Nanocomposites as Substituent of Cement: Structure and Mechanical Properties

Abstract

To date, the scientific research in the field of recycling of construction and demolition wastes was focused on the production of concrete, cements, and bricks. The attainment of these products was limited to the addition of suitable binder contents, such as lime or cement, compaction, and possibly heat treatment, without a concrete recycling method. In this paper, new cement materials consisting of 2.5 weight% composite and originating from construction and demolition waste powder, were prepared and investigated in view of applications in the construction industry as a substituent of cement. The materials with recycled powder from construction and demolition wastes were characterized by X-ray diffraction (XRD), infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy. The XRD data indicate vitroceramic structures with varied crystalline phases. The NMR relaxometry data show four reservoirs of water associated with bounded water and with three types of pores in the composite construction material. The micro-Vickers hardness was measured to reflect the influence of composite nature in the local mechanical properties of the composite-cement for the mixture with Portland cement and (EC) expired cement.

Full text

Citation: Rada, R.; Manea, D.L.;
Chelcea, R.; Rada, S.
Nanocomposites as Substituent of
Cement: Structure and Mechanical
Properties. Materials 2023,16, 2398.
https://doi.org/10.3390/
ma16062398
Academic Editor: Baoguo Han
Received: 10 February 2023
Revised: 8 March 2023
Accepted: 14 March 2023
Published: 16 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
materials
Article
Nanocomposites as Substituent of Cement: Structure
and Mechanical Properties
Roxana Rada 1, Daniela Lucia Manea 1, Ramona Chelcea 2and Simona Rada 2, 3, *
1Department of Civil Engineering and Management, Faculty of Civil Engineering,
Technical University of Cluj-Napoca, 400114 Cluj-Napoca, Romania
2Department of Physics and Chemistry, Faculty of Materials and Environmental Engineering,
Technical University of Cluj-Napoca, 400114 Cluj-Napoca, Romania
3National Institute of Research and Development for Isotopic and Molecular Technologies,
400293 Cluj-Napoca, Romania
*Correspondence: radasimona@yahoo.com or simona.rada@itim-cj.ro or simona.rada@phys.utcluj.ro
Abstract:
To date, the scientific research in the field of recycling of construction and demolition wastes
was focused on the production of concrete, cements, and bricks. The attainment of these products
was limited to the addition of suitable binder contents, such as lime or cement, compaction, and
possibly heat treatment, without a concrete recycling method. In this paper, new cement materials
consisting of 2.5 weight% composite and originating from construction and demolition waste powder,
were prepared and investigated in view of applications in the construction industry as a substituent
of cement. The materials with recycled powder from construction and demolition wastes were
characterized by X-ray diffraction (XRD), infrared (IR) and nuclear magnetic resonance (NMR)
spectroscopy. The XRD data indicate vitroceramic structures with varied crystalline phases. The
NMR relaxometry data show four reservoirs of water associated with bounded water and with three
types of pores in the composite construction material. The micro-Vickers hardness was measured to
reflect the influence of composite nature in the local mechanical properties of the composite-cement
for the mixture with Portland cement and (EC) expired cement.
Keywords: composite-cement; XRD; IR; NMR; Vickers hardness
1. Introduction
Large quantities of waste are produced during the construction of developments, when
buildings are refurbished and demolished at the end of their lives. The management of
construction and demolition (C&D) wastes results in considerable environmental impacts.
The use of alternative management routes can generate environmental improvements and
cost savings. In the European Union, construction and demolition wastes represent one
third of all solid municipal waste. Construction and demolition wastes contain a wide
variety of materials, such as concrete, bricks, wood, glass, metal, and plastic [1].
The main scientific research was focused on the recycling of construction of concrete,
cement, and bricks [
2
,
3
]. Typically, the main objectives of these studies involve the possibil-
ity of obtaining new products using lime or cement as binders without the involvement of
a recycling technology [4].
The recycling of secondary materials is environmentally beneficial, since in the primary
production of materials, significant amounts of raw materials and energy are used. In this
sense, the separation of metals from construction and demolition wastes and the recycling
of other metal products represent a priority for C&D waste management. Moreover, there
are considerable financial benefits, which already drive the recycling of many materials. In
this paper, two types of construction and demolition wastes, namely, broken glasses and
metals (iron, cash iron, and lead) were tested for recycling.
Materials 2023,16, 2398. https://doi.org/10.3390/ma16062398 https://www.mdpi.com/journal/materials

Materials 2023,16, 2398 2 of 16
Wood waste is the second largest component of construction and demolition after con-
crete. The amount of wood is around 10% of all materials deposited in landfills. The legislation
classifies timber and wood products as a C&D waste component with limited reuse if the ma-
terial is contaminated by environmentally harmful materials. Wood waste from construction
and demolition activities is usually delivered for the processing of boiler fuels and pellets. In
this paper, reutilization of ash from the remaining residue after the combustion of wood waste
in the construction and demolition sector is the topic of the research.
Concrete is widely used as a construction material and requires theconsumption of
a significant amount of cement since it is the main binding material. The production of
Portland cement has a greenhouse effect due to the emission of CO
2
into the atmosphere.
The production of 1 ton of Portland cement results in 1 ton of CO
2
[
5
]. The Hardened
cement paste is the most important component of concrete, which is a porous material
with pore sizes varying from several micrometers to nanometers. The performance of
concrete is derived from the precipitation of calcium silicate hydrate (C-S-H) [
6
]. A detailed
understanding of the C-S-H structure in the cement paste and the mechanisms determining
the concrete properties can directly contribute to the increase in the durability of concrete
materials and the reduction in the CO
2
emissions for combating climate change by recycling.
The possibility of introducing the recycled concrete powder into concrete as a substi-
tute for cement was studied in the last few years [
7
–
9
]. The addition of 5 wt% silica fume
and 20 wt% fly in the cement shows that the abrasion resistance and mechanical property
were increased by about 4–9% [7].
The concept of incorporating nanomaterials in the cement is new and extensively
exploited in the construction industry [
10
]. To date, the incorporation of various nanomate-
rials, including nano-SiO
2
, nano-Al
2
O
3
, nano-TiO
2
, carbon nanotubes, grapheme, and iron
oxide nanoparticles into cementitious materials were demonstrated to accelerate cement
hydration and improve mechanical strength [11–15].
Metallic oxides originally found in cement, such as free CaO (the main accessible
oxide), SiO
2
, Fe
2
O
3
, FeO and Al
2
O
3
can easily react in the long term with the incorporated
nanomaterial [12].
Briefly, during the recent years, many studies were focused on the effect of nanoparti-
cles in the construction materials, especially hardened cement paste, cement mortar, and
concrete. The literature data suggest that a lower content of nanoparticles can improve the
mechanical properties and durability of high performance concrete [
16
]. Despite progress,
the research on nanoparticles-cement is still inadequate [17].
The goal of sustainable development of the built environment is to minimize the
consumption of natural resources and reduce the consumption of cement. In this paper,
the composite materials are preparedas raw materials using the recycled powders from
construction and demolition wastes, such as broken glasses, cash iron, iron, lead, or ash
powder. Metallic powders or waste ash will be incorporated into the glass network;
therefore, the main nanoparticles are enclosed in the glass network and the metallic oxides
of cement are not accessible to produce new crystalline phases. Vickers hardness was
used as an indicator of the changes in the local mechanical properties of the composite-
cement materials. Our aim was to gain a more detailed understanding of the structural and
mechanical properties of the composite-cement materials using XRD, IR and NMR data
and the distribution of Vickers hardness. Identification of the different pore types on the
surface measured by 1H NMR relaxometry was also addressed in this paper.
2. Experimental Procedure
Chemicals used in this paper were waste glasses collected from broken window, waste
powder (recycled powder from construction and demolition wastes, such as cash iron, iron,
lead, or ash powder), NaOH, HCl, gray Portland cement, and expired Portland cement.
Waste cash iron and iron powder were dissolved in 10% H2SO4solution.
Waste glass powder was added to 1N NaOH solution for partial dissolving in a
porcelain capsule placed on a mechanical stirrer. When the glass content was dissolved

