Bulk XRD of the raw cuttlebone sample (Figure 1) indicates that the inorganic matrix is composed just by aragonite, in accordance with previously published data (2). Traces of halite were also detected, probably arisen from seawater. Commercial limestone was composed of calcite, with vestigial amounts of dolomite (20).

The chemical analysis by XRF (Table 1) shows the expected prevalence of Ca in both the raw materials. However, a slightly lower value was obtained for cuttlebone (53.4 wt%) in comparison to limestone (55.2 wt%). Additionally, some amount of Na (0.8 wt%) was found in cuttlebone, which is related to presence of halite identified by XRD and other possible inorganic contaminants. Noticeable contents of Sr (0.3 wt%) and Cl (0.3 wt%) were also found in the cuttlebone, the latter element also associated to halite phase. Magnesium (0.4 wt%) was found in the limestone due to the vestigial dolomite. The loss on ignition was slightly higher for cuttlebone (44.5 wt%) than for the limestone (43.9 wt%) sample. This reflects the different organic matter and calcium carbonate content of the two materials.

The weight loss in the thermogravimetric plot of cuttlebone (Table 2) occurs in three steps: one step (1.0%) up to ~110 ºC due to the release of adsorbed moisture; the second step (3.6%) from ~110 ºC to ~650 ºC due to the degradation of organic matter, within the range reported (2); and, finally a third step (40.8%), between ~650 ºC and ~860 ºC (Figure 2a) corresponding to the thermal degradation of calcium carbonate to calcium oxide. The theoretical calcium carbonate content was calculated to be 92.8%. The presence in the DSC plot of an exothermic peak, at approximately ~300 ºC and of an endothermic peak, at around ~825 ºC (Figure 2b), corroborate the results of the thermogravimetric analysis. Limestone shows the typical thermal behaviour expected for this material: one step of 43.9% weight loss (Figure 2a); and an exothermic peak at ~855ºC for the thermal degradation of calcium carbonate (Figure 2b).

Cuttlebone FTIR spectrum (Figure 3) shows the characteristic bands of aragonite: 1446 and 1462 cm^-1 (ν3 – asymmetric stretching); 1083 cm^-1 (ν1 – symmetric stretching); 853 cm^-1 (ν2 – out-of-plane bending); 713 and 700 cm^-1 (ν4 – in-plane bending). Additionally, a band of small intensity was observed at 1786 cm^-1 probably due to a harmonic vibration (ν1+ν4). A broad band at 1651 cm^-1 was also observed which can be attributed to the C=O stretching of the amide groups of chitin present in the raw cuttlebone (23). The limestone shows a FTIR spectrum as previously reported (20).

Finally, the Raman spectrum (Figure 4) of the cuttlebone exhibited vibrational bands at 1085 cm^-1 (ν1) and 703 cm^-1 (ν4) and translational bands at 285, 271, 259, 248, 205, 180 and 152 cm^-1 which reveal the presence of aragonite, corroborating the results of XRD and FTIR. Two very small-intensity bands were additionally observed at 1573 cm^-1 (probably a combination band) and 1461 cm^-1 (ν3) (24). The Raman spectrum of limestone was previously described (20).

The cuttlebone waste and the limestone sample were calcined at three temperatures: 900, 1000 and 1100 ºC, with 2 h soak time. Figure 5 presents the morphology of the obtained calcium oxide particles. The calcium oxide from limestone presented particles with irregular shape and rough and angular surfaces. The increase of the calcination temperature promoted a general agglomeration of the individual particles. This effect was more pronounced when the calcination temperature was increased from 1000 to 1100 ºC.

The size of the calcined particles of cuttlebone was markedly larger than the size of the calcined particles of limestone, as evident by the comparison of the SEM images of the calcined cuttlebone samples with those of the calcined limestone samples, with an ampliation ten times larger (d, e and f versus a, b and c, in Figure 5). The calcium oxide from cuttlebone presented rounded crystals with smooth surfaces. The agglomeration of particles as well as the disappearance of pores seem to occur between 900 ºC and 1000 ºC, which could indicate a sintering stage. Minor morphological changes were observed between 1000 ºC and 1100 ºC.

