Studying the hydration kinetics and mechanical-microstructural properties of Portland cements made with and without dredged sediment: experimental and numerical approaches

2.1. Raw materials and cement production process in the laboratory

In this study, two ordinary Portland cements (OPC) were made by grinding together clinker and gypsum in a laboratory ball mill. Two different clinkers were produced with and without sediment in the raw meal. The sediment used is fluvial sediment collected from the Air sur Lys region (hereafter ASL) in the North of France. It was homogenized and dried at 105 °C until it reached a constant weight before characterization. Contrary to several previous studies (99. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N. 2022. Recycling of dredged sediment as a raw material for the manufacture of Portland cement – Numerical modeling of the hydration of synthesized cement using the CEMHYD3D code. J. Build. Eng. 48:103871. https://doi.org/10.1016/j.jobe.2021.103871.
, 1010. Aouad G, Laboudigue A, Gineys N, Abriak NE. 2012. Dredged sediments used as novel supply of raw material to produce Portland cement clinker. Cem. Concr. Compos. 34(6):788–793. https://doi.org/10.1016/j.cemconcomp.2012.02.008.
), which used analytical reagents as the raw materials, in this study, the raw materials used were limestone, clays, sand and iron oxide that were extracted from the natural quarries. They were also dried at 105 °C until a constant weight and ground to a smaller particle size than 200 µm before characterization. The chemical composition of all materials obtained using X-ray fluorescence (XRF) analysis is given in Table 1.

Based on the chemical composition of all raw materials as well as the targeted mineralogical composition of clinkers, two raw mixtures were formulated with and without sediment using the moduli values: LSF = 98, SR = 2.6 and AR = 1.45 respectively (2424. Taylor HFW. 1998. Cement chemistry. Second edition. Ed. Thomas Telford. London.
). The proportion of other raw materials in these mixtures must respect the following proportions: $\frac{LM1}{LM2+LM3}$ =1.5 and $\frac{CLAY2}{CLAY1}$ =1.5 as required by the industrial partner. In addition, the analysis of the consistency of raw meal made with different rates of sediment was also performed to determine the maximum incorporable rate of sediment in the raw meal. The raw meals were prepared by mixing the raw materials with the same amount of water (water-to-solid ratio of 0.3) for 5 minutes before being poured onto a flat surface. The consistency of raw meal was determined by measuring the diameter of pates, as illustrated in Figure 1.

Based on the chemical composition of raw materials as well as the consistency of raw pastes, the maximum incorporable rate of sediment in the raw meal is limited to 7.55%. This is very important for assuming an adequate content of water used for raw paste preparation before the clinkering. It should be noted that a greater water content was used, the energy consumption was greater, leading to an increase in the cost of the cement product. The economic efficacy and environmental impact of sediment substitution in cement production will be presented in our future paper. In this study, two cements were produced as follows: the first cement (OPC Ref cement) was produced from conventional raw materials only, and the second, OPC-Sed cement, was made with a maximum substitution rate of sediment up to 7.55%. The composition of two raw meals is given in Table 2.

Two clinkers were produced in this study according to the process described in a previous study (99. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N. 2022. Recycling of dredged sediment as a raw material for the manufacture of Portland cement – Numerical modeling of the hydration of synthesized cement using the CEMHYD3D code. J. Build. Eng. 48:103871. https://doi.org/10.1016/j.jobe.2021.103871.
). After the clinkering, the free lime content (CaO_free) of two clinkers was measured using the Schlafer-Bukolowki method (2525. Schläpfer P, Bukowski R. Untersuchungen über die bestimmung des freien kalkes und des kalziumhydroxydes in zement-klinkern zementen schlacken und abgebundenen hydraulischen mörteln eidgenössische materialprüfungsanstalt an der E.T.H, in Zürich, 63.
). The chemical compositions of clinkers were measured using XRF analysis, while the mineralogical compositions were obtained using the Bogue formula (2626. Bogue RH, Lerch W. 1934. Hydration of portland cement compounds. Ind. Eng. Chem. 26(8):837–847. https://doi.org/10.1021/ie50296a007.
) according to following Equations:

Where:

The chemical-mineralogical compositions of two clinkers are given in Table 3.

