The performance of ultra-lightweight foamed concrete incorporating nanosilica [original]

Vol.:(0123456789)

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Archives of Civil and Mechanical Engineering (2021) 21:79

https://doi.org/10.1007/s43452-021-00234-2

ORIGINAL ARTICLE

The performance ofultra‑lightweight foamed concrete incorporating

nanosilica

MohamedAbdElrahman1,2 · PawelSikora1 · Sang‑YeopChung3 · DietmarStephan1

Received: 20 July 2020 / Revised: 4 October 2020 / Accepted: 16 April 2021

Abstract

This paper aims to investigate the feasibility of the incorporation of nanosilica (NS) in ultra-lightweight foamed concrete

(ULFC), with an oven-dry density of 350kg/m3, in regard to its fresh and hardened characteristics. The performance of

various dosages of NS, up to 10wt.-%, were examined. In addition, fly ash and silica fume were used as cement replacing

materials, to compare their influence on the properties of foamed concrete. Mechanical and physical properties, drying

shrinkage and the sorption of concrete were measured. Scanning electron microscopy (SEM) and X-ray microcomputed

tomography (µ-CT) and a probabilistic approach were implemented to evaluate the microstructural changes associated with

the incorporation of different additives, such as wall thickness and pore anisotropy of produced ULFCs. The experimental

results confirmed that the use of NS in optimal dosage is an effective way to improve the stability of foam bubbles in the

fresh state. Incorporation of NS decrease the pore anisotropy and allows to produce a foamed concrete with increased wall

thickness. As a result more robust and homogenous microstructure is produced which translate to improved mechanical and

transport related properties. It was found that replacement of cement with 5wt.-% and 10wt.-% NS increase the compres-

sive strength of ULFC by 20% and 25%, respectively, when compared to control concrete. The drying shrinkage of the NS-

incorporated mixes was higher than in the control mix at early ages, while decreasing at 28d. In overall, it was found that

NS is more effective than other conventional fine materials in improving the stability of fresh mixture as well as enhancing

the strength of foamed concrete and reducing its porosity and sorption.

Keywords Ultra-lightweight foamed concrete· Cement-based composite· Nanosilica· Microstructure· Compressive

strength· Shrinkage

1 Introduction

Many studies have recently focused on the properties of

different types of lightweight concrete, due to its various

advantages, including excellent sound and thermal insulation

properties as well as its low density [1]. Foamed concrete

(FC) is a type of low density, cement-based lightweight con-

crete, consisting of cement paste with randomly distributed

air bubbles introduced by foaming agents [2]. Foam bubbles

can be produced in cement paste either by mixing a foam-

ing agent with the cement paste or through a pre-forming

process [3]. Foamed concrete is widely used due to its supe-

rior properties, such as self-levelling ability, high insula-

tion properties, filling ability, fire-resistant characteristics

and low cost compared to other types of lightweight con-

crete. FC can also withstand the freezing conditions, and its

performance can be controlled by adjusting the pore struc-

ture with the use of supplementary cementitious materials

* Pawel Sikora

pawel.sik[email protected]

* Sang-Yeop Chung

syc[email protected]

Mohamed Abd Elrahman

[email protected]

Dietmar Stephan

[email protected]

1 Building Materials andConstruction Chemistry, Technische

Universität Berlin, Berlin, Gustav-Meyer-Allee 25,

13355Berlin, Germany

2 Structural Engineering Department, Mansoura University,

Elgomhouria St., Mansoura35516, Egypt

3 Department ofCivil andEnvironmental Engineering, Sejong

University, 209 Neungdong-ro, Gwangjin-gu, Seoul05006,

RepublicofKorea

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(SCMs) [4]. However, it should be taken into account that

the production of FC is highly sensitive to alteration in the

mixture design and production procedure when compared to

the production of lightweight aggregate concrete (LWAC)

with the same density [5]. Kuzielova etal. [6] reported that

the performance of FC can be controlled and improved by

special treatments for foam by using ultrasound and micro-

wave. In addition, Falliano etal. [7] showed that even a cur-

ing condition (in air or in water) plays a spectacular role on

the resulting mechanical properties and fracture energy of

FC. The potential usage of foamed concrete for structural

and nonstructural applications and its mixture design and

production procedures were also comprehensively summa-

rized and reviewed [8].

