Influence of mechanical activation on the behavior of green high-strength mortar including ceramic waste

Limestone cement of the CEM II/A-L 42.5 R type was employed in all high-strength mortar (HSM) blends. Cement has a specific gravity of 3.05, a particle size of 17.99 μm, and a specific surface area of 399 m²/kg. Table 1 details the characteristics of cement that satisfy Iraqi Standard No. 5 [44] and BS EN 197-1 [45]. Figure 1 depicts the distribution of cement particles based on their diameters. The cement’s initial and final setting times are recorded as 2.5 and 3.4 hours, respectively.

Fig. 1.

2.1.1.

Fine aggregate

The high-strength mortar (HSM) formulations utilized through locally available sources include fine aggregate (natural sand) with a particle size ranging from 0.15 to 4.75 mm. The findings of the fine aggregate’s gradation, as depicted in Figure 2, indicate its compliance with Iraqi Standard No. 45 [46].

Fig. 2.

2.1.2.

Ceramic tile waste powder

Crushed ceramic tiles used in flooring (locally available) were broken into small pieces and then ground into powder (Fig. 3) using an electric grinder. Three grinding times (GT) were adopted to study the impact of grinding time (mechanical activation) on the performance of the ceramic tile waste powder (CWP) as an alternative to cement in high-strength mortar; the times used were 10 min, 15 min, and 20 min. The particle size distribution for each grinding time and chemical properties of CWP are presented in Figure 1 and Table 1, respectively. The fineness values of CWP were 542, 588, and 610 cm²/kg for the grinding times of 10 (or GT10), 15 (or GT15), and 20 (or GT20) minutes, respectively.

Fig. 3.

According to Table 1, CWP is mainly made up of 49.05% silica, 11.98% alumina, and 6.5% calcium oxide. The percentages for the remaining components, such as iron and potassium oxide, were less than 2%. Generally, most pozzolanic materials consist of silica-alumina and a smaller percentage of calcium oxide. On the other hand, cement is rich in calcium oxide, which is the primary compound responsible for its properties (hydration).

Figure 1 showed that all grades of grinding CWP resulted in smaller particle sizes than cement. This enhances the filling effect of these grains between sand and cement or even between cement grains. The figure also indicated that the grinding times of 10 and 15 minutes led to almost identical grain sizes, and a grinding time of 20 minutes was slightly finer than either of them. The surface area values shown above match this behavior.

For this investigation, ten HSM mixes were cast, including one control mix and nine additional mixtures in which cement was replaced with CWP. The HSM mixtures were divided into three groups based on grinding time (GT): The GT10 group (10 min GT), the GT15 group (15 min GT), and the GT20 group (20 min GT). Each group consists of three blends in which cement was replaced with CWP in percentages of 10%, 20%, and 30% by weight. The amounts of water, binder, sand, and superplasticizer in each mixture were the same. Table 2 lists the individual components of each mixture.

According to the ACI 116 definition [48], pozzolanic materials are chemically active (by reacting with calcium hydroxide resulting from cement hydration) when they are finely ground. Also, when these materials (with potential pozzolanic activity) are softer than cement, they will have a physical effect of filling the spaces between the cement grains and the aggregate or between the coarser cement grains. Therefore, adopting several grinding grades in this study was done to explore to what extent the increasing fineness of ceramic affects the different properties of high-strength mortar.

The HSM raw ingredients were blended using an electric mixer with two rotational speeds (285 and 140 rpm), which adhered to the ASTM C109 [49] standard. The following procedure was used for the mixing process.

To create the fresh mortar, a blending process was conducted for one minute at a rotational speed of 140 rpm, incorporating the dry components, including cement, sand, and CWP. Following that, the water and superplasticizer were introduced, and the mixing procedure was sustained at a consistent velocity for two minutes. Ultimately, the rotational speed was elevated to 285 rpm, and the moist constituents were subjected to an extra minute of blending.

After the mortar was completely mixed, it was poured into standardized molds with dimensions of 40×40×160 mm³ and 50×50×50 mm³. After around 23 to 24 hours, the molds were removed, and subsequently the hardened samples were submersed in water until the test.

