The Utilization of Waste Marble Dust as a Cement Replacement in Air-Cured Mortar

: The aim of this study is to assess the possibility of utilizing waste marble dust (WMD) as a partial cement substitution in air-cured mortar (ACM). Three di ﬀ erent levels of cement replacement were analyzed: 5%, 10% and 15% by cement weight. The specimens were manufactured in a local laboratory at an air temperature of 22 ± 2 ◦ C and a humidity equal to 20 ± 1%. The chemical and physical properties of ordinary Portland cement and WMD has been found to be the most crucial parameters. A variety of macroscopic tests, such as apparent density, porosity and compressive strength, were proposed in order to explain the e ﬀ ect of utilizing the WMD on the ACM. To conﬁrm the results of the macroscopic properties, thorough microstructural analysis using scanning electron microscopy (SEM) was performed. The obtained results of this study indicate that replacing cement with WMD a ﬀ ects the physical and mechanical properties of air-cured mortar. The apparent density and compressive strength decrease while the porosity increases.


Introduction
The leading factor influencing the quality of cement-based composites and their appropriate adherence is ordinary Portland cement. The manufacturing of ordinary Portland cement requires a meaningful consumption of energy [1,2] and the production of enormous greenhouse gas emissions, including carbon monoxide (CO), carbon dioxide (CO 2 ), nitrogen oxides (NOx), black carbon (BC), and sulfur dioxide (SO 2 ) [3], therefore contributing to environmental pollution [4,5]. Global cement production (4.18 billion tones in 2014) is the third biggest source of carbon dioxide emissions [6]. The average value of CO 2 intensity emissions from all worldwide cement production is 222 kg of CO 2 /t of cement [7].
From an economic point of view, it can be seen to be reasonable to replace a part of ordinary Portland cement with waste mineral dust. In addition, this treatment is an environmentally friendly solution [8][9][10]. Due to an excessive amount of very fine particles in dust, the voids in cement-based materials could be thoroughly filled. The particle size and chemical composition of this by-product allow this material to be treated as an attractive additive in cement-based material technology [11].
Marble has been widely used as a building material in the civil engineering industry [12]. During the mining process and in the polishing of marble stone, marble dust is perceived as a waste material [13]. [13]. These by-products are present in the environment and contribute to pollution [14,15]. The utilization of marble dust reduces the cost of cement-based material production and also decreases the costs of removing it from the environment [16].
The possibility of using waste materials such as marble dust as a partial substitution in the production of mortar and concrete has often been investigated by researchers [5,[17][18][19][20][21][22][23][24][25][26][27][28][29]. Its mechanical properties are the main aspect of the analysis. Consequently, it has been proved that the utilization of waste marble dust (WMD) in cement mortar improves its mechanical and physical properties when subjected to the water-curing condition [10]. The author's previous study showed that the water-curing condition, when compared to the air-curing condition, was profitable for the acceleration of the hydration rate of cement-based materials blended with WMD dust [30]. However, on real construction sites, cement-based materials are mainly prepared and stored in air conditions (in nature) [10].
Based on the performed literature review, there is no study related to the properties of air-cured mortar containing WMD. To fill this knowledge gap, this research studies the effect of the partial replacement of ordinary Portland cement with WMD on the macro and micro properties of mortars stored at an air temperature equal to 22 ± 2°C and a relative humidity of 20 ± 1% by determining the compressive strength and apparent density. In addition, scanning electron microscopy (SEM) analysis was also performed to determine the porosity and Si/Ca ratios of ACM after 28 days of being cured.

