Laterite-Based Low-Carbon Binder Supporting the Circular Economy in Yaoundé, Cameroon

Abstract

This study formulates an efficient, affordable, and low-carbon binder based on locally excavated earth from Yaoundé, offering sufficient mechanical strength and water resistance for rendering applications. Through material characterization, a binary binder composed of Portland cement (PC) and calcined laterite (CL) was developed, reducing the PC content by up to 30%. The mortar used laterite sand with varying fine particle contents in place of river sand, and its mechanical strength and water absorption via capillarity action were evaluated. Due to the porosity of the laterite fines, all mixes were prepared at equivalent workability. The mechanical strength was the same as if the binder solely consisted of PC and reached 11 MPa when the laterite sand contained no fine particles. As the fine particle content increased, the mechanical strength decreased to a minimum value of 4 MPa when raw laterite was used, and the coefficient of water absorption via capillarity action decreased. Overall, the formulated class Wc2 mortar is suitable for rendering applications. The valorization potential of fine particles and coarse aggregates of the crushed mortar was assessed: the crushed mortar fines had pozzolanic properties and could serve as supplementary cementitious materials; the largest particles are suitable for lime stabilization.

1. Introduction

The widespread use of Portland cement in concrete [1]—combined with the depletion of non-renewable mineral resources like sand or gravel, the mining of which impacts human health and natural ecosystems [2]—presents significant challenges and has motivated the development of alternative building systems based on the circular economy.

In Yaoundé, Cameroon, where the population growth rate reached $4.5\%$ in 2020 [3], rapid urbanization has led to the development and saturation of precarious neighborhoods, which mainly include poto poto shelters that are vulnerable to the hydric stress related to the city’s tropical situation [4]. Moreover, sand and gravel extraction in Cameroon severely impacts the environment, particularly through the degradation of mangroves—areas of high ecological importance. Indeed, mangroves contribute to carbon sequestration, among other things. Their alteration leads to harmful consequences: decreased biodiversity, increased coastal erosion, and reduced fish production and soil fertility [5]. In this context, it is important to develop low-carbon binders based on circular waste management in order to protect natural resources and reduce ${\mathrm{CO}}_2$ emissions. Fortunately, to meet this objective, a valuable local resource can be used, namely kaolinite-rich excavated soil, which is available in abundance in 12 quarries with an estimated volume of almost 50,000 ${\mathrm{m}}^3$ [6] and has already shown great promise in the building sector.

Indeed, lateritic soil is primarily used for compressed earth block (CEB) manufacturing. Different types of stabilization have been investigated in the literature [7]: mechanical stabilization, which consists of compacting the soil with an optimum water content [8,9]; physical stabilization through granulometry control [10] or the addition of fibers that act at the macroscopic level [11,12,13]; and chemical stabilization, such as the addition of Portland cement [9,10,14], lime [15], fly ash [12,16], or a geopolymer binder [17,18]. According to the Cameroonian standard [19], the minimum compressive strength required for compressed unstabilized laterite blocks is 2 MPa, while the compressive strength should reach 4 MPa for compressed stabilized earth bricks [14]. The mechanical resistance reported in the literature for unstabilized laterite blocks fulfills the standard requirement; however, such blocks melt when immersed in water [6,8,9]. This fact was verified in a preliminary phase of the current study: the mechanical resistance of CEBs made from lateritic soil extracted in Yaoundé achieved a mechanical strength of 4.77 MPa but failed to withstand 24 h of water immersion. However, with a minimum cement stabilization of 6%, the CEBs combined mechanical strength with water stability [6,8,9]. External stabilization was also investigated: 6 to 10% CEB was applied to the outer part of the block with a thickness varying from 0.75 cm to 2.25 cm, while the core remained unstabilized. According to the study, a stabilization crown 2.25 cm thick with 8% cement met the construction standards in terms of mechanical strength and water resistance [8].

Other valorization axes have been proposed in the literature: lateritic sand can be recycled as fine aggregates in cement concrete [20,21], and the fine fraction of kaolinite-rich laterite, consisting of grains passing through a 100 µm sieve, can be activated via thermal treatment and constitutes an efficient precursor for geopolymerization [22,23,24,25,26,27,28,29,30]. It can also be used as a supplementary cementitious material in cement-based concrete after thermal treatment [31,32,33,34,35,36] or without thermal treatment [37]. In fine river sand mortars, replacing Portland cement with $50\%$ calcined laterite achieved 5.5 MPa strength [32].