Life cycle assessment (LCA) is a holistic method that can be employed to assess the environmental impacts of a product or process. LCA can provide a comprehensive perspective about the sustainability of products and/or processes that significantly aids decision-makers in identifying opportunities to reduce the overall environmental burdens of different products/processes that all lead to the development of eco-friendly products/processes. Therefore, as part of this research, an LCA of each of the mixtures was conducted using the OpenLCA software. The main aim of this section is to provide information regarding the environmental impacts of the new mixtures and directly compare them with the control mixture. The method of the LCA employed in this research is the CML-IA method. The CML-IA method was first introduced in 1992 by the University of Leiden, and its name is derived from the acronym Centrum voor Milieukunde (CML) that refers to the center for environmental science [56]. This method is required by the European EN 15978 and EN 15804 standards [57].

In this research, the ceramic tile waste powder is considered to be a waste material according to the definition of waste materials [58]. The system boundaries used in this research are “cradle to gate” and the scope followed in this research consists of the following:

The targeted product is 1 m³ of mortar.

The results of the flow rate test are illustrated in Figure 4. The findings indicated that substituting cement with CWP enhanced the flowability at varying proportions. This trend was observed across all the grinding time groups and replacement ratios. The GT15, GT10, and GT20 mixtures showed enhancement percentages of 12.52%–16.99%, 1.34%–16.99%, and 11.18%–20.72%, respectively, when replaced at 10%, 20%, and 30%. Gautam et al. [59] achieved similar results with up to 20% cement replacement with bone China ceramic waste. The results also showed that for most ratios, the flow rate increased when the replacement ratio was increased from 10% to 30% at a given grinding time. Except for the GT10/CWP30 mixture, the improvement rate remained constant when the CWP content was increased from 20% to 30% (16.99% higher than the reference mixture). The GT20 group recorded the highest increase in flow rate (20.72% more than the control mix) at 30% substitution of CWP. The increased workability of the fresh mortar containing CWP can be attributed to the additional free water released due to the filling effect of the fine ceramic waste particles within the mortar pastes [60]. Additionally, with increasing grinding, the sharp edges of the particles break off, bringing the shape of the particles closer to spherical [61]. This process reduces the friction between the particles, which ultimately leads to improved workability.

Fig. 4.

The outcomes of the compressive strength test carried out at 28 and 56 days of age for the reference mixture and those containing CWP are displayed in Figure 5. At 28 days, it was observed that the compressive strength did not follow a consistent pattern for each mixing time.

Fig. 5.

The findings revealed that the compressive strength of the GT10 group was not affected up to the replacement rate of 20%. At 30% CWP content, however, the strength decreased by 16.73%. The GT15 group recorded a similar strength to the control mixture at a 10% replacement ratio, but it fell slightly afterwards (declined by 7.38% and 5.58% at 20% and 30% CWP content, respectively). In the case of the GT20 group, the mixture with 20% CWP content showed a slight decrease in compressive strength (4.27%). For replacement ratios of 10% and 30%, however, the strength was lower than the reference sample by 21.81% and 14.93%, respectively.

Some CWP-based mixtures with 10% or 20% content recorded compressive strength equal to or close to the control mixture despite the reduction of cement content (the main binder). This was due to the filling effect of the ceramic particles, as they were smaller in size than the cement particles [62]. The CWP granules can fill the pores between the aggregate and the cement paste, thus increasing the density of the microstructure and contributing to increasing the compressive strength. Moreover, the pozzolanic activity of the CWP also contributed to reducing the strength decline [63]. When the CWP content reaches 30%, however, there is insufficient calcium hydroxide to react with all the CWP particles, resulting in some remaining unreacted. This leads to a reduction in the cement content and an increase in the effective water-to-cement ratio, ultimately decreasing compressive strength [64].

After 56 days, most of the mixtures that showed significant strength reduction at 28 days had regained a significant amount of strength. This behavior was more evident in mixtures containing 30% CWP, regardless of the grinding time. This is because the continuous hydration of cement and the manufacturing of more calcium hydroxide, which chemically reacts with CWP, thus producing more CSH and subsequently reducing the drop in compressive strength [22]. Moreover, it was found that the compressive strength values were convergent for all replacement ratios and grinding times, as the extent of the reduction in compressive strength ranged between 1.19% and 6.68% compared to the control sample. However, the GT15CWP30 mix recorded an increase of 2.08% in compressive strength. These results indicate that increasing the grinding time led to close values in compressive strength at later curing ages (56 days). Previous studies have also identified similar trends in their findings [42, 65, 66].