The CaO_free amount of both clinkers was less than the threshold value of 2% usually required in the cement industry (2727. Anger B. 2015. Caractérisation des sédiments fins des retenues hydroélectriques en vue d’une orientation vers des filières de valorisation matière. Conférence Méditerranéenne Côtière et Maritime Edition 3, Ferrara, Italia (2015). Retrieved from https://www.paralia.fr/cmcm/e03-20-anger.pdf.
). It also meant that the clinkering process was complete and that the cooling process did not lead to the decomposition of C₃S into C₂S and CaO_free. In addition, it can be seen that OPC-Sed clinker contained more C₃S and less C₂S content when compared with the reference clinker. This was principally due to the higher content of CaO and lower content of SiO₂ in OPC-Sed clinker.

The influence of sediment substitution on the chemical composition of mineralogical phases was also investigated using the SEM-EDS technique. The clinker samples were cut into 2 mm thick slices, and then they were impregnated under vacuum using the epoxy. These samples were polished using diamond suspension, with deodorized petroleum used as lubricant. Between each step, isopropanol was used to clear these samples. They were kept for 2 days in a desiccator under vacuum to remove the remaining isopropanol before being coated with carbon to make the surface conductive for SEM-EDS analysis at high vacuum. The Hitachi S-4300SE/N operates with an accelerating voltage of 20 kV, and a current of 0.8 mA was used to measure 100 points for each mineralogical phase in different zones to obtain a representative result. The SEM images of OPC Ref and OPC Sed clinker are shown in Figure 2 (a, b).

The SEM result also indicated that the sediment substitution in the raw meal did not change the distribution and morphology of the phases of clinker. Indeed, the morphology of alite (C₃S) and belite (C₂S) in both clinker in this study is typical morphology for these phase as observed in a previous study (2828. Zaki M, Sharma S, Gurjar SK, Goyal R, Jayadeva Krishnan NMA. 2023. Cementron: machine learning the alite and belite phases in cement clinker from optical images. Constr. Build. Mater. 397:132425. https://doi.org/10.1016/j.conbuildmat.2023.132425.
).

The chemical composition of all four major phases of both clinkers obtained from SEM-EDS analysis is shown in Figure 3 (a, b, c, d) corresponding to C₃S, C₂S, C₃A, and C₄AF, respectively.

Observing the SEM-EDS result, it can be seen that the sediment substitution in the raw meal has not had a significant impact on the chemical composition of all four major phases. Indeed, the ratio of major elements in these mineralogical phases of both clinkers is nearly similar. This is interesting to investigate the cement quality made from the sediment substitution because the uptake of minor elements such as K⁺, Na⁺ or Mg²⁺ could modify the morphology of the mineralogical phase, in particular in the C₃S phase as described in a previous study (99. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N. 2022. Recycling of dredged sediment as a raw material for the manufacture of Portland cement – Numerical modeling of the hydration of synthesized cement using the CEMHYD3D code. J. Build. Eng. 48:103871. https://doi.org/10.1016/j.jobe.2021.103871.
). However, the ratio of major elements in all four major phases is slightly higher than the theoretical values of these phases found in the literature. For example, in the case of C₃S, the Ca/Si ratio obtained from the SEM-EDS analysis is 3.51, and 3.53 for OPC Ref Clinker, and OPC - Sed Clinker respectively, while the theoretical value is 3.0. The difference between the experimental value and the theoretical value can be due to numerous reasons, including the presence of foreign elements or the interaction of phases.

Gypsum was added to obtain a total SO₃ amount of 3.6% in both cements. This was an optimized sulfate content for a plain Portland cement hydration as defined in the NF EN 197-1 standard (2929. Association Française de Normalisation (AFNOR). NF EN 197-1. 2012. Composition spécifications et critères de conformité des ciment courant.
). Based on the chemical composition of gypsum and clinkers presented in Table 1 and Table 3 respectively, the mass proportion of each component of OPC Ref and OPC Sed cements is given in Table 4.

Then, clinker and gypsum were ground using a ball-mill device until they achieved a Blaine surface of 4500 cm2/g.

