1. Introduction
The cement manufacturing process is one of the main contributors to global warming as well as climate change [
1]. In 2021 approximately 4.1 billion tonnes of cement were manufactured [
2] and the total carbon dioxide produced in cement plants was equal to approximately 2.8 billion metric tonnes, namely about 7% of global anthropogenic CO
2 emissions [
3]. Nowadays the cement industry is aimed at lowering the CO
2 emissions to 1.55 billion metric tonnes/year by the end of 2050 [
4]; therefore, cement producers and the researchers are looking for new hydraulic binders manufactured with reduced CO
2 emissions (low-CO
2 cements) [
5,
6,
7,
8,
9,
10,
11,
12,
13,
14]. Low-CO
2 cements can be manufactured using three different modes, that is: I) the utilization of non-carbonated sources of calcium oxide for limestone in the raw mix for the synthesis of Portland clinker [
15,
16]; II) the increased production of ordinary Portland cement (OPC) blended with supplementary cementitious materials (SCMs) [
17,
18]; and III) a wider employment of special binders (SCs), namely cements produced from non-Portland clinkers [
19,
20].
SCs display peculiar technical properties useful in specific fields of application; in addition, their composition can be exploited to give more environmentally friendly features to their production process. Nowadays, there is a noticeable interest towards calcium sulfoaluminate (CSA) cements for their relevant technical properties [
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32] coupled with environmentally friendly features [
33,
34,
35]. CSA cements are obtained by mixing a clinker (derived from heating a meal composed by natural gypsum, bauxite and limestone) with a source of calcium sulfate (e.g. anhydrite, natural gypsum); CSA binders contain calcium sulfoaluminate (3CaO·3Al
2O
3·CaSO
4, ye’elimite) as principal constituent and, on the basis of the burning temperature, type and proportion of raw materials, calcium sulfates, dicalcium silicate (2CaO·SiO
2, belite), calcium aluminium ferrite (brownmillerite, 4CaO·Al
2O
3·Fe
2O
3) calcium sulfosilicate (sulfospurrite, 4CaO·2SiO
2·CaSO
4) and several calcium aluminates.
The CSA cements technical features (e.g. rapid hardening, high impermeability, excellent dimensional stability) are mainly due to the ye’elimite hydration with CaSO
4c which lead the formation of 3CaO·Al
2O
3·3CaSO
4·32H
2O (ettringite) [
20].
Compared to ordinary Portland cement, CSA cements display the following sustainable characteristics: 1) reduced synthesis temperatures (<1350°C); 2) lower limestone requirement (usually <35%) in the clinker-generating raw meal; 3) lower energy for the cement milling; and 4) wider utilization of industrial wastes in the cement manufacturing process [
36,
37,
38,
39,
40,
41,
42,
43,
44]. In order to further decrease the generation of CO
2 and reduce their high costs (mainly depending on the utilization of bauxite), CSA cements can be blended with SCMs [
45,
46,
47,
48,
49,
50,
51,
52,
53].
Water reservoirs store fresh water which is generally piped to electric power stations and/or used for drinking as well as irrigation. The most important issues for these basins are silting phenomena which cause the reduction of the original storage capacity. To recover the original capacity, dredging operations are carried out: they generate large amounts of sediments which are generally landfilled. The research conducted so far to avoid the landfill, has evaluated the utilization of dredged sediments for the manufacture production of lightweight aggregates, Portland cement clinker, bricks, as component for stabilized road-base as well as supplementary cementitious material in blended cements [
54,
55,
56].
This paper aimed at investigating the use of thermally treated (TT) clayish reservoir sediments (RS) as SCM in blended CSA binders; the best treatment temperature (BT) allows the total conversion of crystalline clay phases in amorphous constituents (process of dehydroxylation) [
57]. Four TTRS
BT-based CSA binders were investigated by means of hydration and physical-mechanical tests for curing times falling within the interval 4 hours - 56 days. A plain CSA binder was employed as a benchmark. The effect of TTRS
BT on both hydration evolution and technical behaviour of CSA-blended binders was evaluated by using differential-thermal-thermogravimetric (DT-TG) and X-ray diffraction (XRD) measurements coupled with mercury intrusion porosimetry (MIP), dimensional stability and mechanical tests.
3. Results
The chemical analysis for CSA cement, RS and TTRS
BT is illustrated in
Table 1 which also shows the mineralogical composition of the binder evaluated by the Rietveld method (normalizing the results to only the detected crystalline phases).
The chemical analysis on C_R reveals calcium, aluminium, sulfur and silicon oxides are, in the order its main components. Moreover, the Rietveld evaluation denotes that 4CaO·3Al2O3·SO3 (43.0 mass%), β-2CaO·SiO2, 4CaO·Al2O3·Fe2O3, 3CaO·Al2O3 (tricalcium aluminate) and 2CaO·Al2O3·SiO2 (gehlenite), are, in the order, its main crystalline components; CaSO4, mainly coming from the added natural anhydrite to the clinker, is also present. As far as the reservoir sediments are concerned, from the results of the chemical composition, it is found that silicon and aluminum oxides represent their principal components; additionally, CaO, as also revealed by DT-TG investigation, is present as CaCO3. Therefore, the RS l.o.i. is due to both CO2 and water of the argillaceous minerals.
