Two different zeolites synthetized from a hazardous aluminum waste were used as admixtures in the manufacture of cementitious materials: a NaP-type zeolite synthesized at bench scale by Sánchez-Hernández et al. (44), and a Linde Type-A (LTA) zeolite synthesized at pilot scale by López-Delgado et al. (45). Although most of the characteristics of both zeolites, such as morphology, structure and thermal stability were reported in the referred papers, some other ones of special relevance to understand the final characteristics of the obtained cementitious materials have been determined in this work.

The mineralogical composition of the zeolites was determined by XRD. A Bruker D8 Advance Diffractometer with CuKα radiation was used for X-ray diffraction analysis (XRD). The semi-quantification of the amorphous phase of the zeolites was performed using Diffrac.Suite EVA software. The chemical composition of the zeolites used in this study was determined by X-ray fluorescence (XRF) using a pressed pellet of 10 g of the zeolite without additives in a wavelength dispersive X-ray fluorescence spectrometer (Bruker, S8 Tiger). The loss of ignition (LOI) was calculated by subjecting the samples to heating at 1000 ºC for 1 h. Samples density was determined in triplicate by using the Archimedean method. Textural characterization of the zeolites was conducted by determination of nitrogen adsorption/desorption isotherms at 77 K in an ASAP 2010 Micromeritics equipment. Before the measurements, the samples underwent outgassing at 50 ºC under vacuum for 24 h. The particle size distribution (parameters d₁₀, d₁₆, d₅₀, d₈₄ and d₉₀) of the zeolites was determined by laser diffraction (Dispersion Analyser LUMISizer 610 Berlin, Germany).

Seven different mortar mixes were designed and characterized. CEM I 42.5R according to EN 197-1 and siliceous sand (0/2 mm) were used in all of them. The density of the cement used was 3.1 g/cm³ with a mean particle size of 19.2 µm (particle size distribution: 10% < 1.9 µm, 50% < 14.4 µm and 90% < 44.5 µm). The two zeolites characterized in this study were considered: a NaP-type for the NaP mortars (M_NaP) and a LTA-type for the LTA mortars (M_LTA). Each zeolite was considered in mass percentages of 5%, 10% and 15% of cement content. Previous studies mentioned that above 10% the natural zeolites limited the performance of cementitious materials (15-19). Therefore, the authors aimed to evaluate if this percentage is similar when using synthetic zeolites from aluminum waste. An additional mortar without zeolite was produced as reference mix. Table 1 shows the composition of the mortars fabricated. Both the binder content and the water to binder ratio were maintained constant in all cases.

The consistency of the fresh mortar mixes was determined by the flow table test according to EN 1015-3 and the density content was determined according to EN 1015-6. The consistency was also measured after different times from the start of the mixing process in order to evaluate the maintenance of the consistency over the first 90 minutes. Regarding the mechanical properties. Two determinations of the density, the consistency and the maintenance of the consistency were made for each mortar mix. 12 square section prisms of 40 mm × 40 mm × 160 mm were cast for each mortar mix in order to evaluate the flexural and compressive strength after 2, 7, 28 and 90 days of moist curing (98% RH, 20±2ºC). The flexural and compressive strength of the hardened mortars were determined according to EN 196-1 standard. Every prism was firstly submitted to a three-point bending flexural strength test. The resultant halves of each prism were then subjected to a compressive strength test. The outcome is three flexural and six compressive strength measurements per mix and curing age. The measurement of these mechanical properties has allowed to evaluate the influence of the zeolite type and content on both parameters.

Figure 1 shows the XRD patterns of the two zeolites used in this study and high crystallinity in both samples (NaP = blue, LTA = red) is detected. Apart from the diffraction peaks corresponding to both zeolites, peaks attributable to corundum and spinel phases are detected in both cases. Nevertheless, the background analysis of the corresponding profile by EVA software revealed an amorphous phase content of 26.5 % for zeolite NaP and of 30.6 % for zeolite LTA.