2.2. Characterization methods

2.2.2. Hydration kinetics and mechanical-microstructural development of hydrated cements

 ⌅

Contrary to several previous studies (1111. Faure A, Coudray C, Anger B, Moulin I, Colina H, Izoret L, Théry F, Smith A. 2019. Beneficial reuse of dam fine sediments as clinker raw material. Constr. Build. Mater. 218:365–384, https://doi.org/10.1016/j.conbuildmat.2019.05.047.
, 3232. Dalton JL, Gardner KH, Seager TP, Weimer ML, Spear JCM, Magee BJ. 2004 Properties of Portland cement made from contaminated sediments. Resour. Conserv. Recycl. 41(3):227–241. https://doi.org/10.1016/j.resconrec.2003.10.003.
), which investigated the cement hydration through the hydration heat and compressive strength development only, in this study, the hydration kinetics and mechanical-microstructural properties of OPC Ref and OPC-Sed cements were studied using numerous experimental analyses, including the advancement of hydration and hydration heat of pastes, compressive strength, and porosity evolution of mortar samples.

The advancement of hydration reactions with time could provide information on the hydration behavior of cement. The results obtained in a previous study (3333. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N-E. 2021. Determination of the degree of hydration of Portland cement using three different approaches: Scanning electron microscopy (SEM-BSE) and Thermogravimetric analysis (TGA). Case Stud. Constr. Mater. 15:e00754. https://doi.org/10.1016/j.cscm.2021.e00754.
) indicated that the measurement of chemically bound water was the best method for the determination of the degree of hydration of cement (hereafter DoH) thus, in this study, the DoH of cements was determined using thermogravimetry analysis (TGA) based on the measurement of chemically bound water amounts during cement hydration. Two cement pastes were prepared with a water-to-cement ratio of 0.5, and cured under saturated conditions at a constant temperature of 20 °C until specific ages for measurement. At the time of testing, the cement paste samples were crushed into fine particles (<63 µm), then they were directly analyzed in a TG instrument (Netzsch STA 409). TGA was performed according to the process described in a previous study (3333. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N-E. 2021. Determination of the degree of hydration of Portland cement using three different approaches: Scanning electron microscopy (SEM-BSE) and Thermogravimetric analysis (TGA). Case Stud. Constr. Mater. 15:e00754. https://doi.org/10.1016/j.cscm.2021.e00754.
). The DoH of hydrated cements was calculated according to Equation [5]:

$\alpha \left(t\right)= \frac{W_n\left(t\right)}{W_n\left(\infty \right)}$  [5]

Where:

α(t): Degree of hydration of cement at time t.

W_n(t): Chemically bound water amount of cement paste at time t.

W_n(∞): Ultimate chemically bound water amount corresponding to the complete hydration of cement paste.

The TGA curve and DTG curve of OPC Ref cement paste after 28 days of hydration are presented in Figure 4 (a, b) respectively.

FIGURE 4.  (a) TGA curve of OPC Ref cement paste after 28 days of hydration; (b) DTG curve of OPC Ref cement paste after 28 days of hydration.

Based on the TGA curve and DTG curve, it can be seen that the loss of mass of cement paste in the range of 40 °C to 105 °C corresponds to the mass of evaporable water (free water), while the chemically bound water content is obtained by measuring the loss of mass of the sample between 105 °C and 1000 °C. The Ca(OH)₂ amount could be determined by measuring the mass loss of the sample in the range of 400 °C to 500 °C. Table 6 summarizes the mass loss of each sample corresponding to the different temperature ranges.

TABLE 6.  Mass loss of each sample corresponding to the different temperature range.

Mass loss	2 days		14 days		28 days
	OPC Ref	OPC Sed	OPC Ref	OPC Sed	OPC Ref	OPC Sed
40 °C-105 °C	26.2	26.57	23.01	22.97	21.82	21.68
105 °C-1000 °C	7.87	8.26	11.56	11.74	12.77	13.08
Mass of cement	65.93	65.17	65.43	65.29	65.41	65.24

The chemically bound water amount of cement paste could be determined from the sample mass loss according to Equation [6]:

$W_n\left(t\right)= \frac{{∆m }_{sample }\left(105 ℃-1000 ℃\right)\left(t\right)}{m sample \left(1000℃\right)\left(t\right)}*100$  [6]