Among the valorization methods used for excavated laterite described in the literature, the external stabilization of unstabilized compressed earth blocks appears to be the most promising approach for minimizing the environmental footprint while ensuring water resistance. However, there is a significant gap in the literature regarding this approach, and only a few studies have explored it [8,32]. In this work, the development of an external stabilization solution is investigated in the form of a rendering mortar. Indeed this solution offers a minimized carbon footprint compared to the current CEB stabilization attempts, as only the outer surface is stabilized over a reduced minimum thickness. Therefore, this study explores the feasibility of formulating a sustainable rendering mortar based on locally sourced excavated laterite, with a minimum PC content. The research flowchart outlines the following steps:

• The material characterization of calcined laterite fines was performed to determine the appropriate binder type—either geopolymer or cementitious—for formulation. Indeed, due to the pozzolanic activity expressed after calcination, the preferred valorization axis is the use of calcined laterite fines as partial cement replacement. In the literature, the cement ratio is decreased by up to $50\%$ [32], such that a novelty of this study is a decrease in the cement ratio to the range of 20 to 40%.
• The minimum Portland cement ratio was determined through compressive strength and material characterization tests on binder samples.
• Laterite sand, consisting of laterite particles larger than 100µm and a varying ratio of laterite fines, substituted river sand in the rendering mortar composition, which is a novelty of this work. The efficiency of the formulated mortar was evaluated using compressive strength and water absorption tests.

This approach preserves both non-renewable mineral resources and the mangrove ecosystem and decreases carbon emissions by minimizing the cement content in the binder. Furthermore, the preparation of the raw material is inexpensive and accessible to local construction companies: the steps of the process involve collecting and sieving the excavated laterite and calcination of the fines, followed by deagglomeration.

The minimization of the cement content and the carbon footprint is related to the pozzolanic activity of the laterite fines after appropriate heat treatment. Indeed, in cementitious systems, portlandite Ca(OH)₂ is part of the hydration products of Portland cement, formed through the hydration reaction of the clinker constituents belite, ${\mathrm{C}}_2$S, and alite, ${\mathrm{C}}_3$S, as shown in the reactions expressed in Equations (1):

$$\begin{array}{c}{\mathrm{C}}_2\mathrm{S}+\mathrm{H}\to \mathrm{CSH}+\mathrm{CH}\hfill \\ {\mathrm{C}}_3\mathrm{S}+\mathrm{H}\to \mathrm{CSH}+\mathrm{CH}\hfill \end{array}$$

(1)

where CH stands for calcium hydroxide Ca(OH)₂, CSH indicates calcium silicate hydrate, and H indicates water according to cement chemist notations. CH reacts with the amorphous oxides ${\mathrm{SiO}}_2$, ${\mathrm{Al}}_2$${\mathrm{O}}_3$ and ${\mathrm{Fe}}_2$${\mathrm{O}}_3$ of pozzolanic materials in the presence of water, as shown in the pozzolanic reaction presented in Equation (2):

$${(\mathrm{SiO}}_2,{\mathrm{Al}}_2{\mathrm{O}}_3,{\mathrm{Fe}}_2{\mathrm{O}}_3)+\mathrm{CH}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CSH},\mathrm{CAH},\mathrm{CASH}$$

(2)

where CSH, CAH, and CASH stand for calcium silicate hydrate, calcium aluminate hydrate, and calcium alumino-silicate hydrate, respectively.

To close the loop, promising recycling strategies for the developed rendering mortar are presented: fines as pozzolanic material and aggregates as a sand substitute in mortar formulations.

2. Materials and Methods

2.1. Materials

2.1.1. Raw Materials

Laterite was collected in Yaoundé (GPS coordinates 343′19.1″ N and 1132′00.0″ E), crushed in a ceramic mortar and sieved with 100 µm of BS. Two sieving methods were used: dry sieving helped achieve rapid partial sieving of raw laterite, whereas wet sieving enabled the raw laterite to be fully sieved. The non-passing fraction was used as lateritic sand, whereas fines with particle sizes smaller than 100 µm entered the binder formulation. Laterite fines were calcined in a Solo 111-23/13/40 electric furnace at temperatures ranging from 500 to 750 °C for 4 h. After calcination, the laterite fines were cooled in the open air and sieved to BS 100 µm for deagglomeration.

Portland cement was purchased from a local Yaoundé supplier. Grade CEM II B-P 42.5 R (CEM) was available at an affordable price and was therefore used for the present investigations. This grade of cement contains 65% to 79% clinker, 21% to 35% natural pozzolana, and 0% to 5% of other constituents [38]. To evaluate the pozzolanic activity of the calcined laterite, KFN Nekapur hydrated calcium hydroxide was used.

