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Hegyi, A.; Lăzărescu, A.; Ciobanu, A.A.; Ionescu, B.A.; Grebenişan, E.; Chira, M.; Florean, C.; Vermeşan, H.; Stoian, V. Geopolymer Composites with Self-Cleaning Capability. Encyclopedia. Available online: (accessed on 21 June 2024).
Hegyi A, Lăzărescu A, Ciobanu AA, Ionescu BA, Grebenişan E, Chira M, et al. Geopolymer Composites with Self-Cleaning Capability. Encyclopedia. Available at: Accessed June 21, 2024.
Hegyi, Andreea, Adrian-Victor Lăzărescu, Adrian Alexandru Ciobanu, Brăduţ Alexandru Ionescu, Elvira Grebenişan, Mihail Chira, Carmen Florean, Horaţiu Vermeşan, Vlad Stoian. "Geopolymer Composites with Self-Cleaning Capability" Encyclopedia, (accessed June 21, 2024).
Hegyi, A., Lăzărescu, A., Ciobanu, A.A., Ionescu, B.A., Grebenişan, E., Chira, M., Florean, C., Vermeşan, H., & Stoian, V. (2023, August 10). Geopolymer Composites with Self-Cleaning Capability. In Encyclopedia.
Hegyi, Andreea, et al. "Geopolymer Composites with Self-Cleaning Capability." Encyclopedia. Web. 10 August, 2023.
Geopolymer Composites with Self-Cleaning Capability

By geopolymer, it is meant that type of amorphous, alumino-silicate cementitious material which can be synthesized by the polycondensation reaction between a geopolymeric material and alkali polysilicates. This process is called geopolymerization. This innovative technology allows for the transformation of alumino-silicate materials into products called geopolymers or inorganic polymers. Geopolymers, therefore, represent a material developed as an environmentally friendly alternative for the construction industry, but also as a solution for exploitation, reintroduction into the economic circuit of some industrial wastes and by-products, the most common being fly ash, slag kaolin and metakaolin mostly activated with alkaline solutions based on Na2SiO3 in combination with NaOH.

self-cleaning cementitious composites geopolymer TiO2 nanoparticles physical-mechanical performance

1. Physical-Mechanical Characteristics of Geopolymer Composites

The Si/Al ratio is a significant factor affecting the degree of crystallization and reaction with alkali activators [1], forming amorphous to semi-crystalline phases. Both polysialate-siloxo (Si/Al = 2) and polysialate-diloxo (Si/Al = 3) provided good strength of geopolymers. Polysialate-siloxo (Si/Al = 2) appears to be formed faster and has lower compressive strength than polysialate-diloxo (Si/Al = 3). The monomeric group of [SiO(OH)3], [SiO2(OH)2]2− and [Al(OH)4] normally forms later than Si and Al species, because small aluminium silicate oligomers can enhance geopolymer formation [2]. Research tends to indicate that metakaolin-based geopolymers exhibit satisfactory compressive strength with a Si/Al ratio of 1.9 ÷ 3, while the appropriate ratio for fly-ash-based geopolymers is about 2 ÷ 4 [3][4]. Due to the large availability of fly ash, and the need to reintroduce it into the economic circuit, in line with the principles of the Circular Economy, much research has been directed towards making these geopolymers using fly ash. In this context, the issue of integrating affordable eco-efficient solutions into the value chains is a challenge, while combating climate change and reducing the negative impact on the environment by reducing waste and by-products from industry, reducing cement consumption and thus reducing greenhouse gas emissions, etc., while ensuring the development of advanced innovative materials. Class F fly ash has been identified as the most suitable raw material for geopolymer materials due to its reactivity and availability. The mass ratio of SiO2 and Al2O3 in Class F fly ash is between 1.7–4.0, while the amorphous content is generally higher than 50% [5][6][7]. Class C fly ash has a calcium content of 15–40%. It offers a different geopolymer structure compared to class F fly ash due to the increased calcium content. Temuujin et al. (2013) [7] reported that class C fly ash has self-cementing properties, which, through alkaline activation, allow it to harden at room temperature. Having a low calcium content, Class F fly ash, according to the literature studied, was preferred for producing geopolymer binders because the high amount of calcium may impact the polymerization process which may lead to changes in the microstructure of the final product [7][8][9][10][11].