The ultimate chemically bound water amount of cement was estimated using the method proposed by NIST (National Institute of Standards and Technology) as given in the work (3434. Bullard JW, Evans DL, Bond PJ. 2003. The virtual cement and concrete testing laboratory consortium. Annual Report 2003.
). The principle is to estimate the ultimate chemically bound water amount of individual phase when it totally reacts with water. In addition, in a previous study (3333. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N-E. 2021. Determination of the degree of hydration of Portland cement using three different approaches: Scanning electron microscopy (SEM-BSE) and Thermogravimetric analysis (TGA). Case Stud. Constr. Mater. 15:e00754. https://doi.org/10.1016/j.cscm.2021.e00754.
), we used three different methods to determine the degree of hydration (DoH) of cement pastes. The results indicated that the measurement of the ultimate chemically bound water amount is the best method for determining the DoH of Portland cement. For this reason, in this study, the DoH of both cement pastes was determined by measuring the chemically bound water amount. Based on the ultimate chemically bound water amount of individual phase, we can estimate the ultimate chemically bound water amount of cement according to Equation [7]:

$W_n\left(\infty \right)= W_{nC3S}\left(\infty \right)*\left(\%C_{3}S\right)+ W_{nC2S}\left(\infty \right)*\left(\%C_{2}S\right)+ W_{nC3A}\left(\infty \right)*\left(\%C_{3}A\right)+ W_{nC4AF}\left(\infty \right)*\left(\%C_{4}AF\right)+ W_{n{C}_{a}Ofree}\left(\infty \right)*\left(\%C_a{O}_{free}\right)$  [7]

Where:

W_nC3S(∞), W_nC2S(∞), W_nC3A(∞), W_nC4AF(∞), W_nCaOfree(∞) is the ultimate chemically bound water amount of C₃S, C₂S, C₃A, C₄AF and CaO_free respectively.

From the mineralogical composition of clinker and the mass proportion between gypsum and clinker in both cements given in Table 3 and Table 4 respectively, the mineralogical composition of both cements could be easily computed. Table 7 presents the mineralogical composition as well as W_n(∞) value of both cement.

TABLE 7.  Mineralogical composition and Wn(∞) value of both cements.

Phase	Wphase(∞) per gram	Mineralogical composition		Wn(∞) of cement (g/100 g of anhydrous cement)
		OPC Ref	OPC - Sed	OPC Ref	OPC - Sed
C3S	0.24	57.45	59.68	13.79	14.32
C2S	0.21	14.69	11.41	3.08	2.39
C3A	0.4	8.52	9.38	3.41	3.75
C4AF	0.37	8.56	8.63	3.16	3.19
CaOfree	0.33	1.49	1.54	0.49	0.51
Gypsum	-	9.29	9.36	-	-
Total		100	100	23.93	24.17

The heat release during hydration was monitored using isothermal conduction calorimetry performed at a constant temperature of 20°C. For this measurement, cement paste was prepared with a water-to-cement ratio of 0.5 (W/C=0.5) according to the process described in the previous study (77. Chu DC, Amar M, Kleib J, Benzerzour M, Damien B, Abriak N, Jaouad N. 2022. The pozzolanic activity of sediments treated by the flash calcination method. Waste Biomass Valorization. 13:4963–4982. https://doi.org/10.1007/s12649-022-01789-8.
, 99. Chu DC, Kleib J, Amar M, Benzerzour M, Abriak N. 2022. Recycling of dredged sediment as a raw material for the manufacture of Portland cement – Numerical modeling of the hydration of synthesized cement using the CEMHYD3D code. J. Build. Eng. 48:103871. https://doi.org/10.1016/j.jobe.2021.103871.
). After mixing for 2 min at 1600 rpm, 10g of paste was poured into a cell, which was then sealed and placed in the calorimeter. The heat liberated from the cement hydration reactions was measured for up to 70h.

X-ray diffraction (XRD) analysis was used to identify the anhydrous cement phases as well as the hydration products of two cement pastes over time. The hydration of cement pastes was stopped using the solvent exchange method (isopropanol solvent) (3535. Scrivener K, Snellings R, Lothenbach B. 2016. A practical guide to microstructural analysis of cementitious materials. Ed. Taylor & Francis Group. Crc Press Boca Raton F.L. USA.
). Cement pastes were immersed in isopropanol for 3 days, then stored in a desiccator under vacuum for 7 days. The hydration-stopped pastes were ground to obtain a smaller particle diameter than 40 µm. The powders were analyzed using a Bruker D2 Advance diffractometer, which is equipped with Cu radiation, λ = 1.5406 Å. The measurement was performed at 40 kV-40 mA with the 2θ angle scanned from 5° to 80°.