2.1.2. Pozzolanic Activity of Calcined Laterite

The pozzolanic activity was determined by measuring the mechanical resistance of equal-weight mixtures of lime and calcined laterite with calcination temperatures varying between 500 °C and 750 °C, designated as CL-500-CH to CL-750-CH, and a water-to-binder mass ratio of 0.65. The pastes were cast in sealed cylindrical molds 20 mm in diameter and 40 mm in height as shown in Figure 1, stored at room temperature, and demolded after 7 days. The samples were cured at room temperature in plastic bags containing wet compresses. The samples were heated at 50 °C for 24 h prior to material characterization. For each formulation, 3 samples were prepared.

2.1.3. Binder Pastes

Table 1. Binder pastes’ % composition by mass.

Label	CEM II	Calcined Laterite	w/b
CEM100	100	0	0.45
CEM20CL80	20	80	0.57
CEM30CL70	30	70	0.57
CEM40CL60	40	60	0.57

summarizes the composition of the blends that were cast and analyzed throughout this study.

The water-to-binder mass ratio, referred to as w/b, was determined for each set of samples to ensure standard workability of the mixes as defined by the standard EN 196-3 [39]. The aspects of CEM100 and CEM30CL70 binder pastes before molding are presented in Figure 2.

Binder pastes were prepared according to the EN-196-1 standard [40]. The binder pastes were cast into sealed cylindrical molds 20 mm in diameter and 40 mm in height, stored at room temperature, and demolded after 7 days of aging. Afterward, they were cured at room temperature in plastic bags. The samples were heated at 50 °C for 24 h before material characterization. For each formulation, 3 samples were prepared.

2.1.4. Mortar Pastes

Mortar pastes were prepared with one part binder mixed with three parts of “lateritic sand”, which was comprised, alternatively, as follows:

• Excavated laterite containing $70\%$ mass fine particles, labeled LAT 70/30;
• Partially sieved laterite that contained $47\%$ by mass fine particles, labeled LAT 47/53;
• Totally sieved laterite-containing particles with grain sizes larger than 100 µm, labeled LAT 0/100.

The water-to-binder mass ratio was adjusted to achieve standard workability for all formulations as defined in the EN 196-1 standard [40].

Table 2. Description of mortar paste constituents (% composition by mass).

Label	Binder % Weight Composition		Lateritic Sand (% Particle Size Distribution)		w/b
	CEM II	Calcined Laterite	<100 µm	>100 µm
CEM100-LAT 47/53	100	0	60	40	1.35
CEM40- LAT 47/53	40	60	60	40	1.35
CEM40- LAT 70/30	40	60	70	30	1.98
CEM40- LAT 0/100	40	60	0	100	0.88

details the weight percent composition of the binder, the laterite sand granulometry, and the water content, measured as the ratio between the mass of water (w) and the mass of binder (b).

Mortar pastes were prepared in a planetary mixer according to the EN 196-1 standard [40]. The blend was cast into 40 mm × 40 mm × 160 mm prismatic molds. The samples were covered with a plastic foil for 24 h as shown in Figure 3 and then demolded. The prisms were sealed in plastic bags for 28 days of aging. Then, 3 h prior to mechanical testing, the samples were placed in ambient laboratory conditions. For each formulation, 3 samples were prepared.

2.1.5. Reuse Potential of End-of-Life Ground Mortar

The hardened CEM40-LAT 0/100 mortar, cured for 28 days, was used to assess the potential for closing the life cycle loop of the developed formulation. For this reason, the mortar was ground using an A077-01 micro-deval abrasion machine at a speed of 100 r.p.m. for 30 min.

The following mass composition and water-to-binder mass ratio were used: $75\%$ lime and $25\%$ mortar fines, referred to as CH75MF25, with a water-to-binder ratio of 0.5. The mix was cast and stored as described previously in the binder paste paragraph. For each formulation, 3 samples were prepared.

Three prismatic samples of ground mortar with particle sizes smaller than 4 mm stabilized with $15\%$ hydrated lime were cast and stored as described previously in the mortar paste paragraph. The water content, measured as the ratio between the mass of water and the mass of binder, was 0.30.

2.2. Characterization Methods

A raw laterite granulometry curve was obtained according to the ISO 17892-4 standard [41]. Wet sieving of raw laterite enabled us to quantify the fine particle mass ratio. The particle size distribution of laterite fines, with diameters smaller than 100 µm, was investigated using a Malvern panalytical mastersizer 3000. Ethanol was used as a dispersant, and a refractive index of 1.58 was assumed.