During geopolymerization, fly ash reacts with the alkaline medium and specifically with aqueous solutions of polyisalates, leading to the formation of geopolymer materials comprising alumino-silicate-hydrate (A-S-H) gel [9]. Fly-ash-based geopolymers have demonstrated good mechanical strength and enhanced durability [9][10][12].
Several additions introduced at preparation in the geopolymer composite can have beneficial effects, improving the characteristics of the material in the fresh state but especially in the hardened state. Thus, literature indicates beneficial effects of nano-SiO2 in small amounts (max. 10 wt% relative to the amount of FA) on workability, polymerization reaction, mechanical strength and durability [13][14][15][16][17][18], and the use of MgO powder as an addition can improve aspects related to compressive strength, workability, drying shrinkage and porosity [19]. However, all these should be analyzed in relation to the oxide composition and specific characteristics of fly ash used as the main raw material.
The concentration of the alkali activator has a significant effect on the compressive strength of the geopolymer. The test age and the temperature at which curing (heat treatment) of the geopolymer takes place are other variables that influence the compressive strength of the geopolymer. However, with all the still existing uncertainties, research has shown that a sufficient concentration of activator is required during geopolymerization, as the NaOH concentration has a greater influence on the compressive strength values than on the curing time [20][21][22]. A common alkaline solution used for making fly-ash-based geopolymers consists of sodium silicate and sodium or potassium hydroxide, respectively, a solution with molar concentrations of sodium hydroxide between 7 and 10 M. The combination of NaOH and sodium silicate (Na2SiO3) is the most suitable for the alkaline activator because sodium silicate contains partially polymerized and dissolved silicon, which reacts more easily. Research shows that geopolymers with higher strength, for the same type of fly ash, are obtained when the ratio (Na2SiO3/NaOH) is between 0.67 and 1.00 [23], with a molar concentration of sodium hydroxide (NaOH) of at least 8–10 M [8][24].
In this direction, Al Bakri et al. [25] studied the effect of Na2SiO3/NaOH ratio on the compressive strength of fly-ash-based geopolymer, showing that by increasing the Na2SiO3/NaOH ratio from 0.6 to 1.00, the compressive strength increased, with the maximum compressive strength obtained for Na2SiO3/NaOH ratio equal to 1.00 [26][27]. Morsy et al. (2014) [28] studied the effect of Na2SiO3/NaOH ratios of 0.5, 1.0, 1.5, 2.0 and 2.5 on the strength of fly-ash-based geopolymer mortar. The highest value of compressive strength was obtained when the ratio was equal to 1.0 and increasing the Na2SiO3/NaOH ratio caused a decrease in compressive strength. Álvarez-Ayuso et al. (2008) [29] showed that when the NaOH concentration is higher, geopolymerization can be achieved even without soluble sodium silicate.
Rangan et al. [30] proposed 0.2 < Na2O/SiO2 < 0.28 and 15 < H2O/Na2O < 17.5 as optimum oxide ratios in the activation solution to achieve improved performance in geopolymer concrete. Provis et al. [31] stated that if the activator to binder mass ratio is between 0.6 ÷ 0.7 and the activator has a SiO2/Na2O ratio in the range of 1 ÷ 1.5, the resulting geopolymer binder imparts better mechanical properties. Another study showed that as the ratio of sodium hydroxide to sodium silicate increased from 1 to 2.5 and the molar concentration of NaOH increased from 8 to 16 M, the compressive strength of fly-ash-based geopolymers increased, i.e., the highest compressive strength was obtained for NaOH (16 M) concentration and sodium hydroxide/sodium silicate ratio in the range of 1.5 to 2.0 [32].