Figure 1 displays the XRD patterns for RS together with its corresponding thermally treated (at 750°, 830° and 900°C) materials.
Muscovite, kaolinite, quartz and calcite are the key-crystalline components for RS; XRD patterns for the TTRS display: I) the disappearance of kaolinite already at 750°C; II) the reduction of muscovite with temperature; and III) the presence of alumoakermanite (identified for the first time at 830°C) whose main peak increases as temperature rises. Consequently, 830°C (BT) represents the best temperature for avoiding the generation of novel unwanted phases coupled with the disappearance of phases already present.
The Blaine specific surface areas (EN 196-6) are 4500 for CSA and 3750 cm2/g TTRSBT.
Figure 2 reports the DT results for C_R, C_15, C_25 and C_35 cured at 4 hours and 1, 28 and 56 days. From the data published on [
58], the following endothermal effects (in the order of increasing temperature) were assigned to ettringite (E) and Al(OH)
3 (aluminium hydroxide, AH).
Except for the peak of CaCO3, no relevant thermal (exo/endo) effects are present over 500°C; for the examined cement pastes the endothermal dehydration peaks, attributed to E and AH (already detectable after 4 hours of hydration), are respectively observed at 164°±5°C and 287°±4°C.
Overall, the thermogravimetric analyses reveal that the hydration behaviour is basically due to the reaction of 4CaO·3Al
2O
3·SO
3 with CaSO
4. The hydration behaviour of the systems was also assessed by means of the determination of the bound water content (CBW,
Figure 3); it was evaluated considering the weight loss values to 500°C and normalized to 100 g dry cement.
From
Figure 3 it can be argued that the 4 systems follow an analogous trend during the first 24 h; at that period the hydration rate is quite high because of the fast ettringite formation; from 7 days of curing, the curves exhibit an almost constant value.
XRD patterns (
Figure 4) almost confirm the indications obtained from both DT analyses and bound water results; therefore, concerning the hydration products, ye’elimite and calcium sulfate fast react and 3CaO·Al
2O
3·3CaSO
4·32H
2O concentration grew up to 28 days of aging; the aluminium hydroxide main peak is also found in all the hydrated systems.
Furthermore, belite was virtually not involved in the hydration process, owing to both the higher reaction kinetics of calcium sulfoaluminate and the related rapid water consumption. Quartz signals, already present in the TTRS
BT samples, are detected in all the investigated CSA-blended cements.
Figure 5 shows the evolution of ettringite
vs. curing time. Elevated quantities of ettringite already forms after 4 hours of hydration in all the systems; after 2 days, the 3CaO·Al
2O
3·3CaSO
4·32H
2O concentration reaches almost its maximum value and, since then, sliglthly rises until 28 days of curing.
The curves related to the expansion (under water)–shrinkage (in air) tests are reported in
Figure 6.
Both in air and submerged under water, the behaviour of the three blended cements was almost similar to each other and to that of the reference system. When aged in air it is evident a constant shrinkage until nearly 2 weeks, when a lowest value is attained (-0.18%, -0.13%, -0.08% and -0.07% for C_R, C_15, C_25 and C_35, respectively); afterwards, the shrinkage results are almost consistent. The highest values of expansion under water, reached after approximately 20 days of aging, are included in the range 0.06-0.17%.
In
Figure 7 the plots of the porosimetric tests for the pastes of the four examined cements are reported; its left and the right side respectively represent the derivative and cumulative pore volume (CPV) curves related to the intruded mercury for calcium sulfoaluminate-based cements towards pore radius for aging periods ranging comprised in the interval 16 hours-56 days. For C_R (
Figure 7 (A)), all the derivative plots display a unimodal distribution [
20].
Over the aging period, the CPV as well as the threshold pore radius slightly reduce (from 142 to 123 mm3/g and from nearly 25 to 16 nm, respectively). Compared to C_R, the TTRSBT-based cements show a similar behaviour; in fact, a unimodal pore size distribution is observed at any examined aging period. Moreover, for C_15, the CPV decreases of nearly 30% passing from 201 (at 16 hours) to 140 mm3/g (after 2 days); at longer aging times it further reduces (about 100 mm3/g after 56 days of hydration). Similarly, the pore width ranges from 274 (at 16 hours) to 120 nm (at 56 days). For C_25 and C_35, the CPV respectively lowers of approximately 39% and 16% (going from 207 to 118 mm3/g and from 190 to 160 mm3/g, respectively) in the same time interval; compared to C_15, C_25 and C_35 exhibit a pore size distribution oriented towards higher radii at all the investigated curing times, being respectively comprised in the intervals 315-160 nm and 400-240 nm.