In order to qualify the reactivity of the laterite fines, the water demand test, a semi-quantitative test giving an indirect measure of the specific surface area of the particles, was performed according to the EN 196-3 standard [39]: The water demand corresponds to a probe insertion depth of 63 mm into the binder paste with a load of 1 kg. The ratio of the corresponding amount of water to the mass of powder is referred to as the water demand. A photo of the paste used to assess water demand of CL is presented in Figure 4.

The oxide content was measured with an S2 PUMA Brucker spectrometer.

The diffraction patterns were obtained using the Bruker D2 Phaser diffractometer with a Cu tube (30 kV, 10 mA) and a Ni filter. The recordings were performed for $2\theta$ ranging from 10° to 55° (increment of 0.02 s; time per step of 0.5 s).

A Spectrum Two Perkin Elmer spectrometer was used to analyze raw laterite.

A thermo-gravimetric analysis was conducted using the Netzsch TG 209 F3 Tarsus thermo-microbalance under a nitrogen atmosphere. The temperature ranged from 20 to 950 °C with a heating rate of 5 °C/min, followed by a 60 min period of 950 °C.

The compression resistance of cylindrical and prismatic samples was assessed using a Walter+Baï LFM 400 Press according to the EN 196-1 standard [40].

Water absorption tests via capillarity action were performed according to the EN 15801 standard [42].

3. Results and Discussion

3.1. Raw Material Analysis

3.1.1. Particle Size

Wet sieving of raw laterite enabled us to assess the proportion of fines with a particle size smaller than 100 µm, which amounts to $70\%$ by mass. Figure 5 shows the particle size distribution of raw excavation laterite [41].

Granulometry and specific surface area are key parameters driving pozzolanic activity. The particle size distribution parameters of $d_{10}$, $d_{50}$ and $d_{90}$ measured for raw and calcined laterite fines are reported on

Table 3. Key granulometry parameters of fine laterite particles smaller than 100 µm.

	${\mathit{d}}_{10}$ (µm)	${\mathit{d}}_{50}$ (µm)	${\mathit{d}}_{90}$ (µm)
Raw laterite fines	1.53	12.0	36.4
CL	3.24	22.0	55.8

. The average particle size $d_{50}$ obtained for crude laterite fines and calcined laterite fines at 750° was 12.0 and 22.0 µm, respectively. The average particle size is smaller for crude laterite than for calcined laterite. This is related to an agglomeration effect reported elsewhere [24,28]. The $d_{50}$ values obtained for crude and calcined laterite fines are higher than the $d_{50}$ value of metakaolin, which is 4.35 µm [43].

Water demand, which is an indirect measure of the specific surface area, is 0.52 for crude laterite fines, 0.63 for calcined laterite fines and 0.27 for CEM. The higher water demand obtained for calcined laterite indicates that calcination increased reactivity but also porosity. Due to the high value of water demand of calcined laterite, the water-to-binder ratio was carefully determined for each formulation in order to achieve suitable workability.

3.1.2. XRF

XRF analysis results of CEM and CL are presented in

Table 4. XRF analysis results of raw materials—major oxide composition by mass (%).

	${\mathbf{SiO}}_{\mathbf{2}}$	${\mathbf{Al}}_{\mathbf{2}}$${\mathbf{O}}_{\mathbf{3}}$	${\mathbf{Fe}}_{\mathbf{2}}$${\mathbf{O}}_{\mathbf{3}}$	CaO	MgO	${\mathbf{SO}}_{\mathbf{3}}$	${\mathbf{TiO}}_{\mathbf{2}}$	${\mathbf{Na}}_{\mathbf{2}}$O
CL	$45.43$	$34.20$	$15.76$	$0.07$	$0.49$	$0.23$	$2.15$	$0.00$
CEM	$23.06$	$6.55$	$7.71$	$49.2$	$4.26$	$4.62$	$1.36$	$1.19$

. CL exhibits high contents of ${\mathrm{SiO}}_2$, ${\mathrm{Al}}_2$${\mathrm{O}}_3$, and ${\mathrm{Fe}}_2$${\mathrm{O}}_3$, with the summation of the three oxide contents reaching $95.4\%$ of the total oxide composition. Loss of ignition was measured and reached $3.1\%$; therefore, the class N natural pozzolana requirement, for which the oxide summation must reach $70\%$, was largely met [37,44]. Upon calcination, these free and reactive oxides are responsible for the pozzolanic activity of laterite and enable us to consider low cement ratios in this study.

