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Citation: Wang, G.; Ye, Z.; Sun, T.; Mo,
Z.; Wang, Z.; Ouyang, G.; He, J.; Deng,
Y. Comparison of Curing Conditions
on Physical Properties, Mechanical
Strength Development, and Pore
Structures of Phosphogypsum-Based
Cold-Bonded Aggregates. Materials
2024,17, 4971. https://doi.org/
10.3390/ma17204971
Academic Editor: Carlos Leiva
Received: 10 September 2024
Revised: 3 October 2024
Accepted: 9 October 2024
Published: 11 October 2024
Copyright: © 2024 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
Comparison of Curing Conditions on Physical Properties,
Mechanical Strength Development, and Pore Structures of
Phosphogypsum-Based Cold-Bonded Aggregates
Guiming Wang 1, Zhiyi Ye 2, Tao Sun 3,4,5,* , Zhenlin Mo 6, Ziyan Wang 1, Gaoshang Ouyang 1, Juntu He 1
and Yihua Deng 1
1School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China;
guimingw@hotmail.com (G.W.); zy_w@whut.edu.cn (Z.W.); oygs@whut.edu.cn (G.O.);
hejt0813@163.com (J.H.); 18973455749@163.com (Y.D.)
2International School of Materials Science and Engineering, Wuhan University of Technology,
Wuhan 430070, China; yezhiyi@whut.edu.cn
3State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology,
Wuhan 430070, China
4Wuhan University of Technology Advanced Engineering Technology Research Institute of Zhongshan City,
Zhongshan 528437, China
5Marine Building Materials and Civil Engineering Center, Science and Education Innovation Park of Wuhan
University of Technology in Sanya, Sanya 572000, China
6China Southwest Geotechnical Investigation and Design Institute Corporation Limited,
Chengdu 610052, China; 10195358@cscec.com
*Correspondence: sunt@whut.edu.cn; Tel.: +86-180-6411-1505
Abstract: This study compared the physical properties and mechanical strength development of
PCBAs with water, sealed, standard, and open ambient air curing over 28 days to find a suitable
curing method for the production of phosphogypsum-based cold-bonded aggregates. The types and
relative amounts of hydration products, microstructural morphology and pore structure parameters
were characterized utilizing XRD, TGA, FTIR, SEM and nitrogen adsorption methods. According
to the results, water curing leads to rapid increases in single aggregate strength, reaching 5.26 MPa
at 7 d. The standard curing condition improved the 28 d mechanical strength of the aggregates by
19.3% over others by promoting the generation of hydration products and the transformation of the
C-S-H gel to a higher degree of polymerization and by optimizing the pore structure. Further, PCBAs
achieved an excellent solidification of phosphorus impurities under all four curing conditions. This
work provides significant guidance for selecting an optimized PCBA curing method for industrial
production.
Keywords: phosphogypsum-based cold-bonded aggregates; curing condition; humidity; pore
structure; specific surface area
1. Introduction
Phosphogypsum-based cold-bonded aggregates (PCBAs) are mainly produced by
the hydration reaction of phosphogypsum (PG, defined as a solid waste produced in the
process of wet production of phosphoric acid) and ground granulated blast-furnace slag
(GGBS, defined as a by-product with pozzolanic activity) activated by alkali-activators
to generate hydration products, which encapsulate and cement the excessive PG to form
cementite and produce strength [
1
,
2
]. Compared with other solid waste-based cold-bonded
aggregates, PCBAs have the advantages of low production cost, a simple preparation
process and broader application prospects [
3
]. The use of PCBAs can reduce the mining of
crushed rocks, which reduces the occupation of land by PG, and the harmful impurities in
PG are cured, which reduces the destruction of the ecological environment [
4
]. Since then,
Materials 2024,17, 4971. https://doi.org/10.3390/ma17204971 https://www.mdpi.com/journal/materials
Materials 2024,17, 4971 2 of 21
some studies have increased the PG doping to 45% or even higher to be used as road base
materials, pre-products and so on, and it has been more widely used [5].
Concrete is the world’s largest volume of materials used in which the aggregate
occupies up to 70% of the volume of concrete. PG can be prepared as a cold-bonded
aggregate to prepare concrete with low production cost, which can reduce the use of
natural coarse aggregates [
6
,
7
]. In a previous study, the preparation of PG into cold-bonded
aggregates for the preparation of phosphogypsum-based concrete provided a feasible
way to consume large amounts of PG [
2
]. The results of previous studies have shown
that the 7-day compressive strength of the high-content phosphogypsum-based concretes
reached 9.3–19.4 MPa, which was about 39.6% to 59.2% of the 28 d compressive strength [
7
].
The PCBA and concrete prepared therefrom showed great solidification capability for
phosphorous and heavy metals [1,2,7].
The cold-bond function relies on hydration of GGBS activated by alkali-activators
and sulfate to build cementitious properties [
8
–
11
]. Further, structures of hardened PCBAs
are established by wrapping excessive PG particles with sufficient hydrates of ettringite
and C-S-H gel, thus offering reliable strength [
12
,
13
]. However, due to the relatively low
content of cementitious components and interference of impurity ions, PCBAs have poor
early hydration properties with slow strength evolution, which shows prominent curing
regime dependence, according to our previous research [
1
,
2
,
7
]. Due to the significant
increase in the amount of PG, the hydration products become less in comparison, causing a
looser structure, and the hydration reaction process in the gelling material will be more
significantly affected by external conditions [14].
A notable influencing factor is the ambient curing moisture and improper curing meth-
ods, such as insufficient water, which are disadvantages to the hydration progress. Further,
premature exposure to air (CO
2
) leads to the decomposition of the already formed ettrin-
gite [
15
–
17
]. Thereafter, a comparative study on curing conditions affecting PCBA property
evolution is desired so as to guide combined curing method selection for optimizing
PCBA production.
PCBAs successfully achieve the large-scale consumption of PG and make contributions
to alleviating the shortage of natural aggregate. Compared with conventional sintered
aggregates, the production and employment of PCBAs characterize low-cost, low-CO
2
emission with far less energy demand. Moreover, unlike natural aggregates whose proper-
ties remain constant, the structure and properties of PCBAs change gradually as hydration
proceeds. Ettringite is a primary hydration product of PCBAs, whose production and
crystal growth are affected by internal factors such as ion concentration in pore solution
(i.e., sulfate, aluminates, alkaline ions, etc.), as well as the external environment, such as
humidity [
9
,
18
,
19
]. Moreover, it has been reported that the hydration process of cementi-
tious materials varies under different curing methods [
20
–
22
] in which water is present in
different forms in the material before and after hardening [
8
,
23
]. During the early stage of
the hydration reaction, multiple ions need to enter the aqueous solution in order to react;
then, water is physically and chemically bonded to the hydration products of ettringite and
C-S-H gel [
9
,
12
,
24
]. The excess water exist in the form of free water in the capillaries [
25
].
As far as PCBAs are concerned, the ratio of water-to-powder during granulation is limited
to 0.14–0.16, which indicates a very low water content level. However, the water content
in the aggregates (including the internal water and the water in the environment) plays a
significant role in strength development during hydration.
Another important aspect of PCBAs is the pore structure as a result of the pelletiza-
tion process, powder particle size, water evolution and spatial accumulation of hydration
products, which is also influenced by the curing conditions at the initial stage of hydra-
tion [
20
,
26
]. It is believed that the internal pore structure is responsible for the macroscopic
properties and mechanical characteristics of hardened materials [
11
]. As an example, pores
can act as channels of moisture transport and CO
2
diffusion, and the microstructure evolu-
tion due to hydrate accumulation and carbonation can also affect pore structure, thereby
impacting the macro performance [15].
Materials 2024,17, 4971 3 of 21
For the curing condition, Tajra [
27
] et al. suggested that relative humidity significantly
affects the mechanical strength of aggregates, and that core–shell aggregates produce the
fastest strength development at 99% RH. Manikandan et al. [
19
] carried out a series of
studies on the modes of maintenance of aggregates and investigated the rate of hydration
reaction of the aggregates and found that both autoclave and steam curing significantly
reduced the time of curing. The mechanical strength of lightweight aggregates cured in
steam for 10 h can increase rapidly for a short period of time up to more than 85% of
the strength of 28 d water curing. It can be concluded that the effect of humidity on the
mechanical properties of cold-bonded aggregates is significant. Particularly, industrial-
scale aggregate production often requires lower cost and simple process, with the same
for storage conditions. From a comprehensive consideration of convenience and cost, the
optional curing methods include sealed curing, water curing, standard curing and natural
curing. Sealed curing completely isolates the aggregate from the surrounding environment
but may be difficult to implement. Curing with water can facilitate strength growth but
it may cause potential pollution of the water because of the phosphorus impurities [
28
].
For standard curing, it is necessary to maintain a strict temperature and humidity control
environment. Natural curing is the simplest and most economical curing option but the
low alkali content easily leads to carbonation, which is unpleasant for stable ettringite
growth, despite there some research indicating that using accelerated CO
2
curing for cold-
bonded aggregates can sequestrate 3.5–4.1 wt.% CO
2
and the produced environmentally
friendly by-products [
29
]. Furthermore, consideration should be given to the changes in
the structure and properties of PCBAs under natural curing conditions.
