3.2.1. Dynamic Modulus of Elasticity
The results of the Dynamic modulus of elasticity test carried out at 28, 90 and 180 days are displayed in
Figure 3.
The modulus of elasticity increased from 28 to 90 days in the mortars CL and CL_10Mk, for all types of curing conditions. On the other hand, all CL_20Mk registered a decrease in the value of DME with increasing age. It is also notable that humid curing delayed the DME of the reference air lime mortar at 1 month, and both maritime and standard curing were more adequate for that mortar. On the contrary, maritime curing seemed to optimize the DME of mortars with 10% replacement by Mk (CL_10Mk) at young ages; however, there was a decrease of DME from 3 to 6 months, although this was not so significant for the other curing conditions.
Mortars with 20% Mk replacing lime achieved the highest DME at 1 month, decreasing after 3 and 6 months; however, DME was always higher for these mortars.
Comparing the dynamic modulus of elasticity obtained in the present study with the results obtained by previous studies, it is possible to see that Andrejkovičová et al. [
20], for mortars with 20% Mk, obtained very similar results compared to those present in
Figure 3. Based on Loureiro et al. [
21], the contents of hydraulic products of the air lime–Mk mortars with 20% Mk in comparison to the CL mortars justify the increase of the DME by the improvement of the cohesion and bonding between the particles. However, this increase was not observed in mortars with 10% substitution (
Figure 3), which suggests that a higher Mk content is necessary for the hydration products to compensate for the lime replacement. Comparing the results at 90 days with mortars tested at 60 days by Faria & Martins [
24], the values were in the same range. In Faria & Martins [
24] study, the higher values were registered in mortars with 20% replacement of CL by Mk, in conditions with high RH and accelerated curing with CO
2.
Faria et al. [
25], at 28 days (a short curing period when considering lime-based mortars) and with humid curing, obtained higher values of DME with percentages of 30% and 50% of Mk. Ferraz et al. [
26], for mortars with 0% of Mk, obtained an upward trend in elasticity modulus values from 28 to 90 days, especially for 30% Mk substitution, which reached the highest value of 3850 MPa. However, in the present study, the maximum value was achieved with 20% Mk at 28 days, with nearly 5000 MPa of DME, and a decrease from 28 to 90 days occurred, contradicting the results obtained by Ferraz et al. [
26].
3.2.2. Flexural and Compressive Strength
The flexural and compressive strength tests performed at 28, 90, and 180 days can be seen in
Figure 4 and
Figure 5, respectively.
It is possible to observe, in maritime and standard curing mortars, that the values of flexural strength increased from 28 to 90 days and decreased from 90 to 180 days. Regarding humid curing, this trend was only seen in the mortar with 10% Mk.
Both the maritime and the standard curing seemed beneficial for the reference air lime mortars, particularly after 1 month. However, there was a decrease from 3 to 6 months that did not occur with the humid curing. This curing seemed to delay the initial flexural strength of the reference and 10% Mk mortars. However, for the reference mortar, values drastically increased after 3 months.
Mortars with 20% replacement of lime by Mk achieved the highest flexural strength, particularly with the standard curing. Except for the reference mortar with humid curing, where flexural strength increased with age, and the mortar with 20% Mk with humid curing, where it decreased with age, there was always an increase from 1 to 3 months and a decrease for 6 months.
Gameiro et al. [
19], for 30%, and Andrejkovičová et al. [
20], for 20% of lime substitution by Mk, also obtained a decrease of flexural strength with age. This was possibly due to mortars’ shrinkage with microcracking and also due to the increase of the water/binder ratio. However, although there was a decrease in values, mortars with the presence of Mk, specifically with substitutions of 20% or more, always presented higher values of mechanical strengths over time, compared to reference mortars, which seemed to be induced by the occurrence of a pozzolanic reaction.
Figure 5 shows the result obtained for the compressive strength, and the highest values were obtained with the mortars where Mk replaced 20% of lime, for all the curing conditions and ages. However, the decrease with age was much less significant for standard curing mortars in comparison to maritime and humid curing mortars. For the reference air lime mortar, the compressive strength increased with time from 1 to 6 months, achieving the highest results with standard curing. With maritime curing, the results at 1 month were higher in comparison to the humid curing. This behaviour was also present in Pavlík & Užáková [
27], where these authors obtained a significant decrease in compressive strength with the age for mortars of up to 365 days, with a binder:aggregate of 1:1 (vol.) and 50% substitutions of CL by Mk. However, the values obtained for these modified mortars were much higher than those obtained in reference mortars, showing one more time very significant mechanical improvement with Mk substitutions with mortar age. This decrease in mechanical strength can be explained by the fact that lime mortars that were cured in highly humid conditions, with feeble access to CO
2, had a reduced carbonation rate. This uncarbonated lime in mortars that were in humid curing conditions justifies the decrease in compressive strength. The same behaviour was registered by Arizzi & Cultrone [
28] in all mortars, with the exception of 10% substitution of lime by Mk, when an increase in compressive strength was obtained from 28 to 180 days.