In general, foamed concrete is produced with a den-

sity in the range of 500–1600kg/m3. However, at a den-

sity < 500kg/m3, it tends to be unstable due to the high

foam content and low cement paste content, which signifi-

cantly limits its applications [3]. At such low densities, it is

generally designated as ultra-lightweight foamed concrete

(ULFC) and requires more attention than conventional con-

crete, when selecting its constituents, as well as in the mix-

ing process, to improve its stability, strength and physical

characteristics.

It is now available to produce and characterize foamed

concrete with ultra-low plastic densities (< 500kg/m3), in

particular, as a result of ongoing development in the field of

concrete technology, which has included the introduction of

new evaluation techniques including X-ray micro-computed

tomography (micro-CT) [9], X-ray diffraction (XRD) and

scanning electron microscopy (SEM) [10]; this, in turn, ena-

bles to understand the role of SCMs and various designing

processes towards optimizing the mixture designs. However,

numerous research and engineering experiences have shown

that ultra-lightweight foamed concrete can be unstable, par-

ticularly as its density decreases [8]. Kuzielova etal. [11]

reported that even a small variation in density could affect

significantly the compressive strength of specimens in the

case of ULFCs with a bulk density below 200kg/m3. Chung

etal. [12] also confirmed that with decreasing the density of

ULFC the amount of pores of larger diameter were notice-

ably increasing. In addition, at low densities, ULFC suffers

from low strength and is highly susceptible to cracking [8].

As a result, the applications and design of FC are some-

what limited and requires very precise producing process to

achieve a material with the desired low density.

To date, many comprehensive studies on the effects of

various chemical admixtures, additives and fibers on the

properties of lightweight concretes, have been performed.

Chung etal. [13] used micro-CT to evaluate the effect of

pore size, distribution and shape on physical and mechani-

cal properties of foamed concrete and they found that with

increasing the density of concrete the solid content increase

which keep the spherical shape of the pores and make them

more isotropic. Abd Elrahman etal. [14] compared three dif-

ferent approaches towards incorporating air voids in cement

pastes by adding aluminum powder, air-entraining agent, and

hollow microspheres and figured out the significant differ-

ences between the compressive strength of cement-based

composites according to air-entraining admixtures. Zhihua

etal. [15] reported that cracking phenomenon of FC can

be partially mitigated by the replacement of cement with

15–20wt.-% of ultrafine blast furnace slag powder as well

as the introduction of polypropylene fibers. Similarly Fal-

liano etal. [16] suggested an approach to overcome the low

bending strength of ULFC with different densities (400, 600,

800kg/m3) by using polymer fibers and their results showed

that the addition of fibers increased the flexural capacity of

the beams, especially for the low-density specimens. A com-

prehensive evaluation of the effects of type of foaming agent

was also performed highlighting the importance on choos-

ing the proper foaming agent as well as curing conditions

towards optimizing the production process of ULFC [17].

Huang etal. [18] evaluated the methods towards hindering

the collapse and air-voids escape phenomena during foam

concrete production by adding thickening agent and foam

stabilizing emulsion. Jones etal. [19] presented the possi-

bility of the production of low embodied carbon dioxide FC

containing a high volume of fly ash.

