This work aims to synthesize new cementitious materials (binders) using marble powder, rice husk ash, activated laterite and NaOH solution by applying low energy process. The binder was used to stabilize solid precursors (laterite and pozzolan). To achieve this objective, calcium–silicate–hydrate (CSH) was first synthesized at different temperatures (26, 50, 80 and 100 °C). The best physical–mechanical properties were chosen to produce iron–calcium–aluminium–silicate–hydrate [Fe–C(A)SH] at different concentrations of sodium hydroxide solution: 4, 5, 6 M. Finally, the formulated binder at 6 M of NaOH solution was used to stabilize laterite and pozzolans at the following proportions 20%, 30%, 40% and 50%. The samples were characterized after 28 days of curing at room temperature. FT-infrared spectroscopy, X-ray diffraction, and environmental scanning electron microscope ESEM-EDS permitted to confirm the formation of CSH, and Fe–C(A)SH. The mechanical test used to evaluate the performance showed that the incorporation of 10% iron-rich laterite into CSH increased the strength up to 42.93 MPa and the addition of Fe–C(A)SH in the laterite/pozzolans increased the compressive strength of the final product (15.34 and 15.8 MPa for laterite and pozzolan, respectively). The highest concentration (6 M) increases the alkalinity and reduces the efficiency of silicate polymerization affecting the final structural compound. From the results, low-energy Fe–C(A)SH-based cement and stabilized compounds appeared promising for the development of sustainable infrastructures.
Introduction
Generally, most buildings are made of concrete whose principal component is Portland cement. However, studies have shown that the production of this Portland cement requires large amounts of energy and emits CO2 into the environment which is harmful to human health. According to Bameri et al. [1], annual cement production worldwide has reached more than 4.2 billion tonnes and is anticipated to grow continuously. As the production of cement increases, the pollution also increases by the emission of carbon dioxide which represents 8–10% of gas emissions in the world [2‐4]. The calcination of limestone (CaCO3) to obtain lime (CaO) released more than 50 wt% of carbon dioxide in the manufacturing process to produce cement, the rest of the CO2 emitted results from the burning fuel to provide the thermal energy necessary for calcination that takes place at the temperature around 1400 or 1500 °C. For the production of one tonne of cement, an average of 100–110 kWh of electricity is consumed, which represents 30–40% of the cement production costs [5, 6]. Then, the need for low-energy processes for local materials valorization is imperative in addressing the global quest for reducing the CO2 emission and enormous energy needed by ordinary Portland cement processing. In this view, new types of environmentally friendly cementitious binders (low energy consumption, low carbon dioxide emission) can then be applied to construction such as geopolymers-based binder [6] and blast furnace slag, fly ash-based alkali-activated material [7], and glass fibres from waste composites [8].
In 1957, Kalousek showed that the reaction between kaolin and CSH gives a structure more stable (CASH) than CSH due to the substitution of Si4 + by Al3+ [9]. Many other works have been done using the insertions of aluminium into the C–S–H structure by varying the Ca/(Al + Si) ratio [7, 9, 10]. The results show that the incorporation of aluminosilicate precursors into CSH enhances strength under several environmental conditions according to [11‐13]. To study the impact of iron, iron sources like laterite can be incorporated into a C(A)SH due to the affinity between Al3+ and Fe3+. Laterite and lateritic soil are raw materials abundantly available in tropical [14] areas. They appear as an alternative aluminosilicate precursor to replace conventional fly ash, metallurgical slag or other industrial wastes or by-products in a binder or alkali-activated material [15, 16]. According to [11, 14], laterite is formed by the laterization of mother kaolinitic rock with the substitution of partial kaolinite by iron mineral phases. Additionally, laterite is composed of amorphous kaolinite in which a high proportion of Al3+ is replaced by Fe2+ or Fe3+.
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The nature of the laterite plays an important role in the reactions to it. Kaze et al. [17] observed that the best calcination temperature of laterite is lower than 600 °C, and above such temperature, there is the sintering of particles which affects the dissolution consequently the physico-mechanical properties of laterite-based geopolymers due to the formation of microcracks. In the same way. Kamseu et al. [6] show that laterite is more reactive when cured by the hydrothermal method. The incorporation of iron-rich laterite into the CSH (calcium–silicate–hydrate) phase of cement presents a compelling theoretical framework that warrants further exploration and debate. Kamga et al. [18], when studying the evolution and microstructure of iron-rich laterite-cement based composite observed that the pozzolanic reaction that follows the bonding of laterite with cement are responsible for the formation of CASH and Fe–C(A)SH phase which are gismondine and stratlingite. They also found that the presence of the original phase as goethite can easily dissolve by the alkalinity of cement [18]. This makes it evident that most of the mineral’s phases formed will have iron in their structure. Theoretically, iron compounds have been shown to influence the hydration process by acting as catalysts for the formation of stable crystalline phases within the CSH structure. This innovative approach offers potential advancements in cementitious materials, with implications for strength, durability, and sustainability.