Materials 2023,16, 2398 3 of 16
with 1N HCl solution, a stoechiometric amount of waste powder was added to the solution.
The experiment was performed with constant stirring speed at 50
◦
C for 15 min, and
continued at 100
◦
C for 5 min. Finally, the temperature was increased to 250
◦
C and the
composite product was obtained.
Composite-cement materials were prepared with gray Portland cement and, in every
mixture, 2.5 weight% of cement amount was substituted by the composite product. The
composition of the cement-based materials with recycled powder from construction and
demolition wastes is listed in Table 1. The water to cement ratio was 0.3:1 for the validated
cement and 0.4:1 for the expired cement, respectively.
Table 1. Description of composite and composite-cement materials.
Notation of
Composite
Material
Description of
Composite
Material
Notation of
Composite-
Validated Cement
Materials
Description of
Composite-Cement
Materials
Notation of
Composite-
Expired Cement
Materials
Description of
Composite-Expired
Cement Materials
IMixture of glasses
and iron powder CI
2.5 weight% of
cement was
substituted by I
composite
ECI
2.5 weight% of
cement was
substituted by I
composite
CMixture of glasses
and cash iron
powder CC
2.5 weight% of
cement was
substituted by C
composite
ECC
2.5 weight% of
cement was
substituted by C
composite
LMixture of glasses
and lead powder CL
2.5 weight% of
cement was
substituted by L
composite
ECL
2.5 weight% of
cement was
substituted by L
composite
AMixture of glasses
and ash CA
2.5 weight% of
cement was
substituted by A
composite
ECA
2.5 weight% of
cement was
substituted by A
composite
G Reference glass C Reference mix, 100%
cement EC Reference mix, 100%
expired cement
The samples were characterized by X-ray diffraction using a Smart Lab Rigaku diffrac-
tometer, with a monochromator of graphite for the Cu-K
α
radiation (
λ
= 1.54 Å) at room
temperature. The data were collected in the 2
θ
range 10–60
◦
with scanning step size of 0.01
◦
and step time of 0.1 s per step. For phase identification, the material was carried out using the
Match! software. The PDF-2 database was used for the identification of crystalline phases.
The Fourier Transform InfraRed (FTIR) spectra of the glasses were obtained in the
350–2000 cm
−1
spectral range with a JASCO FTIR 6200 spectrometer (JASCO, Tokyo, Japan)
using the standard KBr pellet disc technique. The spectra were carried out with a standard
resolution of 2 cm−1.
The samples were subjected to nuclear magnetic resonance (NMR) measurements us-
ing a low field Bruker Minispec NMR spectrometer (Bruker, Billerica, MA, USA) operating
at 19.69 MHz proton frequency.
A Nova microdurimeter equipped with the microscope was used to measure the
micro-Vickers hardness. A penetrator with diameter D was operated with a load F of
0.3 kgF
for an interval of 15 s. After unloading the penetrator, the diagonals of the trace
of pyramidal contour were measured and the values of HV hardness (expressed in MPa)
were determined with the device software.

Materials 2023,16, 2398 4 of 16
3. Results and Discussion
3.1. X-ray Diffraction Analysis of the Composite Materials
X-ray diffractograms of the broken glasses and prepared composite materials are
shown in Figure 1. The XRD pattern reveals two halos that are characteristic of the amor-
phous structure of the waste glass powder. The analysis of XRD data proves that the
presence of different crystalline phases depends on the doping nature.
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
ANovamicrodurimeterequippedwiththemicroscopewasusedtomeasurethe
micro‐Vickershardness.ApenetratorwithdiameterDwasoperatedwithaloadFof0.3
kgFforanintervalof15s.Afterunloadingthepenetrator,thediagonalsofthetraceof
pyramidalcontourweremeasuredandthevaluesofHVhardness(expressedinMPa)
weredeterminedwiththedevicesoftware.
3.ResultsandDiscussion
3.1.X‐rayDiffractionAnalysisoftheCompositeMaterials
X‐raydiffractogramsofthebrokenglassesandpreparedcompositematerialsare
showninFigure1.TheXRDpatternrevealstwohalosthatarecharacteristicofthe
amorphousstructureofthewasteglasspowder.TheanalysisofXRDdataprovesthatthe
presenceofdifferentcrystallinephasesdependsonthedopingnature.