Moreover, although the compressive strength for a mixing time of 15 min gave slightly positive results, it can be noted that varying the grinding times of the CWP within the range of 10 to 20 min did not lead to a significant difference in the compressive strength, especially at later ages (56 days). This behavior was observed for the corresponding proportions with different grinding times. This indicates the possibility of being satisfied with a specific grinding time in order to reduce the energy consumption associated with increasing the grinding time.

The density results are displayed in Figure 6. The results demonstrated that the substitution of cement with CWP, within a range of 10% to 30% at various grinding times, did not affect the density of the hardened HSM significantly. Similar behavior (no significant change in mortar density) was also recorded previously when replacing 20% of the cement with ceramic waste [67]. The highest reduction in density was only 2.64%, which was observed in the GT30CWP10 mixture. Replacing CWP with cement did not significantly affect density despite its lower specific gravity compared to cement. This may be because the fine particles of CWP enhanced the compactability of the mortar through the filling effect, thereby contributing to the densification of the microstructure of HSM. This effect overcame the reduction in density resulting from the presence of CWP. Moreover, CWP particles’ internal curing may improve density [68, 69]. Water in the fresh mix supplied by CWP helps build dense CSH gel. This dense CSH gel maximizes the mix density [68, 70].

Fig. 6.

The results of the absorption test are presented in Figure 7. The findings indicate that, for the GT10 group, the mixture containing 10% CWP showed a slightly higher absorption rate (2.02%) than the reference mix. Meanwhile, it was observed that the sample with 20% CWP content had a slight decrease of 1.67% compared to the control sample. However, the high replacement rate (30%) increased the mortar absorption by about 11.3%.

Fig. 7.

Furthermore, the behavior of the GT15 and GT20 groups was similar, as the highest absorption rate was observed with low content (10%). Additionally, water absorption tends to decrease with increasing CWP content, recording a lower absorption rate of 4.31% and 7.66% at the CWP content of 30% for the GT15 and GT20 groups, respectively. The improvement can be explained by the filling effect of CWP particles, due to their small size in comparison to cement (packing effect), as well as their pozzolanic activity, which enhances the resistance to water absorption [71]. It led to the closure of the permeable pores inside the mortar, improving its resistance to water absorption compared to the control specimen. Moreover, according to the literature [72, 73], if the absorption values are less than 10%, the mortar or concrete is considered to be durable. The water absorption values in this study were less than 6%. Therefore, the HSM produced is considered to be durable.

Figure 8 displayed the findings of the UPV examination. It was found that the UPV values of the control mix and the mixture containing CWP as a substitute for cement were equal, indicating that the velocity of the ultrasonic pulse was not affected by the substitution. It is important to note that UPV is influenced by the density and modulus of elasticity of materials [74]. On the other hand, based on the literature [75], the concrete or mortar quality can be categorized depending on the range of pulse velocity values. For instance, excellent, good, doubtable, poor, and very poor quality are associated with a pulse velocity range of more than 4500, 3500–4500, 3000–3500, 2000–3000, and less than 2000 m/s, respectively [76]. The UPV values obtained in this study were higher than 4670 m/s, indicating that the produced mortar is of excellent quality.

Fig. 8.

The findings of the study on the dynamic elastic modulus are presented in Figure 9. The research indicated that using CWP with different grinding degrees was either comparable to or slightly lower than the reference sample (without replacement). Kulovaná et al. also observed a slight decrease in elastic modulus after the replacement of cement with CWP [77]. The elastic modulus values for all HSM mixtures were between 42 and 43.9 GPa. The lowest value of modulus of elasticity was observed in the GT10CWP30 mixture, which was 4.34% less than the reference mixture. The elastic modulus values for the GT15 group decreased with increasing CWP content, but the reduction was within the range of 1.83% to 3.12%. The decrease in the dynamic elastic modulus did not exceed 2.41% for GT20 group mixtures. Despite the dilution effect due to the reduction of cement by 30%, the modulus of elasticity did not decrease significantly. These findings can be interpretated according to Kuan et al. [78] who stated that the Al₂O₃ and SiO₂ present in the ceramic powders underwent a pozzolanic reaction with the hydration products of the cement to form a more compact calcium aluminate and hydrated C-S-H gel. As a result, the cementitious materials were able to be more effectively filled with regenerated coagulation, leading to an improvement in the bonding performance between the aggregate and the cement paste.