The mechanical development was investigated by measuring the compressive strength of standard mortars. Samples (40 x 40 x 160 mm³) were prepared according to the NF EN 196-1 standard (3636. Association Française de Normalisation (AFNOR). NF EN 196-1. 2016. Méthode d’essai des ciments- Partie 1: Détermination des résistance.
) and cured under saturated conditions at a constant temperature of 20 °C. The compressive strength of mortars was measured after 1, 2 and 28 days of curing, respectively using an INSTRON 5500R device that has a maximum applicable load of 150 kN and a displacement rate of 144 KN/min.

The microstructure development of hydrated cement over time was monitored using the Mercury Intrusion Porosimetry (MIP) method. For MIP measurement, mortar samples were cut into small pieces of 1 x 1 x 1 cm³ and immersed in isopropanol for 3 days to stop the hydration. Samples were then stored in a desiccator for at least 7 days before measurement. In this study, the total porosity and pore size distribution of samples were measured after 1 and 28 days of hydration using an Autopore IV device. The instrument has a maximum applicable pressure of 200 MPa, and is able to detect a minimum pore diameter of 6 nm.

In addition, the pore structure could be studied measuring the capillary porosity according to BS 1881 Pt. 124 (3737. British Standards BS 1881: Part 124. 1988. British standard testing concrete. Part 124. Methods for analysis of hardened concrete.
) based on the DoH of cement. The calculated capillary porosity of mortar was obtained according to Equation [8]:

$P= \frac{\frac{W}{C}-0.36\infty \left(t\right)}{\frac{W}{C}+0.32}*V{\left(\%\right)}_{cement paste in the mortar}$  [8]

Where

P is the calculated capillary porosity of mortar

α(t) is the degree of hydration of cement.

V(%) is the percentage of cement paste by volume in the mortar.

W/C is the water-to-cement ratio of mortar.

3.1. Hydration kinetics

Figure 5 presents the model prediction and the experimental result for DoH of OPC Ref and OPC-Sed pastes. The result indicated that the DoH of OPC-Sed cement was always slightly higher than the reference cement at any time of testing. The DoH of OPC Ref and OPC-Sed cements increased with time and reached 0.81 and 0.815, respectively after 28 days of hydration. This confirmed that the sediment substitution level of 7.55% in the raw meal did not impact the reactivity of the mineralogical phases of cement.

For the modeling result, in general, CEMHYD3D provided a good prediction of DoH for both cements when compared with the experiment. As the experimental result, the DoH of OPC-Sed cement predicted in the CEMHYD3D was also slightly higher than that of OPC Ref cement. However, it can be seen that for the two cement pastes, the DoH predicted in the CEMHYD3D was slightly higher than that measured by the experiment with the DoH values of 0.835 for OPC Ref cement and 0.852 for OPC-Sed cement after 28 days of hydration. This can be attributed to the difference between the cement particle size measured by the experiment and the simulation in the model as indicated in Table 7. Indeed, in cement hydration, the hydration rate of coarser cement is generally lower than that of finer cement (4040. Bentz DP, Ferraris CF, Galler MA, Hansen AS, Guynn JM. 2012. Influence of particle size distributions on yield stress and viscosity of cement-fly ash pastes. Cem. Concr. Res. 42(2):404–409. https://doi.org/10.1016/j.cemconres.2011.11.006.
).

The cumulative hydration heat of OPC Ref and OPC-Sed cements, measured by isothermal conduction calorimetry and modeled in CEMHYD3D is shown in Figure 6. The experimental result indicated that the hydration kinetics of both cements were nearly similar when we compared the curve shape as well as the cumulative heat. As a result of the DoH measurement, the hydration heat of OPC-Sed cement was slightly higher than that of OPC Ref cement. Indeed, after 72h of hydration, the cumulative hydration heat of OPC Ref cement was 291.75 (KJ/kg), while OPC-Sed cement achieved 310.84 (KJ/kg).