The main objective of this study is to synthesize CSH binders by hydrothermal method at different temperatures (26, 50, 80, and 100 °C) with NaOH solution at different concentrations, then, target the best temperature to incorporate iron aluminosilicate (laterite) to CSH to obtain Fe–C(A)SH. The second objective of this work is to stabilize laterite and pozzolans with the Fe–C(A)SH binder having the best physic mechanical properties.
Materials and experimental method
Raw materials and chemical composition
The laterite, composed of iron–alumina–silica was collected from the deposit at Montee des Soeurs located in Yaounde Cameroon. After crushing, laterite was ground for 4 h in a drum mill (VICENTINI, Officine-Fonderie di Covazzole, Italy) to have fine powders with particles below 100 µm. The ground powder was calcined at 600 °C for 4 h in a programmable electric furnace with a thermal treatment rate of 5 °C per minute (ISUNI, type MN 51 A).
Natural pozzolan used in this study was collected in the Galim West region of Cameroun. The collected pozzolan was firstly oven-dried at 105 °C for 2 days to remove moisture and subsequently ground and sieved below 100 µm.
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Marble powder is an industrial waste material collected from Figuil. The powder was sieved below 100 µm and calcined at 850 °C for 24 h in a programmable electric furnace (ISUNI, type MN 51 A) using a heating rate of 5 °C per minute. The calcined calcium oxide was sealed in plastic paper to limit the carbonization.
The filler/aggregate used is quartz sand (QS) which was collected from the Sanaga River located in the centre region of Cameroon. The obtained normalized quartz sand has a specific gravity of 2.55 with particle sizes in the range of 60–100 µm.
Rice husk was collected from the Ndop Northwest region of Cameroon. The silica source, rice husk ash (RHA) was obtained from the calcination of rice husk at 600 °C for 2 h.
The activating NaOH solutions of 4, 5, and 6 M were obtained by dissolving pellet caustic soda, laboratory grade (98%, Sigma Aldrich, Italy). In distilled water, the alkali solution was stored and cooled to room temperature.
According to the results in Table 1, the ratios of Al/Si, Al/Fe, and Fe/Si of the laterite used in the study were 0.85, 0.66, and 1.29, respectively, indicating its suitability as a source of cementitious materials. The natural pozzolans analysed showed a high silica content of 41.36%, relatively low alumina content of 15.31%, and 12.88% FeO–Fe2O3. Tchakounte et al. (2013) found that the ratio values of Al/Si, Al/Fe, and Fe/Si for natural pozzolans were 0.44, 1.87, and 0.23, respectively, further supporting their potential as sources of cementitious material. These findings emphasize the importance of precursor nature in determining its suitability as a source of cementitious materials.
Table 1
Chemical compositions of the raw materials from XRF
Oxide
RHA
Laterite
Marble powder
Natural pozzolans
Quartz sand
SiO2
90.54
25.12
0.9
41.36
49.69
Al2O3
0.53
17.96
0.2
15.41
13.23
Fe2O3
0.57
42.50
–
12.88
5.76
TiO2
2.55
1.21
–
3.04
1.38
CaO
0.59
0.07
95.5
7.98
1.75
Na2O
0.17
–
–
2.22
K2O
2.31
0.12
–
0.90
2.13
MgO
0.37
–
2.8
6.45
–
SO3
0.17
0.04
–
–
0.83
ZrO2
0.01
0.05
–
–
0.17
MnO
0.53
0.06
–
0.2
0.05
P2O5
1.05
0.18
–
0.48
0.19
LOI
1.2
11
–
9.31
–
Total
99.4
97.94
99.4
100.10
75.65
In addition, the marble powder was reported to contain 95.5 wt% of calcium oxide [19]. In addition, the SiO2 content of rice husk (RHA) and sand was found to be 90.54 wt% and 49.69 wt%, respectively.
In summary, the results show that the laterites and natural pozzolans investigated in this study exhibit good properties for use as cementitious materials. The specific ratios of Al/Si, Al/Fe, Fe/Si, and chemical elements such as silica content play an important role in determining the suitability of the material as a source of cement.
