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Applied Nanoscience (2020) 10:2627–2638

https://doi.org/10.1007/s13204-019-00993-8

ORIGINAL ARTICLE

The effects ofseawater onthehydration, microstructure andstrength

development ofPortland cement pastes incorporating colloidal silica

PawelSikora1,2 · KrzysztofCendrowski3 · MohamedAbdElrahman1,4 · Sang‑YeopChung1 ·

EwaMijowska3 · DietmarStephan1

Received: 12 December 2018 / Accepted: 21 February 2019 / Published online: 4 March 2019

Abstract

This contribution investigates the effects of seawater and colloidal silica (NS) in the amounts of 1, 3 and 5wt%, respectively,

on the hydration, strength development and microstructural properties of Portland cement pastes. The data reveal that seawater

has an accelerating effect on cement hydration and thus a significant contribution to early strength development was observed.

The beneficial effect of seawater was reflected in an improvement in compressive strength for up to 14 days of hydration,

while in the 28days compressive strength values were comparable to that of cement pastes produced with demineralized

water. The combination of seawater and NS significantly promotes cement hydration kinetics due to a synergistic effect,

resulting in higher calcium hydroxide (CH) production. NS can thus react with the available CH through the pozzolanic

reaction and produce more calcium silicate hydrate (C-S-H) gel. A noticeable improvement of strength development, as

the result of the synergistic effect of NS and seawater, was therefore observed. In addition, mercury intrusion porosimetry

(MIP) tests confirmed significant improvements in microstructure when NS and seawater were combined, resulting in the

production of a more compact and dense hardened paste structure. The optimal amount of NS to be mixed with seawater,

was found to be 3wt% of cement.

Keywords Colloidal silica· Nanoparticles· Portland cement· Seawater· Hydration· Compressive strength

Introduction

The incorporation of admixtures and additives to cemen-

titious formulations enables the production of sustainable

cement-based composites, by reducing cement consumption

on the one hand and improving certain properties, especially

the durability of composites, on the other. In the last two

decades the incorporation of nanomaterials (NMs), mainly

as cement admixtures, for improving the mechanical and

durability properties of cement-based composites in con-

struction, has gathered noticeable attention of research-

ers and industry. Much attention has especially been paid

to the incorporation of silica (SiO2) nanoparticles, with

some commercial products already available on the mar-

ket. It has been reported that nanosilica (NS) have a sig-

nificant impact on accelerating the hydration process of

cement (Singh etal. 2011; Land and Stephan 2012; Skoc-

zylas and Rucińska 2018a), refining the pore structure of

cement matrices (Sikora etal. 2016), improving early and

long-term mechanical properties (Skoczylas and Rucińska

2018b), as well as improving the durability performance

Submitted to special issue NANO-2018 within the 6th

International Conference “Nanotechnologies and Nanomaterials”

NANO-2018.

* Pawel Sikora

pawel.sik[email protected]

1 Building Materials andConstruction Chemistry, Technische

Universität Berlin, Gustav-Meyer-Allee 25, 13355Berlin,

Germany

2 Faculty ofCivil Engineering andArchitecture, West

Pomeranian University ofTechnology Szczecin, Al. Piastow

50, Szczecin70-311, Poland

3 Nanomaterials Physicochemistry Department, Faculty

ofChemical Technology andEngineering, West Pomeranian

University ofTechnology Szczecin, Al. Piastow 45,

Szczecin70-311, Poland

4 Structural Engineering Department, Mansoura University,

Elgomhouria St., Mansoura35516, Egypt

2628 Applied Nanoscience (2020) 10:2627–2638

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of cementitious composites (Du etal. 2014; Heikal etal.

2016; Marushchak etal. 2017). The beneficial effect of silica

nanoparticles is attributable to three synergistic effects: (i)

the filler effect—nanosized silica particles fill the voids in

the cement matrix, decreasing paste porosity and optimiz-

ing the distribution of the pore structure; (ii) the nucleation

effect—ultrafine silica nanoparticles act as the nucleation

sites (kernels) for the hydration reaction of the cement;

(iii) the pozzolanic effect—through the reaction of silica

nanoparticles with calcium hydroxide (CH), the amount of

calcium silicate hydrate (C-S-H) gel increases (Singh etal.

2011, 2013; Abd El-Aleem and Ragab 2014; El-Didamony

etal. 2016; Маrushchak etal. 2016; Xu etal. 2016; Rupas-

inghe etal. 2017). The effects of NS on the properties of

cement-based composites have been investigated in detail

in recent years and many review papers are available (Singh

etal. 2013; Safiuddin etal. 2014; Aggarwal etal. 2015;

Silvestre etal. 2015; Shah etal. 2015; Bastos etal. 2016;

Sikora etal. 2018).