Vora and Dave [33] concluded that an increase in the sodium hydroxide/sodium silicate ratio and a higher molar concentration of sodium hydroxide result in higher compressive strength. However, Sindhunata et al. [34] reported that a soluble silicate content (SiO2/M2O > 2, where M is alkali ion) may reduce the reactivity in alkali-activated mixtures, as the concentration of cyclic silicate species may inhibit further condensation of aluminium ions. Therefore, higher concentration is judged to give rise to stronger ion pair formation and provide a more complete and faster polycondensation process of the particle interface [26][35], improving the dissolution of silicon and aluminium-containing materials in the presence of activators [36], but too high concentration could lead to an increase in the coagulated structure [37], causing lower workability with rapid curing behavior [38].
The mass ratio of alkaline solution to raw material is widely used in geopolymer synthesis to define both alkaline dosage and water content. In most cases, fly ash was used and the ratio was called alkali activator to fly ash ratio (AA/FA). Barbosa et al. [39] analyzed the effect of AA/FA ratio on strength development using 10 M NaOH solution as alkaline solution with AA/FA ratio = 0.34 ÷ 0.46. It was observed that the compressive strength increased when the AA/FA ratio increased to 0.40. Too high of an AA/FA ratio could lead to a decrease in concentration as more sodium carbonate was formed and obstructed the geopolymerization process [40]. It was found that, depending on the type of alumina-silicate source materials, the recommended AA/FA ratio could be between 0.35 ÷ 0.50 to have good compressive strength as well as good workability [41][42].
In terms of heat treatment temperature, research conducted on geopolymer paste and mortars for a temperature in the range (30 °C and 90 °C) demonstrated an increase in the overall geopolymerization reaction, leading to an increase in compressive strength from an early age [43]. However, exposure of the material to a treatment temperature above 90 °C will result in a geopolymer with a porous structure due to the rapid loss of water from the mixture, which will result in a possible decrease in mechanical performance [44]. Therefore, heat treatment is considered to improve mechanical properties, but the optimum temperature that improves the geopolymerization process and leads to the development of a suitable geopolymer microstructure is in the range 60 ÷ 75 °C [45].
It is very difficult to accurately identify all the factors influencing the properties of alkali-activated geopolymer materials and to quantify the influence of each. According to the literature, the main factors influencing the properties of geopolymer materials are: type of raw materials and their Si and Al percentage content; Si and Al ratio; type of alkaline activator used; molarity of NaOH or KOH solution used; Na2SiO3/NaOH mass ratio; duration and temperature of heat treatment; test age of the geopolymer material [46][47][48][49][50][51][52][53][54][55][56].
In the case of geopolymers, temperature and curing time play a significant role in the kinetics of the unfolding of chemical reactions: at low temperatures the evaporation of geopolymer precursors and water molecules occur simultaneously, preventing the formation of voids and cracks within the material, thus increasing the compressive strength [57][58]. This suggests that a longer curing time at low temperatures is preferable for the synthesis of geopolymer with higher compressive strength.

2. Influence of TiO2 Adding Nanoparticles in Geopolymer Composites

It should be noted that, as reported by several research [59][60][61][62][63][64], a problem in producing these composites is the way of incorporation of NT, as this is relatively difficult to ensure a homogeneous dispersion, NT tending to agglomerate. Another controversy is in terms of self-cleaning capacity, with Guerrero et al. [62] indicating that 1 wt% NT is sufficient and Yang et al. [63] indicating an optimum requirement of 10 wt% NT.
Geopolymer concrete has been used in various applications such as waste management [47], civil engineering [64][65], cements and concretes [66][67], building retrofit [68] and as an alternative binder to replace OPC [68][69][70]. Applications have been extended to pavements [71][72][73][74] and building facades [65][71][74] and special function coatings [75]. Aguirre-Guerrero et al. [76] explored the potential of hybrid geopolymer coatings as a protective coating for reinforced concrete structures subjected to a marine environment. Sikora et al. [59] studied the applicability of geopolymer mortar as a coating to protect concrete from chemical attack and corrosion. Self-cleaning facades are gaining momentum in the construction industry. The applicability of self-cleaning facades is due to improved performance in brightness and reflectivity without the need for frequent maintenance [60][61].
