*2.2. Preparation of Geopolymeric Mixtures*

The geopolymeric materials (GP) were prepared using the molar ratios SiO2/Al2O3 = 2.5 and K2O/SiO2 = 0.28 based on results from previous research [14]. A potassium hydroxide (KOH) analytical-grade reagent and commercial potassium silicate (K2SiO3) distributed by Pan American Chemicals (SiO2 = 26.38%, K2O = 13.06%, H2O = 60.56%) were dosed to obtain the activator solution module (Ms: SiO2/K2O = 0.76) required in the geopolymer mixture. The percentage of addition of TiO2 as a function of the cementant was varied in three levels, 0 wt %, 5 wt % and 10 wt %. The liquid/solid ratio (L/S) was also varied in three levels, 0.35 (dry consistency), 0.40 (mean consistency), and 0.45 (fluid consistency). In total, nine mixtures per system were made, each with two repetitions, for a total of 18 mixtures. Table 2 presents the blend specifications employed in this research.


**Table 2.** Compositions of the mixtures.

The mixing process of the solid components (MK and titanium dioxide) and liquids (activator) was performed in a HOBART Vulcan 1249 mixer. The solids were homogenized for 15 s, and the activating solution (KOH + K2SiO3) was added and mixed for 3 min at low speed, followed by 2 min at medium speed and, finally, 1 min at low speed until a homogeneous paste was obtained. The geopolymeric paste was then casted into cubic silicone moulds of dimensions 20 mm × 20 mm × 20 mm. The obtained samples were cured at room temperature (25 ◦C) for 24 h and then demoulded and taken to a humidity chamber (relative humidity >90% and 25 ◦C) until the test age was reached (seven and 28 days).

#### *2.3. Physical and Mechanical Properties*

In fresh conditions, the setting time and fluidity were determined. The setting time was determined using a Vicat needle, following ASTM C191. The fluidity was measured following the procedures described in ASTM C230. A modification to the test was that a cone of smaller volume or minislump with dimensions of 57 mm (high) × 38.20 mm (diameter) was employed on the flow table prior to the 25 strokes standardized in the guidelines. The flow diameters reported were measured using a digital caliper.

In the hardened state, the compressive strength was evaluated at ages of seven and 28 days using an INSTRON 3369 universal test machine with a capacity of 50 kN at a deformation rate of 1 mm/min. In each case, a minimum of three specimens were tested. The density, pore volume, and water absorption capacity of each material were also determined following the procedures detailed in ASTM C642-13. It should be noted that the drying of the sample was performed at 60 ◦C for 48 h.

#### *2.4. Microstructural Analysis*

The following techniques were used for the microstructural study of the blends: Electron microscopy, FTIR, and XRD. A JEOL scanning electron microscope (JSM-6490LV, high vacuum of <sup>3</sup> × <sup>10</sup>−<sup>6</sup> Torr, Peabody, MA, USA) was employed. The equipment has an INCAPentaFETx3 Brand Oxford Instruments model 7573 detector. The samples were metalized with gold in a Denton Vacuum Desk IV tank.

#### **3. Results and Discussion**

### *3.1. Fluidity and Setting Time*

Figure 4 shows the diameters measured after subjecting the different systems evaluated to the flow table test. As expected, as the L/S ratio of the system was increased, the fluidity increased, regardless of the TiO2 content. However, in all cases, the samples with no TiO2 addition had a higher fluidity percentage. For a fixed L/S ratio, increased TiO2 content reduced the workability of the samples; using 10 wt % with respect to the cement decreased the fluidity by as much as 30.13%. Duan et al., found similar results when measuring flowability in fly ash geopolymer mortars and reported that the addition of 3% and 5% TiO2 nanoparticles decreased the fluidity of the material by factors of 21.86% and 31.12%, respectively [8].

Figure 5 shows the effect of the TiO2 additions on the setting time. It is evident that both the higher liquid content (L/S ratio) and the addition of TiO2 particles affect the setting time, although their effects have opposite signs. By increasing the percentage of addition up to 10 wt % TiO2 particles, the setting time was decreased by 20.7%, 31.1%, and 27.6% for the dry, medium, and fluid consistency materials, respectively. The reduction of the setting time of the geopolymers with addition of TiO2, at a constant L/S ratio, can be attributed to a nucleation and filling effect that contributes to accelerate the

degree of reaction of the geopolymer. It is also evident that, by increasing the mixing water content, the concentration of ions in the aqueous solution decreases, decreasing the gel formation process, in addition to leading to a greater availability of water in the system to be evaporated for the formation of the consolidated structure, which is reflected in an increase in the setting time [15].

**Figure 4.** Fluidity of the various systems evaluated.

**Figure 5.** Setting times of the various materials evaluated.

#### *3.2. Physical and Mechanical Properties*

The obtained results regarding density, absorption, and porosity for the different mixtures can be observed in Figure 6a,b, and those of mechanical resistance to compression are presented in Figure 7. In general, the density fluctuated between 1230 and 1376 kg/m3, with no significant dependence on the L/S or the percentage of added TiO2 observed (Figure 6a). The results obtained for the density are in accordance with those reported by Liew et al. who evaluated the effect of using different L/S ratios on the density of geopolymers based on MK and found that the system of medium consistency

had the highest density, followed by the more fluid system and, finally, the drier one [16]. The authors concluded that increased liquid content can promote the speed of dissolution of the Al and Si species of the precursor, but it obstructs the processes of polycondensation; in contrast, a small amount of liquid does not favour dissolution and, therefore, generates increased viscosity [16]. Zuhua et al. also evaluated the role of water in the geopolymerization of materials based on MK using the calorimetry technique, noting that increased water reduced the rate of geopolymerization owing to the lower concentration of ions present in the dissolution processes; they also mentioned that water promotes hydrolysis processes, but decreases the rate of polycondensation [15].