The amount of cement produced in 2016 reached 4.20bil-

lion tonnes (Xiao etal. 2017), which corresponds to around

25billion tonnes in estimated concrete production and,

moreover, production is growing rapidly year by year. This

translates directly to the amount of freshwater required to

produce the concrete and represents a serious environmental

threat in many parts of the world, where the availability of

freshwater is a scarce resource (Etxeberria etal. 2016a). The

search for alternative sources of water has therefore gathered

the noticeable attention of researchers and authorities. While

reusing recycled water from concrete production has already

been widely established, there is still no agreement regard-

ing how to efficiently use seawater to develop durable and

strong concrete (Xiao etal. 2017). However, desalination is

highly energy consuming, making desalted water relatively

expensive. The use of raw seawater would therefore be a

good solution, especially due to the fact that concrete, as the

most produced man-made material in the world, has a high

potential for incorporating great amounts of seawater, thus

saving huge amount of freshwater.

It is widely agreed that the chemical composition of water

can interfere with the setting of cement and might have an

adverse effect on its strength and durability (Kucche etal.

2015). Various standards, such as ASTM C1602 or EN 1008,

therefore give strict guidelines regarding the properties of

water which can be used for concrete production. Accord-

ing to EN 1008, salt water can be used for concrete without

reinforcement or other embedded metals, but it is generally

not suitable for the production of reinforced or prestressed

concrete. The maximum chloride content for prestressed

concrete or grout; reinforced concrete or embedded metal;

concrete without reinforcement or embedded metal should

be lower or equal to 500mg/L, 1000mg/L, and 4500mg/L,

respectively. The typical chloride content in seawater is

around 19,400mg/L, with Eastern Mediterranean and Mid-

dle Eastern seawater exceeding even 21,000mg/L. As a

result, reinforced concrete mixed with seawater will have

a high risk of corrosion, induced by the high concentra-

tion of chlorides, which is known to be the major cause

of steel damage (Bertolini etal. 2002). To overcome these

obstacles various solutions have been proposed, including:

mixing with pozzolan materials (ground granulated blast

furnace slag, fly ash, silica fume), mixing with corrosion

inhibitors or replacing conventional reinforcements (made

of steel) with corrosion-resistant reinforcement (Otsuki etal.

2012; Lim etal. 2015; Xiao etal. 2017; Tawfik etal. 2018).

Furthermore, silica nanoparticles have been introduced as

successful admixtures for mitigating the chloride-induced

corrosion possibility in concrete (Jalal etal. 2012).

Nevertheless, seawater can be incorporated in various

non-reinforced concrete elements, including concrete blocks,

pavements or lightweight concretes, where reinforcement is

not necessary. The case study undertaken by Etxeberria etal.

(2016a) has shown that seawater can be successfully incor-

porated as a mixing water to produce concrete dyke blocks,

while, Chen etal. (2017) have demonstrated the use of sea-

water in producing fibre-reinforced polymer-confined con-

crete, as an alternative solution to producing steel-free struc-

tures, without the corrosion risk. The life cycle cost analysis

undertaken by Younis etal. (2018) has shown that the incor-

poration of seawater with recycled concrete aggregate along

with glass fibre-reinforced polymer (GFRP) rebars, might be

a good alternative for the production of structural concrete

with improved long-term economic performance.

Several studies have agreed that the salt quantity in sea-

water can improve the early strength of concrete, with later

strength remaining similar or slightly higher in comparison

to concrete made with tap water (Demir etal. 2010; Wegian

2010; Otsuki etal. 2012; Abdel-Magid etal. 2016; Etxeber-

ria etal. 2016a). The chloride ion (Cl−) content of seawa-

ter contributes to the acceleration of the cement hydration

process, thus decreasing the setting time of cement (Govin-

darajan and Gopalakrishnan 2011) and fastening its harden-

ing (Fernanda etal. 2017). In addition, a decrement in the

porosity of the cement matrix and the absorption capacity of

concrete have been reported (Etxeberria etal. 2016a; Adiwi-

jaya etal. 2017). Study of Shi etal. (2015) on the synergistic

effect of metakaolin (MK) and seawater, has shown that a

combination of pozzolanic admixture along with seawater

has a noticeable effect on the hydration process and the final

mechanical and microstructural properties of concrete. This

phenomenon has been attributed to the increased CH content

available for pozzolanic reaction with MK, as a result of the

reaction of cement with seawater, thus resulting in a higher

amount of C-S-H gel formed in the concrete.

Despite the established success of using nanosilica (NS)

as admixture in cement-based composites, there has been no

2629Applied Nanoscience (2020) 10:2627–2638

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study focused on the potential application of seawater as a

mixing water for the production of such cement-based com-

posites. As silica nanoparticles are one of the most reactive

commercially available pozzolanic materials, their combina-

tion with seawater might be a considerable step towards the

production of durable and sustainable cementitious compos-

ites with significant mechanical performance.