Research reports on the influence of NT introduction in the geopolymer matrix on micro- and macro-structural characteristics, physical-mechanical and durability performances are found in a limited number of literatures. The literature indicates possibilities to induce self-healing capacity on geopolymeric materials using ZnO, TiO2, WO3 or Fe2O3 nanoparticles, with preference for NT and ZnO due to their high stability and low toxicity [77][78][79][80][81][82][83][84][85]. However, although they represent an environmentally friendly alternative in the construction industry, the properties of geopolymer materials with self-healing capacity are not yet sufficiently investigated so that they can be exploited.

2.1. NT Influence on Geopolymer Paste Properties

Experimental research conducted by Ambikakumari Sanalkumar and Yang [86] for geopolymer produced using metakaolin and containing 1–10% nano-TiO2 (mass percentages reported to the mass of metakaolin) showed that the addition of nano-TiO2 induces reduction in the flowability of the geopolymer paste. Such behavior could be associated with the high surface area of nano-TiO2 particles, which creates the water demand of the mixtures [87]. In addition, the existence of very fine particles brings stronger cohesive van der Waals forces inside the particles, resulting in flocculated and agglomerated particles, which also makes the fluidity of the geopolymer paste in its fresh state become lower. Specifically, the fluidity of the fresh geopolymer paste decreases, relative to the fluidity of the control fresh geopolymer paste (without nanoTiO2) by 3.22%; 5.65%; 25%, and by 37% when 1%, 2%, 5% and 10% nanoTiO2 are introduced into the geopolymer paste (mass percentages relative to the amount of metakaolin).
Research by Duan et al. showed that in the case of replacing 1%, 3% or 5% fly ash with nano-TiO2 (massive percentages relative to the amount of ash), the effects are also significant. Thus, a reduction in the workability of the nano-TiO2-containing geopolymer is reported, the workability decreasing by 7.9% for the case of 1% nanoTiO, and by up to 20% if 5% fly ash is substituted with nano-TiO2.

2.2. Influence of NT on the Properties of Geopolymer in Hardened State

Mechanical properties: Several studies have reported that the inclusion of TiO2 in geopolymer binders induces certain effects on the mechanical properties of the resulting binder. Duan et al. [88] conducted a study to examine the impact of nano-TiO2 inclusion on the physical and mechanical properties of a fly-ash-based geopolymer. The study showed that the incorporation of nano-TiO2 particles increases compressive strength and carbonation resistance, reduces the drying shrinkage of the geopolymer [88]. The addition of nano-TiO2 densifies the microstructure of the geopolymer matrix, in contradiction to a study [87] that reported that the inclusion of micro TiO2 particles does not improve the mechanical properties of the geopolymer.
In terms of density, experimental research [87] reported an increase of up to 12% in density and a reduction of up to 41% in the volume of pore sizes ranging from 2 nm to 5 μm, concomitant with an increase in nano-TiO2 content. These manifestations are thought to be due to the filling of fine pores by TiO2 nanoparticles, thus inducing changes in the geopolymer at the microstructural level. Because of the densification of the material, but not only, as expected, the compressive strength at 7 days was improved by up to 41% (compared to the compressive strength of the control sample without nano-TiO2) as the amount of nano-TiO2 used was increased. Samples stored and tested at longer than 7 days (14 days, 28 days) also showed increases in compressive strength but at lower levels. This behavior shows that, unlike cementitious composites, where the increase in compressive strength occurs as a result of hydration-hydrolysis processes taking place over time (28 days), in the case of geopolymer binders the strength performance is obtained as a result of aluminosilicate dissolution processes and formation of the specific three-dimensional structure, and a densification of the material induces beneficial effects.
Research by Duan et al. showed that in the case of replacing 1%, 3% or 5% fly ash with nano-TiO2 (massive percentages relative to the amount of ash), in terms of drying shrinkage, the presence of nano-TiO2 in the geopolymer paste has a beneficial effect in reducing this indicator. This behavior is attributed to the possibility of filling the pores of the geopolymer paste with nanoparticles, which leads to densification of the material.