Figure1.X‐raypatternsofthebrokenglasses(G)andpreparedcompositematerials(C—mixtureof
glassesandcashiron,I—mixtureofglassesandironpowder,L—mixtureofglassesandlead
powder,A—mixtureofglassesandash).
Fortheglassdopedwithcashiron(denotedasC),theformationofFeCl2⸱(H2O)2with
amonoclinicstructurewasdetected.Withtheintroductionofironcontentintheglass
network,theamountofFeCl2(H2O)2crystallinephasedecreasesandthepresenceofFe3O4
crystallinephasewithacubicstructurewasevidenced.
IntheleadcompositedenotedwithL,themainphaseofCaPbO3crystallinephase
withanorthorhombicstructurewasaccompaniedbythesecondaryphaseofPbCl2crys‐
tallinephase.
Bydopingwithash,twophasesofCaCO3andKClwiththemostintenselineswere
notedintheXRDdata.
TheaveragecrystallitesizeofparticleDwasdeterminedbytheDebyeScherrer
equation[18],consideringthatλistheX‐raywavelength(0.154nm),βisthebroadening
ofthediffractionpeakinradians(fullwidthathalfmaximumofthepeak),andθisthe
Figure 1.
X-ray patterns of the broken glasses (G) and prepared composite materials (C—mixture of
glasses and cash iron, I—mixture of glasses and iron powder, L—mixtureof glasses and lead powder,
A—mixture of glasses and ash).
For the glassdoped with cash iron (denoted as C), the formation of FeCl
2·
(H
2
O)
2
with
a monoclinic structure was detected. With the introduction of iron content in the glass
network, the amount of FeCl
2
(H
2
O)
2
crystalline phase decreases and the presence of Fe
3
O
4
crystalline phase with a cubic structure was evidenced.
In the lead composite denoted with L, the main phase of CaPbO
3
crystalline phase with
an orthorhombic structure was accompanied by the secondary phase of PbCl
2
crystalline phase.
By doping with ash, two phases of CaCO
3
and KCl with the most intense lines were
noted in the XRD data.
The average crystallite size of particle D was determined by the Debye Scherrer equa-
tion [
18
], considering that
λ
is the X-ray wavelength (0.154 nm),
β
is the broadening of the
diffraction peak in radians (full width at half maximum of the peak), and
θ
is the diffraction
angle for maximum peak in radians. The average crystallite size of prepared compos-
ite particles for different dopant types is extracted using the Debye Scherrer equation
and is shown in Table 2.
The particle size in the prepared composites increases with the doping of lead and ash.
The values of obtained particle size in the composite for the high intensity peak confirm
the nanostructure properties. Literature data on the particle size effect of recycled brick
powder from the blended cement paste show that the reduction in particle sizes improves
the pore structure [
19
]. The higher particle sizes in the samples doped with lead and ash

Materials 2023,16, 2398 5 of 16
can have a significant impact on porosity and, at later stages, transport the properties of
cement structure.
Table 2. Average size of particle D of the high intensity peak of prepared composites.
Composite Material
Bragg Angle, θ(Degree)
Corresponding to the Main
Crystalline Phase
β(Radian ×10−3)The Average Crystallite Size
of Particle D (nm)
C (cash iron) 15.95 (FeCl2(H2O)2) 2.790 54.04
I (iron) 35.38 (Fe3O4) 2.792 63.59
15.92 (FeCl2(H2O)2) 1.958 76.88
L (lead) 31.07 (CaPbO3)
24.77 (PbCl2)1.919
1.958 88.07
81.43
A (ash) 29.39 (CaCO3)
28.38 (KCl) 2.199
1.759 75.56
93.55
3.2. FTIR Spectra of the Composite Materials
Infrared spectra of broken glasses and prepared composite materials are presented
in Figure 2. The analysis of IR data indicates specific domains of silicate units (800 and
1300 cm
−1
) and structural units of the metallic ions (370 and 550 cm
−1
). The first region of
IR spectra situated between 370 and 550 cm
−1
can be attributed to the stretching vibrations
of metal (Me)—oxygen bonds in the [MeO
n
] structural units, where n = 4 and 6 overlapped
with the stretching vibrations in the silicate glass network [
20
,
21
]. For sample I, this region
corresponds to the stretching vibrations of the Fe-O bonds with iron cations located in
octahedral and tetrahedral sites, respectively and the formation of Fe
3
O
4
crystalline phase,
in accordance with XRD data. For sample L, the intensity of the IR band centered at
about 460 cm
−1
corresponds to the stretching vibrations of Pb-O bonds in the [PbO
4
]
structural units.
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
diffractionangleformaximumpeakinradians.Theaveragecrystallitesizeofprepared
compositeparticlesfordifferentdopanttypesisextractedusingtheDebyeScherrer
equationandisshowninTable 2.
Tabl e 2.Avera gesizeofparticleDofthehighintensitypeakofpreparedcomposites.
CompositeMaterial
BraggAngle,θ(Degree)
CorrespondingtotheMainCrystalline
Phase
β(Radian×10−3)TheAverageCrystalliteSizeof
ParticleD(nm)
C(cashiron)15.95(FeCl2(H2O)2)2.79054.04
I(iron)35.38(Fe3O4)2.79263.59
15.92(FeCl2(H2O)2)1.95876.88
L(lead)31.07(CaPbO3)
24.77(PbCl2)
1.919
1.958
88.07
81.43
A(ash)29.39(CaCO3)
28.38(KCl)
2.199
1.759
75.56
93.55
Theparticlesizeinthepreparedcompositesincreaseswiththedopingofleadand
ash.Thevaluesofobtainedparticlesizeinthecompositeforthehighintensitypeak
confirmthenanostructureproperties.Literaturedataontheparticlesizeeffectofrecy‐
cledbrickpowderfromtheblendedcementpasteshowthatthereductioninparticle
sizesimprovestheporestructure[19].Thehigherparticlesizesinthesamplesdoped
withleadandashcanhaveasignificantimpactonporosityand,atlaterstages,transport
thepropertiesofcementstructure.
3.2.FTIRSpectraoftheCompositeMaterials
Infraredspectraofbrokenglassesandpreparedcompositematerialsarepresented
inFigure2.TheanalysisofIRdataindicatesspecificdomainsofsilicateunits(800and
1300cm−1)andstructuralunitsofthemetallicions(370and550cm−1).Thefirstregionof
IRspectrasituatedbetween370and550cm−1canbeattributedtothestretchingvibrations
ofmetal(Me)—oxygenbondsinthe[MeOn]structuralunits,wheren=4and6over‐
lappedwiththestretchingvibrationsinthesilicateglassnetwork[20,21].ForsampleI,
thisregioncorrespondstothestretchingvibrationsoftheFe‐Obondswithironcations
locatedinoctahedralandtetrahedralsites,respectivelyandtheformationofFe3O4crys‐
tallinephase,inaccordancewithXRDdata.ForsampleL,theintensityoftheIRband
centeredatabout460cm−1correspondstothestretchingvibrationsofPb‐Obondsinthe
[PbO4]structuralunits.