The goal of this study is to therefore evaluate the syner-

gistic effects of commercially available colloidal silica (NS)

and seawater, towards the production of sustainable cement-

based composites. To characterize the effects of seawater

and NS on the cement hydration process, strength develop-

ment and microstructure cement pastes with different NS

contents (1, 3 and 5wt%) were prepared. Specimens with

demineralized water were used as reference samples.

Materials andmethods

Materials

The cement pastes were prepared on the basis of Portland

cement CEM I 42.5R (conforming to the EN 197-1 stand-

ard). Two types of water were used: demineralized (as a

reference) and artificial seawater (35.00‰ of salinity),

prepared according to ASTM D1141-98. To obtain a reason-

able workability for the selected cement pastes, polycarbox-

ylate ether (PCE)-based superplasticizer (BASF ACE430)

was used.

Nanomaterial properties andcharacterization

A commercially available colloidal silica suspension (NS),

with a density of 1.4g/cm3 and containing 20wt% of solid

material, was used. The external (liquid) phase in the NS

suspension was further considered to be a part of the mixing

water used for the cement paste preparation (see “Cement

paste mixture design and specimen preparation”). High-reso-

lution transmission electron microscope (TEM) micrographs

are presented in Fig.1a–c. These clearly show the spherical

shape of silica nanoparticles with sizes ranging from 10 to

140nm. The size distribution of the silica nanospheres is

presented in Fig.1d. The EDX spectrum presented in Fig.1e

shows that the sample was composed of Si and O atoms.

The carbon and copper signals present in the spectrum come

from the holey carbon TEM grid. The X-ray diffraction

(XRD) patterns of the sample (Fig.1f), show a single broad

peak at around 23° that comes from the amorphous phase

of NS. The amorphous phase of NS plays an important role

Fig. 1 TEM images (a–c), size distribution (d), EDX (e) and XRD (f) analysis of colloidal silica

2630 Applied Nanoscience (2020) 10:2627–2638

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in kinetics of cement hydration, as it is highly advantageous

for cementitious reactions (Singh etal. 2011).

Figure2 shows the particle size distribution (PSD) of

the colloidal silica and cement samples from laser dif-

fraction analyses. Demineralized water and isopropanol

were used as dispersants for the NS and cement, respec-

tively. The details of this technique have been described

by Hackley etal. (2004). The results of laser granulom-

etry indicated a noticeable difference between the PSD

of the cement and of the NS. In the case of the NS, a

narrow peak with average particles sizes of 181nm was

detected, while in the cement broad PSD with a D90 value

of 43µm was observed. The results presented in Figs.1d

and 2 illustrate that particle size ranges observed with

TEM and laser diffraction methods were slightly different.

This inconsistency between the results obtained with dif-

ferent techniques has been widely reported in the literature

(Bretsnajdrova etal. 2010; Quercia etal. 2013; Tuoriniemi

etal. 2014; Babick 2016). One of the reasons for the differ-

ence is associated with the difficulty incompletely break-

ing particle agglomeration during dispersion in aqueous

media (Quercia etal. 2013). Nevertheless, both methods

clearly confirmed that NS particles were much finer than

the cement particles.

Cement paste mixture design andspecimen

preparation

Eight types of cement pastes, containing 0, 1, 3 and 5%

(by weight of cement) of NS, divided into two groups con-

taining demineralized water (DW) and artificial seawa-

ter (SW), were produced. The samples were designated

as DW0, DW1, DW3, DW5 and SW0, SW1, SW3, SW5,

where the letters indicate the type of water used, while the

digits represent the wt% of NS.

The cement pastes were mixed using a mixer conform-

ing to EN 196-1, with a water-to-cement ratio (w/c) equal

to 0.5. In order to maintain an equal w/c in all speci-

mens, the mixing water was reduced proportionally to the

amount of liquid contained in the NS suspension. The NS

was incorporated into the cement paste as an admixture

(not exceeding 5wt%) and as a result the cement content

remained fixed in all the cement pastes, in accordance

with EN 934-2. Due to the high water absorption of NS

particles, specimens containing 5wt% (DW5 and SW5)

required the addition of 0.5wt% of PCE superplasticizer in

order to obtain a reasonable workability. Table1 presents

the compositions of the cement paste mixtures.

The samples were mixed according to the following

procedure: 30s of slow mixing, 1min of fast mixing, a

1min break, 1min of fast mixing. Next, the fresh paste

was poured into oiled moulds to form samples with a size

of 20 × 20 × 20mm3. The specimens were cured for 24h

in a climate chamber with a relative humidity of 95% at

room temperature (20 ± 1°C). After 24h of curing, the

specimens were demoulded and cured in the same climate

chamber for up to 27days.