In terms of the performance of the hardened composite, there is an increase in the compressive strength at 7 days after peening, compared to the control sample, by more than 4% in the case of substitution of 1% fly ash with nanoTiO2 and by more than 17% in the case of 5% nano-TiO2. This increase in compressive strength is also evident when testing specimens at lower or higher ages, i.e., 1 day, 3 days after pouring or 28, 56, even 90 days after pouring. However, in agreement with other reports in the literature, the intensity of the effect on the compressive strength at early ages is noted, with the highest increases in compressive strength recorded compared to the control sample 24 h after casting, respectively, 7.1% for 1% nano-TiO2 and 51% for 5% nano-TiO2, a sign that these particles act as a densification spinner and induce microstructural changes.
According to Zulkifli et al. [89], the geopolymer made from metakaolin with NT content shows a much more homogeneous, compact microstructure with low porosity compared to the control sample made without NT. Syamsidar et al. [90] present results on a geopolymer material made by heat treatment at 50 °C based on class C fly ash from Bosowa Power Plant Jeneponto, Suth Sulawesi alkali activated, SiO2/Al2O3 = 3; Na2O/SiO2 = 2 and H2O/Na2O = 10, in which 5%, 10% or 15% NT (mass percentage relative to the amount of fly ash) were introduced. Tests on specimens matured up to 7 days showed a much more compact and smooth surface appearance, without apparent porosity or surface defects, with apparent density increasing slightly with increasing %NT (2.85%, 3.1% and 4.76% increase in apparent density for 5%, 10% and 15% NT samples, compared to the apparent density of the control—0% NT). The compressive strength did not show a continuous increase as %NT increased, with a maximum recorded for the 10% NT composite, i.e., an increase of 12.36% for the 5%NT sample, 53.74% for the 10%NT sample and 45.64% for the 15%NT sample, suggesting the need to identify the optimal NT content interval in the geopolymer matrix in order to obtain the best compressive strength. XRD diffractograms performed for the NT geopolymer showed that, at microstructural level and compared to the control geopolymer (0%NT), no new crystallization phases are identified in the structure, therefore NT added during the preparation does not react with the constituents of the geopolymer, like the behavior of NT in cementitious matrices. Supporting the results obtained in the compressive strength evaluation, SEM analysis of the specimens indicates numerical reduction and decrease in microcracks opening for the case of NT specimens compared to the control geopolymer. Additionally, SEM analysis indicates a good adherence of the NT in the geopolymer matrix. In terms of resistance to H2SO4 action (1M, immersion 3 days), the strong reaction with CaO in fly ash is indicated with the formation of gypsum crystals (CaSO4-2H2O), which is favored by the existence of NT in the geopolymer matrix. The formation of these crystals will induce internal stresses in the composite matrix which will favor the degradation process of the material, therefore, the use of a CaO-rich fly ash is not favorable for the use of a geopolymer composite with NT if it is intended for use in an acidic environment. Regarding the self-healing capacity, it was analyzed on specimens immersed in red clay solution showing that after removal from the solution, the surface of the specimens remained clean, without adherent red clay particles.
Guzman-Aponte et al. [91] showed that the inclusion of up to 10 wt.% NT did not influence the development of calcium silicate hydrate gel but did not indicate in detail the effect of NT addition on the physico-mechanical properties of GP.
Duan et al. [88], analyzing the influence of NT on the physico-mechanical characteristics of geopolymer paste prepared by alkaline activation of fly ash (alkaline activator prepared based on Na2SiO3 and NaOH), showed that 5 wt%, NT relative to the amount of fly ash, compared to the geopolymer control sample without NT, contributes to an increase in the compressive strength both at early ages and 28 days after casting, to obtain a more compact microstructure with less microcracks and improves the carbonation resistance of the composite. Duan et al. contribute by their research, and given that, reports on the carbonation resistance of nano-TiO2-containing geopolymer are rare. Thus, it is shown that, as a result of these microstructurally induced changes, the depth of carbonation decreases as the number of nanoparticles substituting fly ash increases, the effect being even more evident compared to the control geopolymer, the longer the duration of exposure to carbonation. These results agree with the results reported by Sastry et al. [92], which indicate the increase in compressive strength of alkaline-activated fly-ash-based geopolymer for 2.5 wt% NT and Yang et al. respectively, [91] who, for the case of geopolymer made by alkaline activation of slag indicate the role of NT on several factors, namely, on geopolymer formation reactions, reduction in microcracks, cracks and improvement of compaction at the microstructural level.
