Development of a new ecological material based on Moroccan industrial wastes for road construction

The Black Steel slag (Ss) and phosphogypsum (PG) are industrial wastes produced in Morocco. In order to reduce these two wastes and to evaluate their pozzolanic reactivity in the presence of water, they were incorporated into bentonite (B) mixed with lime (L). The studied mixtures (BLW, BL-PG-W and BL-PG-Ss-W) were analyzed by X-ray diffraction, Infrared spectroscopy, Raman spectroscopy and SEM/EDX analysis. Compressive strength tests were performed on hardened specimens. The results obtained show that the hydration kinetics of the BL-W and B-L-PG-W mixtures are slow. The addition of PG to a bentonite-lime mixture induces the formation of new microstructures such as hydrated calcium silicate (C-S-H) and ettringite, which increases the compressive strength of the cementitious specimens. The addition of the Ss to a mixture composed by 8%PG and 8%L-B accelerates the kinetics of hydration and activates the pozzolanic reaction. The presence of C2S in the slag helps to increase the mechanical strength of the mixture B-L-PG-Ss. The compressive strength of the mixtures BL-W, BL-PG-W and BL-PG-Ss-W increases from 15 to 28 days of setting. After 28 days of setting, 8% of Sc added to the mixture 8% PG-8%L-B is responsible for an increase of the compressive strength to 0.6 MPa.


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presence of calcium silicate (Ca2SiO4), Tricalcium silicate (Ca3SiO5), tetracalcium aluminoferrite (Ca4Al2Fe2O10) and other mineral components provide to the steel slag hydraulic setting properties [10,11], allowing an increase of compressive strength of dredged marine clays [12]. Steel slag is also successful used as an aggregate in the cement industry [13]. It helps to delay the hydration mechanism of concrete and reduces their mechanical strength [14]. Likely slag activates the hydration of C2S and C3S [15]. In addition, the occurrence of Fe2O3 in the steel slag decreases the expansion of clay-based material and increases its mechanical strength [16]. In particular, the use of steel slag as aggregate for metakaolinitic-cement enhances its mechanical properties [17]. In addition steel slag is used in others fields such as civil engineering, mainly to stabilize the swelling behavior of clays in road infrastructure [18].
Recently, several studies aimed at valorizing PG to improve road pavements and stabilize swelling behavior of soils have been realized [23][24][25][26][27][28]. PG was also used as aggregates for bricks masonry materials [29,30] and for the production of cement [31,32]. Gu and Chen [33] attested that the addition of PG and fly ash increased compressive strength of the loess-PG-fly ash mixture due to the formation of nanosilicates gel (C-S-H) gel and ettringite (AF-t). Furthermore PG promotes the carbonization rate of steel slag [34].
Bentonite, mainly composed of calcium and sodium montmorillonite, is among the most used clays in civil engineering due to its high adsorption capacity [35]. At room temperature, at high pH, and in the presence of water and lime (CaO), bentonite undergoes geoplymerization like a Portland cement through complex pozzolanic reactions.
Our study aims to determine the influence of the addition of black steel slag and phosphogypsum on the mechanical performance of bentonite-based mortar with the presence of lime. This study will also strive to understand the different reaction mechanisms and to identify the hydrated microstructures after hardness process using several analytical complementary techniques (XRD, IR, RAMAN and SEM/EDX).

Raw materials
The raw bentonite was sampled from the Trebia deposit in the Nador area (North-East Morocco). Phosphogypsum (PG), produced by OCP at Jorf Lasfar (El jadida, Morocco).

Experimental procedure
The collected samples were ground and sieved to a diameter less than 250 µm. Three mortar specimens were prepared with Ss, PG, L and B according to the proportions reported in Table 1. The prepared specimens are cylindrical with 3.4 cm in diameter and 6.8 cm in height according to ASTM Standard D1621. The test tubes contain 80 g of the solid mixed with 38.4 ml of the distilled water having a W/S ratio of 46% [27].
The mechanical resistance of the specimens was monitored after 15 and 28 days of setting. For the ES3, EP3 and EL3 test pieces, the mineralogical composition was determined by XRD after 3, 7, 15 and 28 days of setting. All test tubes were immersed in distilled water for 4 hours and the pH of the solution was measured.  and differential scanning calorimetry (DSC) by heating the samples from 20 to 1000 °C at a uniform temperature ratio of 10 °C/min.

Characterization of raw materials
The physical properties of raw bentonite are shown in Table 2     Bentonite+lime+PG+Ss after 15 and 28 days of hydration.   Figure 5a shows the infrared spectra of raw bentonite and samples EL3, EP3 and ES3. The absorption band at 3700-3600 cm -1 is due to stretching of the OH band of water adsorbed [17]. When 8% of PG is added, a peak at 3411 cm -1 appears which is attributed to the symmetrical and asymmetric stretching of the OH bond of hydrated water in the gypsum molecule CaSO4.2H2O [26].

Infrared and Raman results
Stretching and bending vibrations of SO4 of PG are also observed at 1102 cm -1 and 669 cm -1 , respectively. The most intense band at 992 cm −1 is attributed to asymmetric Si-O-Si stretching and the band at 518 cm −1 is due to the vibrational mode of Si-O-Si bending in calcium silicate (Ca2SiO4) found in steel slag [40].  The mechanism of C-S-H chains formation involves three steps according to Monnin [45]:

DSC analysis
As shown in figure 8, the DSC analysis of EL3 mixture showed four endothermic peaks associated with a loss of mass. The first weight loss occurs between 85 and 156 °C, is attributed to the water molecules adsorbed to the Ca-Mt or Na-Mt.
The addition of 8% of PG in EP3 and ES3 mixtures shows two small endothermic peaks at 128 °C and 152 °C, which the later corresponds to a loss of water molecules from ettringite crystal in the samples [46]. The endothermic peak at 470-545 °C can attributed to the decomposition of calcium hydroxide to the lime with following reaction: Ca(OH)2  CaO + 2H2O The fourth weight loss occurs at 650-800 °C, which corresponds to the decomposition of calcite [46,47].