Fig. 2 Particle size distributions (PSD) of cement and colloidal silica

(NS)

Table 1 The composition of

cement paste mixtures [g] Sample desig-

nation

CEM I 42.5R NS suspension Demineralized

water (DW)

Seawater

(SW)

Superplasticizer

DW0 100 – 50 – –

DW1 100 5 46 – –

DW3 100 15 38 – –

DW5 100 25 30 – 0.05

SW0 100 – – 50 –

SW1 100 5 – 46 –

SW3 100 15 – 38 –

SW5 100 25 – 30 0.05

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Methods

Isothermal calorimetry

For evaluating the hydration kinetics with calorimetry, 5g

of water was mixed with NS and superplasticizer (if appli-

cable). Next, 10g of cement was placed in a calibrated

plastic container and mixed with 5g of the suspension.

Afterwards, the samples were mixed, and the hydration

heat of the samples was measured for a period of 168h.

The heat flow and cumulative heat flow were determined.

In addition, for a detailed analysis of the effects of seawa-

ter and NS on the hydration kinetics of cement, the accel-

eration rate of cement hydration was calculated, using the

following equation (Eq.1):

where Hi corresponds to cumulative heat (normalized by

cement mass) of the selected sample and Hc corresponds

to the cumulative heat (normalized by cement mass) of the

DW0 reference paste.

Thermogravimetric analysis (TGA)

To study the phase change during the cement hydration

process, selected cement pastes were examined with the

use of thermogravimetric (TG) analysis. The crushed sam-

ples from the hardened cement pastes were taken at 3, 7

and, 28days and then ground. Isopropanol was used to

stop any hydration reactions of the powdered samples and

then the samples were well dried. In the TG procedure, the

sample was first held at 25°C for 20min, and heated from

25 to 1000°C at 10.00°C/min.

Afterwards, the mass drop in the thermogravimetric

curve at temperatures between 400 and 500°C, which

indicates the loss of water from CH, was calculated. The

amount of CH in the specimen was calculated directly

from the thermogravimetric curves, using the following

equation (Eq.2):

where WLCH corresponds to the percentage mass loss attrib-

utable to CH dehydration and where MWCH and MWH are

the molecular weights of CH (74g/mol) and water (18g/

mol), respectively (Scrivener etal. 2016).

(1)

Acceleration rate

−H

DW0

×100%

(2)

(%) = WLCH (%) ×

Compressive strength

A compressive strength test of cement pastes was car-

ried out on the 20 × 20 × 20mm3 cubes after 1, 3, 7, 14

and 28 days of curing, using six samples of each mix.

The mean value and standard deviations were taken into

consideration.

Mercury intrusion porosimetry (MIP)

For the characterization of the pore structure, mercury intru-

sion porosimetry (MIP) was applied. The MIP method is

commonly used to characterize the pore structure in porous

materials, due to its simplicity, quickness and wide pore

diameter measuring range (Strzałkowski and Garbalińska

2017). In order to obtain information about the pore size dis-

tribution of the cement matrix, a MIP test was performed on

small-cored samples taken from the specimens. The speci-

mens were immersed in isopropanol after 28days of curing,

to stop hydration and then freeze-dried before testing.

Characterization techniques

The morphology and chemical composition of the NS sam-

ple was analysed with a high-resolution transmission elec-

tron microscope (TEM) (FEI Tecnai G2 F20 Twin) together

with energy dispersive X-ray (EDX) spectroscopy. The crys-

tallographic analysis of the NS sample was conducted on a

Philips diffractometer using Cu Kα radiation. Laser diffrac-

tion (Malvern Mastersizer 2000, UK) was used to analyse

the particle size distribution (PSD) of the colloidal silica and

cement samples. The hydration heat of the cement pastes

was characterized by isothermal conduction calorimetry

with the use of a TAM Air 3 (TA Instruments) calorim-

eter. TG measurements were performed with the use of a

TG 209, Tarsus F3 (Netzsch) instrument under a nitrogen

atmosphere, at a flow rate of 250mL/min. The intrudable

pore volume of hardened samples was determined by Pascal

140 and 240 series (Thermo Scientific) mercury intrusion

porosimeter. Mercury density was 13.5450g/mL, surface

tension was taken as 0.48N/m, while the selected contact

angle was 140°.

Results anddiscussion

Hydration kinetics

Variations in the kinetics of the hydration processes of the

cement pastes are presented in Fig.3. In addition, to obtain

a comprehensive understanding of the calorimetry study, the

acceleration rate of cement hydration, calculated from the

cumulative heat (Eq.1), is presented in Table2.

