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 (9, 10), 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 (24). The proportion of other raw materials in these mixtures must respect the following proportions: L M 1 L M 2 + L M 3 =1.5 and C L A Y 2 C L A Y 1 =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 (9). After the clinkering, the free lime content (CaO_free) of two clinkers was measured using the Schlafer-Bukolowki method (25). The chemical compositions of clinkers were measured using XRF analysis, while the mineralogical compositions were obtained using the Bogue formula (26) according to following Equations:

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 (27). 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 (28).

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 (9). 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 (29). 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.

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 (40).

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 (43-45). 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 (44, 45). Hemi-carboaluminate (10.8°-2θ) and a low amount of mono-carboaluminate (11.7°-2θ) (46-49) 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 (39). Moreover, calcite can to stabilize ettringite against the conversion to mono-sulfoamuminate (39, 49). 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 (43). 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.

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 (29). 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 (50); (2) high pressures of mercury in the MIP method might have damaged the pore structure leading to the increase in porosity (51).

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 (51). 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.