3.1.3. XRD

Figure 6 shows the XRD pattern of crude laterite and laterite calcined at 500°, 600°, and 750°, referred to as CL-500, CL-600, and CL-750. Kaolinite can be detected through the diffraction peaks located at 12.5°, 20°, 21.2°, 25.3°, 35°, 38.4°, and 45.5°, which disappear upon calcination. This indicates the dehydroxylation of the clay and, consequently, the release of the reactive oxides SiO₂, Al₂O₃, and Fe₂O₃. After calcination, goethite to hematite transformation occurs. Goethite reflections are detected at 33.5° and 36° in crude laterite patterns, and hematite is visible at 19.8°, 33.3°, and 35.7° in calcined laterite patterns. Quartz reflections are visible at 20.8°, 25.2°, 26.7°, 39.4°, 42.5°, and 46° [23,45,46].

3.1.4. FTIR

The FTIR spectra of crude and calcined laterite are presented in Figure 7. The presence of kaolinite is detected via a set of well-resolved bands that disappear or shift upon calcination [35,46,47,48,49,50,51,52,53,54]:

• OH stretching bands located at 3686, 3651, and 3610 ${\mathrm{cm}}^{-1}$. These bands disappear upon calcination—this accounts for the dehydroxylation of laterite.
• Si-O-T(Si,Al,Fe) deformation bands located at 1119, 1028, 1001. Upon calcination, these narrow bands are replaced with a broad peak, located at 1036 ${\mathrm{cm}}^{-1}$, 1045 ${\mathrm{cm}}^{-1}$, and 1051 ${\mathrm{cm}}^{-1}$ when the calcination temperature is 500°, 600°, and 750°, respectively—this peak is ascribed to Si-O bond and the shift to a higher wavelength when the calcination temperature increases is a sign of the increasing amorphization degree of the newly formed phase.
• Al-OH and Fe-OH bands located at 909 ${\mathrm{cm}}^{-1}$ and 684 ${\mathrm{cm}}^{-1}$, respectively, that disappear upon calcination—this accounts for the dehydroxylation of laterite.
• Quartz is visible through the Si-O vibration bands located at 793 ${\mathrm{cm}}^{-1}$ and 463 ${\mathrm{cm}}^{-1}$.
• The narrow peak at 731 ${\mathrm{cm}}^{-1}$ is related to Si-O and Al-O-Si vibration modes—upon calcination this peak transforms to a broad peak located at 777 ${\mathrm{cm}}^{-1}$, which is related to Si-O bonding.
• The band located at 526 ${\mathrm{cm}}^{-1}$ is related to an Al-O-Si bond; upon calcination, this band transforms into a broad band ascribed to Al-O vibration located at 533 ${\mathrm{cm}}^{-1}$, 545 ${\mathrm{cm}}^{-1}$, and 558 ${\mathrm{cm}}^{-1}$ when the calcination temperature is 500°, 600°, and 750°—the shift to a higher wavelength indicates the change in Al coordination in the newly formed amorphous phase.
• The bands located at 408 ${\mathrm{cm}}^{-1}$ and 463 ${\mathrm{cm}}^{-1}$ are related to the Fe-O bond of geothite; upon calcination, these bands disappear because of the transformation of goethite to hematite.
• The broad band located at 430 ${\mathrm{cm}}^{-1}$, which appears upon calcination, is ascribed to Fe-O and Si-O bonds in the amorphous phases.

The FTIR analysis results confirm the dehydroxylation of the clay and the formation of amorphous oxides with varying degrees of amorphization, which increases with the calcination temperature. Indeed, the progressive shift of the Si–O stretching band from 1036 ${\mathrm{cm}}^{-1}$ to 1051 ${\mathrm{cm}}^{-1}$ with increasing calcination temperature correlates with the degree of amorphization and structural reorganization in the kaolinite-derived phase: it is related to a change in aluminum coordination, from the octahedral configuration of kaolinite to the presence of four- and five-coordinated aluminum in amorphous metakaolin [55,56].

3.1.5. DTG

Figure 8 presents the DTG analysis of crude and calcined laterite for calcination temperatures ranging from 500 to 750 °C. The crude laterite curve exhibits three peaks, as already reported elsewhere [57,58]:

• The first peak between 25 and 100 °C, which corresponds to the loss of adsorbed water;
• The second peak between 225 °C and 325 °C, which corresponds to the decomposition of organic matter and of goethite to hematite;
• The peak between 400 °C and 600 °C, which corresponds to the dehydroxylation of kaolinite.

The kaolinite dehydroxylation peak appears on the crude laterite curve, whereas it completely disappears when the calcination temperature is higher than 600 °C.