Therefore, this paper focused on the effects of four common curing conditions, includ-
ing water, sealed, natural and standard curing on the macroscopic physical and mechanical
properties of PCBAs, as well as the microstructural evolution. SEM, XRD, TGA, FTIR
and low-temperature gas adsorption methods were used to characterize the hydration
products and pore structure of PCBAs. Furthermore, the phosphorus leaching of PCBAs
was assessed since PG possessed potential phosphorus leaching pollution [
28
]. It is the first
time that a large amount of PCBA is prepared using disk pelletizing technology and placed
in four different curing environments to study the relationship between its macroscopic
mechanical properties and microstructure. The findings of this are expected to solve the
problem of the lack of suitable curing conditions and curing time for industrial production
and provide an economic guarantee for industrial production.
2. Raw Materials and Methods
The experimental procedures of this study are shown in Figure 1. In this study, PCBA
was prepared and placed in four different curing conditions to evaluate the physical and
mechanical properties.
Materials2024,17,xFORPEERREVIEW4of23
Figure1.Experimentalproceduresinthisstudy.
2.1.RawMaterials
Freshoriginalphosphogypsumwasdriedat40°Cformorethan24handthensieved
throughasievewith0.6mmsquarehole,obtainingPGpowderwithanappropriatepar-
ticlesizeforfurtherhydration(D50=33.87µm).TheordinaryPortlandcementandGGBS
wereproducedbyHuaxinCementCo.,Ltd(Huangshi,China)andHubeiXinyeSteelCo.,
Ltd(Huangshi,China).Thechemicalcompositionofthreerawmaterialswasdetermined
byXRF,andthephysicalperformancesalsowereevaluatedinTab le1andFigure2.Phos-
phogypsumismainlycomposedof81.59wt.%gypsumperSO3content,smallamountof
quarandphosphorus.
Figure2.Particlesizedistributionofphosphogypsum,ordinaryPortlandcementandgroundgran-
ulatedblast-furnaceslag.
AccordingtoGB/T18046-2017[30],thehydrationreactivityofGGBSwasevaluated
withanactualactivityindex,aswellasmasscoefficientcalculatedasEquation(1)[2].The
activityindexofslagreached86%and99%at7dand28d,respectively,exceedingthe
Figure 1. Experimental procedures in this study.
Materials 2024,17, 4971 4 of 21
2.1. Raw Materials
Fresh original phosphogypsum was dried at 40
◦
C for more than 24 h and then sieved
through a sieve with 0.6 mm square hole, obtaining PG powder with an appropriate
particle size for further hydration (D
50
= 33.87
µ
m). The ordinary Portland cement and
GGBS were produced by Huaxin Cement Co., Ltd. (Huangshi, China) and Hubei Xinye
Steel Co., Ltd. (Huangshi, China). The chemical composition of three raw materials was
determined by XRF, and the physical performances also were evaluated in Table 1and
Figure 2. Phosphogypsum is mainly composed of 81.59 wt.% gypsum per SO
3
content,
small amount of quartz and phosphorus.
Table 1. Chemical composition and physical properties of phosphogypsum, ordinary Portland
cement and ground granulated blast-furnace slag (wt.%).
Compositions (wt%) PG GGBS Cement
SiO28.34 29.57 21.75
Al2O31.27 14.02 6.45
Fe2O30.58 1.39 3.24
CaO 26.66 41.27 58.02
MgO 0.11 8.24 2.25
SO337.95 2.48 2.47
TiO20.13 0.95 0.51
P2O50.80 0.07 0.25
MnO / 0.55 /
F 1.30 / /
LOI 21.70 0.09 3.80
Physical properties
Specific gravity 2.28 2.84 2.98
Specific surface area (m2/g) 0.50 1.38 1.79
D10 (µm) 6.09 1.52 1.18
D50 (µm) 33.87 10.87 10.59
D90 (µm) 100.57 27.85 33.78
Materials2024,17,xFORPEERREVIEW4of23
Figure1.Experimentalproceduresinthisstudy.
2.1.RawMaterials
Freshoriginalphosphogypsumwasdriedat40°Cformorethan24handthensieved
throughasievewith0.6mmsquarehole,obtainingPGpowderwithanappropriatepar-
ticlesizeforfurtherhydration(D50=33.87µm).TheordinaryPortlandcementandGGBS
wereproducedbyHuaxinCementCo.,Ltd(Huangshi,China)andHubeiXinyeSteelCo.,
Ltd(Huangshi,China).Thechemicalcompositionofthreerawmaterialswasdetermined
byXRF,andthephysicalperformancesalsowereevaluatedinTable1andFigure2.Phos-
phogypsumismainlycomposedof81.59wt.%gypsumperSO3content,smallamountof
quarandphosphorus.
Figure2.Particlesizedistributionofphosphogypsum,ordinaryPortlandcementandgroundgran-
ulatedblast-furnaceslag.
AccordingtoGB/T18046-2017[30],thehydrationreactivityofGGBSwasevaluated
withanactualactivityindex,aswellasmasscoefficientcalculatedasEquation(1)[2].The
activityindexofslagreached86%and99%at7dand28d,respectively,exceedingthe
Figure 2. Particle size distribution of phosphogypsum, ordinary Portland cement and ground
granulated blast-furnace slag.
According to GB/T 18046-2017 [
30
], the hydration reactivity of GGBS was evaluated
with an actual activity index, as well as mass coefficient calculated as Equation (1) [
2
]. The
Materials 2024,17, 4971 5 of 21
activity index of slag reached 86% and 99% at 7 d and 28 d, respectively, exceeding the
requirements of the national standard of 70% and 95%. Additionally, the calculated Kwith
value of 2.04 means that GGBS used in this study possessed high activity and contributed
to the persistent hydration of PCBAs.
K=mCaO +mMgO +mAl2O3
mSiO2+mMnO +mTiO2
(1)
2.2. Mix Proportion and Preparation Process
There has been some discussion [
2
] regarding PCBAs with PG proportions between
60 wt.% and 80 wt.% exhibiting satisfactory macro performance, in which the bulk density,
water absorption, compression strength and softening coefficient meet the specifications of
GB/T 17431.1 [
31
]. As a result of the significant reduction in degree of hydration, physical
and mechanical performance deteriorates rapidly with the further incorporation of PG. To
ensure the strength development and durability of PCBAs in this study, the PG, GGBS and
cement content are restricted to 80 wt.%, 15 wt.% and 5 wt.%.
In this study, PCBAs were prepared using the stirring granulation method, divided
into two steps. Firstly, the process of “nucleation”, which affected the whole granulation
process as well as the performances of aggregate, was the most important step. Another
characteristic was the growth of aggregate balls, during which the aggregate had formed
preliminarily. Next, the coated powder and the initial PCBA balls were fused into one
as the aggregate grew under the influence of adhesive water. The whole granulation
process was carried out in a disk granulator with an outer diameter of 50 cm, an inner
diameter of 49 cm, a depth of 16 cm, a fixed inclination of 45
◦
and a speed of 43 r/min and
kept constant.
Phosphogypsum, ground granulated blast-furnace slag and cement were weighed
according to 80:15:5 mass ratio in the disk granulator and mixed evenly. The preparation of
PCBAs is divided into two steps:
1.
Seeding. Take 200 g of the mixed powder and pour it into the uniformly rotating disk
granulator; use the atomizing spray can to spray a small amount of water to make
the powder in the disk slightly wet. After all the powder is wet, continue to add the
power mixture; the wet powder in the disk will agglomerate to form the seed pellets.
Alternate steps of adding powder and sprinkling water. When the seed pellets are still
fragile, a rolling and curtain-dropping movement will make them constantly collide
with the force squeeze to become dense.
2.
Size enlargement. After the formation of seeds, a small amount of atomized water
is sprayed into the disk to moisten the surface of these microspheres. Then, a small
amount of powder is evenly sprinkled into the disk to make the newly added powder
evenly adhere to the surface of seeds, and the powder is fused with the aggregate
microspheres under the effect of water adhesion. Then, under the action of gravity,
the pellets fall along the arc of the disk surface to the bottom of the pelletizing disk.
During the movement, the pellets are continuously compacted from the inside out
by the compression of the inner wall of the pelletizing disk and by their own gravity.
The process is repeated several times until the pellet diameter increases to 8 mm. The
size is controlled by sieving.
The amount of water used for granulation is 14–15 wt.% of the powder mass. The
formed fresh pellets were transferred to sealed containers for 1 d at room temperature,
and then placed in natural condition, sealed condition, water immersion condition and
standard condition.