Preparation of alkali-activated materials
Synthesis of calcium–silicate–hydrate (CSH) binder
The CSH binder was prepared by direct reaction between marble powder {calcium hydroxide [Ca(OH)2]} and Rice Husk Ash [amorphous silicon dioxide (SiO2)]. Ca(OH)2 and SiO2 were added with different ratios (Ca/Si = 1, Ca/Si = 2, Ca/Si = 3) and then mixed for 4 min, followed by the addition of NaOH solution at the concentration 4, 5 and 6 M in the proportion of liquid/solid maintained at 0.265. Finally, the river sand (sand collected from the Sanaga River, Cameroon, with particle size below 100 µm) was added with the proportion paste/sand of 1/2. The homogenous slurry obtained was ball milled for 15 min before being poured in a cylindrical mould (with dimensions of 40 cm length and 20 cm width) at room temperature. The moulds were carried out in an oven 2 h later; the moulded pastes were then cured by steam curing at four different temperatures (room temperature (26 ± 3), 50, 80, and 100 °C) maintaining the relative humidity above 80% in a water bath for 3 h. The cooling continued in the water bath down to room temperature. The reaction product with different concentrations was demoulded and sealed into plastic paper to limit the carbonization. The CSH binder samples were aged for 28 days at ambient temperature before determining the compressive strength. It should be noted that the role of this step is to see the ability of the binder to bind sand. Samples were labelled CaSXM where a is a ratio Ca/Si and X is the molar concentration of NaOH solution.
Synthesis of iron–calcium–alumina–silicate–hydrate binder
Fe–C(A)SH binder was synthesized (standard EN 934) by a reaction between iron-rich laterite and calcium–silicate–hydrate (CSH). For the three Ca/Si ratios (Ca/Si = 1, 2, 3), iron–alumina–silicate source (laterite) was added at different percentages: 5, 7.5, 10, 12.5, and 15%; the two were mixed for 4 min followed by the addition of alkaline solutions. The obtained slurry was ball milled for 15 min before being moulded in a cylindrical mould (with dimensions of 40 cm length, 20 cm width), cured at room temperature for 2 h followed by applying a steam curing at 50 °C maintaining the relative humidity above 80% in a water bath for 3 h. The cooling continues in the water bath down at room temperature. Meanwhile, the concentration of NaOH solution, the proportion of paste/sand and the liquid/solid ratio were not changed. The Fe–C(A)SH binder was remoulded and sealed into a plastic paper to limit the carbonization during the curing extension up to 28 days before characterization.
Development of Fe–C(A)SH structural compounds
Two types of solid precursors were stabilized using Fe–C(A)SH binder namely laterite and natural pozzolans. Fe–C(A)SH binder was mixed to laterite and pozzolans at the proportion of 20, 30, 40, and 50% followed by the addition of 6 M NaOH solution in a proportion of liquid/solid equal to 0.265; finally, river sand was the double proportion of binder. The homogenous slurry was then milled in a porcelain mortar for 4 min and ball-milled for 15 min before moulded in a cylindrical mould. The paste was maintained (waiting time) at room temperature for 2 h and steam curing at the temperature of 50 °C keeping the relative humidity above 80% in a water bath for 3 h. The stabilized laterite and pozzolans were demoulded and sealed into plastic paper to limit carbonization. The curing continued for up to 28 days at ambient temperature before characterization.
Characterization methods.
Structural characterizations
X-ray diffraction (XRD) was used to investigate the phases evolution under the influence of the Ca/Si molar ratio and the alkalinity in CSH, Fe–C(A)SH, and stabilized compound. Samples were ground using a mortar and pestle and passed through 100 µm. The obtained powder was packed into a low-background sample holder. Experiments were performed using an analytical copper pro diffractometer (Bruker) with a copper target λKα1 = 1.5405 Å. The working conditions of the diffractometer were 40 kV and 40 mA. Measurements were made from 5° to 70°, 2θ at the rate of 10°/min with a step size of 0.02° 2θ. The diffraction pattern was analysed using the mean of EVA 21.0 software (Brucker).
FTIR was performed by a mean spectrophotometer NICOLET 6700 FTIR series, operating in an attenuated total reflectance (ATR) using a diamond crystal. The infrared spectra were digitally recorded in the range from 4000 to 500 cm−1 using a powder finely ground and sieved at 100 µm. FTIR provided detailed information on the local atomic structure of CSH, Fe–C(A)SH, and structural compound.
Setting time
The setting time was determined using the Vicat’s apparatus. The needle, 1.00 ± 0.005 mm in diameter, to determine the initial and the final setting times at which no indentations could be seen on the surface of the specimen and the time was measured using a digital chronometer. This test was carried out in the laboratory with a relative humidity of 70% room temperature of 26 °C and performed according to the EN196-3 standard (BS EN 196-3, 1990).
Mechanical characterization
The compressive strength was measured with an Instron 1995 mechanical testing machine with a displacement of 4 mm/min. For each hardened binder and structural composite cured for 28 days, a total of 3 specimens were tested for compressive strength, and the average result was presented. The strength is given by Eq. (1).