Above mentioned studies have shown that the inclusion of

supplementary cementitious materials (SCMs) has a signifi-

cant effect on controlling the mechanical, transport and dura-

bility performance of ultra-lightweight foamed concretes;

these effects can mainly be attributed to a refinement of the

pore and void structures, as well as to the improvement of

foam bubble stability when a proper SCM is included in the

mixture. Jones etal. found that [19] replacement of cement

with a high volume of fly ash (up to 70%) reduces the aver-

age bubble size of the FC and increase the thickness of the

bubble walls. Similar findings were found by Hilal etal. [20]

in relation to silica fume used as a SCM. Another approach

to improve the stability of foam was proposed by Xin etal.

[21], where silica fume was added to the foam prior mixing

it with cement. As a result of the chemical interaction of

foam and silica fume, the stability of foam was improved.

However, despite its beneficial effects on the properties of

ULFCs, the incorporation of SCMs can also cause unde-

sirable drawbacks, such as increased shrinkage and late

strength development. Jones and McCarthy [22] reported

that due to high drying shrinkage strain and relatively low

tensile strength and stiffness performance straight substitu-

tion for normal- weight concrete is not possible. Roslan etal.

[23] found that ULFC containing 15wt.-% and 30wt.-% of

cement replacement with fly ash exhibited half and one-

forth of compressive strength, respectively, after 7days of

curing when compared to pristine concrete. However, after

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28days of curing compressive strength of FC containing fly

ash is usually found to be comparable or higher than that of

pristine FC [24]. Raj etal. [25] have reported that the dry-

ing shrinkage of foamed concrete is 10 times higher than

that of conventional concrete. The addition of typical SCMs

can be successfully applied to reduce shrinkage to a certain

extent, due to the low amount of hydration heat produced

[26]. The drying shrinkage of foamed concrete can also be

reduced significantly by decreasing the pores connectivity

[27]. Incorporation of lightweight aggregates can reduce the

drying shrinkage without a significant increase in the density

of foamed concrete [28]. Abd Elrahman etal. [29] reported

that fly ash addition can reduce the drying shrinkage of ultra-

lightweight foamed concrete with dry density lower than

500g/m3 by about 30%. It was also reported that silica fume

improves early strength but can have a negative effect on

drying shrinkage [25]. Therefore, in selecting SCMs, careful

consideration must be given to the target property that is to

be improved [8].

In recent years, nanomaterials (NMs) have attracted

spectacular attention as admixtures for improving the per-

formance of cement-based composites. Due to their ultra-

fine particle size, as well as their high chemical reactivity,

they act as fillers with physical and chemical influence in

cementitious systems, thus refining microstructure and

accelerating the cement hydration process. There have been

some notable contributions in the field of the modification

of lightweight aggregate concretes (LWACs) with NMs [30],

but evaluations of the effects of NMs on the properties of

foamed concretes are still limited. Studies on the effects of

carbon nanotubes (CNTs) on the properties of ULFCs have

shown that the use of CNTs contributes significantly to early

strength development and 28d strength. It was also reported

that CNTs can affect the improvement in the uniformity

of air void structures [31]. Luo etal. [32] reported a 27%

improvement in the compressive strength of ULFC, with a

density of 300kg/m3, through the addition of a low dosage

of CNTs, together with a reduction in the average pore diam-

eter. Zhang and Liu [33] presented that better dispersion of

CNTs within the ULFC can be facilitated by the introduction

of low dosages of nano-Ce(SO4)2, thus specimens exhibit

better mechanical performance. Li etal. [34] have analyzed

the effects of nano-montmorillonite on foamed concrete

containing a high volume of fly ash, reporting an improve-

ment in the compressive strength and microstructure of FC.

Other researchers have used nanomaterials to stabilize the

foam before mixing it with cement slurry [2, 35]. The use

of nanomaterials improves foam stability, leads to less or

no collapsing of foam bubbles, refines pores and improves

compressive strength at early ages. Moreover, the incorpora-

tion of nanoparticles has been found to significantly improve

compressive strength, due to a narrow pore size distribution,

compared to normal foamed concrete [2].

