3.1.1. Bulk Density (Volumic Mass)
The density of the mortars under study is depicted in
Figure 5 as findings.
It is clear that adding natural fibers to a mortar causes the material’s density to drop. This quality is crucial since it makes no sense to make a given building material resistant if it is very heavy [
39]. It is a good idea to lighten mortars by utilizing natural fibers because doing so significantly reduces the structure’s dead weight [
40]. On the contrary, it is desirable that mortars have a higher density of mass in the fresh state. It is known that mortars with a higher density of mass in the fresh state have a higher standard of performance in application and execution, which leads to a higher saving of material; thus, a balance must be found between fiber addition and density loss.
The measured values of the mortar density are listed in
Table 7 and illustrated in
Figure 5.
It can be observed that the bulk density of the reinforced mortars gradually decreased when the dosage of fibers was increased. This decrease compared to the control sample, ranging from 0.40% (B5 0.4 kg/m
3 of fibers) to 12.02% (H15 4 kg/m
3 of fibers), was caused by the fibers’ alveolar structure and the increased porosity from adding fibers. Given that the fibers’ density was lower than that of the mortar, this was to be expected. The creation of voids at the interfaces between the fibers and the solid matrix as a result of air bubbles being trapped by fibers during the mixing process can also be used to explain these findings [
41]. Due to the large porosity of the resulting material’s fibers, this reduction is also largely attributable to them [
42]. Equivalent results were found by Siham Sakami et al. [
41], in which the bulk density decreased by up to 14.68% for 5% of alfa mass content when using a reinforcement of alfa fibers.
Another conclusion that can be drawn from the graph (
Figure 5) is that soft fibers embedded in mortar have a much more significant drop in volumetric mass than rigid fibers, especially at maximum dosage.
This can be explained by the fact that the AR is much higher for soft fibers. This higher AR means that the fibers are slenderer, so more fibers are required for the same dosage, so that, as the number of fibers increases, there are fewer aggregates and the density decreases. Another immediate consequence of a higher AR is a greater loss of workability and a major increase in occluded air with soft fibers compared to with rigid fibers. On the other hand, there should be a greater shrinkage control with soft fibers; however, in the shrinkage section it can be seen that the difference is not remarkable: although the flax FRS had the least shrinkage, the bamboo FRS had a minimal shrinkage difference compared to this one.
Evidence can be gathered through the analysis of
Figure 5 and
Table 7 to identify whether the low density of the mortar is caused by the inclusion of the fibers or the development of pores. When the density of the fiber-reinforced screed was compared to that of a control sample free of fibers, it was observed that the density of the soft FRS with a greater dose of 4 kg/m
3 was lower. This suggests that the fibers were likely to blame for the low density. Although the density of the control sample was always higher, the other FRS densities were pretty close to the latest, suggesting that the creation of pores may be the main reason for the low density except in the case of high dosages of the soft fibers.
It is preferable for mortars to have a larger mass density when they are fresh. It is well known that mortars with higher mass densities when they are fresh perform better when placed and used, which leads to a higher saving of material. In turn, it can be concluded that the incorporation of fibers is beneficial.
3.1.2. Control of Air Content
Figure 6 and
Table 8 show that the values of air entrapped increased with an increase in the quantities of the vegetal fibers.
Additionally, it has been shown that the soft fibers (flax and hemp) had higher values of entrapped air than the rigid fibers (miscanthus and bamboo). Thus, the structural arrangement of the fiber strands and lower densities than the cement led to higher values of entrapped air. The vegetal reinforced composites had lower unit weights than that of the control mortar due to the increase in air content and the lower weights of the fibers.
The air content of vegetal fiber mortar composites depends on the fiber volume, workability, and mixing methods, all of which influence air dispersion from fresh mortar samples [
39]; as we can see in
Figure 6, flax and hemp had the highest values of air entrapped with the biggest dosage of 4 kg/m
3, corresponding to a lower workability and difficulties with the soft fibers during the mixing because they were more abundant and slender (AR ratio higher than the rigid fibers), which meant that the fibers agglutinated and clumped into each other.