The water absorption and the bulk density were carried out using the Archimedes method according to ASTCM. This consists of immersing the specimens in water at ambient temperature for 24 h and comparing the humid weight (mh), dry weight (md) and suspended weight (ms) according to Eqs. (2 and 3). These measurements were carried out according to the Archimedes method using an electric balance with a sensitivity of ± 0.001 g. These samples were first dried in an oven at 64 °C until a constant mass was reached and their properties were evaluated.
The morphology and the detailed features of the microstructure of the samples were observed using an ESEM-EDX (Quanta, 200, FEI). After the mechanical test was completed, fine pieces were deposited into the sample holder (aluminium stap) which received initially the carbon liquid that allowed to fixed particles of CSH, Fe–C(A)SH, and stabilized compounds. The cylindrical specimens are gold-coated (10 nm thickness) using a stupper working in high pressure for ESEM performing. This metallization with gold avoids an important accumulation of charges on the surface of the samples and reduces the penetration depth of the beam thus improving the quality of the image. Once the sample is placed in a room analysis, the secondary vacuum is realized and the scanning by an electron beam is performed. The electron-matter interaction causes different reactions (diffraction, diffusion, and secondary electron emission). Images are obtained by collecting the secondary or backscattered electrons emitted by the surface of the material. The acceleration voltage used was 40.0 kV at an approximate working distance of 10 mm.
Results and discussion
Phase evolution during the synthesis of C–S–H, Fe–CASH, and stabilized compounds
Figure 1 presents the selected infrared spectra of calcium–silicate–hydrate (Fig. 1a), calcium–silicate–hydrate with 10% of laterite (Fig. 1b), and structural compounds (laterite and natural pozzolan stabilized (Fig. 1c) cured at 50 °C. The broad band's absorption peaks at 3069, 3329 cm−1 (Fig. 1a and b) are assigned to the stretching O–H bond at the surface of the silicate-hydrate (Si–OH, and H2O [20, 21]. In the case of stabilized compound (Fig. 1c), these chemicals bound appear at 3310 cm−1. The absorption bands appearing at 1639, 1643, and 1640 cm−1 on the infrared spectra reported in the different spectra are ascribed to the bending H–O–H vibration (of interlayer absorbed H2O molecule) of the coordinated water, possibly of the interlayer absorbed H2O molecule [22, 23]. The absorption peak (Fig. 1a–c) at 1412, 1404, and 1414 cm−1 correspond to the stretching vibration of C=O confirming the presence of carbonate groups namely calcium carbonate and calcite (CO32−) from the reaction between unreacted sodium hydroxide and CO2 in the atmosphere [6]. The absorption band appearing at 969 cm−1 can be attributed to the asymmetric stretching vibration Si–O bon, characterizing the mainly C2S5M and C3S6M in the calcium–silicate–hydrate (Fig. 1a). The weak absorption peak at 653 cm−1 can be ascribed to the symmetric vibration of amorphous Si–O–Si (internal deformation of SiO4) [22]. The main characteristic peaks of the C–S–H binder (series C2S5M and C3S6M) are located in the range between 500 and 1210 cm−1 [19]. It is noticed that the absorption band belonging to the vibration mode of Si–O–T (T=Al, Si, Fe…) shifted to lower values from 1067 to 1029 cm−1 when the concentration of the NaOH increases from 4 to 6 M. This shift towards lower wavenumber is because the formation of the new reaction product known as C–S–H phase is more pronounced at 6 M. The new phase C–S–H could have formed from the depolymerization of Si–O–Si and repolymerization to Si–O–Ca [23]. The appearance of the peak at 785 cm−1 (Fig. 1b) can be ascribed to the Al–O stretching vibration in AlO4 and Si–O–Al, while those at 690 and 580 cm−1 are attributed to the stretching vibration of Si–O–Si, Si–O–Al and Fe–O bonds [11, 24]. The absorption bands located at 870–874 cm−1 (Fig. 1b and c) show the vibration modes of Si–O–Al, and Si–O–Fe bonds belonging to the Fe–C–A–S–H binder and structural composite [6]. The regions with bands from 607 to 965 cm−1 characterized the Si–O–Al, Si–O–Fe, and Si–O–Si vibrations. Using increasing concentrations of 4, 5, and 6 M of NaOH solution significantly modified the level of polymerization and affected the silicate chains present in the matrix. A similar trend was observed on the infrared spectra of C–S–H where the absorption band shifted to a lower value when the concentration increased. This is attributed to the structural reorganization of AlO45− and Fe2O3 [6, 22, 25]. Meanwhile when the quantity of alumina oligomer and iron oligomer increases, the Si–O stretching vibration is weak. Moreover, the inclusion of aluminium or iron had an impact on the deformation and bending vibration of SiO44− showing that there was no significant change when Al or Fe was substituted for Si into the calcium–silicate–hydrate synthesized.