The BET specific surface area of the limestone calcium oxide samples decreased with increasing calcination temperature: 10.7 m²/g at 900 ºC, 5.2 m²/g at 1000 ºC and 2.3 m²/g at 1100 ºC. This agrees with the SEM microscopy results shown above, with smaller particles providing higher specific surface area. For the calcined cuttlebone, the magnitude of the BET values was lower: BET specific area was fifteen times lower at 900 ºC (0.7 m²/g), five times lower at 1000 ºC (1.0 m²/g) and two times lower at 1100 ºC (1.1 m²/g). This is in agreement with the great particle size difference between the two materials, shown by SEM. The larger and more aggregated particles of calcium oxide from cuttlebone promote a lower specific surface area. However, it is noteworthy that when calcination temperature was increased to 1100 ºC, the BET values of both calcined materials approached each other. Again, this could be an evidence of the sintering stage.

Wet slaking tests were performed for the calcined cuttlebone samples and the calcined limestone. For the calcined limestone the temperature of 1000 ºC was chosen, since this is the temperature usually used as reference in laboratory experiments. In the case of cuttlebone waste it was decided to change the calcination temperature, once for the tests with sample calcined at 1000 ºC the reactivity was not very high. Thus, temperature was increased and decreased of 100 ºC relatively to the reference temperature. For the calcined limestone a remarkably high reactivity was already obtained for the calcination at 1000 ºC (Figure 6), as expected (20).

The behavior of CaO from limestone was already described elsewhere (20). Briefly, this material is classified in the R5 class (reached 60 ºC in 25 s). The calcium oxides from cuttlebone were much less reactive (the sample calcined at 1000 ºC reached 60 ºC in 13:09 min:s). In agreement with the t60 values, the tu for 80% of the reaction was significantly longer for the calcined cuttlebone samples, showing that the hydration process is much slower. The values obtained in the reactivity test are in the same range of those obtained for razor and scallop seashells previously reported (20). The parameters T’maximum, Tmaximum and Tu, 80% reaction were similar for the calcined limestone (1000 ºC) and the calcined cuttlebone at 1000 ºC and 1100 ºC (Table 3).

Apparently, increasing calcination temperature from 1000 to 1100 ºC for cuttlebone seems to slightly retard the hydration curve (Figure 6). In average, the values for t60 and tu, 80% reaction were approximately 1:30 min:s higher when using the cuttlebone waste calcined at 1100 ºC (Table 3). However, a statistical analysis by ANOVA showed that the differences between the two tested temperatures are not significant: F calculated = 1.16 and F3;3;95% confidence level = 9.28; t calculated = 0.92 and t6;95% confidence level = 2.45. These results indicate that changing the calcination temperature to 1100 ºC does not promote an increase of reactivity in the wet slaking. Probably, the similar particle morphology and specific surface area (mentioned above) provide a similar wet reactivity, as well.

When calcining cuttlebone at 900 ºC some difficulties were found in obtaining reproducible wet slaking curves. The referred temperature could be insufficient to achieve calcium oxide particles with homogeneous physicochemical properties. The temperature of 900 ºC is also very close to the offset of CaCO₃ degradation temperature (~860 ºC) determined by thermogravimetry (Figure 2a). Notwithstanding, it was not observed a positive effect of changing the calcination temperature from 1000 to 900 ºC in the overall reactivity. Instead, the average wet curve was retarded (Figure 6) and the corresponding t60 and tu, 80% reaction even seemed to increase in comparison to the sample calcined at 1000 ºC. The statistical comparison between the t60 results at 900 and 1000 ºC, however, provided a F value of 0.86 and a t value of 1.24, from which it can be inferred that there are no statistically significant differences.

The lower reactivity of cuttlebone versus limestone must be related with their larger particles, lower specific surface area and more agglomerated structure, as it was evidenced by the SEM micrographs.