Among the NMs, silica nanoparticles (SiO2) are the mate-

rial which is considered to be the most promising material

for application in cementitious composites with few com-

mercially available products on the market. While knowl-

edge regarding the effects of nanosilica (NS) on the prop-

erties of normal-weight cementitious composites is widely

established, little research has so far been conducted to

evaluate the mechanisms of the effects of nanosilica (NS)

on the properties of ULFCs. Therefore, this study aims

at shedding light on the design and performance of ultra-

lightweight foamed concretes (with a density ~ 350kg/m3)

modified with nanosilica. To realize this goal comprehensive

evaluation of the compressive strength, drying shrinkage,

thermal conductivity and the sorption of foamed concrete

were performed. In addition, X-ray microcomputed tomog-

raphy (µ-CT) was adapted to characterize the wall-thickness

and pore structure incorporating a probabilistic method. The

results obtained were compared with selected mixtures uti-

lizing conventional SCMs, such as silica fume and fly ash.

As an outcome reported experimental results were linked

with the microstructural characteristics obtained by µ-CT

and scanning electron microscopy (SEM) studies.

2 Materials andmethods

2.1 Materials

Ordinary Portland cement, CEM I 52.5 R complying with

EN 197–1, was used in this research (HeidelbergCement

AG, Germany). Three different fine materials were selected

and used as SCMs: class F fly ash (FA) (Baumineral, Ger-

many), silica fume (SF) (Sika, Germany) and nanosilica

(Levasil CB8, Nouryon, Sweden) in slurry form (NS). The

same silica nanoparticles were used in or previous studies

[30, 36] and have been comprehensively characterized in

the work [36]. Transmission electron microscope (TEM)

micrographs (Fig.1a–c) shows spherically shaped silica

nanoparticles, with a energy dispersive X-ray (EDX) spec-

trum (Fig.1e) and X-ray diffraction (XRD) patterns (Fig.1f)

confirming its high purity and amorphicity. Table1 sum-

marise basic properties of NS suspension used in this study.

Table2 shows the chemical and physical properties of

the cement, fly ash and silica fume, while Fig.2 depicts the

particle size distribution of these materials. Since nanosilica

was used in slurry form, the amount of the liquid phase was

deducted from the effective water (w/b). In most foamed

concrete mixtures, according to the authors’ knowledge,

fine aggregate (sand) is used to reduce volumetric changes

and concrete shrinkage. However, this addition can cause a

significant increase in the density of the foamed concrete.

Therefore, in this research, lightweight fine sand (expanded

glass, Liaver), was used in a 0.1–0.3mm fraction, with a

Archives of Civil and Mechanical Engineering (2021) 21:79

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density of 0.8g/cm3, to facilitate the production of con-

crete with an ultra-low density. The water absorption of the

lightweight aggregates used was determined to be 12wt.-%,

with an additional amount of water, equal to the absorption

of fine lightweight sand, included in the effective water. A

foaming agent, with a density of 1.05g/cm3 (Lightcrete 400)

and produced by Sika Germany, was used to produce the

foam. In addition, superplasticizer compatible with the used

foaming agent has been implemented to achieve the required

consistency. To improve the homogeneity and stability of the

foamed concrete mixture, a viscosity-enhancing admixture

(Sika Stabilizer ST3) was adopted, to prevent segregation

in the fresh mixture.