Fig. 1
FTIR spectra of a calcium–silicate–hydrate as a function of the NaOH concentration, b iron–calcium–aluminium–silicate–hydrate with 10% of laterite as a function of the NaOH and c stabilized precursors (laterite and pozzolans) with iron–calcium–aluminium–silicate–hydrate at 6 M concentration of NaOH cured at 50 °C
×
The X-ray patterns of C–S–H and Fe–C–A–S–H binders synthesized at different Ca/Si molar ratios (2 and 3) with various NaOH concentrations (4, 5 and 6 M) and structural composites (laterite and pozzolans) are shown in Fig. 2a–c. In addition to some initial minerals phases present in raw materials, it is observed the formation of newly formed phases such as tobermorite [Ca5Si6O16(OH)2.4H2O] and calcium–silicate–hydrate (C–S–H). C–S–H gel phase have also been reported [26]. However, it was also noticed the reflection peaks of calcite suggest it is not completely dissolved with reactive silica from rice husk ash (RHA). Moreover, the reflection peaks of Fe–C–A–S–H and C–A–S–H phases are found in Fig. 2b, suggesting that iron and aluminium from indurated laterite could have incorporated successfully into the C–S–H binder and formed Fe–C–A–S–H (gismondine and stratlingite). For the stabilized pozzolans (Fig. 2c), the diffractograms are dominated by the reflection peaks of SiO2 (quartz) followed by the new peak located at 2θ equal to 8 and 10° representing phase like Ettringite [Ca6Al2(SO4)3(OH)12]; In case of the stabilized laterite, the main crystalline mineral phases are fayalite (Fe2SiO4) and quartz (SiO2).
Fig. 2
X-ray diffraction patterns of a calcium–silicate–hydrate as a function of the NaOH concentration, b iron–calcium–aluminium–silicate–hydrate with 10% of laterite as a function of the NaOH and c stabilized precursors (laterite and pozzolans) with iron–calcium–aluminium–silicate–hydrate at 6 M concentration of NaOH cured at 50 °C
×
It is noticed that applying a steam curing for 3 h is favourable for the formation of semi-crystalline C–S–H (mix of poorly crystallized tobermorite and SiO2) and Fe–C–A–S–H phases [18] (Fig. 2b and c). One can note the high intense reflection peaks of disordered C–S–H between 28° and 32° in 2θ characterizing binder and stabilized compound that contains a semi-crystallized tobermorite, poorly crystalized tobermorite and jennite [24]. A lower crystallinity C–S–H gel and amorphous in the range 22–30° suggest the iron–alumina–silica gel that was formed. It was also observed that the alkalinity improved the dissolution of SiO2, Al2O3, and FeO–Fe2O3 (contained in laterite) at 50 °C. The reflection peaks belonging to crystalline minerals such as quartz, hematite (goethite) and kaolinite did not disappear even when a high alkalinity at 6 M was applied means that these mineralogical phases were not totally amorphous. Additionally, an excess release of Si species in the alkalinity solution would have increased the pH of the whole system and might have hindered the site nucleation of C–S–H, and Fe–C–A–S–H crystallite as also reported by literature [25]. High alkalinity could increase the dissolution of C–S–H, Fe–C–A–S–H binder, and stabilized compounds. In all the series (C2S and C3S), the presence of amorphous silicate allows the formation of poor tobermorite; this is due to the excess of Si which would have effectively hindered the nucleation.
Physical and mechanical properties
Setting time
The setting time of ball-milled C–S–H paste at 50 °C, Fe–C–A–S–H with 10% of iron-rich laterite and stabilized precursors paste are shown in Fig. 3. The initial setting time of C–S–H, Fe–C–A–S–H and stabilized precursors (laterite and pozzolans) were 50, 38, 30, and 53 min, respectively, and the final setting time were 69, 50, 47, and 70 min, respectively. It is noted that the setting time of the pastes tended to decrease with the incorporation of iron-rich aluminosilicate into CSH. Calcined laterite released Si-oligomer, Fe-oligomer, and Al-oligomer into the C–S–H matrix, thus reducing the time to set. Such a reactive phase reacts with CaO and SiO2 to form Fe–CASH. The low setting time of stabilized laterite could be explained by the reactive phase and crystalline phase present into the precursor, contrary to stabilized pozzolan. The raw laterite contains an amorphous phase that is less reactive. The high setting time of stabilized pozzolan is due to the saturation and Si ions in the matrices.