The characteristic parameters of four curing conditions are listed in Table 2. Natural
curing was carried out with the aggregate in an ambient air at 20
±
2
◦
C indoors, named
P8N. Water curing was the curing of the aggregate by immersing it in water at
20 ±2◦C
,
named P8W. Seal curing was the plastic wrapped against air and curing at 20
±
2
◦
C,
named P8S. Standard curing was carried out in a permeable mesh bag and placed in a
Materials 2024,17, 4971 6 of 21
standard curing room at a temperature of 20
±
2
◦
C and a relative humidity of 90% or more,
named P8M. In this paper, the aggregate was cured in a permeable mesh bag and placed in
a standard curing room. In order to ensure the same and constant temperature, we used air
conditioning to control the temperature.
Table 2. Characteristic parameters of the four curing conditions.
Sample Label Curing Condition Relative Humidity Temperature
P8W Water curing 100% 20 ±1◦C
P8S Sealed curing <5% 20 ±2◦C
P8N Natural curing >50% 20 ±2◦C
P8M Standard (moist) curing >90% 20 ±2◦C
2.3. Testing Methods
The bulk density, water absorption, cylinder compressive strength and softening
coefficient of the aggregates were tested according to national standard GB/T 17431.2 [
32
].
It was recommended that PCBAs should be dried at 45
◦
C before testing in order to attain a
constant weight. The testing method of cylinder compressive strength is shown in Figure 3a.
Materials2024,17,xFORPEERREVIEW7of23
Figure3.Testi ngmethodsofmechanicalstrength:(a)cylindercompressivestrength;(b)singlepellet
compressivestrength.
2.4.MicrostructureTestingScheme
Priortothemicrostructureandphasetests,PCBAscuredfor28dwerecrushedtoa
sizelessthan75µmanddriedundervacuumat40°Cfor24h.Samplepreparationshould
berapidandpreventcarbonation.ThesamplesweremeasuredwithanEmpyreanX-ray
diffractometerwithacoppertargetX-raytubeandoperatedat40mAand40kV.X-ray
paernwascollectedoverarangeof5°to70°.
TheThermogravimetricAnalysis(TGA)spectrarecordedatNeschSTA449F3(Selb,
Germany)weremonitoredinthetemperaturerangeof25to900°Cwithaheatingspeed
of10°C/minundera20mL/minflowofN2.
Amixtureofsamplepowderandpotassiumbromidewaspressedintotabletsata
ratioof1:100usingaFouriertransforminfraredreflection(FTIR)instrument(Nexusspec-
trometer,ThernoNicolet,Madison,WI,USA.)inthemid-infraredregionfrom4000cm−1
to400cm−1.Themicrostructureofthematerialatthemicronlevelcanbeobservedwith
scanningelectronmicroscope(SEM),whichenablesthestructureandmorphologicaldis-
tributionofthecrystalstobedeterminedmoreaccurately.ThemicrostructureofPCBAs
wasexaminedusingaZeissUltraPlusscanningelectronmicroscope(Oberkochen,Ger-
many)operatingatlowvacuumand10–15kVacceleratingvoltage.TheSEMhadareso-
lutionof1.0nmwithamagnificationof12–100,000×.Duetothepoorelectricalconductiv-
ityofPCBAs,thesamplesurfaceswereplatedwithplatinum(Pt)invacuumfor600s
beforethetest.
Gasadsorptionwasacommonmethodfordeterminingthespecificsurfaceareaand
porestructureofporousmaterials[34–36].TheASAP2460specificsurfaceandporevol-
umeanalyzer(ASAP2460,Micromeritics,Atlant a,GA,USA)wasusedforthelow-temper-
aturenitrogenadsorptionexperiments,withaporesizerangeof0.4–200nmandamini-
mumporevolumeof0.0001mL/g.Sampleswerefirstdegassedbyvacuumdryingat40
°Cfor24hinavacuumovenatavacuumlevelof0.06MPa,andthen2.0gofthesample
wastakenasadsorbentwithhigh-puritynitrogenandtheamountofnitrogenadsorbed
at77Katdifferentrelativepressuresp/p0wasmeasured.Theadsorption/desorptioniso-
thermwasploedusingtherelativepressurep/p0asthehorizontalcoordinateandthe
adsorptionamountperunitmassofsampleastheverticalcoordinate.Thespecificsurface
areaandthetotalporevolumeofthesampleweredeterminedbasedontheBETequation
usingtheadsorptionbranchasthedatasource.Theporesizedistributionwascalculated
usingtheBJH(Barre–Joyner–Halenda)modelbasedontheKelvinequationwiththede-
sorptionbranchasthedatasource.
Figure 3. Testing methods of mechanical strength: (a) cylinder compressive strength; (b) single pellet
compressive strength.
The aggregates used in the single aggregate strength test are small spherical aggregates
with a diameter of 8 mm. Single aggregate strength was obtained by a measurement of
the crushing load of aggregates shown in Figure 3b. It was recommended to record the
maximum value of the aggregate when it was damaged by the pressure load as 50 N/s
between parallel plates. The measurement should be repeated for more than twenty
samples, and the average should be taken. According to Equation (2), the strength of a
single aggregate was introduced [33]:
σ=2.8 ×P
π×d2(2)
where dwas the diameter of aggregates (mm), Pwas the broken force when aggregates
were crushed (N).
2.4. Microstructure Testing Scheme
Prior to the microstructure and phase tests, PCBAs cured for 28 d were crushed to a
size less than 75
µ
m and dried under vacuum at 40
◦
C for 24 h. Sample preparation should
Materials 2024,17, 4971 7 of 21
be rapid and prevent carbonation. The samples were measured with an Empyrean X-ray
diffractometer with a copper target X-ray tube and operated at 40 mA and 40 kV. X-ray
pattern was collected over a range of 5◦to 70◦.
The Thermogravimetric Analysis (TGA) spectra recorded at Netzsch STA449F3 (Selb,
Germany) were monitored in the temperature range of 25 to 900
◦
C with a heating speed of
10 ◦C/min under a 20 mL/min flow of N2.
A mixture of sample powder and potassium bromide was pressed into tablets at a
ratio of 1:100 using a Fourier transform infrared reflection (FTIR) instrument (Nexus spec-
trometer, Therno Nicolet, Madison, WI, USA.) in the mid-infrared region from 4000 cm
−1
to
400 cm
−1
. The microstructure of the material at the micron level can be observed with
scanning electron microscope (SEM), which enables the structure and morphological distri-
bution of the crystals to be determined more accurately. The microstructure of PCBAs was
examined using a Zeiss Ultra Plus scanning electron microscope (Oberkochen, Germany)
operating at low vacuum and 10–15 kV accelerating voltage. The SEM had a resolution
of 1.0 nm with a magnification of 12–100,000
×
. Due to the poor electrical conductivity of
PCBAs, the sample surfaces were plated with platinum (Pt) in vacuum for 600 s before
the test.
Gas adsorption was a common method for determining the specific surface area and
pore structure of porous materials [
34
–
36
]. The ASAP 2460 specific surface and pore volume
analyzer (ASAP 2460, Micromeritics, Atlanta, GA, USA) was used for the low-temperature
nitrogen adsorption experiments, with a pore size range of 0.4–200 nm and a minimum
pore volume of 0.0001 mL/g. Samples were first degassed by vacuum drying at 40
◦
C for
24 h in a vacuum oven at a vacuum level of 0.06 MPa, and then 2.0 g of the sample was taken
as adsorbent with high-purity nitrogen and the amount of nitrogen adsorbed at 77 K at
different relative pressures p/p
0
was measured. The adsorption/desorption isotherm was
plotted using the relative pressure p/p
0
as the horizontal coordinate and the adsorption
amount per unit mass of sample as the vertical coordinate. The specific surface area and
the total pore volume of the sample were determined based on the BET equation using the
adsorption branch as the data source. The pore size distribution was calculated using the
BJH (Barrett–Joyner–Halenda) model based on the Kelvin equation with the desorption
branch as the data source.
2.5. Phosphorus Leaching Test
2.5.1. Static Leaching Test
The PCBAs cured for 28 d were vacuum-dried at 45
◦
C for 24 h, about 100 g of
PCBAs was collected and mixed with deionized water at a ratio of 5 L/kg and left at room
temperature (23
±
2
◦
C) for 28 d [
2
]. The soluble phosphorus content of PCBAs and original
PG was measured with an ultraviolet spectrophotometer after color development.
2.5.2. Acetic Acid Leaching Test
The acetic acid leaching test was leached using the horizontal vibration method with
reference to standard HJ 557-2010 [
37
]. The PCBAs and fresh PG were crushed, and a
mixture of 20 L/kg of acetic acid was placed in a conical flask. About 10 g of PCBA particles
were collected after grinding and crushing in the mortar. The leachate was filtered through
a 0.45
µ
m microporous membrane and the phosphorus content was measured with an
ultraviolet spectrophotometer after digestion and color development.
3. Results and Discussion
3.1. Physical Properties
3.1.1. Bulk Density
The bulk density of PCBAs at 7 d, 14 d and 28 d under different curing conditions
is shown in Figure 4. The bulk density of the dried PCBAs prepared by cold bonding
technology is less than 1100 kg/m
3
, which meets the requirement of GB/T 17431.1 that
the density of lightweight aggregates should not exceed 1200 kg/m
3
. Density increases
Materials 2024,17, 4971 8 of 21
with curing age, hydration products increase, densification increases and density increases.