Materials
The following materials were used to manufacture the mortars: ordinary Portland cement CEM I 42.5 R, produced by Biskria ciment, containing 95% clinker with 5% gypsum for the setting regulation. As the partial ordinary Portland cement replacement, waste marble from the aggregate production of marble stone in the CHATT/FilFila quarry of the Skikda region was used. Before use, it was ground to the proper size in order to obtain a WMD fineness in the ordinary Portland cement fineness range (between 3000 to 4000cm 2 /g) [30].
Sand from the Biskra region (Oued-Djedi River) with a particle size between 0 and 5 mm was used as fine aggregate. It was first cleaned and fractionated with the use of a sieving machine to receive a 0/2 particle size ( Figure 1) according to the standard EN 196-1 [31]. Tap water was used in the manufacturing of the mortars.  Table 1 presents the mineralogical and chemical composition, as well as the physical properties of the ordinary Portland cement, WMD and sand used in the study. Figure 2 presents the X-ray diffraction (XRD) spectrum of these materials.  Table 1 presents the mineralogical and chemical composition, as well as the physical properties of the ordinary Portland cement, WMD and sand used in the study. Figure 2 presents the X-ray diffraction (XRD) spectrum of these materials.    Figure 3 presents the particle size distributions of WMD that determined using a Malvern Mastersizer 2000 analyzer (liquid: Hydro 2000MU). According to this figure, the particles in WMD smaller than 80 µm, 30 µm and 5 µm represent about 90%, 50% and 10% of the total material volume, respectively [10].  Figure 3 presents the particle size distributions of WMD that determined using a Malvern Mastersizer 2000 analyzer (liquid :Hydro 2000MU). According to this figure, the particles in WMD smaller than 80 µm, 30 µm and 5 µm represent about 90%, 50% and 10% of the total material volume, respectively [10].

Methods
As a partial replacement of the ordinary Portland cement, WMD was used. The replacement levels were 0%, 5%, 10% and 15% by weight of the cement in the mortar. The proportion of the mortar mixtures is shown in Table 2. The mortar mixtures were manufactured with a 3:1 (by mass) sand to cement ratio in accordance with the standard EN 196-1 [31]. The constant 0.5 water to binder ratio was assured according to ASTM C270-14a [32]. The flow test was repeated 3 times for each replacement ratio and determined using the following formula: Where: E: Flow expressed as a percentage (%), : Final diameter (cm), : Initial diameter (cm).

Methods
As a partial replacement of the ordinary Portland cement, WMD was used. The replacement levels were 0%, 5%, 10% and 15% by weight of the cement in the mortar. The proportion of the mortar mixtures is shown in Table 2. The mortar mixtures were manufactured with a 3:1 (by mass) sand to cement ratio in accordance with the standard EN 196-1 [31]. The constant 0.5 water to binder ratio was assured according to ASTM C270-14a [32]. The flow test was repeated 3 times for each replacement ratio and determined using the following formula: where: E: Flow expressed as a percentage (%), D f : Final diameter (cm), After determining the flow value for each sample, three prisms with dimensions of 40 × 40 × 160 mm, and three cubes of 50 mm for each replacement ratio, were prepared and then packed to avoid water evaporation. They were then put to cure in standard laboratory conditions Sustainability 2019, 11, 2215 5 of 14 at 22 ± 2 • C and a relative humidity of 20 ± 1%. After 24 h, the samples were demolded and left to cure in a standard laboratory with the same conditions. Different tests were performed, such as, apparent density and compressive strength at 3 days, 7 days, 28 days, and 65 days and scanning electron microscopy analysis after 28 of being cured in standard laboratory.
The apparent density was determined according to Kirgiz [33], and the compressive strength was determined according to ASTM C 109/C109 M-9 [34]. Three cubes of 50 mm for each replacement ratio were prepared to measure the average.
The apparent density was determined using the following formula: where: The compressive strength was determined using the following formula: where: The scanning electron microscopy analysis was performed after 28 of being cured in standard laboratory on samples extracted from the mid-prisms. These samples were cold-mounted in an epoxy resin and they were wet grinded using abrasive papers to make them flat. After grinding, the samples were polished with diamond paste (with dimensions of 6µm and 1µm). The cleaned samples were covered with a conductive path made of copper tape and they were then carbon coated to obtain electrical conductivity and to prevent electrical charging of the samples. A conductive layer with a thickness of 30 nm was applied in a vacuum of 1.3 × 10 −4 Pa (10 −6 Torr) with the use of the thermal evaporation method and by using sharpened graphite rods [10]. Finally, the samples can be analyzed using scanning electron microscopy to determine their microstructural properties. The JEOL JSM-6610A microscope with JED-2300 energy-dispersive X-ray spectroscopy (EDX) detector were used.