Fig. 3
Setting time of binder, CSH and Fe–CASH, and structural composite: SL = stabilized laterite, SP = stabilized pozzolans
×
Compressive strength
Figure 4 illustrates the results of the compressive strength of C–S–H, Fe–C–A–S–H based binder and stabilized compounds with 20, 30, 40, and 50% of Fe–C–A–S–H. It is observed that the compressive strength of C–S–H binder (Fig. 4a) increases with the curing temperature up to 50 °C as well as the concentration of NaOH (from 4 to 6 M) and then decreases from 50 to 100 °C. The maxima recorded on C–S–H binder cure at 50 °C are 32.28, 35.77, and 39.37 MPa when applied 4, 5, and 6 of NaOH solution, respectively. Such improvement between 26 and 50 °C is likely linked to the combined synergetic effect of increased alkaline solution and temperature that activate the CaO and favour the dissolution of SiO2 resulting in the intensive formation of the C–S–H phase. Consequently, the C–S–H binder ensures the important cohesion between different components within the matrix. At elevated temperature (beyond 50 °C), the hydration and evaporation of water are accelerated very early; consequently, the dissolution rate is very high due to the Arrhenius effect [27], and the polymerization and polycondensation are limited due to the absence of liquid phase. Meanwhile, at room temperature at 50 °C, the hydration reaction is slow and the diffusion-limited becomes effective when the molar ratio of CS, C2S and C3S is transformed into calcium–silicate–hydrate justifying the high performance achieved.
Fig. 4
a Compressive strength of synthesized calcium–silicate–hydrate (the compressive strength increases with the molar ratio of Ca/Si). b Compressive strength of synthesized iron–calcium–aluminium–silicate–hydrate (the compressive strength increases with the molar ratio of Ca/Si) c Compressive strength of stabilized precursors
×
Figure 4b presents the impact of the NaOH concentration on the compressive strength of consolidated Fe–C(A)SH binder samples. It is observed that the compressive strength increases with the addition of calcined laterite up to 10% and then decreases when incorporated at 12.5, and 15%. The maxima of mechanical performance recorded on Fe–C–A–S–H composite made with 10% of thermally activated laterite are 35.91, 39.33, 42.93 MPa when applied 4, 5, and 6 of NaOH solution. This performance could be attributed to the increased Na2O content that allowed the high dissolution of calcined laterite and silica (from RHA, sand) leading to the formation of a dense and strong matrix ensuring higher strength and stiffness [28]. After 10%, the reduction in mechanical strength could be also attributed to the saturation of the whole system with Al, Si and Fe-species from incorporated laterite. A similar observation was made by Balamuralikrishnan et al. when incorporating 10% alccofine into mortar cement [29]. Those elements not being incorporated into the cementitious matrix would have weakened the strength development resulting in a less dense matrix justifying the decreased strength. It is important to note that applying the steam curing at 50 °C avoids the abrupt departure of water and thus enhances the mechanical properties of the Fe–C–A–S–H composite. Meanwhile, the dissolution of amorphous compounds into the alkaline solution improves the chemical bonds, Si–O–Fe, Si–O–Al, Si–O–Ca, and the formation of major species such as Fe–C–S–H and minor species, namely ferrosialate and ferrisialate. The presence of iron in indurated laterite allowed dense matrix and better polycondensation. The presence of iron in indurated laterite contribute to the formation of stronger cementitious materials by promoting the formation of stable crystalline phases, which in this case are gismondine [CaAl2Si2O8·4(H20)], stratlingite (S) [Ca2Al2(SiO2) (OH)10·2.5H2O], Fe–C–A–S–H. These combinations are capable to develop optimum densification and compactness [30]. Results significatively similar from what has been achieved by [18]. The nature of laterite regarding the mineralogical composition are not the only factors that influence the properties of laterite–CSH composites. The CSH content also has an important role in terms of the degree of cohesion between the particles. In fact, CSH content at 6 M is significant to achieve enough cementing agents and more reactions with the disordered and amorphous components of the laterite.
Figure 4c illustrates the compressive strength of stabilized laterite and pozzolan with 20, 30, 40 and 50% of Fe–C–A–S–H binder containing 10% of calcined laterite. It is observed that the strength increases with the percentage of binder up to 40% for laterite and up to 50% for pozzolan, respectively. From 20 and 30%, the amount of binder is not sufficient to embed all the particles within the matrix. When the percentage of binder increases up to 40 and 50%, the stabilized laterite and pozzolan present high strength due to several reasons: (i) the Fe–C–(A)–S–H binder ensured better connectivity between particles, (ii) the addition of a binder to the soil increase the pH [31] that could have improved the solubility of siliceous, aluminous, iron compounds, and (iii) the Fe–C–(A)–S–H binder is completely converted into C–S–H, C–A–H, C–F–H (calcium ferrite hydrate). The type of soil to stabilize had an impact on the compressive strength as reported by Kassim [32].