The chemical composition of both zeolites is collected in Table 2. They consist principally of SiO₂, Al₂O₃ and Na₂O, even though several compositional differences are observed. Thus, although both zeolites correspond to low Si/Al ratio zeolite type, the Si/Al ratio in zeolite LTA is 0.71, that is lower than the value 1.15 for zeolite NaP. It is worth noting the TiO₂ content in zeolite LTA (2.44 wt.%), could partially replace the SiO₄ tetrahedra in the crystal lattice. In this way, the (Si+Ti)/Al increases up to 0.79. Similarly, the K content is higher in zeolite LTA, this indicating a partial substitution of the Na⁺ exchange cations. These two low Si/Al molar ratio zeolites have a high cation exchange capacity due to the saturation of the negative charge compensation cations introduced by Al (Na⁺, K⁺...), which are very easy to exchange, especially in aqueous media. Moreover, they are very hydrophilic due to dipolar interactions with the electrostatic fields located between the charge compensating cations and the negative charges in the lattice.

The physical and textural characteristics of both zeolites, such as density, particle size distribution, specific surface area (S_BET), external surface area (Sext), and pore volume, are listed in the Table 3. The initial pH values are also included. The density and pH value of NaP are higher than those of LTA. Both zeolites exhibit a particle size distribution ranging from 0.1 to 60 µm, with slightly smaller particles observed for the LTA zeolite (see Figure 2). Analysis reveals that 50% of the particles are smaller than 18 µm for the NaP zeolite and 5 µm for the LTA zeolite, indicating a larger particle size for NaP compared to LTA. Considering the 16th to 84th percentiles, the particle size range for NaP zeolite is between 11.0 ± 0.8 µm and 30 ± 2 µm, while for LTA, it ranges from 1.92 ± 0.03 µm to 11 ± 3 µm. Accordingly, LTA zeolite exhibits a greater particle size dispersion than NaP zeolite. Furthermore, for NaP, 90% of the particles measure under 35 µm, twice the size observed for LTA at 12 µm. As for the specific surface area, even though the value is quite similar for both zeolites, the value of micropore volume is much higher for the LTA zeolite, i.e. this zeolite exhibits a higher microporosity.

The potential use of the fabricated zeolites in cementitious materials has been evaluated by characterizing the influence of their incorporation on the fresh and the hardened state of mortars. Regarding the fresh state, density, consistency and “open time” at 0, 15, 30, 60, 75 and 90 minutes have been measured. Open time refers to the time that the mortar mix maintains an acceptable consistency; this parameter is important in order to evaluate the incorporation of zeolites in concrete technology since a very low maintenance of the consistency over time could make difficult their scalability to the real market.

Figure 3 shows the density values measured in the fresh state of the fabricated mortars. The density values obtained are very similar in all cases but slightly higher in mortars with zeolites. Furthermore, as the zeolite content increases, the fresh density of the mortar slightly increases too. The observed performance differs from that observed in mortars with natural zeolites in previous studies (46). Nevertheless, Nagrockiene et al. (22) also observed increases in the fresh density of concretes with natural zeolites and the density of the concrete increased as the content of natural zeolite used increased. The reported increases were similar to those obtained in the LTA mortars but lower than the ones obtained in NaP mortars, even though the density of the natural zeolites used was little higher than the density of the zeolites used in the present paper. For example, in the mix with 10% of natural zeolite, the concrete density increased 1% with respect to the reference concrete while this increase is around 3% for the mortars with 10% of NaP-type zeolite and 1% for the LTA mortars with 10% of LTA-type zeolite with respect to the reference mortar. It is also remarkable that in the same works it was shown that at early ages porosity decreased, so it is possible that the synthetic zeolites used in this study promotes a more compacted mixture. Actually, the present results agree with those observed by Girskas et al. (47) in the hardened density values of concretes with zeolites derived from the waste of aluminum fluoride production by low-temperature synthesis; they detected that 10% of zeolite increased 1.5% the density of the concrete at 28 days. However, these authors detected decreases in the fresh density of the same concretes when the same synthetic zeolite content was used in combination with air-entraining admixture while slowly increases were detected without this chemical additive. Thus, possibly the combination of zeolites, both natural and synthetic, with different chemical additives can explain the previously published contradictory results. In this regard, any chemical additive was used in the present study so only the effect of the synthetic zeolite used is taken into account. Furthermore, although the density of the zeolites used is lower than the density of the cement, higher compaction of the mixtures could occur due to the lower particle size of the zeolites with respect to the cement one, thus slightly increasing the density of the resulting fresh mortar mixes.