Properties of Fresh Mortars
The flow of mortars containing WMD is presented in Table 3. Based on the results, it can be stated that with the increase of content, the mortar flow was slightly decreased. The use of WMD in amounts of 5%, 10%, and 15% led to a decrease in the flow by 0.02%, Sustainability 2019, 11, 2215 6 of 14 1.98%, and 2.96% respectively. Due to the small scale of the phenomenon, this decline may be neglected. The results were found to be in the range from 105% to 115%, which complies with ASTM C270-14a [32]. Regarding concrete, a similar observation was noticed in [26]. The authors deduced that the utilization of WMD with a lower fineness does not affect workability. On the other hand, according to [18], the utilization of WMD with a higher fineness in the manufacturing of concrete causes a decrease of workability and an increase of friction.

Properties of Hardened Mortars
In this article, the effects of the utilization of WMD on the properties of hardened ACM were considered. The compressive strength (f cd ) and apparent density (Υ ad ) of ACM were studied. Based on the results, it can be stated that with the increase of content, the mortar flow was slightly decreased. The use of WMD in amounts of 5%, 10%, and 15% led to a decrease in the flow by 0.02%, 1.98%, and 2.96% respectively. Due to the small scale of the phenomenon, this decline may be neglected. The results were found to be in the range from 105% to 115%, which complies with ASTM C270-14a [32]. Regarding concrete, a similar observation was noticed in [26]. The authors deduced that the utilization of WMD with a lower fineness does not affect workability. On the other hand, according to [18], the utilization of WMD with a higher fineness in the manufacturing of concrete causes a decrease of workability and an increase of friction.

Properties of Hardened Mortars
In this article, the effects of the utilization of WMD on the properties of hardened ACM were considered. The compressive strength (fcd) and apparent density (ϒad) of ACM were studied.    Based on the results, it can be stated that with the increase of content, the mortar flow was slightly decreased. The use of WMD in amounts of 5%, 10%, and 15% led to a decrease in the flow by 0.02%, 1.98%, and 2.96% respectively. Due to the small scale of the phenomenon, this decline may be neglected. The results were found to be in the range from 105% to 115%, which complies with ASTM C270-14a [32]. Regarding concrete, a similar observation was noticed in [26]. The authors deduced that the utilization of WMD with a lower fineness does not affect workability. On the other hand, according to [18], the utilization of WMD with a higher fineness in the manufacturing of concrete causes a decrease of workability and an increase of friction.