Water absorption and bulk density
Figure 5 shows the water absorption and bulk density at the concentration of 6 M of C–S–H cured at a different temperature (26, 50, 80, 100 °C) (Fig. 5a), the Fe–C(A)SH binder with different percentages of indurated laterite (5, 7.5, 10, 12.5, 15%) (Fig. 5b) and stabilized composite [stabilized with 20, 30, 40, and 50% of Fe–C(A)SH] (Fig. 5c).
Fig. 5
a Water absorption and bulk density of synthesized calcium–silicate–hydrate (at the molarity of 6 M and Ca/Si = 3 molar ratio). b Water absorption and bulk density of synthesized iron–calcium–aluminium–silicate–hydrate (at the molarity of 6 M and Ca/Si = 3 molar ratio). c Water absorption and bulk density of stabilized compounds at 6 M red: stabilized laterite (40% binder) and black stabilized pozzolan (50% binder)
×
For the hardened C–S–H binder (Fig. 5a), the water absorption decreases from 15.05 to 13.11% between 26 and 50 °C and increases from 13.11 to 17.44% between 50 and 100 °C. The reduction in water absorption recorded at the interval of 26–50 °C suggests the formation of a dense and compact structure. Such a structure would have reduced the fixation of water molecules when samples were immersed in water for the test measurements. The curing of samples beyond 50 °C could have caused the formation of open voids within the matrix which allowed a great retention of water justifying the high values recorded for these specimens. Thus, increasing the curing temperature beyond 50 °C seems to have an impact on the structure evolution of the end products.
An opposite trend was observed in the results of bulk density. The bulk density increases from 1.6 g cm−3 (26 °C) to 1.80 g cm−3 (50 °C) and decreases up to 1.58 g cm−3 at 100 °C. Hence, the open voids formed when applied steam curing at 80 and 100 °C could have these samples a bit lighter, thus less dense compared to those cured at 26 and 50 °C.
In the case of Fe–C(A)SH composite blended with indurated laterite treated at 50 °C, the water absorption decreased with the rise of calcined laterite up to 10%(15.83%) and then increased and reached 16.1%, contrary to bulk density that increase with the incorporation of activated laterite up to 10% (1.66) and decrease and reached 1.64 g cm−3 as illustrated in Fig. 5b. This trend is explained by the dissolution of reactive fraction contained in calcined laterite allowing the extension of Fe–C–(A)–S–H binder resulting in strong and compact structure. The increase in water absorption and the decrease in bulk density beyond 10% of calcined laterite is due to the saturation of the whole system of Fe, Al and Si species that did not integrate the Fe–C–(A)–S–H binder network. Hence, these unfixed species (excess of Al, Fe, Si from calcined laterite) would have leached when the sample was immersed in water rendering this sample lighter.
For the stabilized laterite and pozzolan presented in Fig. 5c, the water absorption decreases when the binder increased from 20 to 50%, whereas the opposite trend is observed on the bulk density. It is noted that laterite and pozzolan showed the lowest water absorption (14.20 and 14%) and higher bulk (1.9 and 1.6 g.cm−3 density) when stabilized with 40 and 50% of Fe–C–(A)–S–H binder. The compactness and the high consolidation of the matrix can explain the result obtained with 40 and 50% of Fe–C(A)SH binder because when the specimens are consolidated, it occurs fewer open voids and pores. Additionally, the high-water absorption and low bulk density at 20 and 30% can be explained by the fact that the amount of binder was insufficient to consolidate the raw precursors and allow the matrix to be less compact and consolidated. The other reason could be the insufficient amount of Al, Fe, and Si species into the Fe–C(A)SH that did not diffuse ions into the matrix. The dissolution of silica (RHA), calcium (marble powder), iron and alumina (laterite) at the concentration of 6 M gives a structure more compact and denser.