Figure 5.
Compressive strength of mortars with 0, 10 and 20% Mk replacing air lime, after 28, 90 and 180 days in maritime, humid and standard curing conditions.
Figure 5.
Compressive strength of mortars with 0, 10 and 20% Mk replacing air lime, after 28, 90 and 180 days in maritime, humid and standard curing conditions.
In general, Gameiro et al. [
19], Andrejkovičová et al. [
20], Loureiro et al. [
21], Faria & Martins [
24] and Faria et al. [
25] (
Table 1) also obtained an increase of flexural and compressive strength with the increase of the percentage of lime substitution by Mk in all mortars. Nevertheless, Gameiro et al. [
19] and Loureiro et al. [
21], who performed tests on mortars with different binder:aggregate ratios, obtained a decrease of mechanical strength with the decrease of the binder:aggregate ratio, which could be induced, as in compressive tests, by mortars shrinkage with microcracking and also due to the increase of the water/binder ratio
Faria et al. [
25], for substitutions of 30 and 50%, concluded that humid curing proved to be unfavorable for mechanical characteristics compared to standard curing, and the same occurred in the present study for 10 and 20%. As in the compressive strength test, this behaviour could also be explained by the lack of contact with CO
2, causing a lower carbonation rate.
Cardoso et al. [
22] concluded that pure lime mortars (CL mortars) have much higher mechanical behaviour than 9% Mk mortars, of nearly more than 80%. In the present study, something similar was found, although in a smaller percentage, with CL and CL_10Mk. It seemed that too low percentages of lime replacement by Mk could have a beneficial effect on the initial hardening period but could lead to mortars with reduced strength, with this behaviour being more visible in the flexural strength tests (
Figure 4).
Unlike other studies, Ferraz et al. [
26] stated that 10–30 wt.% Mk does not promote a significant influence on flexural and compressive strength, especially at 28 days of curing. It is noteworthy that for these authors, reference mortars with 0% Mk had a slight increase with age, but in mortars with Mk, there was also a slight decrease in mechanical characteristics from 90 to 180 days.
3.2.4. Open Porosity and Dry Bulk Density
The obtained results for the open porosity of all the mortars after 28, 90 and 180 days of different types of curing are presented in
Figure 7. It can be observed that the open porosity increased when Mk was incorporated into the mortars. Humid curing presented the highest average value across all ages, and the standard cure showed the lowest average value at 90 and 180 days. Open porosity also decreased in every mortar from 1 to 6 months, except for CL_10 Mk (m), which kept its value from 1 to 3 months. The mortars where Mk replaced 20% of lime registered the highest values, meaning that the total volume of interconnected pores was higher than the reference mortars (CL) and the mortars where Mk replaces 10% of lime.
Loureiro et al. [
21] also obtained similar results in the range of 28% for mortars with substitutions of 0% at 90 days. For 25 and 30% of lime substitution by Mk, results in the range of 27–29% were obtained, similar to those in
Figure 7. Loureiro et al. [
21] obtained the lowest values of porosity in the mortar only with air lime, similar to the results obtained in
Figure 7, where CL (s) had the lowest value at 90 days of age. According to the same author, mortars with Mk were more cohesive than the air lime mortars but had a similar microstructure (in terms of void numbers) to the mortars without Mk.
Faria et al. [
25], for 25%, 30% and 50% of lime substitution by Mk, obtained results in the same range as those present in
Figure 6, with porosity slightly increasing with the increase of Mk. However, Arizzi & Cultrone [
28] declared that the pore system of mortars with and without Mk presented similar open porosity values in the range of 31%. Despite the presence of Mk generating a new family of pores, whose volume increases at increasing Mk amounts [
42], reference mortars also have a low volume of pores in the same range of the radius [
43], translating into minimal differences of porosity.
Figure 8 presents the results of dry bulk density obtained in the present study. It was possible to identify a trend, where bulk density increased from 1 to 6 months in all of the analyzed mortars. The reference mortar showed higher values than mortars where Mk replaced 10% and 20% of the binder, as expected, since mortars CL_10Mk and CL_20Mk (regardless of the curing) obtained higher values in the open porosity test. The variation in porosity and bulk density does not explain the different mechanical results obtained for the tested mortars.
Figure 7.
Open porosity of mortars with 0, 10 and 20% Mk replacing air lime, after 28, 90 and 180 days in maritime, humid and standard curing conditions.
Figure 7.
Open porosity of mortars with 0, 10 and 20% Mk replacing air lime, after 28, 90 and 180 days in maritime, humid and standard curing conditions.