As a consequence of the fast hydration products generation in C_R (revealed by both XRD and DT-TG analyses), a region of lower porosity is quickly stated; therefore, the hydration products lower and separate the inner space. At higher aging periods, the porosity development goes gradually inasmuch as the hydration has approximately stopped. Due to the lack of phases (e.g. calcium hydroxide) able to react with thermally treated reservoir sediments the investigated blended cements, in comparison with C_R, always display a higher pore size distributions.
The total porosity percentage (TP) results
vs. of aging period related to the four investigated cements are displayed in
Figure 8; the histograms evidently display a comparable trend for all cement pastes inasmuch as their TP values lower as curing time increases.
In particular, at all the hydrations periods the plain CSA shows the lowest TP values; moreover, it is evident that the higher CSA cement substitution the higher TP values. At the last investigated period, the total porosity for C_35 (23.2%) was about 13% and 30% higher than those exhibited by C_25 and C_15, respectively.
In
Figure 9 the compressive strength results for calcium sulfoaluminate-based mortars are reported; the strength results for the 3 blended binders were always lower than those for C_R.
As the amount of TTRS in the blends increases, the compressive strength values decreases, thus revealing an almost inert behaviour of the reservoir sediments: this phenomenon is most probably due to the lack of a lime source able to react with the TTRSBT amorphous silica and alumina in the cement pastes.
Figure 1.
XRD patterns for RS and the corresponding samples burnt from 750° to 900°C. Aa=alumoakermanite ((Ca,Na)2(Al,Mg,Fe++)(Si2O7)); A=anorthite (CaAl2Si2O8); C=calcite (CaCO3); K=kaolinite (Al2Si2O5(OH)4); M=muscovite (KAl2(Si3Al)O10(OH,F)2); Q=quartz (SiO2).
Figure 1.
XRD patterns for RS and the corresponding samples burnt from 750° to 900°C. Aa=alumoakermanite ((Ca,Na)2(Al,Mg,Fe++)(Si2O7)); A=anorthite (CaAl2Si2O8); C=calcite (CaCO3); K=kaolinite (Al2Si2O5(OH)4); M=muscovite (KAl2(Si3Al)O10(OH,F)2); Q=quartz (SiO2).
Figure 2.
DT curves for cement pastes (a) C_R, (b) C_15 (b), (c) C_25 and (d) C_35 at 4 hours, 1, 28, and 56 days of hydration. E=ettringite; AH=aluminium hydroxide.
Figure 2.
DT curves for cement pastes (a) C_R, (b) C_15 (b), (c) C_25 and (d) C_35 at 4 hours, 1, 28, and 56 days of hydration. E=ettringite; AH=aluminium hydroxide.
Figure 3.
Chemical bound water calculated by TG until 56 days of aging (g/100 g of unhydrous cement) for C_R, C_15, C_25 and C_35 cements pastes vs aging time.
Figure 3.
Chemical bound water calculated by TG until 56 days of aging (g/100 g of unhydrous cement) for C_R, C_15, C_25 and C_35 cements pastes vs aging time.
Figure 4.
XRD patterns of C_R (A), C_15 (B), C_25 (C) and C_35 (D) hydrated for 4 (down) hours and 2 (middle) and 28 (up) days. A=anhydrite; E=ettringite; G=aluminium hydroxide; Q=quartz; Y=ye’elimite.
Figure 4.
XRD patterns of C_R (A), C_15 (B), C_25 (C) and C_35 (D) hydrated for 4 (down) hours and 2 (middle) and 28 (up) days. A=anhydrite; E=ettringite; G=aluminium hydroxide; Q=quartz; Y=ye’elimite.
Figure 5.
Ettringite concentration (evaluated by means of the Rietveld analysis) for C_R, C_15, C_25 and C_35 evaluated in the interval 4 hours-28 days.
Figure 5.
Ettringite concentration (evaluated by means of the Rietveld analysis) for C_R, C_15, C_25 and C_35 evaluated in the interval 4 hours-28 days.
Figure 6.
Expansion/shrinkage plots for the four CSA systems (cured in water and in air).
Figure 6.
Expansion/shrinkage plots for the four CSA systems (cured in water and in air).
Figure 7.
Derivative (right) and cumulative (left) mercury intruded volume vs. pore radius for C_R (a), C_15 (b), C_25 (c) and C_35 (d) hydrated for 16 hours and 2, 28, 56 days.
Figure 7.
Derivative (right) and cumulative (left) mercury intruded volume vs. pore radius for C_R (a), C_15 (b), C_25 (c) and C_35 (d) hydrated for 16 hours and 2, 28, 56 days.
Figure 8.
TP for C_R, C_15, C_25 and C_35 pastes hydrated for 16 hours, 2, 28 and 56 days.
Figure 8.
TP for C_R, C_15, C_25 and C_35 pastes hydrated for 16 hours, 2, 28 and 56 days.
Figure 9.
Results of compressive mechanical tests of C_R, C_15, C_25 and C_35 at different curing times.
Figure 9.
Results of compressive mechanical tests of C_R, C_15, C_25 and C_35 at different curing times.
Table 1.
Chemical and phase compositions (solely for C_R) of the employed cement components, mass %.
Table 1.
Chemical and phase compositions (solely for C_R) of the employed cement components, mass %.