There is no significant difference in bulk density between various curing conditions.
Materials2024,17,xFORPEERREVIEW8of23
2.5.PhosphorusLeachingTest
2.5.1.StaticLeachingTes t
ThePCBAscuredfor28dwerevacuum-driedat45°Cfor24h,about100gofPCBAs
wascollectedandmixedwithdeionizedwaterataratioof5L/kgandleftatroomtem-
perature(23±2°C)for28d[2].ThesolublephosphoruscontentofPCBAsandoriginal
PGwasmeasuredwithanultravioletspectrophotometeraftercolordevelopment.
2.5.2.AceticAcidLeachingTe st
Theaceticacidleachingtestwasleachedusingthehorizontalvibrationmethodwith
referencetostandardHJ557-2010[37].ThePCBAsandfreshPGwerecrushed,andamix-
tureof20L/kgofaceticacidwasplacedinaconicalflask.About10gofPCBAparticles
werecollectedaftergrindingandcrushinginthemortar.Theleachatewasfiltered
througha0.45µmmicroporousmembraneandthephosphoruscontentwasmeasured
withanultravioletspectrophotometerafterdigestionandcolordevelopment.
3.ResultsandDiscussion
3.1.PhysicalProperties
3.1.1.BulkDensity
ThebulkdensityofPCBAsat7d,14dand28dunderdifferentcuringconditionsis
showninFigure4.ThebulkdensityofthedriedPCBAspreparedbycoldbondingtech-
nologyislessthan1100kg/m3,whichmeetstherequirementofGB/T17431.1thattheden-
sityoflightweightaggregatesshouldnotexceed1200kg/m3.Densityincreaseswithcur-
ingage,hydrationproductsincrease,densificationincreasesanddensityincreases.There
isnosignificantdifferenceinbulkdensitybetweenvariouscuringconditions.
Figure4.BulkdensityofPCBAsat7d,14dand28d.
3.1.2.Wat e rAbsorption
Asacriticalindicatoroftheperformanceofmanufacturedaggregates,the1hand24
hwaterabsorptionratesofPCBAsat7d,14dand28dunderthefourcuringconditions
Figure 4. Bulk density of PCBAs at 7 d, 14 d and 28 d.
3.1.2. Water Absorption
As a critical indicator of the performance of manufactured aggregates, the 1 h and 24 h
water absorption rates of PCBAs at 7 d, 14 d and 28 d under the four curing conditions are
evaluated, as Figure 5shows. A declining trend in the 1 h and 24 h water absorption rates is
noticed in all samples with the prolonging of curing age, which even reaches 7.8% and 8.2%
in the PCBAs with sealed curing at 7 d, indicating that the low ambient humidity is not
favorable for hydration hardening and structural development. Furthermore, the 1 h and
24 h water absorption rates are the lowest in the P8S after standard curing for 14 d, at 3.5%
and 4.9%, respectively. In contrast, the other samples display similar levels of 24 h water
absorption. A similar drop in water absorption was observed between 4.2% and 4.7% for
all four PCBAs at 28 d. The lowest water absorption at 7 d in P8N and the highest at 28 d
indicates that carbonation leads to the decomposition of the hydration products [
15
,
38
,
39
],
leading to an increase in the pore structure and an increase in water absorption; both 1 h
and 24 h water absorption is the lowest at 14 d.
Considering the similar absorption behavior of P8W, P8N and P8M at 7 d, it can be
assumed that the standard curing conditions are the most suitable for hydration hardening
and the fastest structural development of the PCBAs. There were no aggregates that
exceeded the 10% limit for 1 h water absorption of aggregates set by GB/T 17431.1. The
water absorption of aggregates gradually decreases with an increase in the curing period as
a result of hydration reactions within the aggregates [
40
]. As the GGBS gradually dissolves,
the generation of hydration products precipitates and optimizes the pores, increasing the
density of the pastes [1].
Materials 2024,17, 4971 9 of 21
Materials2024,17,xFORPEERREVIEW9of23
areevaluated,asFigure5shows.Adecliningtrendinthe1hand24hwaterabsorption
ratesisnoticedinallsampleswiththeprolongingofcuringage,whichevenreaches7.8%
and8.2%inthePCBAswithsealedcuringat7d,indicatingthatthelowambienthumidity
isnotfavorableforhydrationhardeningandstructuraldevelopment.Furthermore,the1
hand24hwaterabsorptionratesarethelowestintheP8Safterstandardcuringfor14d,
at3.5%and4.9%,respectively.Incontrast,theothersamplesdisplaysimilarlevelsof24
hwaterabsorption.Asimilardropinwaterabsorptionwasobservedbetween4.2%and
4.7%forallfourPCBAsat28d.Thelowestwaterabsorptionat7dinP8Nandthehighest
at28dindicatesthatcarbonationleadstothedecompositionofthehydrationproducts
[15,38,39],leadingtoanincreaseintheporestructureandanincreaseinwaterabsorption;
both1hand24hwaterabsorptionisthelowestat14d.
Figure5.Wat erabsorptionratesat7d,14dand28d:(a)1h;(b)24h.
ConsideringthesimilarabsorptionbehaviorofP8W,P8NandP8Mat7d,itcanbe
assumedthatthestandardcuringconditionsarethemostsuitableforhydrationharden-
ingandthefasteststructuraldevelopmentofthePCBAs.Therewerenoaggregatesthat
exceededthe10%limitfor1hwaterabsorptionofaggregatessetbyGB/T17431.1.The
waterabsorptionofaggregatesgraduallydecreaseswithanincreaseinthecuringperiod
asaresultofhydrationreactionswithintheaggregates[40].AstheGGBSgraduallydis-
solves,thegenerationofhydrationproductsprecipitatesandoptimizesthepores,increas-
ingthedensityofthepastes[1].
3.2.MechanicalProperties
3.2.1.CylinderCompressiveStrengthandSofteningCoefficient
AsseeninTable3,thecylindercompressivestrengthofthePCBAsdifferedsignifi-
cantlywithregardtothecuringconditions.ApartfromtheP8Nundernaturalcuring,all
PCBAsmeettherequirementsofGB/T17431.1-2010[31],withcylindercompressive
strengthsexceeding6.5MPa.ThepreparedPCBAsarefarstrongerthanthelimitedvalue
of5–6.5MPainGB/T17431.1–2010[31]andslightlyhigherthanotherindustrialsolid
wastecold-bondaggregates(4.5MPato12.59MPa)thatwerereportedinRefs.[11,41].
Bothstandardcuringandwatercuringareconducivetothedevelopmentoftheir28ddry
cylindercompressivestrength,whichreaches11.3MPainthesampleunderwatercuring.
Asignificantcontributiontoitshydrationreactionmaybeduetothepresenceofsufficient
waterinthecuringenvironment.PCBAswithanaturalcuringconditionhadacylinder
Figure 5. Water absorption rates at 7 d, 14 d and 28 d: (a)1h;(b) 24 h.
3.2. Mechanical Properties
3.2.1. Cylinder Compressive Strength and Softening Coefficient
As seen in Table 3, the cylinder compressive strength of the PCBAs differed signif-
icantly with regard to the curing conditions. Apart from the P8N under natural curing,
all PCBAs meet the requirements of GB/T 17431.1-2010 [
31
], with cylinder compressive
strengths exceeding 6.5 MPa. The prepared PCBAs are far stronger than the limited value
of 5–6.5 MPa in GB/T 17431.1–2010 [
31
] and slightly higher than other industrial solid
waste cold-bond aggregates (4.5 MPa to 12.59 MPa) that were reported in Refs. [
11
,
41
].
Both standard curing and water curing are conducive to the development of their 28 d
dry cylinder compressive strength, which reaches 11.3 MPa in the sample under water
curing. A significant contribution to its hydration reaction may be due to the presence of
sufficient water in the curing environment. PCBAs with a natural curing condition had a
cylinder compressive strength of 7.2 MPa, which was lower than that for PCBAs with a
sealed curing condition of 8.6 MPa, as well as the development in the softening coefficient.
Table 3. Cylinder compressive strength and softening coefficient at 28 d.
Sample Label Cylinder Compressive
Strength (Dry Status)/MPa
Cylinder Compressive
Strength (Saturated Surface
Dry Status)/MPa
Softening Coefficient
P8W 11.3 10.1 0.89
P8S 8.6 7.5 0.87
P8M 10.1 8.8 0.87
P8N 7.2 6.2 0.86
In comparison, the development of PCBA cylinder compressive strength was more
favorable when water curing was used as opposed to standard curing. A sealed curing
condition led to slower PCBA strength development, while a natural curing condition led
to unfavorable results attributed to water in the aggregate evaporating with time, which
affects the hydration process, and some alkaline substances in the system were carbonated.