Subaer et al. [93] analyze the thermo-mechanical properties of geopolymer composites made from alkali-activated metakaolin with soil. Na2SiO3 + NaOH, dispersive reinforced with 1–2 wt% carbon fibres and with NT applied as surface coating. The results showed that the geopolymer represents a good adhesion incorporating NT, but since NT are not included in the composite matrix and represent only a surface coating, their influence on the thermo-mechanical performance of the composite is not noticeable, carbon fibers having a more significant influence.
Mohamed et al., analysing the effect of TiO2 on the performance of alkaline activated meta-halosite based geopolymers with potassium hydroxide and potassium silicate-based activator, indicate that additions of 2.5%, 5%, 7.5% or 10% nanoparticles (mass percentage) reduce the total pore size (total porosity) by up to 49% compared to the control geopolymer, proportional to the amount of NT used. An increase in tensile strength of up to 78% is also reported, proportional to the amount of nano-TiO2 used. All this leads, in agreement with other reports in the literature, to the conclusion that NT has a beneficial densification effect at the microstructural level, but it is noted that rutile TiO has a stronger effect than anatase TiO.
SEM-EDS analyses reported by Bonilla et al. [94] indicate the possibility of obtaining more homogeneous, smooth, compact surfaces with a reduced number of cracks compared to the control sample. In terms of physical-mechanical properties, a slight increase in density was observed, but, probably due to the heterogeneous distribution and agglomeration of nanoparticles both in the NT and nano-ZnO cases, the compressive strength decreased significantly by more than 2.5 times compared to the control.
Self-cleaning capacity and biocidal capacity: The self-cleaning performance of nano-TiO2 modified geopolymer as a potential building material has been rarely reported in the literature [91]. However, as with cementitious composites, this self-cleaning ability is the sum of two main mechanisms: the ability to modify the surface’s hydrophilicity and the ability to decompose organic molecules and even microorganism cells through redox reactions. The most common methods of evaluation from this point of view are oriented towards the evaluation of surface hydrophilicity (induction of superhydrophilicity of the surface), the evaluation of the decolorization capacity of rhodamine B or methylene blue, respectively, the evaluation of resistance in the presence of an environment contaminated with microorganisms. Experimental research by Ambikakumari Sanalkumar and Yang [86] showed an improvement of up to 15% in total solar reflectance (TSR) for nanoTiO2-containing geopolymers compared to the control sample. Additionally, an improvement in hydrophilicity and surface self-healing ability is indicated and obtained on the one hand, as a result of the photoactivation of nano-TiO2 and, on the other hand, as a result of the use of NaOH in the alkaline activation of the raw materials to obtain the geopolymer, since it is known that NaOH has a strong decomposition effect on organic molecules [95][96].
In agreement with Loh et al. [97], it is estimated that incorporation of NT into fly ash or kaolin-based composites has the effect of increasing the photocatalytic activity of NT. Moreover, even in the absence of light, based on the MB decolorization test and tests using microbiological techniques, antifungal properties were demonstrated.
Zailan et al. [98] report a review on the induction of self-cleaning ability by introducing 2.5%, 5% and 7.5% NT into the geopolymer matrix and evaluating/demonstrating this performance using rhodamine B (RhB) and methylene blue (MB) staining tests, respectively. They also analyze the influence of ZnO nanoparticles on the performance of geopolymer prepared based on fly ash (FA) class F from CIMA plant Perlis, Malaysia and alkaline activator based on Na2SiO:NaOH, 12 M = 2.5:1.0 in which 2.5; 5; 7.5 and 10, wt%, ZnO nanoparticles were introduced. The results of the research on specimens matured up to 28 days showed a reduction of the compressive strength by approx. 29–54% compared to the control sample (0% ZnO nanoparticles), depending on the amount of nano-ZnO used, a behavior also supported by microstructural analysis by XRD and Sem which reveals changes in the crystallization phases. In terms of surface self-healing ability, based on the methylene blue (MB) staining test, a continuously increasing stain discoloration is recorded over time (over the evaluation time of 150 min UV exposure) and in relation to the amount of nano-ZnO used. This phenomenon is explained by a mechanism similar to the one presented for NT and confirms the possibility of inducing photocatalytic character on geopolymer not only using NT but also using nano-ZnO.
