3.1. Clinkers Characterization
One of the factors used to assess the quality of clinkers obtained by the calcination of raw mixes is free lime content. Higher values of free lime in clinkers could be detrimental in that they could determine an important volume expansion during the hardening of cement and, consequently, an important reduction in the mechanical strength; these types of cements are classified as unsound [
36]. Therefore, it is recommended to keep the free lime content in clinker below 2% [
30,
31,
32,
37].
The free lime content assessed by chemical methods on the clinkers obtained by the calcination of raw mixes based on clay or marl, with sand or waste glass admixtures, are presented in
Figure 3.
As expected, the increases in the calcination temperature and plateau duration determine the decrease in free lime content, both in reference clinkers (Rc and Rm) and clinkers resulting from the raw mixes with WG content (Gc and Gm). For the raw mixes with waste glass, the calcination temperature can be reduced by 20 °C, i.e., the clinkers Gc and Gm have a free lime content below 2% when the calcination is performed at 1430 °C with a 30 min plateau. This decrease is explained by the lower melting point of soda-lime glass (below 1000 °C); the addition of this type of waste glass to cement raw mixes decreases the eutectic temperature of these compositions [
24].
Figure 4 shows the XRD patterns of the Rc and Rm clinkers obtained at 1450 °C with a 30 min plateau and of clinkers with WG (Gc and Gm) calcined at 1400 °C with a 20 min plateau and at 1430 °C with a 30 min plateau. As can be seen, in all clinkers, the crystalline phases detected by XRD are alite (3 CaO·SiO
2), belite (2 CaO·SiO
2), calcium iron aluminum oxide Ca
2(Fe0
,654Al
1,346)O
5 and 3 CaO·Al
2O
3, which are the main mineralogical phases usually assessed in Portland clinkers.
Interestingly, these analyses did not provide information regarding the formation of new phases with alkali content in the clinkers with WG, as suggested in other studies [
7,
24]. The same mineralogical compounds identified in reference clinkers are present in the XRD patterns of Gc and Gm clinkers. One can assume that alkali ions brought into the system by the waste glass addition are incorporated (solid solutions) in the main mineralogical phases, i.e., alite, belite, calcium iron aluminate and tricalcium aluminate [
30].
Figure 5 presents the phase amounts resulting from the Rietveld refinement performed on the samples in which the WG was introduced into the raw mix based on clay and the raw mix based on marl. One can notice a clear decrease in C
3S amount in the specimens with waste glass content as compared with the references, correlated with a small increase in C
2S content.
SEM and EDS analyses performed on clinkers provide supplementary information regarding their microstructure. The SEM analyses of the two reference clinkers with sand and clay or marl as raw materials are presented in
Figure 6.
One can notice the porous structure of clinker particles (some pores are indicated by arrows in
Figure 6a,d). At higher magnifications (
Figure 6b,c,e,f), one can assess the main specific phases, namely alite (3CaO·SiO
2), being euhedral particles with clear edges and smooth faces [
38,
39], and belite (2CaO·SiO
2), being particles with round shapes [
39,
40] partially covered with solidified melt (
Figure 6f), as well as an interstitial phase (calcium aluminate and calcium ferrite aluminate phases) with a specific elongated “honeycomb” structure [
39].
The EDS elemental maps presented in
Figure 7 confirm the wide distribution of calcium in all phases, of silicon in the euhedral and rounded particles, and of aluminum and iron mainly in the interstitial phases. One can also notice in the analyzed micro-area the presence of potassium localized mainly in the interstitial phases.
The SEM images of the clinkers obtained by calcination at different temperatures and plateaus of the raw mix with clay and WG are presented in
Figure 8.
Both clinkers contain porous particles (
Figure 8a,d). At higher magnifications, one can assess the alite and belite particles partially covered with the solidified melt, as well as the interstitial phase (calcium aluminate and calcium aluminate ferrite phases) with the previously described morphologies.
The elemental mapping of Gc clinker obtained by thermal treatment at 1430 °C with a 30 min plateau (
Figure 9) shows the presence of Na (in low amounts), along with Ca, Si, Al and Fe. Sodium seems to be distributed in all phases present in the analyzed micro-area, which could confirm its presence in the silicate and aluminate phases [
41].
The SEM images of clinkers obtained by the calcination of raw mixes with WG and marl are presented in
Figure 10.
The microstructure is similar to the one assessed for the clinkers with clay, having porous particles; moreover, at higher magnifications, the alite, belite and interstitial phases are identified in the studied micro-area.
Based on the information presented so far, the clinkers with waste glass (Gc and Gm) were obtained by the calcination of raw mixes at 1430 °C with a 30 min plateau, and the reference clinkers (Rc and Rm) were obtained by the calcination of raw mixes at 1450 °C with a 30 min plateau.
The oxide composition of these clinkers was assessed according to the methods presented in the EN 196-2 standard [
33]. Based on these values, LSF, the silica ratio and the alumina ratio, were calculated [
41]. The alkali content is expressed as Na
2O equivalent. The results are presented in
Table 2.
As can be seen from
Table 2 and
Figure 5, all clinkers fulfill the requirements of the EN 197-1 standard [
42]:
- -
The sum of C3S and C2S (assessed by the Rietveld method) is between 79.20% and 84.80%, which represents more than two-thirds of the mass of clinker.
- -
The value of the CaO to SiO2 ratio is between 3.12 and 3.15 (the standard requires a minimum value of 2).
- -
MgO content is between 1.6% and 1.99%, much lower than the maximum admissible value of 5%.
The alkali content of the clinkers with WG is higher than those with sand, which was expected given the higher alkali content in the waste glass (see
Table 1).