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A noticeable difference in the hydration kinetics of DW0

and SW0 paste can be observed (Fig.3a). The replacement

of distilled water with seawater accelerated the rate of hydra-

tion. The exothermic peak appeared approximately 100min

earlier in SW0, as compared to DW0 and the peak value was

~ 19% higher in comparison to the DW0 paste. The amount

of heat released (Table2) was 19.2% higher after 12h, in

respect to DW0. Furthermore, the disparity between the

cumulative heats of the SW0 and DW0 specimens gradually

decreased with time. However, even after 3days of hydra-

tion, the released heat value of SW0 was still ~ 7.7% higher.

The total cumulative heat values of both specimens were

comparable after 7 days of hydration. This confirms the find-

ings in previous works, which have argued that the presence

of chloride ions in seawater promotes cement hydration in

the early hours. NaCl, MgCl2, CaCl2 have an accelerating

effect on cement hydration, especially on calcium silicates

(mainly tricalcium silicate—C3S), which results in faster for-

mation of the C-S-H phase and decreases the setting time

of the composite (Govindarajan and Gopalakrishnan 2011;

Etxeberria etal. 2016b; Fernanda etal. 2017; Parthasarathy

etal. 2017).

The introduction of NS resulted in a significant increase

in reaction rates, from the very beginning of the hydration

process (Fig.3b). It can be seen that the height of the early

rate peak had increased and that the time required to reach

the maximum rate was simultaneously reduced. However,

the reaction rate was highly dependent on the NS content. In

the case of low NS dosages (DW1 and SW1) the exothermic

peak was reached only slightly faster by both specimens. In

addition, the exothermic peak of DW1 was actually slightly

lower as compared to DW0, while in case of SW1 a clear

increment in the exothermic peak value was observed, as

compared to SW0. After 12h of hydration (Table2), the

cumulative heat of specimen SW1 was 34% higher than that

of the DW0 specimen, while in the case of DW1 only a 3%

increment in cumulative heat was reported, as compared

with DW0.

The increment in the NS content resulted in a higher

hydration rate, which stands in agreement with the liter-

ature (Yu etal. 2014), up to a certain amount of NS. As

reported by Land and Stephan (2012) and Skoczylas and

Rucińska (2018a), the combination of Portland cement and

NS causes a considerable increase in hydration heat, due

Fig. 3 Comparison of the heat flow of cement pastes without NS admixture (a) and heat flow curves of all cement pastes (b) after up to 36h of

hydration

Table 2 The acceleration

rates of cement hydration, in

comparison to DW0, after up to

7 days of hydration

Sample des-

ignation

Acceleration rate

12h (%) 1day (%) 2days (%) 3days (%) 5days (%) 7days (%)

DW0 0.0 0.0 0.0 0.0 0.0 0.0

SW0 19.2 11.6 9.3 7.7 2.6 0.7

DW1 3.0 − 0.7 − 1.4 − 1.4 − 2.0 − 1.9

SW1 34.0 18.6 13.4 9.8 3.9 2.1

DW3 17.9 5.0 2.0 1.5 − 0.9 − 1.2

SW3 44.4 16.8 9.2 4.6 − 0.8 − 1.8

DW5 35.3 9.3 3.6 0.7 − 3.7 − 4.7

SW5 30.3 7.3 0.6 − 4.6 − 10.2 − 11.9

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to the nucleation effect of silica nanoparticles, resulting in

faster formation of CH and more dynamic consumption of

C3S during the binding period. A gradual increment of the

exothermic peak was therefore observed, in specimens con-

taining 3 and 5wt% of NS. It is important to stress that

DW5 and SW5 reached the exothermic peak slightly later

than the corresponding DW3 and SW3 specimens, due to the

presence of PCE superplasticizer, which is widely known to

exhibit a retarding effect on cement hydration (Reales and

Filho 2017). The highest exothermic peak value was reached

by specimen SW5, followed by SW3. However, the SW3

specimen reached the exothermic peak in the shortest time

from among all the specimens; 205min and 104min faster

than in the cases of DW0 and SW0, respectively. After 12h

of hydration, the cumulative heat of SW3 was 44.4% higher

than that of the DW0 specimen, while DW3 exhibited an

increment of only 17.9%. After up to 7days of hydration,

the cumulated heat released reached comparable values for

most mixtures, excluding specimens with a high NS content

(DW5 and SW5), where a slight decrease in the cumulative

heat value was observed.

Based on the results presented here, it can be concluded

that seawater has a noticeable effect on increasing the

hydration rate in the early stages of hydration and that this

effect is even more pronounced when seawater is mixed with

NS. As has been reported by Li etal. (2015) and Parthasar-

athy etal. (2017), the formation of CH in the early stages

is promoted as a result of the acceleration of the hydration

process by the chlorides in seawater. CH is therefore con-

sumed at the early stage of hydration, due to the very high

pozzolanic activity of NS, resulting in the production of a

C-S-H phase. Similar findings have been reported by Shi

etal. (2015), where seawater was mixed with metakaolin,

another highly reactive pozzolan. To support this conclu-

sion, the evolution of the CH content in specimens with the

highest reaction rates, from the DW and SW groups, was

analysed with the use of a thermogravimetric technique.

