1. Introduction
According to the European standard NF EN 197-1 [
1], “Cement is a hydraulic binder, i.e., a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens using hydration reactions and processes and which, after hardening, retains its strength and stability even under water”.
The cement is made of Portland clinker, calcium sulfate to control the setting and many minerals additions (slag, fly ashes, pozzolanic materials…etc.) [
1]. Cement production starts with extracting and grinding finely raw materials such as limestone and clay to obtain a raw meal. The fine powder is then heated to a sintering temperature of 1450 °C in a cement kiln [
2,
3]. In 1990, cement production emitted 0.783 kg of CO
2 to produce 1 kg of cement [
4]. However, with the effort from academics and the construction sector by introducing different processes to reduce the CO
2 impact. The method used impacts different stages of the cement production process, the most commonly used technique being the substitution of clinker with other materials (fly ash, slag, limestone, etc.) [
5,
6,
7,
8,
9], the energy efficiency and alternative fuels [
4,
10,
11,
12] and carbon capture and storage [
4,
10]. Due to all those improvements, in 2017, cement production emitted 0.667 kg of CO
2 to produce 1 kg of cement. However, the effort must continue to achieve carbon neutrality in the construction sector, and this is why the CEMBUREAU launched the “5C approach” to work more actively on the CO
2 emissions and energy consumption of the construction sector including the cement production [
13].
Our research project, “clinkerization by indirect mechanosynthesis” aims to reduce the cement production process’s energy consumption and CO2 emissions.
This article is the continuation of the previous paper [
14], which is part of our research project mentioned above. This project aims to produce clinker and cement using the mechanosynthesis process.
The mechanosynthesis is a high-energy ball milling process; the material to be milled is put in a container with balls or bars and will be subjected to repetitive welding/fracturing movement. Due to this movement, metastable or nanocrystalline phases can be produced and also transform crystalline phases into amorphous phases [
15,
16,
17].
With mechanosynthesis, we can reduce the size and combine solids from the micrometer scale of a powder particle. At the same time, it allows the size of the crystallites (the grains composing a particle) to be reduced to a nanometric size, resulting in nanostructured materials.
Mechanosynthesis can be used in three different ways [
18]; the first way is called direct mechanosynthesis, which allows the mechanical activation of materials introduced (one or more simultaneously) into the mill for a short or long period. For cementitious materials, direct mechanosynthesis is carried out for the mechanical activation of by-products for the clinker substitution. This activation improves mechanical performance, especially at a young age, and allows more by-products to be added [
19,
20,
21]. Different studies used this process to activate by-products for cementitious materials [
19,
20,
22,
23,
24,
25] or geopolymeric materials [
26,
27,
28]. Using planetary ball milling for mechanical activating slag and fly ashes and then substituting 50% CEMI cement, Bouchenfa et al. [
20] showed that wet milling with the optimal PCA (process control agent) improved the compressive strength of (50% CEMI/50% milled Fly ashes) for 28 and 90 curing days with +2.88% and +11.03%, respectively, than the reference (100% CEMI). Moreover, the authors [
20] found that for (50% CEMI/50% milled slag), the compressive strength increased by 16.1%, 20.9% and 40.7% for 7, 28 days and 90 curing days in comparison to reference. The authors attributed this increase to the pozzolanic reaction activity.
Alex et al. [
26] studied the influence of the high-energy grinding atmosphere (air or CO
2) on the geopolymers prepared from milled slag. The authors observed that both air and CO
2 milled slag found good geopolymerization behaviour leading to the high compressive strength of the geopolymer products. According to these studies, using mechanosynthetically activated materials shows improved mechanical performance of cementitious or geopolymeric materials. It has also been shown that activated fly ash improves the performance of bio-based mortars [
29].
The second way, known as indirect mechanosynthesis, consists of combining mechanical activation (short milling time) with another process. The second process can be a heat treatment (mechanically activated annealing (M2A), mechanically activated sintering (MAS)) [
30,
31,
32] or another milling (double mechanical alloying (dMA)) [
15]. The method used in this study is mechanically activated annealing, which is generally used in the production of refractory materials or ceramics [
33]. However, according to the state of the art and the information at our disposal, this method has never been used to produce clinker or cement.
The third way is called mechanochemistry, which refers to any process that uses grinding to trigger chemical reactions. The uses of mechanochemistry include all chemical reactions caused by mechanical activation, such as exchange reactions, reduction/oxidation reactions, decomposition of compounds and phase transformations [
34,
35]. For example, mechanochemistry is used to prepare a ready-to-use pregeopolymer powder that only needs water to prepare a geopolymer paste [
36].
This article is a continuation of a previously published paper [
14]. In this part of the project, the indirect mechanosynthesis process was carried out for the laboratory preparation of clinker from a premix (limestone/kaolinite) to obtain clinker production at a temperature of 900 °C.
In the previous study, 80% limestone and 20% clay were used without taking into account the different indicators used by cement manufacturers: lime saturation Factor (LSF), Silica Modulus (SM) and Alumina Modulus (AM). The results obtained showed that the clinker was of a belitic nature with a significant amount of lime. The cement showed a false set with a very high heat release. That is why in this study, a correction is carried out by adding raw kaolinite to the industrial premixes, considering cement indicators (LSF, SM, AM) for producing clinker and cement at temperatures of 900 °C and 1200 °C.
Structural (XRD, PSD) and microstructural studies (SEM) of clinker and cement manufactured by indirect mechanosynthesis were carried out. The hydraulic properties of these cements were also tested.
4. Conclusions
The previous work published in 2020 shows that clinkerization by mechanosynthesis allows us to produce clinker with heat treatment of 900 °C. In this work, we have continued the manufacture of cement and clinker using an indirect mechanosynthesis process by introducing before milling.
It was found that the cements produced are belitic cements. Phase quantifications by TOPAS software showed us that the belite is generally around 70%, with a more significant amount produced using 10% (92.29%) at 1200 °C. It was also noted that there was a significant production of C4AF in cements produced at 900 °C. The presence of belite is confirmed by the SEM analysis, with spherical particles characteristic of belite particles.
Regarding the mechanical performances of the cements produced by indirect mechanosynthesis, it is remarked that in general, the performances at 7 curing days of the pastes produced with our cements are weaker with 10% 3.60 and 7.60 MPa at 900 °C and 1200 °C, respectively, than the pastes produced with commercial CEM I and CEM III cements (23.03 and 19.14 Mpa, respectively).
The structural analysis of the cementitious pastes produced with our process showed that at 7 days, the quantity of remaining belite is high, resulting in a lack of reactivity. In addition, the production of portlandite is also low, which corroborates the lack of reactivity of C2S and the low presence of C3S in our cements. Finally, the important presence of ettringite in our cements is due to the hydration of C3A and C4AF in the anhydrous cement.
The next step of the study will be to work on the tempering of the products after the thermal treatments because we suspect that during the slow cooling of the product, transformations in the different structures are produced.