Figure 2.
Fourier transform infrared (FTIR) spectra of waste glass powder and prepared
composite materials.
The high intensity bands region located between 800 and 1300 cm
−1
is assigned to the
stretching vibrations of the Si-O bonds in varied silicate structural units. By doping, this
region differs from features of the host matrix since it appears as new shoulders and char-
acteristic IR bands, which indicates a drastic depolymerization of silicate network. These
IR bands were assigned to symmetric Si-O stretching vibrations of silicate units containing
[SiO
4
] tetrahedral units with zero, one, two, three, and four non-bridging oxygens, namely,

Materials 2023,16, 2398 6 of 16
tectosilicate, disilicate, metasilicate, pyrosilicate, and orthosilicate units denoted as Q4, Q3,
Q2, Q1, and Q0, respectively [
22
]. These IR features allow for thedistinguishment of bands
located at about 1100–1250 cm
−1
, 1000–1100 cm
−1
, 900–1000 cm
−1
, near 900 cm
−1
, and
near 850 cm
−1
of each dominant at the tectosilicate, disilicate, metasilicate, pyrosilicate,
and orthosilicate units.
The IR bands are characterized by a broad asymmetric profile with shoulders and new
bands between 800 and 1300 cm
−1
depending on the composition of the composite. For
samples C and I, this profile has two shoulders centered at about ~1010 and 1090 cm
−1
and
the intensity of the band located at about 925 cm
−1
was decreased. This evolution suggests
a rearrangement of the host matrix, leading to the formation of chains or ring structures,
namely, disilicate (Q3) and metasilicate (Q2) units.
By doping with lead content, the maximum of this profile shifts toward higher
wavenumber values (~1110 cm
−1
) and the intensity of this band is stronger than its ana-
logues. This feature corresponds to the Q4 unitswith four oxygen atoms per silicate of
tetrahedrons overlapping with the stretching vibrations of Pb-O bonds in
the [PbO4] structural units.
More significant differences are visible in the case of the addition of ash content in the
host matrix when the presence of a new IR band centered at about 875 cm
−1
, including
orthosilicate units was evidenced.
The observed features of the profile situated between 800 and 1200 cm
−1
clearly
indicate the progressive depolymerization of the silicate network by doping. Silicate
tetrahedral units have different degrees of polymerization, namely, for doping with
C and I
,
the formation of short ring structures, such as pyrosilicate and metasilicate units, by
the addition of L amounts to the presence of tectosilicate units, and for ash content the
orthosilicate units were evidenced. The effect of silicate network depolymerization increases
by doping with lead and ash.
The prominent IR band centered at about ~3435 cm
−1
is assigned to the H-O stretch-
ing vibrations from adsorbed water molecules on the surface of the sample and the
presence of stretching vibrations of the Si-OH bonds. The IR band centered at about
1600 and 2900 cm−1
is assigned to the H-O-H bending vibrations and hydroxyl units [
23
].
For cast-iron (C) and iron (I) composite materials, these IR bands were gradually enhanced
due to the presence of a higher number of Si-OH units.
The prominent IR band centered at about ~1450 cm
−1
corresponds to the CO
3−2
carbonate ions and the formation of CaCO
3
crystalline phase in accordance with XRD data.
By doping of the glass network with different contents of wastes, structural modifica-
tions in the silicate network and in the process of water absorption were evidenced. The
higher amounts of adsorbed water were evidenced in C and I samples, which have an
important role in the hydration process of cement paste.
3.3. FTIR Spectra of the Composite-Cement Materials
In the Portland cement, the major anhydrous phases are alite, Ca
3
SiO
5
(denoted as
C
3
S) and belite, Ca
2
SiO
4
(denoted as C
2
S), while the minor phases are calcium aluminate,
Ca
3
Al
2
O
6
, calcite, CaCO
3
, gypsum, CaSO
4
, ferrite, Ca
2
(Al, Fe)
2
O
5
. By the reaction between
cement and water, varied hydration products, namely, calcium silicate hydrates, C-S-H,
portlandite, Ca(OH)
2
, ettringite, calcium monocarboaluminate, or calcium monosulphoalu-
minateare obtained [
24
]. The C-S-H consists of polymerized silica and calcium ions with
water incorporated.
The formation of calcium silicate hydrate, C-S-H, occupies 50% ofthe fully hydrated
cement paste. The C-S-H is a non-stoichiometric material with poor crystalline phase, which
is the main hydration product and the most important component in the cement. The C
3
S
and C
2
S are responsible for strength development performances in Portland cement from
the short term up to months (C3S phase) and long term up to years (C2S phase).
The absorption intensities situated between 970 and 1100 cm
−1
are due to the C-S-H
calcium silicate hydrate. The dip in the IR bands situated between 800 and 970 cm
−1
can

Materials 2023,16, 2398 7 of 16
be assigned to the dissolution of the C
3
S alite clinker phase, which can be correlated with
the formation of C-S-H.
FTIR spectra of the validated and expired cement (EC) materials are plotted in Figure 3.
The inspection of the IR bands located between 800 and 1100 cm
−1
indicates the formation
of C-S-H in the validated cement material due to the increase in the intensity of these bands.
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phoaluminateareobtained[24].TheC‐S‐Hconsistsofpolymerizedsilicaandcalcium
ionswithwaterincorporated.
Theformationofcalciumsilicatehydrate,C‐S‐H,occupies50%ofthefullyhydrated
cementpaste.TheC‐S‐Hisanon‐stoichiometricmaterialwithpoorcrystallinephase,
whichisthemainhydrationproductandthemostimportantcomponentinthecement.
TheC
3
SandC
2
SareresponsibleforstrengthdevelopmentperformancesinPortlandce‐
mentfromtheshorttermuptomonths(C
3
Sphase)andlongtermuptoyears(C
2
S
phase).
Theabsorptionintensitiessituatedbetween970and1100cm
−1
areduetotheC‐S‐H
calciumsilicatehydrate.ThedipintheIRbandssituatedbetween800and970cm
−1
can
beassignedtothedissolutionoftheC
3
Saliteclinkerphase,whichcanbecorrelatedwith
theformationofC‐S‐H.
FTIRspectraofthevalidatedandexpiredcement(EC)materialsareplottedinFig‐
ure3.TheinspectionoftheIRbandslocatedbetween800and1100cm
−1
indicatesthe
formationofC‐S‐Hinthevalidatedcementmaterialduetotheincreaseintheintensity
ofthesebands.

