## 4 Capillary Water Absorption Results  
In this section physical properties related to water absorption are evaluated considering the two mortar types (MNS and MHS) at different high temperature levels. In this regard, Figure 7 presents the Capillary water Absorption (CA), in kg/m\({}^{2}\), of the two mortar mixtures measured at different time steps, in hours, and for different pre-conditioning thermal treatments, in degrees Celsius. The results in Figure 7 show that complete saturation was almost reached in 24 h all specimens. It can be observed in Figure 7a that CA-24 ranged from 4.54 (at 20 \({}^{\circ}\)C) to 22.12 kg/m\({}^{2}\) (at 600 \({}^{\circ}\)C) by considering increasing temperature levels for the MNS samples, while in Figure 7b CA-24 ranged from 0.58 (at 20 \({}^{\circ}\)C) to 18.26 kg/m\({}^{2}\) (at 600 \({}^{\circ}\)C) for MHS ones at increasing temperatures. From these results, it can be noticed that, by considering the same level of temperature conditioning, normal strength mortar MNS specimens possess a much higher capillarity water absorption than the MHS samples. Actually, higher levels of capillary absorption can be directly correlated to higher volume of accessible porosity. In this sense, the \(w/b\) ratio plays a fundamental role in driving the absorption behavior of the composites, especially for those cases conditioning at low temperatures, i.e., 20 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, where the water absorption is driven by only the open porosity of the specimens. In other words, at low temperatures of conditioning, mortars with high values of \(w/b\) are characterized by high capillary absorption and accessible porosity. Moreover, the results also demonstrate that temperature effects hugely affect the water absorption of the mortars. Although this effect is relatively much higher on MHS (since the damages due to thermally-induced cracks are more evident in high strength mortars) than on MNS, it is significant on both composites. This means that physical properties of mortars are very sensitive to medium to high temperature levels and consequently the mortar durability performance is greatly affected by thermal effects. Figure 6: Relative weight loss (Diff.%) for MNS and MHS. Finally, the values of accessible (open) porosity (\(p\)) evaluated as described in UNE-EN 1015-18 (2003) [36], are shown in Figure 8. Particularly, Figure 8a shows the \(p\)(%) values for both MNS and MHS at several temperature levels (20 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C). The values of \(p\)(%) varied from 9.39% (at 20 \({}^{\circ}\)C) to 19.07% (at 600\({}^{\circ}\)) for MNS, and from 3.02% (at 20 \({}^{\circ}\)C) to 17.26% (at 600 \({}^{\circ}\)C) for MHS. As expected, the mixtures with higher \(w/b\) ratio (i.e., MNS mixtures) are characterized by higher values of accessible porosity than those of lower \(w/b\) ratio (i.e., MHS mixtures). It can also be observed in Figure 8a that \(p\)(%) increased almost linearly for MNS, while for MHS the increasing of \(p\)(%) was more evident from low-to-medium temperatures (up to 300\({}^{\circ}\)). This particular physical behavior of MHS specimens can be attributed to a higher development of microcracks in high strength mortars when temperature begins to rise. On the other hand, Figure 8b reports the dimensionless \(p^{*}\), defined as \(p^{*}=p(\%)/p_{20^{\circ}\text{C}}(\%)\), with \(p_{20^{\circ}\text{C}}(\%)\) the porosity of the samples at 20 \({}^{\circ}\)C. This figure mainly helps to comparatively understand the effects of high temperatures in both normal and high strength mortars. Particularly, it shows how thermal treatments more affect MHS than MNS mortars, producing in the MHS ones a greater increase of the \(p^{*}\) value measured at certain high temperature compared with \(p^{*}\) measure at room temperature (20 \({}^{\circ}\)C).