Furthermore, it can be seen that the model prediction for the cumulative hydration heat was in nearly perfect agreement with the experimental measurement for both cement pastes. Indeed, the cumulative hydration heat predicted in the model was 283.80 (KJ/kg) for OPC Ref cement and 305.44 (KJ/kg) for OPC-Sed cement after 72h of hydration. The model also indicated that the heat liberated from the hydration of OPC-Sed cement was greater than that of OPC-Ref cement, with a cumulative heat of 435.64 (KJ/kg) for OPC Ref cement and 460.51 (KJ/kg) for OPC-Sed cement after 28 days of hydration. However, the mode appeared to slightly overestimate the heat released at very early ages (several minutes), while slightly underestimating it within 20-30 h for both cement pastes. The difference at very early ages was very likely due to the capability of an isothermal calorimetry device which was able to measure the hydration heat after approximately 5 minutes only.

The heat evolution curve and hydration progress of all individual cement phases of both cement pastes were studied to better understand the contribution of each cement phase to the hydration heat peak. The heat flux curve liberated from the hydration of both cements is also shown in Figure 7, while the hydration progress of all individual cement phases as a function of time predicted in the CEMHYD3D is highlighted in Figure 8 (a, b) corresponding to OPC Ref and OPC-Sed cement pastes respectively.

The hydration heat curve of both cements may be distinguished in several stages, and in each stage, the contribution of individual cement phases on the hydration heat could be clearly investigated.

In general, the hydration progress of individual cement phases over time simulated in CEMHYD3D for the two cement pastes was in accordance with the experimental observations (43-4543. Bullard JW, Jennings HM, Livingston RA, Nonat A, Scherer GW, Schweitzer JS, Scrivener KL, Thomas JJ. 2011. Mechanisms of cement hydration. Cem. Concr. Res. 41(12):1208–1223. https://doi.org/10.1016/j.cemconres.2010.09.011.
44. Zhang Z, Scherer GW, Bauer A. 2018. Morphology of cementitious material during early hydration. Cem. Concr. Res. 107:85–100. https://doi.org/10.1016/j.cemconres.2018.02.004.
45. Richardson IG. 1999. Nature of C-S-H in hardened cements. Cem. Concr. Res. 29(8):1131–1147. https://doi.org/10.1016/S0008-8846(99)00168-4.
). For the calcium silicate phases, the model prediction indicated that the C₃S hydration started much earlier and faster than that of the C₂S phase. Indeed, more than 45% of the C₃S phase has reacted after 1day and about 90% after 28 days, while for the C₂S phase, the hydration rates were 20% and 68%, respectively at the same time. For the aluminate phases (C₃A and C₄AF), it can be seen that the hydration kinetics of the C₃A and C₄AF phases were similar, however the hydration rate of the C₄AF was much lower than that of that of the C₃A phase at long-term.

The hydration products of OPC Ref and OPC-Sed cements as a function of time identified using XRD analysis are shown in Figure 9 (a, b) respectively. The XRD patterns indicated that all hydration products of hydrated OPC-Sed cement were similar to those formed in hydrated OPC Ref cement. In the two cement pastes, the Ca(OH)₂ amount has increased with a decrease in the calcium silicate amount (C₃S, C₂S). However, the hydration rate of C₃S was much higher than that of the C₂S phase when we observed the peak magnitude of both phases. After 28 days of hydration, total gypsum and C₃A phases were consumed, while C₄AF phase was still detected. It appeared in accordance with the modeling result as shown in Figure 8 (a, b), which also indicated that the hydration kinetics of cement phases were not identical.

