4.1. Mercury Intrusion Porosimetry
The total porosity values obtained for the studied specimens at 90 days can be observed in Table 2
. In the exposure medium column, “no attack” refers to samples hardened in laboratory conditions and “attack” refers to those exposed to the sulphuric acid attack.
First, the total porosityof all the mortars had similar magnitude, independently, in the acid attack. In relation to those stored in optimallaboratory conditions (without attack), the lowest total porosity values were noted for the specimens with nanosilica. This result shows a slightly beneficial effect of this addition in terms of porosity, which agrees with another study in which nanosilica has been used [28
]. The control and ethyl silicate specimens had practically the same total porosity when not exposed to the acid attack. This result was expected in view of the fact that the ethyl silicate was applied by spraying on the surface of the sample, whereas the rest of the admixtures analysed in this work were incorporated during the setting of the specimens. Therefore, the effects of the ethyl silicate in the total porosity of the global material were less noticeable, especially when it was not exposed to harmful conditions. Finally, for this optimalcondition, the highest total porosity corresponded to the mortars with zinc stearate.
Regarding the total porosity of the mortars exposed to the sulphuric acid attack, the lowest values were noted for the control specimens, followed by the nanosilica ones. On the other hand, the highest values of this parameter were observed for the zinc stearate and ethyl silicate samples.
With respect to the effects of the abovementioned attack on the total porosity of each material, small variations of this parameter were observed. In the case of control and zinc stearate mortars, total porosity showed slightly lower values for the samples exposed to the sulphuric acid attack, compared to the optimal condition ones. On the contrary, for the ethyl silicate specimens, this parameter was higher when subjected to the harmful condition. For nanosilica mortars, the attack hardly had an effect on total porosity.
The pore size distributions in fixed diameter intervals—<10 nm, 10–100 nm, 100 nm–1 µm, 1–10 µm, 10 µm–0.1 mm, and >0.1 mm—were also studied. These distributions for the studied mortars can be observed in Figure 2
. In addition, the volume of each pore interval for the studied samples is compiled in Table 3
. Regarding the control mortars, the volume of pores with diameters higher than 100 nm hardly changed between the samples exposed to the sulphuric acid attack and those hardened in the optimal condition. On the other hand, a noticeable decrease in the volume of smaller pores (diameters lower than 100 nm) in the control samples subject to the acid attack was observed, compared to those without the attack. Therefore, the previously described reduction of total porosity for control mortars exposed to the attack could be related to the abovementioned decrease of overall volume of pores of diameter ranges lower than 100 nm. In view of this, it can be pointed out that the sulphuric acid attack produced a less-refined microstructure in control mortars, with a predominance of pores of higher sizes. This could be indicative of the damage causedby this attack, which entails a progressive dissolution of Portlandite and decomposition of C-S-H phases [31
], as well as the formation of expansive products [27
], which can cause microcracking of the pore network of the material, reducing its refinement (increasing the proportion of higher pores) and making it more accessible to aggressive substances.