A high content of alkalis in clinker/cement can negatively influence the properties of the concrete produced with this cement due to potential reactions between the alkalis released by the cement during hydration and the reactive silica which could be present in aggregates (alkali–silica reaction, ASR). Thus, to reduce the effect of potential ASR, careful selection of the aggregates, which should not contain reactive silica minerals, is recommended.
3.2. Influence of Waste Glass on the Burnability of Raw Mixes
The burnability of raw mixes is an important technological aspect. It provides information regarding the energy consumption necessary to transform the raw mix into clinker [
36,
41,
43]. The burnability is influenced by the composition of raw materials and raw mixes (LSF, SR and AR values, the content of minor components) as well as the particle size distribution of raw mix (especially the coarse fraction) [
41].
The burnability can be assessed based on experimental results (free lime content in clinkers calcinated at different temperatures and plateaus) or can be expressed by calculated indexes, based on oxide and Bogue compositions [
27,
43,
44]. In
Table 3 are some of these indexes, their usual (recommended) values and the values calculated for the studied clinkers.
As can be seen, all indexes calculated for the raw mixes based on clay or marl, with or without WG, are in the ranges specific for clinkers with adequate burnability.
A higher value of the percentage of liquid phase (
Fliq) indicates an easier burning of the raw mix [
43]. As can be seen from
Table 3, the raw mixes in which sand was substituted with waste glass have higher values for this index, confirming their better burnability, indicated also by the free lime content in clinkers calcined at various temperatures and plateaus.
Another important parameter for the assessment of raw mix burnability is the heat of reaction, which can be calculated with the formula proposed by Onoda [
43], shown in
Table 4.
The values presented in
Table 4 confirm the positive effect exerted by the replacement of sand with waste glass, i.e., a reduction in the heat of reaction compared to reference clinkers. Since most soda-lime glasses have a melting point below 1000 °C, the addition of this type of waste glass to cement raw mix decreases the eutectic temperature of these compositions [
24].
The reduction in the calcination temperature, and implicitly, the reduction in the theoretical consumption of heat necessary for the clinker formation, have positive effects on the environmental impact of clinker and cement production. This allows a decrease in fuel consumption, thus reducing the production costs and at the same time contributing to the reduction in associated CO2 emissions.
3.3. Cement Properties
The main characteristics assessed for cement, as stipulated in the norm EN 197-1, are:
- -
Chemical characteristics: loss on ignition (LOI), insoluble residue, sulfate and chloride contents; and
- -
Physical and mechanical properties: initial setting time, soundness and compressive strengths assessed on mortars after 2, 7 and 28 days of curing.
Given the high alkali content of clinkers with waste glass, the alkali content of cements resulting from the intergrinding of clinkers with 5% gypsum (CEM I) and with 5% gypsum and 30% slag (CEM II/B-S) was also assessed by the method specified in the EN 196-2 standard.
The chemical characteristics of these cements are presented in
Table 5. The cements obtained from clinkers with glass waste (Gc and Gm) meet the chemical conditions stipulated in the EN 197-1 standard, the determined values being below the maximum limits imposed by this norm.
The use of granulated blast furnace slag as a grinding addition for the preparation of CEM II/B-S cements based on Gc and Gm clinkers (slag partially substituting clinker in cement composition) leads to a decrease in the alkali content, as compared with the corresponding CEM I cements. Furthermore, the alkali content in CEM I cements obtained from Gc and Gm clinkers is smaller as compared with the alkali content of corresponding clinkers (see
Table 2), while the CEM II/B-S cements have an alkali content 40% smaller as compared to the corresponding clinkers. Additionally, slag addition to the cement could mitigate the potential harmful alkali–silica reactions [
23].
The physical characteristics of cements are presented in
Table 6 and the mechanical characteristics are presented in
Figure 11.
All cements fulfill the conditions imposed by the EN 197-1 standard for these properties (
Table 6). The presence of waste glass in the raw mixes does not significantly affect the initial setting time and soundness of cements. The cements based on marl seem to have a slightly longer initial setting time as compared with those based on clay for both types of cement, CEM I and CEM II/B-S.
The mechanical properties, i.e., the values of compressive strength, presented in
Figure 11, correlated with the values of initial setting times (
Table 6), leading us to the following conclusions:
- -
All CEM I can be classified in the 42.5 R class; and
- -
In the case of CEM II/B-S cements, the one obtained from raw mix with clay (CEM II/B-S Gc) can be classified in the 42.5 N class, while those obtained from raw mixes with marl are included in the 32.5 R class.
The compressive strength values are correlated with the mineralogical composition of the cements assessed by the Rietveld method (
Figure 5), i.e., the decrease in the C
3S amount in the clinkers with waste glass content (Gc and Gm) explains the lower compressive strength recorded for these cements (compared to the references). The partial substitution of clinker with slag determines, as expected, a reduction in compressive strength values, more important for a short hardening time (2 days).
The effects of the use of waste glass, clay and marl as raw materials on the environmental impact (CO
2 emissions) of clinker and cement production are presented in
Table 7. The CO
2 emissions were calculated based on the theoretical calculation of the greenhouse gas (CO
2) emissions resulting from the limestone decarbonation process.
From the data presented in
Table 7, one can notice a reduction in CO
2 emissions with 11.32 and 12.70 kg CO
2/t clinker when waste glass replaces sand in the raw mix composition. The higher values obtained for the raw mixes with marl (as compared to those with clay) are due to the limestone content of marl (see
Figure 1). Moreover, the CO
2 emissions associated with CEM I production decrease by 5% and those associated with CEM II/B-S production decrease by 35%, with reference to the CO
2 emissions associated with the production of 1 t of clinker with WG.