In addition, it can be seen that in the two cement pastes, C-S-H gels were not identified by XRD analysis due to their amorphous structure (4444. Zhang Z, Scherer GW, Bauer A. 2018. Morphology of cementitious material during early hydration. Cem. Concr. Res. 107:85–100. https://doi.org/10.1016/j.cemconres.2018.02.004.
, 4545. Richardson IG. 1999. Nature of C-S-H in hardened cements. Cem. Concr. Res. 29(8):1131–1147. https://doi.org/10.1016/S0008-8846(99)00168-4.
). Hemi-carboaluminate (10.8°-2θ) and a low amount of mono-carboaluminate (11.7°-2θ) (46-4946. Antoni M, Rossen J, Martirena F, Scrivener K. 2012. Cement substitution by a combination of metakaolin and limestone. Cem. Concr. Res. 42(12):1579–1589. https://doi.org/10.1016/j.cemconres.2012.09.006.
47. Matschei T, Lothenbach B, Glasser FP. 2007. The role of calcium carbonate in cement hydration. Cem. Concr. Res. 37(4):551–558. https://doi.org/10.1016/j.cemconres.2006.10.013.
48. Matschei T, Lothenbach B, Glasser FP. 2007. The AFm phase in Portland cement. Cem. Concr. Res. 37(2):118–130. https://doi.org/10.1016/j.cemconres.2006.10.010.
49. De Weerdt K, Haha M, Ben Le Saout G, Kjellsen KO, Justnes H, Lothenbach B. 2011. Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash. Cem. Concr. Res. 41(3):279–291. https://doi.org/10.1016/j.cemconres.2010.11.014.
) were identified, while mono-sulfoaluminate (9.8°-2θ) was not detected. In Portland cement hydration, mono-sulfoaluminate was produced from the reaction between ettringite and excess of C₃A phase according to Equation [10], while hemi-carboaluminate and mono-carboaluminate were formed from the reaction between ettringite and calcite as described in Equation [11] and Equation [12] respectively (3939. Bentz DP. 1997. Guide to using CEMHYD3D: A three-dimensional cement hydration and microstructure development modelling package. National Institute of standards and technology. Retrieved from https://www.researchgate.net/publication/237062671_Guide_to_using_CEMHYD3D_A_three-dimensional_cement_hydration_and_microstructure_development_modelling_package.
). Moreover, calcite can to stabilize ettringite against the conversion to mono-sulfoamuminate (3939. Bentz DP. 1997. Guide to using CEMHYD3D: A three-dimensional cement hydration and microstructure development modelling package. National Institute of standards and technology. Retrieved from https://www.researchgate.net/publication/237062671_Guide_to_using_CEMHYD3D_A_three-dimensional_cement_hydration_and_microstructure_development_modelling_package.
, 4949. De Weerdt K, Haha M, Ben Le Saout G, Kjellsen KO, Justnes H, Lothenbach B. 2011. Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash. Cem. Concr. Res. 41(3):279–291. https://doi.org/10.1016/j.cemconres.2010.11.014.
). Thus, the presence of these phases in the two cement pastes could be attributed to the following reaction series. First, the CO₂ in the atmosphere reacted with Portlandite to form calcite (CaCO₃). Then, a part of the calcite reacted with ettringite to form hemi-carboaluminate and mono-carboaluminate.

The XRD patterns of OPC Ref and OPC Sed paste after 28 days of hydration are shown in Figure 10. It can be seen that the magnitude of the peaks in both XRD patterns is nearly similar. This result also confirmed that the sediment substitution did not influence the hydration behavior of cement, as indicated in the previous analyses.

The amount evolution of hydration products over time predicted in the CEMHYD3D is presented in Figure 11 (a, b) for OPC Ref and OPC-Sed cements respectively. For the two cement pastes, the modeling result also indicated an increase in Portlandite content with a decrease of anhydrous calcium silicate amount (C₃S, C₂S) as shown in Figure 8 (a, b) and Figure 9 (a, b). Contrary to the XRD analysis result, CEMHYD3D could predict the content of C-S-H gels formed in the two hydrated cements. As expected, the C-S-H gels were the primary hydration product with a volume fraction greater than 50% in the two cement pastes after 28 days of hydration. The model result showed that the formation of Ca(OH)₂ started later than that of C-S-H and it started to form at the end of the induction period as observed in the experiment (4343. Bullard JW, Jennings HM, Livingston RA, Nonat A, Scherer GW, Schweitzer JS, Scrivener KL, Thomas JJ. 2011. Mechanisms of cement hydration. Cem. Concr. Res. 41(12):1208–1223. https://doi.org/10.1016/j.cemconres.2010.09.011.
). However, it can be seen that in the CEMHYD3D, the content of ettringite decreased with increasing the content of mono-sulfoaluminate when the total gypsum was consumed. Additionally, the presence of a combination of hemi-carboaluminate and mono-carboaluminate hydrates was not predicted. This was contrary to the result obtained in the XRD analysis. The difference between the experiment and the model result can be explained the fact that the CEMHYD3D did not consider the reaction between CO₂ and Ca(OH)₂, which led to the formation of calcite, which had two effects on the cement hydration as indicated above.

