentration on Zeta Potential