Figure3.FTIRspectraofvalidatedandexpiredcementmaterialsperformedat28daysaftertheir
preparationintheregionbetween(a)400and1600cm
−1
and(b)1600and4000cm
−1
.
TheintensityofIRbandscenteredatabout1070cm
−1
andintheregionbetween
1350and1550cm
−1
canbeattributedtotheCO
3−2
carbonateionsandCaCO
3
,respective‐
ly.Theintensitiesofthesebandswereincreasedfortheexpiredcement.
AnewIRbandcenteredatabout855cm
−1
appearsintheexpiredcementduetothe
Ca‐Obond[25].
TheIRbandcenteredatabout3640cm
−1
correspondstotheCa(OH)
2
,whichis
formedasasilicatephaseinthedissolvedcement[26].Inthevalidatedcement,thein‐
tensityofthisbandwasenrichedsuggestingtheformationofC‐S‐H.
TheFTIRspectraofthesimplecementandcomposite‐cementmaterialsperformed
at28daysaftertheirpreparationaredisplayedinFigure4.Fortheexpiredcementand
composite‐expiredcementmaterials,theIRspectracanbeseeninFigure5.
Figure 3.
FTIR spectra of validated and expired cement materials performed at 28 days after their
preparation in the region between (a) 400 and 1600 cm−1and (b) 1600 and 4000 cm−1.
The intensity of IR bands centered at about 1070 cm
−1
and in the region between 1350
and 1550 cm
−1
can be attributed to the CO
3−2
carbonate ions and CaCO
3
, respectively. The
intensities of these bands were increased for the expired cement.
A new IR band centered at about 855 cm
−1
appears in the expired cement due to the
Ca-O bond [25].
The IR band centered at about 3640 cm
−1
corresponds to the Ca(OH)
2
, which is formed
as a silicate phase in the dissolved cement [
26
]. In the validated cement, the intensity of
this band was enriched suggesting the formation of C-S-H.
The FTIR spectra of the simple cement and composite-cement materials performed
at 28 days after their preparation are displayed in Figure 4. For the expired cement and
composite-expired cement materials, the IR spectra can be seen in Figure 5.
The IR bands characteristic of the sulphate originally from Portland cement are found
in the range between 1100 and 1200 cm
−1
due to the stretching vibration of S-O bonds in
the SO
4−2
units [
9
]. Ettringite is an hydrous calcium aluminum sulphate mineral and has a
characteristic peak of SO4−2vibrations centered at about 1115 cm−1[27].
The strong IR band centered at about 1100 cm
−1
corresponds to the antisymmet-
ric stretching vibration of the Si-O-Si linkages, while the broad band situated at about
950 cm−1reflects the stretching vibrations of the Si-OH [26,27].
Introduction of iron in the CI stone causes the increase in intensity of these IR bands.
In all composite-expired cement materials, the characteristic feature of this band increases
in strength and intensity.

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Figure4.FTIRspectraofcementandcomposite‐cementmaterialsperformedat28daysaftertheir
preparationintheregionbetween(a)400and1600cm−1,(b)800and1250cm−1,and(c)2500and
4000cm−1.

Figure 4.
FTIR spectra of cement and composite-cement materials performed at 28 days after
their preparation in the region between (
a
) 400 and 1600 cm
−1
, (
b
) 800 and 1250 cm
−1
, and
(c) 2500 and 4000 cm−1.
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Figure4.FTIRspectraofcementandcomposite‐cementmaterialsperformedat28daysaftertheir
preparationintheregionbetween(a)400and1600cm−1,(b)800and1250cm−1,and(c)2500and
4000cm−1.

Figure 5. Cont.

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Figure5.FTIRspectraofexpiredcementandcomposite‐expiredcementmaterialsintheregion
between(a)400and1600cm−1,(b)800and1200cm−1,and(c)2500and4000cm−1performedat28
daysaftertheirpreparation.
TheIRbandscharacteristicofthesulphateoriginallyfromPortlandcementare
foundintherangebetween1100and1200cm−1duetothestretchingvibrationofS‐O
bondsintheSO4−2units[9].Ettringiteisanhydrouscalciumaluminumsulphatemineral
andhasacharacteristicpeakofSO4−2vibrationscenteredatabout1115cm−1[27].
ThestrongIRbandcenteredatabout1100cm−1correspondstotheantisymmetric
stretchingvibrationoftheSi‐O‐Silinkages,whilethebroadbandsituatedatabout950
cm−1reflectsthestretchingvibrationsoftheSi‐OH[26,27].
IntroductionofironintheCIstonecausestheincreaseinintensityoftheseIR
bands.Inallcomposite‐expiredcementmaterials,thecharacteristicfeatureofthisband
increasesinstrengthandintensity.
Fortheexpiredcement,theintensityoftheIRbandslocatedbetween800and1200
cm−1wasincreasedforalltestedsamples.
TheuseofC‐S‐Hseedingdemonstratestheimprovementoftheperformanceto
strengthsincethemicrostructureofthehardenedcementisfineandthehydrationpro‐
cesswasaccelerated[28].Introductionofironcontenttothestructureofvalidatedce‐
mentcausestheincreaseintheintensityoftheIRbandcenteredatabout970and3640
cm−1(Figure4),showingtheformationofC‐S‐HandCa(OH)2,respectivelyinthenet‐
workstructure.Fortheexpiredcement,theintensityoftheIRbandcenteredatabout970
cm−1increasesforalldopedsamples(Figure5).TheIRbandcorrespondingtothe
Ca(OH)2increasesbydoping,withtheexceptionofaddingleadcontentintheexpired
cement.
ThecharacteristicfeatureoftheIRbandlocatedatabout855cm−1wasdiminished
fortheexpiredcementanditsintensityattainsthemaximumvalueforthesamplesECA
andECC.
TheanalysisofIRdataconfirmsthatnonewbandsinthecomposite‐cementmate‐
rialsappearedduetotheadditionof2.5weight%ofcompositeproductandthewater
associatedbandswasshiftedslightlytolowerwavenumbers.
3.4.NMRRelaxometryInvestigationsoftheComposite‐CementMaterials
Thedevelopmentinthemicrostructureofcomposite‐cementmaterialsduringhy‐
drationwasevaluatedbasedonthesurfaceareameasuredby1HNMRrelaxometryin‐
vestigations.TheechotrainsmeasuredbyCPMG(Carr‐Purcell‐Meiboom‐Gill)pulse
Figure 5.
FTIR spectra of expired cement and composite-expired cement materials in the region
between (
a
) 400 and 1600 cm
−1
, (
b
) 800 and 1200 cm
−1
, and (
c
) 2500 and 4000 cm
−1
performed
at 28 days after their preparation.
For the expired cement, the intensity of the IR bands located between 800 and
1200 cm−1was increased for all tested samples.
The use of C-S-H seeding demonstrates the improvement of the performance to
strength since the microstructure of the hardened cement is fine and the hydration process
was accelerated [
28
]. Introduction of iron content to the structure of validated cement
causes the increase in the intensity of the IR band centered at about 970 and 3640 cm
−1
(Figure 4), showing the formation of C-S-H and Ca(OH)
2
, respectively in the network
structure. For the expired cement, the intensity of the IR band centered at about 970 cm
−1
increases for all doped samples (Figure 5). The IR band corresponding to the Ca(OH)
2
increases by doping, with the exception of adding lead content in the expired cement.
The characteristic feature of the IR band located at about 855 cm
−1
was diminished
for the expired cement and its intensity attains the maximum value for the samples
ECA and ECC.
The analysis of IR data confirms that no new bands in the composite-cement materials
appeared due to the addition of 2.5 weight% of composite product and the water associated
bands was shifted slightly to lower wavenumbers.
3.4. NMR Relaxometry Investigations of the Composite-Cement Materials
The development in the microstructure of composite-cement materials during hydra-
tion was evaluated based on the surface area measured by
1
H NMR relaxometry investiga-
tions. The echo trains measured by CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence
of the simple cement paste and composite-cement pastes performed at 28 days after their
preparation are shown in Figure 6a. The full decay of the CPMG normalized echoes of the
expired cement and composite-expired cement materials performed at 28 days after their
preparation are shown in Figure 6b.