Microstructure of CSH, Fe–CASH and stabilized precursors
SEM–EDS micrographs of C–S–H and Fe–C–A–S–H binders
The micrographs of synthesized C–S–H (Fig. 6a) cured at 50 °C show a progressive densification of structure with the increase in the molarity of NaOH solution from 4 to 6 M. Such improvement could be attributed to the increased the dissolution and provided reactive silica from RHA and released more Si species that react will CaO contained in waste marble powder to form the C–S–H binder phase. Figures 6a show the progressive reduction of the porosity or open voids and the extent of the cementitious phase with the increase in the concentration of sodium hydroxide solution. It seems that it is obviously difficult only with ESEM to identify the phases formed. For this reason, a semi-quantitative EDX analysis at 03 points was done on a sample. The EDS of CSH shows the presence of different ions present in the microstructure as illustrated in Fig. 6a(iii). The densification observed in Fig. 6a(iv, v) at 6 M is justified by the presence of the reactive silica (provided from river sand and RHA) that improved the potential of the silica oligomers prompt to form N–S–H contributing to densify the structure. In addition, the dissolution and the reactivity are more effective at the concentration of 6 M compared to that of 4 M [33]. A dense rim of hydration product formed, mostly composed of C–S–H, is seen to have formed around coarse and unreacted particles, as marked with a circle in Fig. 6a. When incorporating 10% of indurated laterite, the micrographs represented in Fig. 6b show capillary pores; this is due to the entrapped air bubbles into binder during the manufacturing process. The air bubbles came from the exothermic reaction during the mixture of iron-rich laterite and alkaline solution, as reported by Kaze [34]. Iron-rich laterite dissolve into the alkaline solution allowing the formation of Fe3+ ions that could be inserted in the C–S–H binder phase. The Fe3+ ion could substitute Si4+ in the binding position due to their chemical affinity; another characteristic of laterite is the crystally Fe3+ that dissolves with quartz (river sand, RHA) into the alkaline solution to increase the mechanical properties of the binder, the same trend was observed by [30] when incorporating steel slag in metakaolin. The presence of iron-rich laterite contributes to the formation of a more interconnected network of CSH gel, leading to a denser and more homogenous cement matrix. This improved microstructure enhances the mechanical properties of the cement as reported in the previous section. It was noticed by EDX (Fig. 6b) that elements such as iron, Al, Si, 0 and Ca are in the majority next to some impurity. It can therefore be concluded that these elements are involved in the formation of the cementitious phases such as CASH and Fe–C–A–S–H. The SEM micrograph in Fig. 6b at 6 M shows the change in the microstructure phase when laterite is incorporated is C–S–H.
Fig. 6
a SEM micrographs of calcium–silicate–hydrate at the concentration 4 M (i, ii), 6 M (iv, v) and EDX (iii). b SEM micrographs of iron–calcium–aluminium–silicate–hydrate at 4 M (i, ii), 6 M (iv, v) and EDX (iii)
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SEM image of structural composite
The SEM images of stabilized laterite and pozzolan with 40 and 50 wt% of the synthesized binder are presented in Fig. 7. The micrographs of stabilized products exhibit a dense structure with few pores and accessible voids within the matrices. When pozzolan is stabilized with Fe–C–A–S–H, the whole system releases more silica and alumina species able to form C–A–H, and C–S–H phases ensuring better cohesion between different components in the matrix [35, 36]. In the case of laterite stabilized products with the same binder using 6 M of NaOH solution at 50 °C, it would notice the dissolution of Fe, Si and Al-species contained in laterite that would react with Fe–C–A–S–H to form ferrisilicate, ferrosialate, and aluminosilicate gel resulting in a dense, compact and homogenous microstructure [6, 37]. Such a densified structure is in agreement with the high mechanical property achieved in this formulation. Despite the development of strong and compact structures in both selected final products, it was noticed the presence of unreacted pozzolan and laterite particles surrounded by the formed binder phase as seen in Fig. 7. The interfaces pastes-aggregates of unreacted laterite and pozzolan with binder and sand is instead positive for the strength [38]. Meanwhile, when pozzolans and binder are mixed with water and ball milled, polycondensation takes place slowly at the initial stage, as the dissolution increases the pozzolanic reaction begins, and this pozzolanic reaction increases with the steam curing at 50 °C. It should be noticed using the ball milled for 15 min improved the reactivity of different mixtures and allowed the formation of binder phases that made the matrix more compact.
Fig. 7
SEM micrographs of stabilized precursors: (i, ii) stabilized laterite (iii, iv) stabilized pozzolans
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Conclusion
This work aimed to synthesize CSH at different temperatures and concentration of NaOH solutions and, then, target the temperature that gave the best physical–mechanical properties to incorporate indurated laterite at different proportions. Finally, choose a binder with high properties to stabilize precursors. The following conclusions can be drawn from results compiled in this contribution:
High temperature impacts negatively the consolidation phase of calcium–silicate–hydrate, formation of open pores, and fast hydration of water and low dissolutions of Ca2+ and Si4+.
The incorporation of iron-rich laterite at 10% into CSH increases the strength.
There are more dissolution of Ca2+, Si4+, Fe3+, Al3+ and Fe2+, at the concentration of 6 M of NaOH solution.
The stabilization of laterite and pozzolans using Fe–C(A)SH increase the properties of the soil.
Stream curing increases the pozzolanic reaction between fine particles.
The successful transformation of industrial waste material into tobermorite, jennite, CSH and Fe–C(A)SH.
Acknowledgements
This project received the contribution of the FLAIR fellowship African Academic of Science and the Royal Society through the funding N◦ FLR/R1/201402 and the contribution of the European Union and OEACP R&I through financial contribution No. PRICNAC-EEPER: MD2022.
Declarations
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.
Informed consent
All participants provided informed consent prior to their participation.
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