Thermogravimetric analysis (TGA)

The thermogravimetric curves of the specimens evaluated

are presented in Fig.4a–c, with the quantitative results of

CH content calculated according to weight loss and stoi-

chiometric considerations (Eq.2) depicted in Fig.4d. The

content of CH can reflect the degree of Portland cement

hydration (Deboucha etal. 2017). It can be seen that after

Fig. 4 TG curves for cement pastes after 3days (a), 7days (b), 28days of curing and d evolution of CH content with time

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3days of hydration, cement paste made with seawater (SW0)

contained more CH than DW0, confirming the accelerating

effect of seawater on the hydration of cement (Li etal. 2015).

After 7 and 28days, the CH content in the SW0 specimen

remained similar, indicating that most of the hydration had

been completed within the first 3days, while an increment

in CH content in the DW0 specimen was observed for up to

28days. In contrast, the CH content in the pastes contain-

ing NS progressively decreased with time, which is in line

with other researchers’ observations (Heikal etal. 2013; Abd

El-Aleem etal. 2014; Ghafari etal. 2014). In addition, it is

evident from these data that the CH content decreased con-

siderably when NS was incorporated into the cement pastes.

As was observed in the previous section, NS can acceler-

ate cement hydration in the first hours of hydration by pro-

viding nucleation sites, which means that more CH is pro-

duced (Xu etal. 2016). However, after 3days of hydration

the CH content in NS-incorporated specimens was much

lower than that of pristine specimens. This is attributable to

the fact that amorphous colloidal silica is a highly reactive

pozzolanic material, which thus reacts very actively with

CH in the pozzolanic reaction in the early ages of cement

hydration. As such, more C-S-H gel is generated (Du etal.

2014; Xu etal. 2016). After 3 days of curing, the CH content

in DW3 was 16% lower than in the DW0 specimen, while a

combination of seawater and NS in specimen SW3 resulted

in a CH content 26% lower than in the corresponding SW0

specimen. As has been reported by other researchers (Hou

etal. 2013; Abd El-Aleem and Ragab 2015; Xu etal. 2016),

although the pozzolanic reaction of NS and cement is usu-

ally almost complete after a few days of curing, disparity in

the CH content keeps increasing with time, which confirms

a slow CH generation and cement hydration of NS-incorpo-

rated cement pastes, at a later age. After 28days of curing

the CH content in specimens DW3 and SW3 was lower by

37% and 40%, respectively, in comparison with the corre-

sponding DW0 and SW0.

Compressive strength

Strength development

Figure5 presents the compressive strength evolution of

cement pastes after up to 28days of curing. Noticeable dif-

ferences in compressive strength values can be observed

from the first day of hydration. After 1day of hydration, all

specimens from the SW group exhibited higher compres-

sive strengths than the corresponding specimens from the

DW group. Specimen SW0 exhibited a 41% higher 1-day

compressive strength than DW0.

The incorporation of NS contributed to further strength

improvements in specimens from the very beginning of

the hydration process, confirming both its nucleating and

pozzolanic effects, as shown by calorimetry and TGA

results; the 1-day strength of specimens SW3 and DW3

was 34.7MPa and 27.7MPa (respectively), while speci-

mens SW0 and DW0 exhibited strengths of 23.0MPa and

16.3MPa, respectively. However, in the case of low NS dos-

ages (specimens DW1 and SW1), its effect was relatively

limited. It is important to stress that all the specimens con-

taining seawater, exhibited over 40% of their final 28-day

strength after 1day of curing, while in the case of the DW

group, the specimens exhibited only 30–36% of their final

28-day strength.

The effect of NS was noticeably pronounced after 2 days

of curing. The DW3, DW5, SW3 and SW5 specimens exhib-

ited compressive strength values almost double to those of

the corresponding DW0 and SW0 specimens. This confirms

the high pozzolanic activity of NS in the early days of hydra-

tion, which was reflected in our TGA study (“Thermogravi-

metric analysis (TGA)”), showing that NS-modified speci-

mens contained significantly lower CH contents after 3days

Fig. 5 Compressive strength of cement pastes after 1, 2, 7, 14 and

28days of curing

2635Applied Nanoscience (2020) 10:2627–2638

1 3

of curing (Fig.4d). Due to the high pozzolanic activity of

NS, specimens with higher dosages of NS (3 and 5wt%)