Properties of Hardened Mortars
In this article, the effects of the utilization of WMD on the properties of hardened ACM were considered. The compressive strength (fcd) and apparent density (ϒad) of ACM were studied.    As shown in Figure 4, by increasing the WMD content at all curing ages, apparent density slightly decreases when compared to the control mortar. The utilization of WMD in the amount ranging from 5% to 15% causes a decrease of the apparent density from 0.30% to 2.01% at all curing ages. This could be caused by the solid density of the WMD, which is lower than that of the ordinary Portland cement [23,25,35], and also by the inherence of ultrafine particles [10]. In addition, due to the storing of samples in air, which causes the evaporation of water, there is an incomplete hydration process and thus a decrease in the apparent density. Nevertheless, the loss in apparent density of the ACM was indiscernible [25,27] and, therefore, omitted. A contrary remarks were found in [36]. The authors indicated that the utilization of WMD as addition to the mortar causes enhancement of density. They attributed that to the density of the WMD which is greater than the density of the mortar. Figure 5 presents the compressive strength development of the ACM with WMD. With the increase of the WMD content up to 15%, compressive strength decreases due to a lack of participation in the hydration process [37].
The decrease in compressive strength proves that the presence of WMD, which does not have pozzolanic properties, reduces the effectiveness of the hydration process [29,38]. The air-curing conditions that caused water evaporation and thus created voids and internal cracks [39], as well as the decrease of apparent density and the significant reduction of the clinker content of the mixture [30], may explain the fall in compressive strength.
Microstructural details of ACM with WMD in amounts of 0%, 5%, 10% and 15% were obtained using scanning electron microscopy analysis. The backscattered electron images (BSE) were gained from mortars cured for 28 days in air conditions ( Figure 6). Each particle in the images were defined using SEM-EDX analysis as illustrated in Figure 7. In addition, the compositions for each ACM containing 0%, 5%, 10%, and 15% of WMD were also determined according to grayscale. The brightness histogram of the back-scattered electron images of ACM was determined using ImageJ software, the histograms are also illustrated in Figure 6.  Scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX) analysis of ACM containing 0% and 15% WMD. Figure 6 shows the microstructure differences of the ACM. As can be seen the ACM with WMD has a higher content of calcium hydroxide (CH). When analyzing the images, it can be seen that adding WMD to cement mortars causes higher porosity in comparison to the control mortar. This could be explained by water accumulating around WMD particles which leads to reduce the water required for the hydration of cement and, therefore, resulting in an increase in the porosity. As can be seen from the mortars containing WMD, the WMD does not appear in the images and the ettringite is too big compared to that of control mortar. This is could be explained by the reaction of calcite provided by WMD with tricalcium aluminate, which led to the formation of calcium carboaluminates [1,21,40]. This new chemical element is introduced in the formation of Ettringite that is why the WMD does not appear in the images and the ettringite is too big. This is confirmed by the SEM-EDX analysis of the samples with 15% WMD and without WMD (Seen Figure 7). The ettringite of the mortar containing 15% WMD appears big than that of control mortar.
As can also be seen from Figure 6, the brightness histogram for each ACM consists of five distinct peaks representing: pores, sand aggregates, hydration products, portlandite and partially hydrated cement grains. By comparison, the highest peak of pores, portlandite and partially hydrated cement grains, and lowest peak of hydration products are appeared in the ACM containing WMD, except the portlandite peak of ACM containing 15% of WMD is the lowest. In addition, there is no visible WMD peak apparently due to two reasons, the WMD is mixed and adherent with the partially hydrated cement grains and the calcite provided by WMD reacted with tricalcium aluminate which led to the formation of calcium carbo-aluminates [1,22,40]; however, the peak of hydration products appeared lower in the ACM containing WMD. This is due to a decrease of C-S-H content, this content representing between 50%-70% of the hydrated products.
The pores content and the percentage of the calcium hydroxide (CH) could be determined based on BSE images and chemical mappings (Figure 8). Microcracks were avoided during the analysis.  Figure 6 shows the microstructure differences of the ACM. As can be seen the ACM with WMD has a higher content of calcium hydroxide (CH). When analyzing the images, it can be seen that adding WMD to cement mortars causes higher porosity in comparison to the control mortar. This could be explained by water accumulating around WMD particles which leads to reduce the water required for the hydration of cement and, therefore, resulting in an increase in the porosity. As can be seen from the mortars containing WMD, the WMD does not appear in the images and the ettringite is too big compared to that of control mortar. This is could be explained by the reaction of calcite provided by WMD with tricalcium aluminate, which led to the formation of calcium carbo-aluminates [1,21,40]. This new chemical element is introduced in the formation of Ettringite that is why the WMD does not appear in the images and the ettringite is too big. This is confirmed by the SEM-EDX analysis of the samples with 15% WMD and without WMD (Seen Figure 7). The ettringite of the mortar containing 15% WMD appears big than that of control mortar.
As can also be seen from Figure 6, the brightness histogram for each ACM consists of five distinct peaks representing: pores, sand aggregates, hydration products, portlandite and partially hydrated cement grains. By comparison, the highest peak of pores, portlandite and partially hydrated cement grains, and lowest peak of hydration products are appeared in the ACM containing WMD, except the portlandite peak of ACM containing 15% of WMD is the lowest. In addition, there is no visible WMD peak apparently due to two reasons, the WMD is mixed and adherent with the partially hydrated cement grains and the calcite provided by WMD reacted with tricalcium aluminate which led to the formation of calcium carbo-aluminates [1,22,40]; however, the peak of hydration products appeared lower in the ACM containing WMD. This is due to a decrease of C-S-H content, this content representing between 50-70% of the hydrated products.