3.2. Pozzolanic Activity of Calcined Laterite

The pozzolanic activity of calcined laterite has been assessed in the literature [55,59]. In order to define an accurate range for cement substitution with calcined laterite, the pozzolanic activity of CL was assessed.

The mechanical strength reported in Figure 9 indicates the pozzolanic reaction. As shown in the material characterization tests, calcination induces the transformation of phyllosilicate groups into reactive oxides capable of reacting with Ca(OH)₂ in the presence of water to form possible CSH, CASH, and CAH hydration products. As shown in Figure 9, the compressive strength increases with calcination temperature. There is a 16% increase in compressive strength when the temperature rises from 500 °C to 600 °C, and a 7% increase when it rises from 600 °C to 750 °C. This is related to the increase in the amorphization degree of the kaolinite-derived phase.

In the literature, lower values of compressive strength have been reported for metakaolin–lime blends, ranging from 8 to 12 MPa for a water-to-binder ratio of 1 [60,61]. This may be related to several causes. First, a lower water-to-binder ratio indicates a better compacity: in the case of CL/lime blends, $w/b=0.65$, while it is higher for metakaolin–lime blends. Secondly, the presence of iron oxides could improve the pozzolanic activity of calcined kaolin [62]. Then, it has been reported that unreacted quartz and iron hydrates in the laterite fines act as microaggregates and improve strength [10].

Material characterization analyses of calcined and non-calcined laterite show that calcined fines exhibit greater pozzolanic activity than metakaolin and that this activity increases with temperature. This is due to the degree of amorphization of the oxides ${\mathrm{SiO}}_2$, ${\mathrm{Al}}_2$${\mathrm{O}}_3$, and ${\mathrm{Fe}}_2$${\mathrm{O}}_3$. These fines therefore appear to be suitable for formulating a low-carbon binder, in which they would be incorporated as a partial replacement for PC. The substitution ratios range from 20% to 40%: these values were chosen to determine the extent to which CEM could be substituted without significantly altering the properties, particularly the compressive strength levels.

For the rest of the study, a calcination temperature of 750° was used, ensuring complete dehydroxylation of kaolinite and a high degree of amorphization.

3.3. Eco-Binder Design

As described in Table 1, the first set of blends is characterized by a water-to-binder ratio of 0.57 for calcined laterite blended cement pastes with cement ratios ranging from $20\%$ to $40\%$ and 0.45 for the blend containing $100\%$ cement. These values ensure similar workability for all blends.

Figure 10 shows the compressive strength of calcined blended laterite with cement ratios ranging from $20\%$ to $100\%$. Pozzolanic activity is revealed for blends containing $30\%$ and $40\%$ of cement: an increase of $30\%$ in compressive strength is indicated between the seventh and twenty-eighth days of curing. The increase in water demand can be related to the pozzolanic reaction [32]. In the bend containing $20\%$ CEM and $80\%$ CL, the mechanical strength is decreased and the pozzolanic activity is not visible: due to the heterogeneous nature of CL, its porosity and low density compared to PC lead to the formation of a weak structure [32,37]. Figure 10 shows that the compressive strength linearly increases with an increase in cement content up to $40\%$. Blends containing $100\%$ CEM on one hand and $40\%$ cement and $60\%$ calcined laterite on the other hand exhibit similar compressive strength for equivalent workability. To achieve the mechanical performance of rendering mortars, the cement content in the developed low-carbon binder should range between $30\%$ and $40\%$.

Figure 11 shows the XRD curves of the pastes containing $30\%$, $40\%$, and $100\%$ cement. As expected, excess portlandite was detected in the cement paste only containing CEM via the reflection peaks located at 18°, 34°, and 47.2°. In the blends containing calcined laterite, these peaks were not visible, meaning that no portlandite was detected: it was consumed through the pozzolanic reaction. The reaction of the excess portlandite and calcined laterite, such as in a $30\%$ replacement of cement with calcined laterite, has been reported elsewhere [32,34]. In the binder composed of 100% cement, the presence of unreacted di- and tri-calcium silicates is visible in a set of low-intensity peaks located at 32.6° (${\mathrm{C}}_2$S) and 28.6°, 29° and 32.4° (${\mathrm{C}}_3$S) [63,64]. The hydration products are not clearly expressed due to their semi-amorphous nature; however, ettringite peaks are visible at 15.8° and 23° and CSH at 29.4°. Unreacted quartz is visible at 20.8°, 25.2°, 26.7°, 39.4°, 42.5° and 46°.