reached over 60% of their final strength, while pristine DW0

and SW0 exhibited only 48.4% and 50.4%, respectively. The

compressive strength of the SW3 specimen, after 2days of

curing, was 200% higher than that of the SW0 specimen,

while DW3 exhibited a strength 188% higher than that of

DW0. In addition, the 2-day compressive strength of the

SW3 specimen was equal to and 11% higher, as compared

to the SW0 and DW0 specimens’ 28-day strength, respec-

tively. Dynamic strength development was observed for up

to 7days of curing. However, as a result of intensive hydra-

tion in the first 2days, the disparity between the specimens

decreased gradually. Nevertheless, after 7days of curing

SW0 exhibited a 23% higher compressive strength than

DW0. After 14days of curing, specimens containing sea-

water reached almost 100% of 28-day compressive strength

(except SW1, which reached 93%), while in case of the DW

group, a similar phenomenon was observed only in the case

of DW3.

After 28days of curing, the corresponding specimens

of each group exhibited relatively comparable strength val-

ues, with only slight variations between them found. Speci-

men SW3 exhibited the highest strength of 83.5MPa, while

specimens DW0 and SW0 exhibited strengths of 50.8MPa

and 56MPa, respectively.

Evaluation oftheaccelerating effects ofseawater andNS

oncompressive strength

To evaluate the synergistic effect of seawater and NS on the

strength development of cement pastes, the relative com-

pressive strength values of the samples in relation to the

control specimen (DW0) were calculated. The results were

calculated for each specimen, as a ratio between the selected

specimen’s strength at a certain day of hydration, divided by

the strength of the DW0 specimen on the same day of test-

ing. The data are depicted in Fig.6.

After 1day of hydration, specimens produced with sea-

water exhibited more than a 40% higher strength than DW0,

while from the second day of hydration the disparity between

pristine SW0 and DW0 specimens started to decrease. Nev-

ertheless, SW0 exhibited over a 20% higher strength than

DW0 for up to 14days. This confirms the accelerating effect

of cement hydration in the presence of seawater, thus notice-

ably increasing early strength development. The use of sea-

water along with NS resulted in significantly increased 1-day

strength and a pronounced effect after 2days. This is attrib-

utable to a higher amount of CH produced in SW specimens,

meaning that more C-S-H gel could be produced as a result

of the pozzolanic activity of NS. After 7days of curing, the

synergistic effect of NS and seawater decreased gradually up

to 28days, at which point specimens with the same NS con-

tents, irrespective of the type of water, exhibited relatively

comparable strength values. The incorporation of NS is ben-

eficial in improving the early strength of cement pastes, as

well as 28-day strength. However, when a low dosage of NS

is incorporated the effect is less pronounced, as was the case

in the DW1 and SW1 specimens. When the amount of NS is

optimal, significant strength improvement can be observed,

as in case of the DW3 and SW3 specimens. Specimens SW3

and DW3 exhibited, respectively, 64% and 52% higher com-

pressive strength values than the DW0 specimen, after 28

days of curing. However, when the amount of NS exceeds

the optimum amount, the effect of NS can decrease or be

neutralized, as was observed in specimens DW5 and SW5

and as has been reported in the studies of other researchers

(Abd El-Aleem etal. 2014; Yu etal. 2014; Abdel Gawwad

etal. 2017; Rupasinghe etal. 2017). Thus, to fully benefit

from the combination of seawater and NS, the incorporation

of an optimal amount of silica seems to be a crucial issue.

Pore size distribution

The obtained pore size distribution plot, covering the pore

size range from around 100µm down to 7nm, is shown in

Fig.7, while values of total porosity, average and median

diameter are summarized in Table3.

Generally, pore size distribution can be divided into three

main size ranges (Aligizaki 2006): mesopores (5–50nm),

middle capillary pores (50–100nm) and larger capillary

pores (> 100nm). It can be clearly seen that the replacement

of demineralized water with seawater has a beneficial effect

on decreasing the porosity of cement paste after 28days of

curing. Total porosity of SW0 was decreased by 12% in com-

parison with DW0. In addition, noticeable refinement of the

pore structure was observed. Both average and median pore

diameter were decreased in SW0 while compared to DW0

Fig. 6 Relative compressive strength of cement pastes after 1, 2, 7, 14

and 28 days in comparison with the DW0 specimen

2636 Applied Nanoscience (2020) 10:2627–2638

1 3

(Table3). In addition, specimen SW0 contained less amount

of large capillary pores than DW0 specimen. As stated by

Neville (2006) this critical interval (middle and large capillary

pores) is mostly responsible for permeability and penetration

of harmful substances into concrete. These observations are

in line with studies of previous researchers showing that the

use of seawater contributes to decrease the total porosity and

refinement of cement matrix (Shi etal. 2015; Adiwijaya etal.