**Figure 6.** (**a**) Density and (**b**) water absorption and porosity of the geopolymeric materials evaluated.

In general, an increased proportion of liquid adversely affects the permeable pore content of the blends (Figure 6b). This higher pore volume for the higher L/S ratio (0.45) is related to the lower mechanical performance of these mixtures. It was observed that the addition of TiO2 particles decreased the pore volume for materials with lower water content or dry consistency (L/S = 0.35), particularly when 5 wt % (GP5Ti) was added, coinciding with increased mechanical strength (Figure 7). This result seems to indicate that the addition of TiO2 particles contributes to a better mechanical performance; however, there is an optimum concentration of TiO2, so in the presence of an excess of the particles, these will act as filler [17].

**Figure 7.** Compressive strength of the various materials evaluated.

Of the different mixtures evaluated, those with the lowest water content (L/S = 0.35) presented higher mechanical performance. Among them, that with 5 wt % TiO2 added exhibited the most superior resistance, a 5.8% increase (50.27 MPa) after 28 days of curing, relative to the reference (GP); however, adding 10 wt % TiO2 yielded the opposite effect, possibly due to the low availability of liquid to hydrate the particles and promote the formation of reaction products, taking into account that there is a greater presence of non-reactive TiO2 particles coexisting in the gel. In contrast, for the materials with L/S = 0.4, it was found that the highest strength was achieved by adding 10 wt % TiO2 (43.25 MPa), yielding a strength 10.3% greater than that of the reference. This effect is attributed to the TiO2 particles having a smaller particle size than the precursor and behaving as nucleation points that enhance the formation of reaction products [17].

#### *3.3. Characterization of the Geopolymer Microstructure*

Figure 8 shows the XRD patterns obtained for the evaluated geopolymer systems. Compared with the MK precursor diffractogram (Figure 2), an amorphous halo between 25◦ and 35◦ can be observed in all cases, which corresponds to the aluminosilicate gel that forms the fundamental binder phase of the geopolymer matrix (KASH) and is responsible for the resistance of the geopolymer [18]. Likewise, the presence of the crystalline phase of the precursor is observed in the GP, which increases with the addition of TiO2 in the material owing to its contribution.

**Figure 8.** XRD pattern of the synthesized geopolymers.

The FTIR spectra of the synthesized geopolymers are shown in Figure 9, in which no large differences between the non-added and those with 5 wt % and 10 wt % TiO2 added are observed. The presence of bands characteristic of precursor materials (Figure 4), in addition to the displacement of the one located at 1089 cm−<sup>1</sup> in MK to 1007 cm−<sup>1</sup> due to the asymmetric stretching of the Si–O–T band (T is a tetrahedron of Al or Si) in KASH gels and a band at 463 cm−<sup>1</sup> corresponding to a lower Si–O–Si band intensity due to the reaction of MK for KASH gel formation, can be observed. The band located at approximately 703 cm−<sup>1</sup> verifies the presence of Ti–O–Ti photoactive species, and it also generates overlap with amorphous Al–O bands, in addition to exhibiting a decrease in its intensity due to gel formation. When comparing these spectra with the MK FTIR spectrum (Figure 3), it is important to highlight the displacement at higher frequencies in the 3436 and 1650 cm−<sup>1</sup> bands, which correspond to OH– and HOH group vibrations, suggesting increased hydrophobicity of the material as a result of the increased TiO2 additive content [19].

**Figure 9.** FTIR spectra of geopolymers.

The surface morphologies of the GP and GP10Ti geopolymers at seven and 28 days of curing evaluated by SEM and the corresponding EDS analysis results are shown in Figures 10 and 11, and the compositions of elements present at the evaluated points are presented in Tables 3 and 4. In general, all samples had a compact and continuous structure with small traces of unreacted MK, which is characterized by preservation of its lamellar structure in layers, mainly for the earliest age of curing; this behaviour was verified via EDS when performing the measurement at point 1 of Figure 10a, in which only high contents of O, Al, and Si, and lower proportions of K, were identified. By increasing the curing age, a greater gel formation is observed in the structure that is identified in the micrographs due to its dark tone, in addition to a lower presence of unreacted MK; in addition, the presence of O, Al, Si, and K is evidenced by the measurement of EDS at point 2 of Figure 10b, suggesting adequate KASH gel formation. It is noteworthy that there is no significant difference between the morphologies of the systems with and without TiO2, but the EDS measurements at point 2 of the sample with 10 wt % added (Figure 11) reveals the presence of O, Al, Si, K, and Ti, the latter being present in a smaller proportion, indicating that the formation of the gel was not impeded by the addition of TiO2 particles and that these coexist in the structure of the material [19].

**Figure 10.** SEM micrographs. (**a**) GP at seven days; (**b**) GP 0 at 28 days; (**c**) GP 10 Ti at seven days, and (**d**) GP 10 Ti at 28 days.

**Figure 11.** SEM micrographs with EDS for sample to which 10 wt % TiO2 was added (28 days).