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sequenceofthesimplecementpasteandcomposite‐cementpastesperformedat28days
aftertheirpreparationareshowninFigure6a.ThefulldecayoftheCPMGnormalized
echoesoftheexpiredcementandcomposite‐expiredcementmaterialsperformedat28
daysaftertheirpreparationareshowninFigure6b.


Figure6.NormalizedCPMGechodecayof(a)cementandcomposite‐cementmaterialsand(b)
expiredcementandcomposite‐expiredcementmaterialsperformedat28daysaftertheirprepara‐
tion.
Thepossibilityofthecharacterizationofwaterreservoirsandtheevolutionofpores
insidethesamplescanbebetteridentifiedusinganumericalLaplaceinversionforCPMG
relaxationcurves.Therelaxationtimedistributions(T2)obtainedbytheinverseLaplace
algorithmofthesimplecementandcomposite‐cementmaterialsperformedat3,7,14,
and28daysaftertheirpreparationarepresentedinFigure7.

Figure 6.
Normalized CPMG echo decay of (
a
) cement and composite-cement materials and
(
b
) expired cement and composite-expired cement materials performed at 28 days after their preparation.
The possibility ofthe characterizationof water reservoirs and the evolution of pores
inside the samples can be better identified using a numerical Laplace inversion for CPMG
relaxation curves. The relaxation time distributions (T
2
) obtained by the inverse Laplace
algorithm of the simple cement and composite-cement materials performed at 3, 7, 14, and
28 days after their preparation are presented in Figure 7.
In the relaxation time, distributions can be identified using four distinct water reser-
voirs inside the samples. The first peak (from the left) of the larger area can be assigned
to the water bounded in the material components. The peaks situated at values of the
higher relaxation time (from left to right) are attributed to the water in small pores, water
in medium pores, and water in large pores [
29
]. For all investigated samples, the largest
amounts of water are evidenced in the water bounded in the material components and few
quantities of water correspond to the small, medium, and large pores. The small pores are
responsible for the strong interactions of the cement material with liquid and gas water [
30
].
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sequenceofthesimplecementpasteandcomposite‐cementpastesperformedat28days
aftertheirpreparationareshowninFigure6a.ThefulldecayoftheCPMGnormalized
echoesoftheexpiredcementandcomposite‐expiredcementmaterialsperformedat28
daysaftertheirpreparationareshowninFigure6b.


Figure6.NormalizedCPMGechodecayof(a)cementandcomposite‐cementmaterialsand(b)
expiredcementandcomposite‐expiredcementmaterialsperformedat28daysaftertheirprepara‐
tion.
Thepossibilityofthecharacterizationofwaterreservoirsandtheevolutionofpores
insidethesamplescanbebetteridentifiedusinganumericalLaplaceinversionforCPMG
relaxationcurves.Therelaxationtimedistributions(T2)obtainedbytheinverseLaplace
algorithmofthesimplecementandcomposite‐cementmaterialsperformedat3,7,14,
and28daysaftertheirpreparationarepresentedinFigure7.

Figure 7. Cont.

Materials 2023,16, 2398 11 of 16
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Figure7.Therelaxationtimedistributionsofcementandcomposite‐cementmaterials(CL:Lead
composite‐cement,CI:Ironcomposite‐cement,CA:Ashcomposite‐cement,CC:Cashironcompo‐
site‐cement)performedat(a)3days,(b)7days,(c)14days,and(d)28daysaftertheirpreparation.
Intherelaxationtime,distributionscanbeidentifiedusingfourdistinctwaterres‐
ervoirsinsidethesamples.Thefirstpeak(fromtheleft)ofthelargerareacanbeassigned
tothewaterboundedinthematerialcomponents.Thepeakssituatedatvaluesofthe
higherrelaxationtime(fromlefttoright)areattributedtothewaterinsmallpores,water
inmediumpores,andwaterinlargepores[29].Forallinvestigatedsamples,thelargest
amountsofwaterareevidencedinthewaterboundedinthematerialcomponentsand
fewquantitiesofwatercorrespondtothesmall,medium,andlargepores.Thesmall
poresareresponsibleforthestronginteractionsofthecementmaterialwithliquidand
gaswater[30].
InthecaseofCIcomposite‐cementmaterialscontainingironpowder,theevolution
ofthewaterpeakinlargeporeswaschangedduring28daysaftertheirpreparation.The
areaofthispeakremainsconstantupto7days,whereasitdisappearssuddenlyfor≤14
daysofhydration.
Figure 7.
The relaxation time distributions of cement and composite-cement materials (CL: Lead
composite-cement, CI: Iron composite-cement, CA: Ash composite-cement, CC: Cash iron composite-
cement) performed at (a) 3 days, (b) 7 days, (c) 14 days, and (d) 28 days after their preparation.
In the case of CI composite-cement materials containing iron powder, the evolution
of the water peak in large pores was changed during 28 days after their preparation.
The area of this peak remains constant up to 7 days, whereasit disappears suddenly for
≤14 days of hydration.
For the CC material, the water in the large pores is exhausted beyond 3 days. By
adding lead powder in the CL paste, the water in the large pores was not evidenced after
3 days. Thereafter, a continuous increase arises in the intensity of this peak until the 28th day.
The peak of water in large pores disappears in the third day and after 28 days, whereas in
the interval between the 7th and 14th day, the peak increased constantly for the CA material.
In all samples, the position of the largest peak classified as water bounded in the
material components shifts to a smaller T
2
relaxation time after 28 days of hydration
comparatively with the simple cement paste.
After 28 days, the CI, CA, and CC composite-cement materials are not evidenced
in water reservoirs of large pores. This evolution suggests that the water is consumed
preferentially from the large pores during the hydration of the cement mixture. The position
of the maximum of the main peak shifts to shorter T2 relaxation times for the CI and CA
samples and moves to longer T2 relaxation times for the CC paste. The differences in the