3.4. Mortar Pastes

3.4.1. Mechanical Resistance

Figure 12 shows the compressive strength measured on days 1 and 28, respectively, of the CEM100-LAT 47/53 and CEM40-LAT 47/53 mortar formulations described in Table 2 obtained with partially sieved laterite as lateritic sand. The mechanical resistance was similar for the two mortar blends. Pozzolanic activity correlates with strength by the consumption of unreacted CH, produced by cement hydration. Consequently, the microstructure porosity is reduced and the packing density is increased. This result shows the efficiency of the developed low-carbon binder for laterite sand mortars.

Figure 13 shows the compressive strength of mortar blend formulations with 40% cement, 60% laterite binder and varying granulometries of lateritic sand, from raw laterite to completely sieved laterite with grain sizes larger than 100 µm, as described in Table 2. The water-to-binder ratio was adapted for each blend in order to obtain the same workability: as the proportion of fines in laterite sand increases, the water demand increases, which decreases the compacity and thus the mechanical strength. However, the compression strength levels show the feasibility of using raw laterite as a substitute for river sand in rendering mortars.

3.4.2. Water Absorption in Capillarity Test

Table 5. Water absorption according to the capillarity coefficient measured for the following mortar formulations: 100% CEM—LAT 47/53, w/b = 1.35; 40% CEM—LAT 47/53, w/b = 1.35; 40 % CEM—LAT 70/30, w/b = 1.98; 40% CEM—LAT0/100, w/b = 0.88.

	Water Absorption According to Capillarity Coefficient
	kg/(${\mathbf{m}}^{\mathbf{2}}$.${\mathbf{min}}^{\mathbf{0.5}}$)
CEM100-LAT 47/53	0.011
CEM40-LAT70/30	0.026
CEM40-LAT 47/53	0.039
CEM40-LAT0/100	0.051

presents the results of water absorption during capillarity tests conducted on the mortar pastes described in Table 2 after 28 days of curing. When partially sieved laterite containing 47% fine particles is used as a substitute for river sand, the capillary water absorption coefficient is lower with a 100% CEM binder than with a binder composed of 40% CEM and 60% CL. The porosity of cement paste is caused by the release of lime during the hydration process. The pozzolanic reaction occurring in binary blends of PC and pozzolans such as CL leads to the consumption of excess CH that results from cement hydration. Consequently, smaller capillary pores are formed, leading to an increase in capillary water absorption coefficient and a decrease in water resistance. When the proportion of fines in the laterite sand increases, the water demand increases, which decreases the compacity: larger capillary voids are formed and in turn, the capillary water absorption coefficient decreases.

For all mortar formulations, the water absorption according to the capillarity coefficient, measured as the slope of water absorption by capillarity curves at the origin, was less than 0.2 kg/(${\mathrm{m}}^2$.${\mathrm{t}}^{0.5})$. This classifies all formulations as class W2 mortars according to the EN998-1 standard, confirming that they are suitable for rendering applications [65].

3.5. Environmental and Economic Advantages of the Proposed Solution

The environmental advantages rely on two main aspects. First the excavation laterite is a local waste material. Secondly, the thermo-mechanical treatment applied to raw laterite consumes less energy than the cement production processes. Indeed, the production of Portland cement generates ${\mathrm{CO}}_2$ emissions due to the chemical reaction occurring during calcination of carbonate, the energy consumption related to the preparation of raw materials including crushing and heating at temperatures ranging from 1400° to 1500°, and the transportation of raw material and finished products [66]. The production of calcined laterite fines, as proposed in this study, reduces the carbon impact for three reasons: firstly, the calcination temperature is lowered to 750 °C; secondly, there are no ${\mathrm{CO}}_2$ emissions during calcination; and thirdly, the energy consumption related to the raw material sourcing and preparation is lower due to the fact that raw laterite is a local waste that only needs crushing and sieving before calcination. For comparison, the carbon emission factor for CEM II is 0.638 kg ${\mathrm{CO}}_2$ eq/kg according to the Ecoinvent database, compared to 0.231 kg ${\mathrm{CO}}_2$ eq/kg for laterite fines sourced from within a 30 km radius and calcined at 750°. These data are consistent with the results reported in the literature [67,68]. Consequently, the proposed binder reduces the carbon emission factor by 38%. The use of waste excavation laterite sand instead of river sand induces an additional saving of 6.6 kg ${\mathrm{CO}}_2$/t [69]. Moreover replacing sand with laterite has the advantage of protecting non-renewable mineral resources and preserving mangrove [5].