2017; Wang etal. 2018). According to Wang etal. (2018), this

effect is attributed to a faster formation of C-S-H phases in the

presence of seawater. Authors reported that seawater encour-

ages the formation of high-surface-area C-S-H matrix phases

in comparison with tap water, thus more C-S-H matrix phases

containing much higher volume fractions of fine C-S-H gel

are developed. Results of our MIP tests are in line with results

of compressive strength (“Compressive strength”) where SW0

exhibited slightly higher compressive strength value than

DW0 after 28days of curing.

The introduction of NS resulted in a significant decrease

of porosity of cement pastes. Incorporation of NS has clearly

decreased the total porosity of cement pastes as well as

noticeably decreased the average and median diameter of

pores (Table3). It can be observed that by addition of NS,

the volume of capillary pores in specimens DW3 and SW3

decreased and the pore structure is more refined than in case

of corresponding pristine pastes. This effect is attributed both

to nano-filling effect as well as higher amount of C-S-H phase

produced due to pozzolanic activity in the cement paste, thus

more refined and compacted microstructure was produced.

This effect is clearly reflected in the mechanical performance

of specimens DW3 and SW3 after 28days of curing (“Com-

pressive strength”). However, the combination of NS and sea-

water was more beneficial for refining the microstructure of

cement paste than mixture of NS and demineralized water. The

total porosity of SW3 was decreased by 33% in comparison

with SW0, while DW3 exhibited only 26% decrement of total

porosity in comparison with DW0. It is important to stress that

mixture of seawater and NS enabled to produce cement paste

with 41% lower total porosity than reference DW0 specimen.

Conclusions

This work has evaluated the effects of colloidal silica (NS),

in the amounts of 1, 3 and 5wt% and the use of seawater as a

mixing water in the hydration process, on the strength devel-

opment and microstructural properties of Portland cement

pastes. Based on the experimental results, the following con-

clusions can be drawn:

• Seawater has an accelerating effect on Portland cement

hydration and thus increases the CH content in the early

ages of cement hydration. However, the cumulative heat

released after 7days of hydration is comparable to that

of cement mixed with demineralized water.

• The use of seawater has a noticeable effect on the early

strength development of cement paste for up to 14days of

hydration. After 28days of curing, only a slight strength

increment is observed when compared with cement paste

made with distilled water.

• Due to the nucleating effect and the high pozzolanic

activity of NS, an acceleration of cement hydration kinet-

ics, along with a significant reduction in CH content from

early ages up to 28 days, were observed.

• The incorporation of NS has a significant effect on acceler-

ating the strength development of cement pastes from the

first day of the hydration process. Therefore, cement pastes

produced with higher dosages of NS (3 and 5wt%) exhib-

ited noticeably increased compressive strengths, after up to

28 days of curing, as compared with plain cement pastes.

• A combination of seawater and NS noticeably increases

hydration process kinetics and promotes the pozzolanic

reaction. Due to an increased content of CH availa-

ble for pozzolanic reaction with NS, in cement pastes

containing seawater, more C-S-H gel is formed; thus

such cement pastes exhibit higher hydration rates and

Fig. 7 Cumulative intruded pore volume versus pore diameter of

cement pastes

Table 3 Porosities, average pore diameter and median diameter of the

DW0, SW0, DW3, SW3 pastes after 28 days of curing

Sample desig-

nation

Total porosity

(vol.-%)

Average pore

diameter (µm)

Median pore

diameter (µm)

DW0 25.12 0.0258 0.0418

SW0 22.20 0.0206 0.0308

DW3 18.56 0.0223 0.0299

SW3 14.87 0.0188 0.0227

2637Applied Nanoscience (2020) 10:2627–2638

1 3

mechanical performance than pastes prepared with

deionized water and NS.

• The use of seawater has a distinct effect on decrement

of total porosity and refinement of the pore structure of

cement paste. Incorporation of NS optimizes the pore

structures and leads to further decrement of total poros-

ity of cement pastes. MIP studies confirmed that the

beneficial effect of NS is more pronounced in cement

pastes mixed with seawater, hence cement pastes pro-

duced with NS and seawater exhibited the lowest values

of total porosity, average and median pore diameter.

• However, the incorporation of an optimal dosage is cru-

cial to fully benefit from mixing seawater and NS. In

this study, 3wt% of NS was established to be an opti-

mal dosage of admixture for producing cement pastes

with the best performance.

Acknowledgements This research was supported by the National Sci-

ence Centre (Poland) within project No. 2016/21/N/ST8/00095 (PREL-

UDIUM 11).

Compliance with ethical standards

Conflict of interest On behalf of all authors, the corresponding author

states that there is no conflict of interest.

Open Access This article is distributed under the terms of the Crea-

tive Commons Attribution 4.0 International License (http://creat iveco

mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribu-

tion, and reproduction in any medium, provided you give appropriate

credit to the original author(s) and the source, provide a link to the

Creative Commons license, and indicate if changes were made.

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