Materials 2023,16, 2398 12 of 16
position of the water bounded in the cement materials can be attributed to the mobility
(dynamics) of the material components [19]. The CL sample has less dynamic behavior.
The relaxation time distributions of the expired cement material and composite-
expired cement materials performed at 3, 7, 14, and 28 days after their preparation are
shown in Figure 8.
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FortheCCmaterial,thewaterinthelargeporesisexhaustedbeyond3days.By
addingleadpowderintheCLpaste,thewaterinthelargeporeswasnotevidencedafter
3days.Thereafter,acontinuousincreasearisesintheintensityofthispeakuntilthe28th
day.
Thepeakofwaterinlargeporesdisappearsinthethirddayandafter28days,
whereasintheintervalbetweenthe7thand14thday,thepeakincreasedconstantlyfor
theCAmaterial.
Inallsamples,thepositionofthelargestpeakclassifiedaswaterboundedinthe
materialcomponentsshiftstoasmallerT2relaxationtimeafter28daysofhydration
comparativelywiththesimplecementpaste.
After28days,theCI,CA,andCCcomposite‐cementmaterialsarenotevidencedin
waterreservoirsoflargepores.Thisevolutionsuggeststhatthewaterisconsumedpref‐
erentiallyfromthelargeporesduringthehydrationofthecementmixture.Theposition
ofthemaximumofthemainpeakshiftstoshorterT2relaxationtimesfortheCIandCA
samplesandmovestolongerT2relaxationtimesfortheCCpaste.Thedifferencesinthe
positionofthewaterboundedinthecementmaterialscanbeattributedtothemobility
(dynamics)ofthematerialcomponents[19].TheCLsamplehaslessdynamicbehavior.
Therelaxationtimedistributionsoftheexpiredcementmaterialandcompo‐
site‐expiredcementmaterialsperformedat3,7,14,and28daysaftertheirpreparation
areshowninFigure8.


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Figure8.Therelaxationtimedistributionsofexpiredcementandcomposite‐expiredcementma‐
terials(ECL:Leadcomposite‐expiredcement,ECI:Ironcomposite‐expiredcement,ECA:Ash
composite‐expiredcement,ECC:Cashironcomposite‐expiredcement)at(a)3days,(b)7days,(c)
14days,and(d)28daysaftertheirpreparation.
AsmallamountofporewaterwaspresentintheECIsampleafter14days,andat28
days,thepeakcorrespondingtothewaterinthelargeporeswasalmostundetectable.
Thepeaksassignedtotheboundedwaterandwaterinthesmallandmediumporeswere
increasedcontinuouslyduringtheobservedperiod.Atrendofshiftingthesepeaksto‐
wardhigherrelaxationtimeswasalsoobserved,indicatinganincreaseinthemobilityof
thecomponentofmaterials.
ForthesampleECC,themajormodificationsintheshapeandtheintensityofthe
waterpeakswereevidencedafter14and28days.At14daysafterthepreparation,the
peaksattributedtotheboundedwaterandwaterinthesmallporeswereoverlapped.Up
to28days,theboundedwaterincreasedagainandadecreaseinthesignalsduetothe
waterinporeswasclearlyobserved.
Upto14days,thepeakscorrespondingtothewaterinthesmall,medium,andlarge
poresintheECLsamplecontinuedtoincreaseandanoverlapoftwopeaks,namely,
waterinthesmallandmediumpores,wasclearlyobservedat28days.
ThepeakattributedtothewaterinsmallporesrisesinsignalandshiftstoalargerT2
relaxationtime,whilethewaterreservoirsinmediumandlargeporesweredecreased
abruptlyuntil28daysinthecaseoftheECAsample.
After28days,thechemicallyboundwaterclearlyexhibitedlongerT2relaxation
timevaluesthantheexpiredcementsample.
Inconclusion,thewaterreservoirsinsidethecomposite‐cementmaterialsaremainly
duetotheconsumptionfromlargeporesinthefirststage.Theprocesscanbeinterpreted
asthemigrationofwaterfromlargetosmallpores.Thereafter,withtheaccelerationof
thehydrationprocess,thewaterinmediumandsmallporescanbequicklyusedforthe
hydrationreaction.Fortheexpiredcementmaterial,thewaterinlargeporeswasevi‐
Figure 8.
The relaxation time distributions of expired cement and composite-expired cement materials
(ECL: Lead composite-expired cement, ECI: Iron composite-expired cement, ECA: Ash composite-
expired cement, ECC: Cash iron composite-expired cement) at (
a
) 3 days, (
b
) 7 days, (
c
) 14 days, and
(d) 28 days after their preparation.