The economic advantages of the proposed binder system have been assessed. The local cost of river sand is XAF 5 per kg, whereas 1 kg of locally produced CEM II BP 42.5 R cement costs XAF 104. Consequently, normalized mortar composed of river sand and CEM II would cost XAF 70,000 per ${\mathrm{m}}^3$. As the preparation cost for calcined laterite is XAF 42 per kg, including sieving and calcination (provided the necessary equipment is available on site, rather than being rented from a local manufacturer, which would considerably increase the price), the formulated rendering mortar would cost XAF 39,000 per ${\mathrm{m}}^3$, enabling 45% savings. Manufacturing a compressed earth brick with dimensions of 206 mm × 105 mm × 48 mm, stabilized with 10% CEM II, costs XAF 20. In contrast, rendering the external face of an unstabilized brick with the same dimensions, using the proposed rendering mortar consisting of raw laterite and a low-carbon binder made up of 40% CEM II and 60% CL, costs XAF 8.43, representing 58% savings.

3.6. Closing the Loop for the Developed Solution

In this section, the potential for reusing the developed mortar is assessed. Ground mortar contains $40\%$ fine particles with a size smaller than 100 µm.

3.6.1. Pozzolanicity of Mortar Fines

Figure 14 shows the compressive strength after 7 and 28 days of curing for CH75MF25 with a water-to-binder ratio of 0.5. Mortar fines expressed pozzolanic behavior: there was a 49% compressive strength increase between 7 and 28 days of curing. This paves the way for the valorization of end-of-life mortar fines as a supplementary cementitious material.

3.6.2. Lime Stabilization of Ground Mortar

Figure 15 shows the compressive strength of ground mortar stabilized with $15\%$ hydrated lime, with a water-to-binder ratio of 0.30, measured after 7 and 28 days of curing. The strength is partly provided by the pozzolanic reaction of the mortar fines and lime, as well as the granular skeleton of the ground mortar, which is thought to be a good alternative to river sand.

3.6.3. Life Cycle of the Developed Solution

Figure 16 shows the expected life cycle diagram of the developed solution. The environmental impact was decreased by minimizing the CEM ratio in the binder. The ground mortar fines show pozzolanic activity and the coarser fraction of ground mortar, with particle sizes larger than 100 µm, can be incorporated into a lime-based rendering mortar, leading to improved compression strength. These concluding remarks indicate the promising valorization potential of the following elements:

• Fines smaller than 100 µm as a partial replacement for calcined laterite, leading to further energy savings;
• Aggregates larger than 100 µm as a partial replacement for lateritic sand.

4. Conclusions

The present work demonstrated the feasibility of reusing excavation earth from Yaoundé to formulate a sustainable, low-cost rendering mortar that does not require specific equipment. A low-carbon binder was formulated using the pozzolanic activity of calcined laterite comprising reactive oxides capable of consuming the excess portlandite (which is a hydration product of Portland cement). The key findings include the following:

• Replacing $70\%$ of cement with calcined laterite yields acceptable mechanical strength in cement pastes;
• Replacing $60\%$ of cement with calcined laterite achieves the same compression strength as a 100% cement paste while maintaining similar workability.

Lateritic sand mortars with a binder formulation containing 40% cement were assessed. They are promising in terms of mechanical strength and water resistance:

• At similar workability, mortars composed of laterite sand and the CEM/CL blend have equivalent compressive strength to the binder composed of only cement.
• The higher the fine content, the lower the absorption according to the capillarity coefficient due to the formation of larger capillary voids.
• Regardless of the fine content in the laterite sand, the mortars were classified as W2 according to capillary water absorption tests.

To close the life cycle loop of the mortar formulation developed in this study, the potential reuse of the ground mortar was assessed:

• Mortar fines are pozzolanic and can enter the binder formulation developed here as a partial replacement of the calcined laterite, leading to further energy savings;
• Larger particles have the ability to substitute laterite sand in the developed rendering mortar or in equivalent mortars.

The following recommendations and limitations are indicated for future work:

• A long-term durability analysis, including abrasion tests, adhesion tests, and dry–wet cycles.
• Evaluate the mortar’s effectiveness in protecting laterite CEBs. Although one laterite CEB sample was tested in this study, the authors recommend systematic testing of water absorption in both stabilized and unstabilized CEBs rendered with the developed mortar. If applicable, the minimal stabilization ratio should be determined to fulfill the local standard requirements in terms of water absorption, which includes the maximum water absorption with an immersion rate of 10% [19].
• The minimal thickness of the rendering, which ensures the local standard requirements in terms of water resistance, should be assessed—the authors have successfully tested 1cm thickness rendering on one CEB sample.
• Conduct an economic and environmental assessment of the proposed low-carbon rendering mortar in comparison to traditional internal CEB stabilization methods and other local building systems used in Yaoundé.