Figure 4 (A) shows the initial consistency measured in the mortars and its evolution over time while Figure 4 (B) shows the percentage of loss of consistency with respect to the initial value over time. Except for M_LTA-5 mortar, the consistency of the mortars with zeolites is lower than the one of the reference mortar during all the test period. Moreover, the higher the zeolite content, the lower the initial consistency measured. This observation agrees with the higher absorption/adsorption of water by zeolites with respect to cement, due to their porosity. In fact, it is clearly detected that the addition of NaP-type zeolite decreases the consistency values in a greater extension than the addition of LTA zeolites. These results do not agree with the S_BET and micropore volume values given in Table 3, since NaP zeolites showed lower values. However, LTA zeolites have lower particle size (see Figure 2) and possibly they can have a filler effect that improve the fluidity of the fresh mortar mix, which agrees with the higher initial consistency measured in M_LTA-5 mortar with respect to REF. This filler effect when using percentages substitution below 10% has been already reported for limestone filler and other raw materials with low particle size (48-51). This performance of LTA mortars is very interesting for construction applications since a more fluid cementitious material makes its placing easier.

Regarding the mechanical properties of the fabricated mortars, Figure 5 shows the evolution of the flexural strength and Figure 6 depicts the evolution of the compressive strength over time. Any significant conclusion is obtained from the flexural strength values but interesting conclusions can be obtained from the compressive strength results. Regarding LTA mortars, they show higher initial compressive strength but a very slow increase is detected from 7 to 90 days of curing, except for M_LTA-5 sample. The higher initial strength showed in LTA mortar agrees with the filler effect previously mentioned. The very low particle size of LTA-type zeolite allows that they acts as nucleation sites from the hydration of the cement hydrates. This effect has been reported in mortars and concretes with limestone filler with similar particle size than LTA-type zeolites (48-51). Moreover, the slightly higher specific surface are of LTA-type zeolite with respect to NaP-type one, can also explain a higher initial reactivity of the formers. Additionally, LTA-type zeolite has particles of TiO₂ (see Table 2) and previous studies have demonstrated the nucleation effect of TiO₂ particles with low particle size (52).

The filler effect is not clearly detected in NaP mortars, possibly due to the higher particle size of NaP-type zeolite. However, NaP-type zeolite clearly increases the long-term compressive strength. Furthermore, this increase is greater when the zeolite content increases, since the mortar with 15% NaP (M_NaP-15) shows the greatest value at 90 days. This, most likely, will be due to pozzolanic reactions of the silica in the zeolites with the portlandite in the cement, which agrees with published results with natural zeolites (13-19). In this sense, in the LTA mortars, the long-term pozzolanic reaction is only clearly detected in the one with the lowest zeolite content (M_LTA-5). Pozzolanic reactions also occur in M_LTA-10 and M_LTA-15 mortars because, if not, their respective compressive strength at 90 days will be lower than the ones measured (considering their lower cement content). In any case, the importance of the pozzolanic reactions in M_LTA-10 and M_LTA-15 mortars is lower than in M_LTA-5 mortar and very much lower than in M_NaP mortars. This agrees with the higher silica content measured in NaP-type zeolite with respect to LTA-type zeolite (see Table 2).

It is noteworthy than, according to these results, while the maximum zeolite content allowed when using LTA zeolite agrees with the percentages already published in order to avoid the decrease of the long-term mechanical performance of cementitious materials (15-19), the maximum content when using NaP zeolite is higher. In this study, the substitution of 15% of cement by NaP-type zeolite has not limit but increase the long-term compressive strength of the fabricated mortars, so possibly higher contents could be used thus increasing the sustainability of the construction materials.