Fig. 9.

The electrical resistivity test is essential because it is linked to the corrosion of the steel reinforcement inside the concrete. The higher the electrical resistivity levels, the better the concrete’s resistance to rebar corrosion. The findings of the resistivity test are illustrated in Figure 10.

Fig. 10.

The results pointed out that all mixtures, except for the 10% replacement ratio for group GR10, had electrical resistivity equal to or less than the control mixture. The resistance of GR10 improved by 5.11%. However, the lowest electrical resistivity was observed at a replacement rate of 20% for all grinding time groups. The lowest value was 11.48 kΩ.cm (for the GT10CWP20 mix) compared to 14.78 kΩ.cm for the reference mixture. The cause of the reduced resistivity of some mixtures may be because electrical resistivity depends on the structural composition of the gaps and the extent of their connection to each other, and not just the size or number of voids [79, 80]. The results also revealed that replacing cement with 30% CWP showed electrical resistance nearly equivalent to the reference mixture. This effect was observed for all groups. These results indicated that the electrical resistivity was not negatively impacted at this ratio, despite the reduction in cement content. This is due to the interaction of CWP with calcium hydroxide resulting from cement hydration and the formation of an additional gel [80]. The intermingling of fine CWP particles within the reduced cement voids within the mixture and improved electrical resistivity [22] also play a role. Moreover, the concrete protection level against corrosion can be classified [63, 81] on the basis of the electrical resistivity as follows: very high, high, low to moderate, low, and very low for the resistivity values of >20,10 to 20, 5 to 10, and < kΩ.cm. In this study, all resistivity values were greater than 10 kΩ.cm. This means that the level of protection against rebar corrosion was high.

Figure 11 shows the results of the percentage of permeable voids. The path of void percentages with different substitution levels followed water absorption results but with different values. This is expected because water absorption depends on the amount of moisture entering through the permeable voids. The results showed that for GT10 mixtures, replacing cement with 10% CWP resulted in void percentages equal to the plain HSM. The voids decreased by 5.63% for the 20% CWP mixture, while they increased by 7.6% when replaced with 30% CWP. For GT15 and GT20, the behavior was similar, with increasing CWP content. The higher the CWP content, the smaller the volume of voids, recording a value of 7.33% and 7.81% less than that of the control mixture when replacing the cement with 30% CWP. Comparable outcomes have been reported in the literature [13]. The penetration of small CWP grains between the cement particles, in addition to reducing the content of soluble calcium hydroxide and its transformation into a solid substance—calcium silicate hydrates (CSH)—due to the pozzolanic reaction, contributed to closing the gaps or reducing their size compared to the mixture without CWP [82]. Similar findings were also recorded previously [68].

Fig. 11.

Figure 12 presents the SEM micrograph of the reference (control) and the GT15CWP30 mixtures. The figure shows that the control mixture had microcracks and air voids, as well as a cleavage in the interfacial transition zone (ITZ) between the aggregate and the cement paste. Moreover, there was inhomogeneity in its matrix. On the other hand, the 30% CWP-based mixture had fewer defects or even none at all. Additionally, the matrix was denser and more compacted, with fewer gaps compared to the reference specimen. This improvement is due to the fine ultra-ceramic granules filling the voids inside the matrix. These granules have a smaller diameter than cement, which helps to improve the microstructure of the mortar. The pozzolanic reaction of CWP also contributes to this improvement by consuming calcium hydroxide and replacing it with denser C-S-H gel.

Fig. 12.

Figure 13 shows the results of the GWP of all the mixtures. As can be seen from Figure 13, all the mixtures with different content of CWP and with different grinding times have showed lower GWP relative to the control mixture. Results also indicated that for the same grinding time, increasing the CWP resulted in decreasing the GWP of the mixtures.

Fig. 13.

The results of the normalization, GWP per the compressive strength of all mixtures at 28 days and 56 days, are provided in Figure 14. The results in Figure 14 clearly state that increasing the age of curing resulted in decreasing the kg CO₂ eq./MPa of all mixtures. According to Figure 14, the best mixtures that provided the lowest kg CO₂ eq./MPa is GT15CWP20 that showed a reduction of the kg CO₂ eq./MPa by about 20% relative to the control mixture.

Fig. 14.