Materials 2024,17, 4971 10 of 21
3.2.2. Single Aggregate Strength
Figure 6illustrates the single-particle strength of PCBAs. Increasing the curing age had
a positive effect on the single-particle strength of the aggregates. As early as 7 days after the
aggregates were cured, the single-particle strength of the water-cured aggregates developed
the fastest. However, a negligible difference is detected between the single-particle strength
for aggregates under standard and sealed curing conditions. Comparing the strength
growth of PCBAs under the three curing conditions, a relatively high strength of 5.26 MPa
is achieved at 7 d in the water curing, followed by very slow strength development at 14 d
and 28 d. The single-particle strength for the P8S under sealed curing reaches 3.61 MPa
at 7 d, and the 14 d-strength increase ratio is greater than that during the later age. For
the aggregates cured under standard conditions, the single aggregate strength at 7 d and
28 d reaches 3.61 MPa and 6.87 MPa, respectively, indicating that this curing method is
capable of showing an excellent effect on potential strength development compared to
other methods. In contrast, the naturally cured samples possess the lowest strength at 7 d
despite growing until curing for 14 d, displaying a sharp inversion at 28 d.
Materials2024,17,xFORPEERREVIEW10of23
compressivestrengthof7.2MPa,whichwaslowerthanthatforPCBAswithasealedcur-
ingconditionof8.6MPa,aswellasthedevelopmentinthesofteningcoefficient.
Table3.Cylindercompressivestrengthandsofteningcoefficientat28d.
SampleLabelCylinderCompressiveStrength
(DryStatus)/MPa
CylinderCompressiveStrength
(SaturatedSurfaceDryStatus)/MPaSofteningCoefficient
P8W11.310.10.89
P8S8.67.50.87
P8M10.18.80.87
P8N7.26.20.86
Incomparison,thedevelopmentofPCBAcylindercompressivestrengthwasmore
favorablewhenwatercuringwasusedasopposedtostandardcuring.Asealedcuring
conditionledtoslowerPCBAstrengthdevelopment,whileanaturalcuringconditionled
tounfavorableresultsaributedtowaterintheaggregateevaporatingwithtime,which
affectsthehydrationprocess,andsomealkalinesubstancesinthesystemwerecar-
bonated.
3.2.2.SingleAggregateStrength
Figure6illustratesthesingle-particlestrengthofPCBAs.Increasingthecuringage
hadapositiveeffectonthesingle-particlestrengthoftheaggregates.Asearlyas7days
aftertheaggregateswerecured,thesingle-particlestrengthofthewater-curedaggregates
developedthefastest.However,anegligibledifferenceisdetectedbetweenthesingle-
particlestrengthforaggregatesunderstandardandsealedcuringconditions.Comparing
thestrengthgrowthofPCBAsunderthethreecuringconditions,arelativelyhighstrength
of5.26MPaisachievedat7dinthewatercuring,followedbyveryslowstrengthdevel-
opmentat14dand28d.Thesingle-particlestrengthfortheP8Sundersealedcuring
reaches3.61MPaat7d,andthe14d-strengthincreaseratioisgreaterthanthatduringthe
laterage.Fortheaggregatescuredunderstandardconditions,thesingleaggregate
strengthat7dand28dreaches3.61MPaand6.87MPa,respectively,indicatingthatthis
curingmethodiscapableofshowinganexcellenteffectonpotentialstrengthdevelopment
comparedtoothermethods.Incontrast,thenaturallycuredsamplespossessthelowest
strengthat7ddespitegrowinguntilcuringfor14d,displayingasharpinversionat28d.
Figure 6. Single-particle strength at 7 d, 14 d and 28 d.
For the single strength of aggregates cured for 28 d, standard curing increased by
23.8% over the sealed curing condition, 19.3% over the submerged curing condition, and
292.6% over natural curing. For 28 d dry cylinder compression strength, standard cured
aggregates showed an increase of 40.3% over natural curing.
Based on the phase assemblage analysis in Figure 7and Table 4, it is clear that the
hydration products, including ettringite (marked in the purple dotted box) and C-S-H gel,
which provide the cementitious ability, have essentially become completely carbonated
and experienced shrinkage, which explains the decrease in mechanical strength [
15
,
42
–
45
].
Materials 2024,17, 4971 11 of 21
Materials2024,17,xFORPEERREVIEW11of23
Figure6.Single-particlestrengthat7d,14dand28d.
Forthesinglestrengthofaggregatescuredfor28d,standardcuringincreasedby
23.8%overthesealedcuringcondition,19.3%overthesubmergedcuringcondition,and
292.6%overnaturalcuring.For28ddrycylindercompressionstrength,standardcured
aggregatesshowedanincreaseof40.3%overnaturalcuring.
BasedonthephaseassemblageanalysisinFigure7andTable4,itisclearthatthe
hydrationproducts,includingeringite(markedinthepurpledoedbox)andC-S-Hgel,
whichprovidethecementitiousability,haveessentiallybecomecompletelycarbonated
andexperiencedshrinkage,whichexplainsthedecreaseinmechanicalstrength[15,42–
45].
Figure7.X-raydiffraction(XRD)spectraofphosphogypsum-basedcold-bondedaggregatesat28
d.
Tab l e 4.Relativeamountsoftwokindsofwaterandcarbonatecontents.
Sample
Label
TotalWeightLoss
(%)
WaterinDihydrate
Gypsum
(%)
Carbonates
Content
(%)
Waterin
Hydrations
(%)
P8W21.18417.737None3.447
P8S20.92617.794None3.132
P8M22.02017.549None4.471
P8N17.24315.9802.818None
Inacomprehensiveanalysisofthesingleaggregatestrengthwithageforeachgroup
ofPCBAs,itwasfoundthatsupplementalwaterprovidedatanearlystagefavorstheir
strengthdevelopment,whereastoomuchwaterwillresultinareductioninlaterstrength
development.Althoughthestrengthofaggregatesdevelopsrapidlyunderthecondition
ofwatercuring,thepotentialleachingmaybethereasonforthelimiteddevelopmentof
mechanicalpropertiesinthelaterstage.Somestudieshavepointedoutthattheshrinkage
causedbytheearlysealedcuringmaybethereasonforthedeteriorationofthemechanical
propertiesofaggregates[46].Standardcuringcreatesanappropriatehumiditywithout
Figure 7. X-ray diffraction (XRD) spectra of phosphogypsum-based cold-bonded aggregates at 28 d.
Table 4. Relative amounts of two kinds of water and carbonate contents.
Sample Label Total Weight Loss
(%)
Water in Dihydrate Gypsum
(%)
Carbonates Content
(%)
Water in Hydrations
(%)
P8W 21.184 17.737 None 3.447
P8S 20.926 17.794 None 3.132
P8M 22.020 17.549 None 4.471
P8N 17.243 15.980 2.818 None
In a comprehensive analysis of the single aggregate strength with age for each group
of PCBAs, it was found that supplemental water provided at an early stage favors their
strength development, whereas too much water will result in a reduction in later strength
development. Although the strength of aggregates develops rapidly under the condition
of water curing, the potential leaching may be the reason for the limited development of
mechanical properties in the later stage. Some studies have pointed out that the shrinkage
caused by the early sealed curing may be the reason for the deterioration of the mechanical
properties of aggregates [
46
]. Standard curing creates an appropriate humidity without
leaching, of which the 28 d-strength is significantly enhanced over those of the other curing
methods. Therefore, the effect of various curing methods on the generation of hydration
products and microstructure construction are investigated further to determine the water
migration and binding mechanism.
3.3. Hydration Products and Microstructure Analysis
3.3.1. Hydration Products
The X-ray diffractometer patterns of PCBAs at 28 d are detailed in Figure 7. The
characteristic peaks at 9.1
◦
for ettringite and 11.6
◦
for gypsum dihydrate can be observed
from the plots. It is noteworthy that the diffraction peak for portlandite is also not ob-
served, as the portlandite would be consumed to produce ettringite due to the excess
gypsum [
2
,
41
,
47
]. In general, the strong diffraction peaks at low angles are important for
reference purposes.
Materials 2024,17, 4971 12 of 21
Compared to P8S and P8W, the relative peak intensity (ettringite/gypsum) of P8M
is the highest, which means the highest hydration degree. After 28 d of natural curing,
no characteristic diffraction peaks are seen for ettringite, nor for calcite, which reacts with
water and CO
2
in the air to form indeterminate calcite and secondary gypsum [
15
]. This is
confirmed by the TG curves that follow.
The TGA curves of the aggregates at 28 d for the four curing conditions are shown in
Figure 8. As the temperature increases, several substances undergo thermal decomposition,
as follows: (1) the removal of adsorbed water from the C-S-H gel interface [
12
,
48
,
49
] and
part of structural water of ettringite within 80–130
◦
C [
12
,
50
]. (2) The dehydration of
gypsum dihydrate is determined in all samples in the thermal range of 130–180
◦
C, which
presents a strong weight loss at the TG curves. (3) Furthermore, PCBAs cured under
natural curing exhibit a very slight weight loss when heated to 620
◦
C, which is the typical
decomposition temperature of calcium carbonate [
15
,
43
]. Based on this, the TGA curve
was segmented into four temperature intervals. The first interval is 25–80
◦
C, the second is
80–130 ◦C, the third is 130–180 ◦C, and the fourth is 600–750 ◦C.