3.2. Mechanical-microstructural development

The compressive strengths of OPC Ref and OPC-Sed mortars measured in the experiment and predicted in the CEMHYD3D are shown in Figure 12. The strengths of OPC-Sed mortar were slightly higher than those of OPC Ref at any time of testing. The compressive strengths of OPC Ref and OPC-Sed mortars were 55.08 MPa and 56.57 MPa respectively after 28 days of curing, and they could be classified as CEM I 52.5 according to the NF EN 197-1 (2929. Association Française de Normalisation (AFNOR). NF EN 197-1. 2012. Composition spécifications et critères de conformité des ciment courant.
). Based on this result, we also confirmed that a sediment substitution rate up to 7.55% had no significant influence on the mechanical properties of hydrated cement.

For the modeling result, it can be seen that there is a perfect agreement between the experiment and prediction for the compressive strengths of mortars at early ages (1, 2 days). As the experimental result, the strengths of OPC-Sed mortar predicted by CEMHYD3D were also slightly higher than those of OPC Ref mortar. However, comparing with the experimental strength obtained at 28 days, the model appeared to slightly overestimate the compressive strength of both mortars. This gap in the prediction can be attributed to the slightly higher value of DoH predicted in the CEMHYD3D compared with the experiment at long-term as shown in Figure 5. Indeed, the compressive strength was predicted using the gel-space ratio theory as described in Equation [5], so increasing the DoH of cement led to increasing the strength of mortar predicted in the CEMHYD3D.

The porosity evolution as a function of time of OPC Ref and OPC-Sed mortars obtained from the experimental measurement and numerical simulation is presented in Figure 13 (a, b) respectively. This experimental result indicated that the porosity decreased with time, from 16.05% at 1 day to 12.76% at 28 days for OPC Ref mortar and from 15.78% at 1 day to 12.88% at 28 days for OPC-Sed mortar. For the modeling result, it can be seen that for the two mortars, the MIP measured porosity was similar to the CEMHYD3D model prediction at early age but much higher at long-term (28 days). This gap could be attributed to the following possibilities: (1) the dimension of pixels in CEMHYD3D was 1 x 1 x 1 µm³, thus this model can measure the pores with a diameter ≥ 1 µm only, while the pore diameter determined by the MIP method included the small pores down to 0.006 µm (5050. Roy DM. Concrete: by Sidney Mindess and J, Francis Young, Prentice-Hall Inc, Englewood Cliffs.
); (2) high pressures of mercury in the MIP method might have damaged the pore structure leading to the increase in porosity (5151. Zhang S, Zhang M. 2006. Hydration of cement and pore structure of concrete cured in tropical environment. Cem. Concr. Res. 36(10):1947–1953. https://doi.org/10.1016/j.cemconres.2004.11.006.
).

Figure 14 presents the pore size distribution of both mortars measured by the MIP method after 1 and 28 days of hydration in order to verify the hypothesis above. As expected, for the two mortars, the pore diameter measured by the MIP method was much smaller than 1 µm, which explained why the MIP measured porosity was much higher than that predicted in the CEMHYD3D. However, the CEMHYD3D provided a good prediction of porosity when compared with the capillary porosity calculated according to Equation [4] for the two mortars. This is likely associated with the pore diameter that could be measured by Equation [4]. Indeed, it allowed determining the capillary porosity only in the size range of 0.01 to 10 µm (5151. Zhang S, Zhang M. 2006. Hydration of cement and pore structure of concrete cured in tropical environment. Cem. Concr. Res. 36(10):1947–1953. https://doi.org/10.1016/j.cemconres.2004.11.006.
). The diameter of these pores was much greater than the smallest diameter of pore determined by the MIP method. Thus, it explained why the porosity predicted in the CEMHYD3D was relatively similar to the capillary calculated porosity. These results indicated that the CEMHYD3D can be used to study the microstructure of Portland cement-based materials, but the porosity was limited to the capillary porosity. In addition, based on the results presented in Figure 9 (a, b) and Figure 10, we could also confirm that a sediment substitution rate up to 7.55% in raw meal had no impact on the durability of hydrated cement when we compared the total porosity as well as the critical diameter of two mortars.