Materials 2023,16, 2398 13 of 16
A small amount of pore water was present in the ECI sample after 14 days, and at
28 days, the peak corresponding to the water in the large pores was almost undetectable.
The peaks assigned to the bounded water and water in the small and medium pores were
increased continuously during the observed period. A trend of shifting these peaks toward
higher relaxation times was also observed, indicating an increase in the mobility of the
component of materials.
For the sample ECC, the major modifications in the shape and the intensity of the
water peaks were evidenced after 14 and 28 days. At 14 days after the preparation, the
peaks attributed to the bounded water and water in the small pores were overlapped. Up
to 28 days, the bounded water increased again and a decrease in the signals due to the
water in pores was clearly observed.
Up to 14 days, the peaks corresponding to the water in the small, medium, and large
pores in the ECL sample continued to increase and an overlap of two peaks, namely, water
in the small and medium pores, was clearly observed at 28 days.
The peak attributed to the water in small pores rises in signal and shifts to a larger
T
2
relaxation time, while the water reservoirs in medium and large pores were decreased
abruptly until 28 days in the case of the ECA sample.
After 28 days, the chemically bound water clearly exhibited longer T
2
relaxation time
values than the expired cement sample.
In conclusion, the water reservoirs inside the composite-cement materials are mainly due
to the consumption from large pores in the first stage. The process can be interpreted as the
migration of water from large to small pores. Thereafter, with the acceleration of the hydration
process, the water in medium and small pores can be quickly used for the hydration reaction.
For the expired cement material, the water in large pores was evidenced in all samples up to
14 days. For the ECC and ECL samples at 28 days, the water was redistributed in the signal
with a shorter relaxation time corresponding to the small and medium pores.
3.5. Vickers Hardness Measurements of the Composite-Cement Materials
To investigate the effect of waste powders on the Vickers hardness values of cement-
based materials with recycled powder, ten different mixtures (five samples consisting of
validated Portland cement and five samples used for the expired cement) were designed.
The influence of composites nature on Vickers hardness values distribution of
composite-cement materials for the mixture with Portland cement and expired cement
is shown in Figure 9.
In Figure 9a, it can be seen that the addition of L, A, C, and I composites in the
validated cement material increases the Vickers hardness values. The maximum hardness
values were attained for the CC and CI composite-cement materials. The IR data show
that the C and I composites have a higher content of water in their structure. The water
bounded to C and I components in the composite-cement materials lead to the difference
in the mobility behavior, which increases the Vickers hardness values.
For the expired cement, the presence of ash-doped composite in cement materials
yields a smaller hardness value than the undoped sample (Figure 9b). The rigidity of water
bounded to material components and less mobility samples can be correlated with the
smaller value of the Vickers hardness, in accordance with NMR investigations. A good
correlation between the experimental hardness value and the mobility behavior of the
components in the composite-cement materials predicted in NMR data was observed.
Briefly, the energy consumption and cost of nanocomposites preparation are lower
than the cement production. The gas emissions of nanocomposite synthesis are significantly
lower than the cement preparation, showing that the recycled nanocomposites are eco-
friendly materials. The blending of nanocomposites in cement as supplementary cementing
material and as a substitute of cement minimizes the consumption of cement and the global
warming. The reprocessing of the waste into nanocomposites is beneficial for solving
the problems of environmental pollution and for reducing the consumption of natural

Materials 2023,16, 2398 14 of 16
resources. Based on the results obtained, we can recommend the prepared composites as a
replacement material for one part of Portland cement.
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dencedinallsamplesupto14days.FortheECCandECLsamplesat28days,thewater
wasredistributedinthesignalwithashorterrelaxationtimecorrespondingtothesmall
andmediumpores.
3.5.VickersHardnessMeasurementsoftheComposite‐CementMaterials
ToinvestigatetheeffectofwastepowdersontheVickershardnessvaluesofce‐
ment‐basedmaterialswithrecycledpowder,tendifferentmixtures(fivesamplescon‐
sistingofvalidatedPortlandcementandfivesamplesusedfortheexpiredcement)were
designed.
TheinfluenceofcompositesnatureonVickershardnessvaluesdistributionofcom‐
posite‐cementmaterialsforthemixturewithPortlandcementandexpiredcementis
showninFigure9.


Figure9.InfluenceofcompositesnatureontheVickershardnessvaluesdistributionofcompo‐
site‐cementmaterialsforthemixturewith(a)Portlandcementand(b)expiredcement,themeas‐
urementswereappliedafter28days.
InFigure9a,itcanbeseenthattheadditionofL,A,C,andIcompositesinthevali‐
datedcementmaterialincreasestheVickershardnessvalues.Themaximumhardness
valueswereattainedfortheCCandCIcomposite‐cementmaterials.TheIRdatashow
thattheCandIcompositeshaveahighercontentofwaterintheirstructure.Thewater
boundedtoCandIcomponentsinthecomposite‐cementmaterialsleadtothedifference
inthemobilitybehavior,whichincreasestheVickershardnessvalues.
Fortheexpiredcement,thepresenceofash‐dopedcompositeincementmaterials
yieldsasmallerhardnessvaluethantheundopedsample(Figure9b).Therigidityof
waterboundedtomaterialcomponentsandlessmobilitysamplescanbecorrelatedwith
Figure 9.
Influence of composites nature on the Vickers hardness values distribution of composite-
cement materials for the mixture with (
a
) Portland cement and (
b
) expired cement, the measurements
were applied after 28 days.
4. Conclusions
In this study, four composites were prepared and tested as a substituent of cement
material. The structure of composite-cement materials was characterized by the analysis of
IR and NMR spectra. The mechanical properties of these materials were determined by
distributions of Vickers hardness.
XRD data evidence the presence of vitroceramic structure with diffraction peaks
characteristic of the varied crystalline phases of the metallic ions. IR data show that
the silicate network connectivity degree was decreased via doping. By doping of the
expired cement, the IR band was enriched in all samples due to the formation of C-S-H.
For the validated cement, the substitution of 2.5 weight% of iron produces an increase
in this band. The formation of C-S-H is accompanied with a finer microstructure and
performance of strength.
The four measured T2 peaks were evidenced in the NMR relaxometry, which consisted
of four water reservoirs in the composite-cement materials. The Vickers hardness values
were used as an indicator of the changes in the local mechanical properties of the composite-
cement materials. The distribution and prediction models of Vickers hardness values
were explored and correlated with the water bounded to the material components in
the composite-cement materials. Based on the findings from this paper, the following
conclusions can be noted: (i) The Vickers harness increases with the addition of 2.5 weight%
metal composite (lead, iron, or cash iron powders) in the cement material, and (ii) for the
ash-doped composite, the Vickers hardness value was lower at the expired cement.

Materials 2023,16, 2398 15 of 16
Author Contributions:
Conceptualization and validation, D.L.M.; writing—original draft prepara-
tion, R.R.; writing—review and editing, S.R.; supervision, D.L.M.; investigation, R.C. All authors
have read and agreed to the published version of the manuscript.
Funding: The APC was funded by Technical University of Cluj-Napoca 57/2021.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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