Materials2024,17,xFORPEERREVIEW13of23
Figure8.TGAcurvesofphosphogypsum-basedcold-bondedaggregatesat28dunderfourcuring
conditions.
Powersetal.[51]pointedoutthathardenedcementpastesconsistofhydrationprod-
ucts,capillarywaterandunhydratedcement,wherethehydrationproductscontainsome
water.Thecapillarywateristhephysicallyboundwaterinsidetheporesandwasre-
movedbeforetheTGAtest.Therefore,thewatercontentinhydrationproductscanbe
consideredanindicatorofthedegreeofhydrationreaction[48],whichisconsistentwith
thetrendofthesingleaggregatestrengthofPCBAsat28d.
ConsideringthatcementandGGBShavehydratedcompletelywithlessofaninflu-
enceonmassloss,wetakethetotalweightlossat750°Casareference,wherethemass
lossrelatedtodifferentsubstancedecompositioniscalculatedandsummarizedinTable
4.Inthisexperiment,theinterval45–750°Cwaschosentocalculatethetotalweightloss,
WTotal;theweightlossbythermaldehydrationofexcessgypsumdihydratebetween130
and180°C,Wg;andthewatercontentofthehydrationproduct,WH=WTotal− Wg.ForP8N,
therewasnolongeraweightchangeinzonesIandII,andthemasslossinzoneIVwas
calculatedtobe2.818wt.%calciumcarbonate.
Theweightlossofwaterinhydrationproductsreaches4.471wt.%inP8M,withan
increasedratioof29.71%and42.75%comparedtoP8WandP8S,respectively.Corre-
spondingly,theweightlossofdihydratedgypsumshowsadecliningtrendwiththecon-
sumptionforthegenerationoferingite.Thisimpliesthatstandardcuringconditions
withsuitabletemperatureandhumiditycontributetohydration,leadingtocontinuous
strengthdevelopment.After28dofnaturalcuring,thereisbasicallynowatermassloss
relatedtothehydrationproduct,whilethemasslossaributedtothecarbonatethermal
decompositionreaches2.818%.CombinedwithXRDanalysis,thecalciumcarbonatepro-
ducedbythecarbonationofPCBAsundernaturalcuringconditionsisaragonite,witha
lowdegreeofcrystallization.Thecompletecarbonizationofthehydrationproductsisthe
mainreasonfortheirlowmechanicalstrength.Intheabsenceofwaterduringthehydra-
tionprocessofGGBS,thereleaseofionsanddissolutionofionsintheporesolutionare
affected,leadingtoarelativelylowerdegreeofhydration.Bycombiningwithalkaline
Figure 8. TGA curves of phosphogypsum-based cold-bonded aggregates at 28 d under four curing conditions.
It can be concluded from the TG curves that the amount of weight loss (mainly
caused by the loss of physically or chemically bound water) is the highest in the standard-
cured sample compared to the sealed-cured and water-cured samples, where the fastest
decomposition rate was noticed within the first period. There is a slight discrepancy
in thermal decomposition rates in each period between the samples under these two
curing conditions on the DTG curves, as well as the total weight loss. Particularly, a weak
thermal decomposition peak associated with the water loss of hydrates is detected in
the DTG curve of P8N, where the mass loss is concentrated in the range of dehydrated
gypsum decomposition.
Powers et al. [
51
] pointed out that hardened cement pastes consist of hydration
products, capillary water and unhydrated cement, where the hydration products contain
some water. The capillary water is the physically bound water inside the pores and was
removed before the TGA test. Therefore, the water content in hydration products can be
considered an indicator of the degree of hydration reaction [
48
], which is consistent with
the trend of the single aggregate strength of PCBAs at 28 d.
Materials 2024,17, 4971 13 of 21
Considering that cement and GGBS have hydrated completely with less of an influence
on mass loss, we take the total weight loss at 750
◦
C as a reference, where the mass loss
related to different substance decomposition is calculated and summarized in Table 4. In
this experiment, the interval 45–750
◦
C was chosen to calculate the total weight loss, W
Total
;
the weight loss by thermal dehydration of excess gypsum dihydrate between 130 and
180
◦
C, W
g
; and the water content of the hydration product, W
H
= W
Total −
W
g
. For P8N,
there was no longer a weight change in zones I and II, and the mass loss in zone IV was
calculated to be 2.818 wt.% calcium carbonate.
The weight loss of water in hydration products reaches 4.471 wt.% in P8M, with an
increased ratio of 29.71% and 42.75% compared to P8W and P8S, respectively. Correspond-
ingly, the weight loss of dihydrated gypsum shows a declining trend with the consumption
for the generation of ettringite. This implies that standard curing conditions with suit-
able temperature and humidity contribute to hydration, leading to continuous strength
development. After 28 d of natural curing, there is basically no water mass loss related
to the hydration product, while the mass loss attributed to the carbonate thermal decom-
position reaches 2.818%. Combined with XRD analysis, the calcium carbonate produced
by the carbonation of PCBAs under natural curing conditions is aragonite, with a low
degree of crystallization. The complete carbonization of the hydration products is the main
reason for their low mechanical strength. In the absence of water during the hydration
process of GGBS, the release of ions and dissolution of ions in the pore solution are affected,
leading to a relatively lower degree of hydration. By combining with alkaline substances,
the erosive CO
2
will weaken the pH value and may even result in the degradation of
hydrates [15,43,44].
Figure 9describes the FTIR analysis profile of PCBAs at 28 d under different curing
conditions, and the characteristic wavenumbers of the bands are listed in Table 5. The
analysis of the FTIR spectra is based on the Lambert–Beer law, where multiple absorption
peaks may usually overlap each other, and therefore, the structural characteristics of the
phases need to be analyzed by the location of the number of characteristic absorption [
52
].
The bands located at 3550 cm
−1
, 3408 cm
−1
, 1685 cm
−1
and 1624 cm
−1
are ascribed to O-H
vibrations in the structural and free water in hydrates and gypsum [
15
,
23
]. The strongest
absorption of O-H vibration is observed in the water-cured sample.
Materials2024,17,xFORPEERREVIEW14of23
substances,theerosiveCO2willweakenthepHvalueandmayevenresultinthedegra-
dationofhydrates[15,43,44].
Figure9describestheFTIRanalysisprofileofPCBAsat28dunderdifferentcuring
conditions,andthecharacteristicwavenumbersofthebandsarelistedinTable5.The
analysisoftheFTIRspectraisbasedontheLambert–Beerlaw,wheremultipleabsorption
peaksmayusuallyoverlapeachother,andtherefore,thestructuralcharacteristicsofthe
phasesneedtobeanalyzedbythelocationofthenumberofcharacteristicabsorption[52].
Thebandslocatedat3550cm−1,3408cm−1,1685cm−1and1624cm−1areascribedtoO-H
vibrationsinthestructuralandfreewaterinhydratesandgypsum[15,23].Thestrongest
absorptionofO-Hvibrationisobservedinthewater-curedsample.
Figure9.Fouriertransforminfraredreflectionspectraofphosphogypsum-basedcold-bondedag-
gregatesat28dunderfourcuringconditions.
Tab le5.CharacterizationofbandsontheFTIRspectra.
Wavenumbers(cm−1)Bands
525Si-Oout-of-planebendingvibration(δSi-O)[53]
605,670S-Obendingvibration[54]
875–878CO32-out-o
f
-planebendingvibration(v2)[55]
925Si-Osymmetrystretchingvibration(v3)[53,56]
970Si-OstretchingvibrationofQ2(C-S-H)[57]
1003Si-OstretchingvibrationofQ3(C-S-H)[50]
1090Si-ObendingvibrationinQ3[52]
1100–1200S-OsymmetrystretchingvibrationofSO4(v3)[54]
1417,1470C-Osymmetrystretchingvibration[52,56,58]
1685,1624H-O-Hbendingvibration[59]
3408H-O-Hstretchingvibration[60]
3550–3554H-O-Hstretchingvibrationingypsum[23]
Figure 9. Fourier transform infrared reflection spectra of phosphogypsum-based cold-bonded
aggregates at 28 d under four curing conditions.
Materials 2024,17, 4971 14 of 21
Table 5. Characterization of bands on the FTIR spectra.