In consensus, Min et al. [99], Gasca-Tirado et al. [100], Zhang and Liu [101] and Luhar et al. [102], Kaya et al. [103], Chen et al. [104] indicate good results in terms of self-healing ability by photocatalytically activated degradation of methylene blue (MB) stains—Zhang and Liu indicating a 93% reduction of methylene blue stain staining for the case of photoactivated alkaline activated fly-ash-based NT-containing geopolymer surface within the first 6 h. However, research shows that part of the reduction in the degree of staining is due to the absorption of the dye into the geopolymer mass, and the rest is due to the degradation of the dye under the action of photoactivated NT [101][102]. Additionally, some research even indicates that GP itself has its own antimicrobial, antifungal and decolorizing capacity of staining substances, but the addition of NT has a strong increasing effect on these properties [97][103][105][106].
Yang et al. [107] report the effects of introducing 10 wt% NT on the performance of a geopolymer matrix made from fly ash from Shenhua Junggar Energy Corporation in Junggar, Inner Mongolia, China. Experimental XRD, SEM, BET analysis and photocatalytic activity results indicated the possibility of uniform distribution of NT in the geopolymer matrix and the influence of NT distribution mode on the specific geopolymer–NT surface area and photoactivity, i.e., the decolorization capacity of MB, which collapsed with increasing distribution homogeneity.
Alouani et al. [108] investigated the ability of geopolymer material produced by alkaline activation of metakaolin as an adsorbent to remove methylene blue. Strini et al. [109] showed that for 3 wt% NT added in geopolymer paste made from fly ash and metakaolin, geopolymer binders can be effective matrices to support photocatalytic activation of NT and induce specific material properties. Relating also to self-cleaning properties, Wang et al. [110] show that 5 wt% NT would be the optimum percentage for maximum MB decolorization effect, but research is insufficient because GP performance is influenced by several factors, as shown in the head above. Finally, and in terms of the influence on the physical-mechanical properties of GP, it is still controversial how much NT is introduced into the GP paste to achieve optimal performance.
Qin et al. [111] indicate the possibility of making superhydrophobic geopolymer surfaces by alkaline activation of blast furnace slag (also Si and Al oxide supplier waste) with reported good results in terms of hydrophobicity and therefore durability, including the fouling resistance capability of the material. In a similar direction of research development, Chindaprasirt et al. [112] indicate the possibility of inducing superhydrophobicity and self-cleaning capability of the surface of a geopolymer made based on alkali-activated fly ash, Na2SiO3/NaOH = 2, but in this case which benefited from a polymeric surface coating. Permatasari et al. [113] indicate the possibility of inducing self-healing performance for the surface of a geopolymer made from Gowa Regency soil deposit laterite, to which a thin film of NT solution coating was sprayed. This method allows for inducing a self-cleaning character, without influencing the flexural strength, with the mention that, once this film is destroyed, the self-cleaning capacity is lost.
In terms of bactericidal effect on K. pneumoniae and P. aeruginosa, Bonilla et al. [114] analyze a geopolymer composite based on powder material consisting of alumino-silicate precursors (85%) and Portland cement (15%), alkaline activated and containing 5 wt%, 10 wt% NT, 5 wt%, 10 wt% nano-ZnO, respectively. Research results demonstrate the development of inhibition halos and bactericidal effect such as a gentamicin antibiotic for both composite paste and composite mortar. In the same paper, the authors also indicate the self-cleaning effect developed by photocativation, an effect that causes rhodamine B, RhB, to decolorize 76.4% after 24 h for NT and over 98% for nano-ZnO.


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