In relation to the ethyl silicate specimens (as occurred with the control mortars), the volume of pores with diameters greater than 100 nm slightly altered between the samples exposed to the acid attack and those hardened in the optimal condition. Nevertheless, for specimens in which this solution was applied, pores with diameters lower than 100 nm showed an increase in volume in contact with the aggressive medium. The rise of total porosity obtained for the ethyl silicate mortars exposed to the attack could thus be due to that increase in volume of pores with sizes <100 nm. It can, therefore, be noted that the pore structure of samples with ethyl silicate became more refined after 90 days of exposure to the sulphuric acid attack, compared to those without, despite being more porous. The global increase in intrusion volume, and consequently in total porosity, for the ethyl silicate exposed to the sulphuric acid condition shows the harmful effects of the attack [2
]. In addition, the rise in volume of finer pores are also indicative of this attack, and represents a part of the pore network in which the attacksdevelops at the studied exposure age. As has been explained, ethyl silicate reacts with the material to which it is applied, producing pozzolanic solid phases [18
]. In the initial steps of the sulphuric acid attack, these pozzolanic products react with the aggressive ions, leading to the formation of ettringite and gypsum [4
] which progressively fill the pores, reducing their sizes. This takes place in part of the microstructure of ethyl silicate samples, as suggested by the pore size distributions obtained (see Figure 2
). In further stages of the attack, once all the pores are completely filled, and with a continuous formation of expansive products, it is expected that the degradation of the microstructure of the material is complete [34
Their behaviourof the zinc stearate specimens with regards to pore size distributions is relatively similar to that observed for the control mortar samples, although with higher values of global intrusion volume and total porosity. The volume of pores with diameters higher than 100 nm wasfound to be very similar for the zinc stearate specimens exposed to the acid attack than for those that were not subject to the attack. On the contrary, for the samples exposed to the acid attack, a reduction in volume of pores lower than 100 nm was observed. Therefore, the acid attack produced a higher proportion of greater pores in the zinc stearate mortars, limitingthe refinement of their microstructure [2
Regarding mortars with nanosilica, minute changes were observed in pore size distribution, when the results obtained for specimens exposed to the optimal condition and the acid attack are compared. The main variations were a slight decrease in the volume of pores <10 nm and a small rise in the volume of pores with diameters in the range of 10–100 nm. In view of this result, it seems that the microstructural effects of the sulphuric acid attack were still emerging at the studied age for nanosilica mortars, particularly with regards to the accuracy and limitations of the mercury intrusion porosimetry technique used to study this type of attack [34
Finally, if the different studied mortars exposed to the acid attack are compared, it is important to emphasise that those with ethyl silicate, zinc stearate, and nanosilica showed a more refined microstructure, with higher proportion of smaller pores (diameters lower than 100 nm) compared to the control mortars, despite their greater global intrusion volume and total porosity.
The results of mercury (Hg) retained after the end of the porosimetry test at 90 days of exposure to the optimal (“no attack”) and sulphuric acid aggressive (“attack”) conditions are compiled in Table 4
. This parameter provides qualitative information on the evolution of the microstructure [37
]—its tortuosity and the progressive closure of pores.
For the mortars and conditions studied in this research, the values of Hg retained had a similar order of magnitude. For the control mortars, this parameter was higher for those subjected to the sulphuric acid attack, compared to those exposed to the optimal condition. This result could be related to the damage caused in the pore network by the acid attack. As has been explained before, degradation of the microstructure due to this attack [4
] results in the formation of microcracks. This microcracking phenomenon modifies the internal surface of the pores, increasing this surface [40
] and consequently the tortuosity of the pore network. This result is in line with the loss of microstructure refinement observed in the control mortars with the attack (discussed earlier).
For the ethyl silicate mortars, a reduction in the retained Hg was noted when subjected to the acid attack, as compared those exposed to the optimal condition. An increase in pore refinement of this type of mortar, subjected to attack by the acid, was observed, with a high percentage of finer pores. This is in agreement with the decrease of Hg reduction under the same conditions and can be explained in relation to the silting of the pores by the formation of ettringite and gypsum, whichare produced by a reaction of the pozzolanic products (caused by the sprayed ethyl silicate) with the aggressive ions [4
]. This process closes the microstructure, reducing its tortuosity, at least until they are filled (when the microcracking phenomenon is predominant).
In relation to zinc stearate mortars, the Hg retained was found to be slightly higher for samples exposed to the acid attack than for those in contact with the non-aggressive environment. This result is in line with the lower microstructure refinement shown by zinc stearate mortars subject to the acid attack. As was with control mortars, this could be due to the formation of microcracks [29
], which increase the pore surface [40
], resulting in a higher tortuosity of the microstructure.
Regarding the nanosilica mortars, those exposed to the acid attack showed slightly lower retained Hg, compared to those stored in conditions that do not have aggressive substances. This result is in line with the total porosity and pore size distribution obtained for these additions. This lower value is indicative of a possible process of pore silting, which could reduce tortuositydue to closure of the microstructure. However, this also suggests that, simultaneous to the abovementioned silting process in pores that still have free space, there could be pores that are completely filled by expansive products. In such a situation, the continuous effect of the attack will form microcracks, breaking the pore walls and increasing the tortuosity of the pore network, subsequently increasing the amount of retained Hg. In global terms, this shows that the reduction of this parameter is relatively small in nanosilica specimens subject to the acid attack, compared to those exposed to the non-aggressive environment.