The pores content and the percentage of the calcium hydroxide (CH) could be determined based on BSE images and chemical mappings (Figure 8). Microcracks were avoided during the analysis. Sustainability 2019, 11, x FOR PEER REVIEW 10 of 14 Figure 8. Scanning electron microscopy (SEM) images of ACM with WMD: calcium hydroxide (blue particles); pores (red particles); sand aggregate (black particles); partially reacted cement grain mixed with WMD (white particles); hydrate products (gray color).
When analyzing data from Figure 8, it can be noted that the increasing amounts of WMD up to 15% cause a growth in the calcium hydroxide (CH) content. In addition, it can also be seen from this figure that the porosity increases with a corresponding increase in the amounts of WMD up to 15%. This could be explained by the trapped air that formed when ordinary Portland cement and sand aggregates are mixed with WMD. They cannot be easily removed because the air tends to be stabilized by WMD. Table 4 presents the porosity and CH results. When analyzing data from Figure 8, it can be noted that the increasing amounts of WMD up to 15% cause a growth in the calcium hydroxide (CH) content. In addition, it can also be seen from this figure that the porosity increases with a corresponding increase in the amounts of WMD up to 15%. This could be explained by the trapped air that formed when ordinary Portland cement and sand aggregates are mixed with WMD. They cannot be easily removed because the air tends to be stabilized by WMD. Table 4 presents the porosity and CH results. Considering the data in Table 4, it can be seen that the increasing amounts of WMD from 0% to 15% led to an increase in porosity and CH content from 7.739% to 8.991% and from 8.391% to 9.201%, respectively.
It is visible from Table 4 that the porosity of the cement matrix increases with corresponding increase in the amounts of WMD. This can be mainly due to the fact that the WMD can trap water (water accumulates around WMD particles). It may lead to the reduction of water required for the hydration of cement particles and therefore resulting an increase in the porosity. In addition, the WMD may accumulate around the cement grains. It can prevent the contact between the cement grains and water and therefore leads to a slight reduction of the hydration products. Finally, this reduction may contribute to an increase in porosity.
This increase in porosity also had an influence on the compressive strength [41]. The utilization of WMD in amounts of 5%, 10% and 15% led to an increase in the porosity of cement matrix in mortar at 28 days by 8.483%, 10.25% and 17.00%, respectively.
On the other hand, the CH content increased in the ACM at 28 days by 6.18%, 8.43%, and 9.65% when using WMD in amounts of 5%, 10%, and 15%, respectively. The increase in CH is accompanied by a decrease of C-S-H.
The replacement of ordinary Portland cement by WMD causes a decrease of C-S-H and an increase of CH content. It may be mainly due to the change in water distribution when the different amounts of WMD are used that leads to an incomplete hydration process. According to our previous studies [10,30], the SEM-EDX analysis indicates that the samples blended with WMD have higher Ca and lower Si content due to the CaCO 3 provided by WMD and lower SiO 2 found in WMD, thus the WMD contributes to the increase in the CH and decrease in the C-S-H.
Finally, the pore size distribution area in the ACM containing different amounts of WMD using ImageJ software was determined, and the results are presented in Figure 9. Considering the data in Table 4, it can be seen that the increasing amounts of WMD from 0% to 15% led to an increase in porosity and CH content from 7.739% to 8.991% and from 8.391% to 9.201%, respectively.
It is visible from Table 4 that the porosity of the cement matrix increases with corresponding increase in the amounts of WMD. This can be mainly due to the fact that the WMD can trap water (water accumulates around WMD particles). It may lead to the reduction of water required for the hydration of cement particles and therefore resulting an increase in the porosity. In addition, the WMD may accumulate around the cement grains. It can prevent the contact between the cement grains and water and therefore leads to a slight reduction of the hydration products. Finally, this reduction may contribute to an increase in porosity.
This increase in porosity also had an influence on the compressive strength [41]. The utilization of WMD in amounts of 5%, 10% and 15% led to an increase in the porosity of cement matrix in mortar at 28 days by 8.483%, 10.25% and 17.00%, respectively.
On the other hand, the CH content increased in the ACM at 28 days by 6.18%, 8.43%, and 9.65% when using WMD in amounts of 5%, 10%, and 15%, respectively. The increase in CH is accompanied by a decrease of C-S-H.
The replacement of ordinary Portland cement by WMD causes a decrease of C-S-H and an increase of CH content. It may be mainly due to the change in water distribution when the different amounts of WMD are used that leads to an incomplete hydration process. According to our previous studies [10,30], the SEM-EDX analysis indicates that the samples blended with WMD have higher Ca and lower Si content due to the CaCO3 provided by WMD and lower SiO2 found in WMD, thus the WMD contributes to the increase in the CH and decrease in the C-S-H.
Finally, the pore size distribution area in the ACM containing different amounts of WMD using ImageJ software was determined, and the results are presented in Figure 9.  As can be seen from Figure 9, 97% of the pore area varied from 20 to 500 µm 2 (the pore diameter varied from 5 to 25 µm). In addition, the increase in amounts of WMD up to 15% led to an increase of pore size area in this range. It may be because 50% of the WMD particles have a size less than 30 µm and thus most of the pores smaller than 30 µm cannot be filled by WMD. These pores represent the spaces previously occupied by water.