Wavenumbers (cm−1) Bands
525 Si-O out-of-plane bending vibration (δSi-O) [53]
605, 670 S-O bending vibration [54]
875–878 CO32−out-of-plane bending vibration (v2) [55]
925 Si-O symmetry stretching vibration (v3) [53,56]
970 Si-O stretching vibration of Q2(C-S-H) [57]
1003 Si-O stretching vibration of Q3(C-S-H) [50]
1090 Si-O bending vibration in Q3[52]
1100–1200
S-O symmetry stretching vibration of SO
4
(v
3
) [
54
]
1417, 1470 C-O symmetry stretching vibration [52,56,58]
1685, 1624 H-O-H bending vibration [59]
3408 H-O-H stretching vibration [60]
3550–3554 H-O-H stretching vibration in gypsum [23]
Combined with the thermal analysis, it is suggested that more water is absorbed in
the PCBAs as free water during the water curing condition. The S-O bending vibrations in
ettringite and gypsum are noticed at 605 cm
−1
and 670 cm
−1
, in addition to S-O symmetric
stretching vibrations of SO
42−
tetrahedra within 1100–1200 cm
−1
. The C-O bands found
near 1400 cm
−1
and 876 cm
−1
, the absorption intensity of which presents stronger and
sharper in P8N, indicates a larger amount of carbonates [
55
,
61
]. Compared to other samples,
the characteristic band position of the Q
2
(Si-O) symmetric stretching vibration within
970–990 cm
−1
in the standard-cured sample shifts towards a larger wavenumber and a
shoulder at 1003 cm
−1
, indicating a generation of C-S-H gel with a higher polymerization
degree [
52
,
57
,
62
]. The starting point of the shoulder at 943 cm
−1
for PCBAs may be
assigned to Si-O
-
terminal bonds, which means a higher Ca/Si ratio of C-S-H [
63
]. The
optimization of the structure of the gel contributes to the construction of cementitious
structure, enhancing the macro performances, which are determined by the strength test
results (Figure 6).
In Figure 10, the microstructure of PCBAs can be clearly seen, where a number of
needle-rods of ettringite and C-S-H gels with a porous honeycomb cluster generated and
gathered in the holes and cracks [
2
,
64
,
65
]. The P8M group exhibited a greater number
of elongated needle-rods of ettringite and C-S-H gels with porous honeycomb clusters
growing between the excess gypsum crystals, which was corroborated with the TGA results.
Especially, there is little ettringite and gels in the P8N, resulting in a weak cementitious
ability for the subsequent microstructure development.
The dissolution of GGBS activated by sulfate and alkaline hydrates originating from
cement hydration has provided sufficient active groups for further polymerization and
precipitation of cementitious phases. As a result of the relatively high level of PG in
hardening pastes, an adhesive bond is established between unreacted particles instead of a
complete encapsulation of the particles. As an important medium for ion dissolution and
transfer, water’s content and exchange rate with the external environment will affect the
formation of hydration products, as well as pore structure and distribution by affecting the
ion concentration and transport rate within the local pore solution.
Where cement and GGBS are cementitious materials in the aggregate, cement takes the
lead in the hydration reaction in the presence of water to produce hydration products such
as portlandite, providing the alkaline conditions to excite the hydration of GGBS under
the combined action of gypsum to form more ettringite and C-S-H gels [
2
,
7
,
29
,
47
]. As the
hydration reaction proceeds, C-S-H gel and ettringite are produced and the PCBAs become
denser, resulting in increasing strength and density decreasing in water absorption [
2
].
Compared to other PCBAs, P8N shows a significant reduction in cold-bonded mechanical
strength and physical properties.
Materials 2024,17, 4971 15 of 21
Materials2024,17,xFORPEERREVIEW16of23
Figure10.Microstructureofphosphogypsum-basedcold-bondedaggregatesat28d:(a)water
cured;(b)sealcured;(c)naturalcured;(d)standardcured.
WherecementandGGBSarecementitiousmaterialsintheaggregate,cementtakes
theleadinthehydrationreactioninthepresenceofwatertoproducehydrationproducts
suchasportlandite,providingthealkalineconditionstoexcitethehydrationofGGBS
underthecombinedactionofgypsumtoformmoreeringiteandC-S-Hgels[2,7,29,47].
Asthehydrationreactionproceeds,C-S-HgelanderingiteareproducedandthePCBAs
becomedenser,resultinginincreasingstrengthanddensitydecreasinginwaterabsorp-
tion[2].ComparedtootherPCBAs,P8Nshowsasignificantreductionincold-bonded
mechanicalstrengthandphysicalproperties.
3.3.2.SpecificSurfaceAreaandPoreSizeDistribution
TheisothermsofallsamplesaredepictedinFigure11.Astherelativepressurein-
creases,themainprocessesofnitrogenadsorptionaremonolayeradsorption,multilayer
adsorptionandcapillarycoalescenceintheorderofporefilling,correspondingtomicro-
pores(<2nm),meso-pores(2~50nm)and,finally,macro-pores(>50nm),andthedesorp-
tionisreversed.Theshapeoftheisothermsreflectsporestructurecharacteristics,suchas
poreshapeparametersandtotalporevolumeofthesample,etc.[34,66,67].Itisimportant
torealizethatmostoftheisothermsarenotdefinedbyIUPACasusualisotherms,which
meanstheyaregenerallyacombinationoftwoormoredifferenttypesofisothermsin
reality[68].
ComparingtheisothermsofPCBAsinthefourcuringconditions,thecurvesalmost
overlapwhentherelativepressurep/p
0
islessthan0.4.Thisindicatesthatthereareasmall
numberofmicroporousholes(<2nm)inthepastes,suchascylindricalporesclosedatone
end,slitporesinparallelplatesorinkbolepores[36].Itisnotevidentthatthemi-
croporousholecontentvariedbetweenthesamplescuredinthedifferentconditions.
Whenarelativepressureofp/p
0
exceeds0.5,theisothermsseparateandformahysteresis
loopwhoseshapeandslopearedependentonthecondition.Ithasbeennoticedthatthe
Figure 10. Microstructure of phosphogypsum-based cold-bonded aggregates at 28 d: (a) water cured;
(b) seal cured; (c) natural cured; (d) standard cured.
3.3.2. Specific Surface Area and Pore Size Distribution
The isotherms of all samples are depicted in Figure 11. As the relative pressure
increases, the main processes of nitrogen adsorption are monolayer adsorption, multilayer
adsorption and capillary coalescence in the order of pore filling, corresponding to micro-
pores (<2 nm), meso-pores (2~50 nm) and, finally, macro-pores (>50 nm), and the desorption
is reversed. The shape of the isotherms reflects pore structure characteristics, such as pore
shape parameters and total pore volume of the sample, etc. [
34
,
66
,
67
]. It is important to
realize that most of the isotherms are not defined by IUPAC as usual isotherms, which
means they are generally a combination of two or more different types of isotherms in
reality [68].
Comparing the isotherms of PCBAs in the four curing conditions, the curves almost
overlap when the relative pressure p/p
0
is less than 0.4. This indicates that there are a
small number of microporous holes (<2 nm) in the pastes, such as cylindrical pores closed
at one end, slit pores in parallel plates or ink bottle pores [
36
]. It is not evident that the
microporous hole content varied between the samples cured in the different conditions.
When a relative pressure of p/p
0
exceeds 0.5, the isotherms separate and form a hysteresis
loop whose shape and slope are dependent on the condition. It has been noticed that the
hysteresis loop area of P8M is slightly larger than that of P8W and P8M. All of the inception
points of these hysteresis loops are located approximately at a middle relative pressure
of 0.50, except for P8N. In other words, there is no abrupt mass desorption process, and
the distribution of pore sizes becomes more evenly dispersed. The area of the hysteresis
loop of the P8M group was slightly larger than that of P8W and larger than that of P8S,
indicating an increasing amount of C-S-H with a higher Ca/Si ratio. Compared with P8W
and P8S, P8M possesses a slightly larger hysteresis loop area, indicating the production of
C-S-H gels with a higher Ca/Si ratio [
68
]. Particularly, a plurality of middle meso-pores
Materials 2024,17, 4971 16 of 21
predominantly existed in C-S-H gels and was shown to be more significantly responsible
for the adsorption of gas in PCBAs.
Materials2024,17,xFORPEERREVIEW17of23
hysteresisloopareaofP8MisslightlylargerthanthatofP8WandP8M.Alloftheincep-
tionpointsofthesehysteresisloopsarelocatedapproximatelyatamiddlerelativepres-
sureof0.50,exceptforP8N.Inotherwords,thereisnoabruptmassdesorptionprocess,
andthedistributionofporesizesbecomesmoreevenlydispersed.Theareaofthehyste-
resisloopoftheP8MgroupwasslightlylargerthanthatofP8Wandlargerthanthatof
P8S,indicatinganincreasingamountofC-S-HwithahigherCa/Siratio.Comparedwith
P8WandP8S,P8Mpossessesaslightlylargerhysteresislooparea,indicatingtheproduc-
tionofC-S-HgelswithahigherCa/Siratio[68].Particularly,apluralityofmiddlemeso-
porespredominantlyexistedinC-S-Hgelsandwasshowntobemoresignificantlyre-
sponsiblefortheadsorptionofgasinPCBAs.
Figure11.Isothermsofthephosphogypsum-basedcold-bondedaggregatesat28dunderfourcur-
ingconditions.
Tabl e 6illustratesthespecificsurfaceareaofPCBAsunderfourcuringconditions
calculatedbytheBETapproaches.PCBAscuredinstandardandwaterconditionpossess
greaterspecificsurfacearea,reaching13.83m2/gand13.38m2/g,respectively.Thismeans
thatsufficientwatercanpromotethehydrationdegreeofpastes,whileexcessivewater
willlaterleadtoaslowhydrationreaction.