The slaking curve of the calcium oxide from cuttlebone exhibited a ‘‘S’’ pattern by opposition to a ‘‘line’’ pattern of the calcined limestone. The hydration rate of the calcined materials is influenced by the two induction periods underlying the slaking process. The slaking of the CaO from limestone is developed in one step, as previously reported (20), with no induction period. The hydration rate of the CaO from cuttlebone could be divided in three steps. As example, for the wet slaking curve of the 1000 ºC calcined sample, the first up to 30 s, the second from 30 s to 5:00 min:s, and the third one from 5:00 to 16:30 min:s, present a hydration rate of 5.1 ºC/min, 1.3 ºC/min and 4.0 ºC/min, respectively.

In the slaking of the calcium oxide samples it was not noted any relevant variation in the thickening of the suspensions.

The lime suspensions were dried to obtain hydrated lime in powder form. For both materials, the characterization by XRD showed portlandite, as main phase, and vestigial contents of calcite. In the case of lime from limestone, traces of brucite were also detected, formed by the hydration of magnesium oxide present in the calcined material (Figure 1).

XRF data (Table 1) showed, as expected, calcium as major element (81.7 wt% for lime from limestone and 76.7 wt% for lime from cuttlebone), obviously related with the presence of portlandite and calcite identified by XRD. The magnesium content was 1.2 wt% for lime from limestone, related with the brucite presence. The presence of strontium in the cuttlebone lime (already noted in the raw material) was confirmed. However, the sodium content as well as the chloride content were remarkably reduced after the calcination and the hydration, pointing out to solubilization during the hydration stage. The values of the loss on ignition were 16 wt% for lime from limestone and 22 wt% for lime from cuttlebone, related to the dihydroxylation of portlandite (and decarbonation of calcite).

Both FTIR spectra (Figure 3) of the limes showed the characteristic O-H stretching band of the portlandite (3637–3640 cm^-1). Bands of carbonate groups of calcite were also observed, including a broad band with maximum at 1420 cm^-1 (ν3). A weak and broad band at 1103 cm^-1 was observed in the lime from cuttlebone, probably indicating the presence of sulfate ions (25), as was previously reported for limes obtained from waste seashells (20). A weak band at 3693 cm^-1 (O-H stretching) due to brucite also occurred in the lime from limestone.

Finally, limes color properties were analyzed. The coordinates for the lime from limestone were L*=97.39, a*=-0.02 and b*=1.21. For the lime from cuttlebone a slightly higher lightness (L coordinate) was obtained (98.22). The values for the coordinates a* (-0.16) and b* (0.09), indicated a negligible shift to the green and to the yellow, respectively. The color results for the lime from cuttlebone, overall, point out to a white tonality material.

The hydrated lime powder from the waste material is in accomplishment with the specifications reported in the building lime standard.

Final considerations should be paid to the incorporation of the cuttlebone waste in the process of building lime production at industrial scale. Although the quicklime from cuttlebone waste was less reactive than the quicklime from limestone, it is not expected that the mixture of cuttlebone waste and limestone would highly reduce the quality of the final produced lime, since the residue would always be incorporated in a small proportion relatively to limestone. This takes into account the lower amounts of available residue, in comparison to the quantities of limestone needed to feed a lime factory: a common Portuguese lime factory needs ~1643 t by day of limestone to reach an average daily production of ~920 t of quicklime (21). In Portugal, there are no cuttlefish-processing companies and consequently an industrial cuttlebone waste is not available. However, the Portuguese Statistical Institute indicates for instance, in 2018, an amount of ~1030 t for landing cuttlefish (26). Assuming that cuttlebone waste represents a maximum of 35% of the total mass caught (27), a value of ~360 t of this waste can be estimated for 2018 (~1 t/day). Therefore, it can be inferred that the referred scenario would allow the total consumption of the cuttlebone waste without predictable loss of the quality of lime. Recycling this type of waste in the production of an ecological binder material will also contribute to a more sustainable construction.