Conclusions
Based on the experimental results of ACM, the following conclusions may be drawn from the work presented in this paper: 1.
The utilization of WMD as a partial replacement of cement does not significantly affect the flow property of fresh mortar, where the results were found to be compatible with ASTM C270. 2.
The macro properties of mortars were affected negatively due to the incorporation of WMD and to the air-curing conditions that caused an incomplete hydration process in which the compressive strength and apparent density decreased, and the porosity increased.

3.
The compressive strength and apparent density of ACM mixed with and without WMD grew with an increase in curing duration and fell with a corresponding increase of WMD.

4.
The use of WMD in an amount of up to 15% caused an increase in porosity. It may be mainly due to the trapped water between WMD particles which reduces the water required for the hydration of cement. In addition, the accumulated WMD around the cement grains can prevent the hydration process. 5.
The utilization of WMD has trapped the air in the mortar and, thus, trapping air creates air voids. 6.
The brightness histogram of ACM indicates that the utilization of WMD contributes to an increase of pores, CH and partially hydrated grains and a decrease of hydration products. 7.
The CH content of the cement matrix in ACM increases with a corresponding increase of WMD. It may be mainly due to the water accumulating around WMD particles that leads to an incomplete hydration process. In addition, the WMD may accumulate around the cement grains and can prevent the contact between the cement grains and water. Therefore, it may lead to a slight reduction of the hydration products. 8.
The pore area in ACM smaller than 500 µm 2 represent 97% of the total pores, and these pores represent the spaces previously occupied by water. The WMD cannot can fill these pores because 50% of its particles are less than 30 µm.
Indeed, the WMD affects negatively the properties of air-cured mortar; however, due to the low solid density of applied WMD compared to the solid density of ordinary Portland cement in construction practice, it may be economically sensible to use of WMD in lightweight cement-based materials used for non-structural elements.