Tab l e 6.Specificsurfacearea,porevolumeandporesizeofphosphogypsum-basedcold-bonded
aggregates.
SampleSBET
(m2/g)
PoreVolume
(cm3/g)
AveragePoreSize
(nm)
MostFrequentPore
Diameter
(nm)
P8W13.380.04868.56.0
P8S9.780.02309.59.5
P8M13.830.05138.98.9
P8N5.660.024015.88.0
Figure 11. Isotherms of the phosphogypsum-based cold-bonded aggregates at 28 d under four
curing conditions.
Table 6illustrates the specific surface area of PCBAs under four curing conditions
calculated by the BET approaches. PCBAs cured in standard and water condition possess
greater specific surface area, reaching 13.83 m
2
/g and 13.38 m
2
/g, respectively. This means
that sufficient water can promote the hydration degree of pastes, while excessive water will
later lead to a slow hydration reaction.
Table 6. Specific surface area, pore volume and pore size of phosphogypsum-based cold-bonded
aggregates.
Sample
SBET
(m2/g)
Pore Volume
(cm3/g)
Average Pore Size
(nm)
Most Frequent Pore Diameter
(nm)
P8W 13.38 0.0486 8.5 6.0
P8S 9.78 0.0230 9.5 9.5
P8M 13.83 0.0513 8.9 8.9
P8N 5.66 0.0240 15.8 8.0
The water and standard curing conditions are beneficial to the hydration reaction,
with the largest specific surface area, followed by water cured and 13.38 m
2
/g under
sealed curing. Lack of water under sealed conditions and without external moisture
supplementation results in a slower hydration process, producing a smaller specific surface
area than the P8M and P8W groups.
The pore size distribution of the PCBAs is shown in Figure 12. Natural curing showed
wide pore size distribution. The same pattern was observed for the average pore size, with
the standard and water-cured PCBAs having a smaller average pore size of 8.9 nm and
8.5 nm, respectively, and a denser structure. For the natural cured aggregates, the average
pore size reaches 15.8 nm, the pore size distribution becomes dispersed and the number of
large pores increases due to the continuous carbonation and decomposition of ettringite
Materials 2024,17, 4971 17 of 21
and the gradual decalcification of the C-S-H gel to form a low Ca/Si C-S-H gel and the
coarsening of the pores [38,43,69].
Materials2024,17,xFORPEERREVIEW18of23
Thewaterandstandardcuringconditionsarebeneficialtothehydrationreaction,
withthelargestspecificsurfacearea,followedbywatercuredand13.38m2/gundersealed
curing.Lackofwaterundersealedconditionsandwithoutexternalmoisturesupplemen-
tationresultsinaslowerhydrationprocess,producingasmallerspecificsurfaceareathan
theP8MandP8Wgroups.
TheporesizedistributionofthePCBAsisshowninFigure12.Naturalcuringshowed
wideporesizedistribution.Thesamepaernwasobservedfortheaverageporesize,with
thestandardandwater-curedPCBAshavingasmalleraverageporesizeof8.9nmand
8.5nm,respectively,andadenserstructure.Forthenaturalcuredaggregates,theaverage
poresizereaches15.8nm,theporesizedistributionbecomesdispersedandthenumber
oflargeporesincreasesduetothecontinuouscarbonationanddecompositionoferingite
andthegradualdecalcificationoftheC-S-HgeltoformalowCa/SiC-S-Hgelandthe
coarseningofthepores[38,43,69].
Figure12.Poresizedistributionofthephosphogypsum-basedcold-bondedaggregatesat28dun-
derfourcuringconditions.
3.4.LeachingBehaviorofPhosphorus
Theleachingbehaviorofphosphorusisevaluatedfortheenvironmentalimpactas-
sessment,asdetailedinTab l e 7.IncomparisonwithfreshPG,PCBAsunderfourcuring
conditionsachieveanexcellentsolidificationofphosphorusimpurities.Duetothealka-
lineenvironmentinPCBAs,phosphoruselementsarefixedintheformofcalciumphos-
phateandcalciumhydrogenphosphate[70],whichinhibitstheleachingofphosphorus
elements,andacidleachingcanproducehigherphosphoruscontentthandeionizedwater
leaching[71].Inaddition,eringiteandC-S-Hgelproducedbyahydrationreactionof
cementingmaterialshavegoodadsorptionandsolidificationeffectsonphosphorusim-
purities.TheleachingconcentrationofPCBAsissignificantlyaffectedbycuringcondi-
tions.Openconditions,includingstandardandnaturalcuringaremorefavorabletothe
solidificationofphosphorusimpuritiesthansealedandwatercuring.Thisissimilarto
previousfindings[1,2].
Tab l e 7.LeachingresultsofPimpurities(mg/L).
Figure 12. Pore size distribution of the phosphogypsum-based cold-bonded aggregates at 28 d under
four curing conditions.
3.4. Leaching Behavior of Phosphorus
The leaching behavior of phosphorus is evaluated for the environmental impact as-
sessment, as detailed in Table 7. In comparison with fresh PG, PCBAs under four curing
conditions achieve an excellent solidification of phosphorus impurities. Due to the alkaline
environment in PCBAs, phosphorus elements are fixed in the form of calcium phosphate
and calcium hydrogen phosphate [
70
], which inhibits the leaching of phosphorus ele-
ments, and acid leaching can produce higher phosphorus content than deionized water
leaching [
71
]. In addition, ettringite and C-S-H gel produced by a hydration reaction
of cementing materials have good adsorption and solidification effects on phosphorus
impurities. The leaching concentration of PCBAs is significantly affected by curing condi-
tions. Open conditions, including standard and natural curing are more favorable to the
solidification of phosphorus impurities than sealed and water curing. This is similar to
previous findings [1,2].
Table 7. Leaching results of P impurities (mg/L).
Sample Acetic Acid Leaching
(mg/L)
Static Leaching
(mg/L)
PG 128.997 4.570
P8W 0.365 0.016
P8S 0.247 0.020
P8N 0.029 0.020
P8M 0.082 0.016
4. Conclusions
Through a comparative study of the properties of PCBAs under different curing
conditions, the following conclusions were obtained.
Materials 2024,17, 4971 18 of 21
Water curing is beneficial to early strength development for the 7 d single aggregate
strength reaching 5.26 MPa. For the single aggregate strength cured for 28 d, standard
curing improved by 19.3% over others, including 292.6% over natural curing.
The curing conditions cause significant differences in the physical and mechanical
properties of PCBAs by affecting the quantity, distribution and accumulation of hydration
reaction products. Standard curing promotes the generation of ettringite and induces a
shift towards more C-S-H gel with increased ratios of 29.71% and 42.75% over water and
sealed-cured methods for 28 d, respectively. The hydration products tend to be generated
in the interstices of gypsum particles and form a strong bond under standard conditions,
while the others are distributed haphazardly, which is not as effective as the standard
condition in consolidating the excess PG particles.
The isotherms of PCBAs under the four curing conditions suggest a slit-type pore
structure. The PCBAs that are standard cured exhibit the maximum BET-specific surface
area of 13.83 m
2
/g, suggesting more gel pore area. The water-cured PCBAs achieve the
smallest average and most frequent pore size, 8.5 nm and 6.0 nm, but the natural-cured
PCBAs exhibit a wide pore size distribution. The PCBAs achieved excellent solidification
of phosphorus impurities compared to the original PG.
Comparing the overall physical and mechanical properties of PCBAs under the four
curing conditions, water curing is recommended for its adaptability to the needs of indus-
trial production with a shorter curing time.
The research in this paper provides an important reference for the industrial produc-
tion and maintenance of phosphogypsum-based cold adhesive aggregates. However, due
to the limitation of research time and conditions, the development law of PCBA perfor-
mance under the combined curing mode has not been studied yet, and further research is
expected in the future.
Author Contributions: G.W.: Writing—review and editing, Resources. Z.Y.: Methodology, Formal
analysis, Investigation, Data curation, Writing—original draft. T.S.: Editing, Resources, Funding
acquisition. Z.M.: Funding. Z.W.: Review and editing, Supervision. G.O.: Methodology, Investigation,
Validation. J.H. and Y.D.: Data analysis and Statistical analysis. All authors have read and agreed to
the published version of the manuscript.
Funding: The authors gratefully acknowledge the financial support from the National Key R&D
Program of China (No. 2021YFC3100805), the National Natural Science Foundation of China (Grant
No. 52378261), the China Construction Engineering Corporation Technology R&D Program Funding
(No. CSCEC-2022-Z-20), the Sichuan Science and Technology Program (No. 2023ZHCG0034), the
Third Batch of Special Fund for Science and Technology Development of Zhongshan City in 2020
(2020-18), and China Scholarship Council.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The original contributions presented in this study are included in the
article; further inquiries can be directed to the corresponding authors (due to privacy).
Conflicts of Interest: Author Zhenlin Mo was employed by the company China Southwest Geotech-
nical Investigation and Design Institute Corporation Limited. The remaining authors declare that
the research was conducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
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