2.2 Mix composition

Seven foamed concrete mixtures were designed and pre-

pared, to achieve a dry target density in the range of

350 ± 50kg/m3. Paste/foam ratio is the most important

parameter controlling the density and stability of foamed

concrete. In this study, it was fixed at 1:3 by volume, for all

the mixes, to achieve the target density. The water/binder

ratio was fixed at 0.40 for all the mixes. Various empirical

models have been proposed to predict the dry density of

foamed concrete [8]. In this study, theoretical dry density

Fig. 1 TEM images (a–b), particle size distribution (c), EDX (e) and XRD (f) analysis of nanosilica. The carbon and copper signals present in

the spectrum come from the holey carbon TEM grid. Reproduced from [36]

Table 1 Properties of silica nanoparticles suspension

* Based on TEM analysis

Particle size* Solid content Density Viscosity pH

10–140nm 50 wt.-% 1.4g/cm38 cP 9.5

Table 2 Chemical and physical

composition of cement, SF and

Material CaO SiO2Al2O3Fe2O3MgO Na2O K2O SO3Density (g/cm3) Surface area

(cm2/g)

CEM I 52.5R 66.2 20.6 3.3 4.9 1.3 0.1 0.43 2.8 3.15 3860

Fly ash 4.8 47.9 21.0 4.6 1.4 0.7 1.1 0.8 2.27 2930

Silica fume 0.2 98.4 0.2 0.01 0.10 0.15 0.2 0.1 2.0 200,000

Archives of Civil and Mechanical Engineering (2021) 21:79

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was calculated based on the following formula, where it was

assumed that 20wt.-% of the cement stone was chemically

bound water, which could not be evaporated by standard

drying below a temperature of 105 ± 5°C (Eq.1) [8]:

where C is the mass of the cementitious materials added (kg/

m3) and A is the mass of the aggregates (kg/m3). Accord-

ing to this formula, 60kg/m3 of fine lightweight aggregate

required a cementitious material content of 240kg/m3, for

all mixtures.

Four dosages were used to study the influence of nano-

silica on the performance of foamed concrete: 1.25, 2.5, 5,

and 10wt.-% of cement. For comparison, a control mix con-

taining pure cement, mix containing fly ash with 25wt.- %

cement replacement and mix containing silica fume with

10wt.-% cement replacement were prepared and also tested.

Table3 presents the compositions of the different concrete

mixtures. Both the amount of mixing water and superplasti-

cizer remained constant in all cases, while the stabiliser con-

tent was adapted to prevent segregation and bleeding of the

(1)

Oven

−dry density

(

kg∕m

=1.2C+

foamed concrete. Foamed concrete compaction is not desir-

able due to the high probability of segregation, destruction

and merging of foam bubbles and because of the resultant

formation of big voids. The concrete mixture was therefore

designed to achieve a consistency class of F4/F5 (according

to EN 206-1), to produce a workable homogeneous mix with

a high filling ability, without the need for vibrations.

2.3 Preparation offoamed concrete

The pre-formed foam in this investigation was produced and

then mixed with cement slurry. An SG S9 foam generator

(Sika Germany) was adapted and used to produce the foam,

at a capacity of 9L per minute and with a pressure of 0.4

bars. Tap water, with a pressure of about 3 bars and com-

pressed air with a pressure of 2 bars, were applied to the

generator, with the foaming agent dosage set at 2wt.-% of

the water. The foam produced had to be stable and, as rec-

ommended by the provider, was to be continuously produced

without pulses. For this purpose, the compressed air pres-

sure was adjusted until the foam was produced uniformly

and steadily. The foam density was measured as 35–40kg/

Fig. 2 Particle size distribution

of the fine materials used

Table 3 Foamed concrete

mixture compositions (kg/m3)

* 50 wt.-% of solid content. The liquid phase was considered to be a part of the mixing water

Mix Cement FA SF NS

(slurry)*

Fine Liaver

aggregate

Water SP ST Paste: Foam

(volume)

C 240 – – – 60 96 1.6 1.0 1:3

FA25 180 60 – – 60 96 1.6 1.2 1:3

NS1.25 237 – – 6 60 93 1.6 1.0 1:3

NS2.5 234 – – 12 60 90 1.6 0.8 1:3

NS5 228 – – 24 60 84 1.6 0.5 1:3

NS10 216 – – 48 60 72 1.6 - 1:3

SF10 216 – 24 - 60 96 1.6 0.8 1:3

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