3.2.5. Capillary Water Absorption
The average capillarity curves of mortars at 28, 90 and 180 days of curing can be seen in
Figure 9. At 28 days, both the maritime and standard curing for the CL mortar presented an increase in the initial rate, but in the total absorbed water absorption, they stabilized at lower values. Mortars with 20% replacement of lime by Mk, concretely CL_20 Mk (m) and CL_20 Mk (h), exhibited the opposite behaviour, presenting a slower absorption rate in the initial phase, but they stabilized at higher values of the total absorbed water. At 90 days and 180 days, it was also visible that mortars where Mk replaced 10% and 20% of the lime had a slower absorption in the first phase, compared to the reference mortars (CL), and they also presented higher values of the total absorbed water. It was also verified in all ages that the mortars submitted to humid curing had an increase in the initial rate and lower values of the total absorbed water, compared to maritime and standard curing. As can be seen in
Table 5, with the introduction of Mk, a decrease in the initial rate existed, shown by the capillarity coefficient (CC), and so did an increase in the total absorbed water, shown by the Capillary saturation value (Cs). At 28 days, the reference mortars (CL) represented the worst cases of the capillarity coefficient, due to the faster ratio of water absorption. At 90 and 180 days, an increase of the CC was visible in humid curing compared to the others. Higher asymptotic values were achieved in CL_10Mk (m) and CL_20Mk (m) at 90 days.
In general, similar results of Cs were obtained by Faria et al. [
25] and Faria & Martins [
24], where the total absorbed water was higher for air lime mortars with Mk, and the capillary coefficient was lower when Mk partially substituted the lime. With the exception of mortars with 20% substitution of lime by Mk and in conditions of humid curing, Faria & Martins [
24] obtained a decrease of total capillary water absorption when Mk was used, contradicting the results presented in
Table 5, for CL_20Mk (h). However, Faria & Martins [
24] and Faria et al. [
25] presented lower values for CC, revealing slower absorption rates in the initial phase of drying.
3.2.6. Drying
Is important to know how mortars lose the water that they have absorbed to better understand their future performance in terms of durability. A high drying capacity can improve a mortar’s durability, avoid the appearance of fungi [
44] and contribute to salts’ transport to the surface. The drying curves of phase 1 and phase 2 at 28, 90 and 180 days, are presented in
Figure 10.
Through the linear segments of the curves by time and by square root of time, it was possible to determine the first drying rate (D1) and the second drying rate (D2), respectively, as shown in
Table 6. D1 is normally associated with the release of water, in the first drying stage, and D2 is related with the release of vapor in a more advanced drying phase.
At 28 days, maritime curing seemed to induce mortars to have the highest drying rates of the 1st phase (D1), meaning an increase in the release of water, in comparison to humid and standard curing conditions. D2 seemed to increase with the addition of Mk at this early age. As an absolute value, the highest ratios of D2 were observed in CL_10Mk(h) and CL_10Mk(s), meaning an increase in the evaporation rate due to improved diffusion. However, mortars with 10% substitution showed inconstant results over time.
Loureiro et al. [
21], analyzing the drying rates of mortars with the same binder:aggregate ratio and 25% substitution of air lime by Mk (
Table 1), achieved very different results when compared to those obtained in
Table 6. According to the same author, mortars with a low binder content show a shorter D2, because that content may have favored the formation of larger pores, which results in a great loss of moisture during phase D1.
Table 6.
Values of drying rates for the first (D1) and second (D2) drying phases of mortars with 0, 10 and 20% Mk replacing air lime, after 28, 90 and 180 days in maritime, humid and standard curing conditions.
Table 6.
Values of drying rates for the first (D1) and second (D2) drying phases of mortars with 0, 10 and 20% Mk replacing air lime, after 28, 90 and 180 days in maritime, humid and standard curing conditions.
Mortar (Curing) | Drying Rate, 1st Phase (D1) (kg/m2·h) | Drying Rate, 2nd Phase (D2) (kg/m2·h0.5) |
---|
28 Days | 90 Days | 180 Days | 28 Days | 90 Days | 180 Days |
---|
CL (m) | 0.144 | 0.162 | 0.178 | 1.710 | 1.335 | 1.326 |
CL_10Mk (m) | 0.163 | 0.166 | 0.176 | 1.472 | 1.534 | 1.639 |
CL_20Mk (m) | 0.138 | 0.193 | 0.173 | 1.822 | 1.695 | 1.473 |
CL (h) | 0.123 | 0.183 | 0.112 | 1.075 | 1.736 | 1.374 |
CL_10Mk (h) | 0.124 | 0.147 | 0.133 | 1.779 | 1.642 | 2.080 |
CL_20Mk (h) | 0.116 | 0.141 | 0.183 | 1.910 | 1.840 | 1.606 |
CL (s) | 0.098 | 0.167 | 0.143 | 1.033 | 1.399 | 1.262 |
CL_10Mk (s) | 0.113 | 0.176 | 0.112 | 1.759 | 1.960 | 2.066 |
CL_20Mk (s) | 0.095 | 0.184 | 0.143 | 1.805 | 1.474 | 1.759 |