H15 with a fiber dosage of 4 kg/m
3 had the highest entrapped air in the fiber composite, with a value of about 5.4%, followed by H5, with 5.2%. Hemp fibers have a lower density due to the structural arrangement of the fiber; hemp is believed to be porous, with a tendency to entrap more air than others. Although it is not the only factor, moisture absorption contributes to the porosity of hemp fibers. Hemp fibers expand when they take in moisture, widening the spaces between the filaments. The fiber’s general porosity rises as a result of this. According to studies, hemp fibers had an equilibrium moisture content (EMC) of 11.6% [
43], flax fibers were roughly 8% [
44], miscanthus fibers were 9% [
45], and bamboo fibers were 6% [
46] at 25 °C and 65% RH. The greater the quantity of entrapped air in a composite the lighter it becomes, which is corroborated by the bulk density results in
Figure 5.
Porosity is referred to as the number of voids by volume in porous material; it is a measure of the volume of air per unit volume of the said material, expressed as a percentage. Furthermore, the addition of fiber to mortar decreases the density and increases porosity [
47,
48]. This accounted for the measurements of density for the fiber composites and the control samples.
The addition of fiber increases porosity [
49] and therefore moisture absorption, which subsequently reduces the compressive resistance of the fiber in the composite [
50].
The air content test results shown in
Figure 6 reveal that, when the fiber dosage was minimal, there was only a very minor impact on the number of pores in the mortar mix. The number of pores grew as the number of fibers in the mortar increased. Similar regularity was observed by Vafaei et al. [
51], the authors concluding that this was caused by air voids trapped in the fresh mixture due to problems with fiber distribution and orientation. In this study, the highest increase in air entrapped (
Table 8) was observed for H5 samples with 4 kg/m
3 of fibers (80% compared to CS), followed by F5 samples with the highest fiber dosage (70% compared to CS); thus, as in the case of volumic mass, soft fibers have a major impact on the occluded air in the cementitious matrix.
It is to be noted that convincing evidence was not found for variations in either volumic mass or occluded air in fibers of nominal length from 5 mm to 15 mm. Accordingly, fibers with a lower AR ratio, such as 5 mm fibers, should perform more effectively due to less occluded air and therefore a higher density, but that is not the case for every dosage.
Regarding the occluded air depicted in
Figure 6, it appears that all fibers had more air than before. These findings suggest the decreasing workability of mortars containing fibers because the material’s workability decreases with increasing air content. Thus, it can be noted that the obtained values of incorporated air are coherent and advantageous in terms of the workability of the mortars because a significant increase in occluded air was not observed until 4 kg/m
3 was reached. Since the air content is directly related to the workability of the mortars, the trend observed proves to be consistent and can be justified by the same reasons given in explaining the workability of the mortars [
52,
53,
54,
55,
56,
57,
58].
3.1.3. Slump Test/Consistency
The slump value of cement composites is an essential factor in measuring the workability of a fiber cement mortar, in which the following factors take an active role: the water–cement ratio, properties of the material, mixing methods, dosages, and admixtures.
The flow results to T0 of reinforced mortars with vegetal fibers in the amounts of 0.4, 0.6, 0.8, 1.2, 2 and 4 kg/m
3 are illustrated in
Figure 7.
The references of the control specimen and with synthetic fibers with a dosage of 0.6 kg/m
3 are included. This reference dosage has been chosen because it is the commercialized dosage by Vicat with which all the results are compared. It should be noted that the flow of the reinforced mortars dropped as the fiber content rose beginning at 0.8 kg/m
3. This decrease can be neglected at the scale of the rheological characteristic of the mortar except for the dosage of 4 kg/m
3 in the case of the soft fibers, where the loss of workability became appreciable. Due to the interlocking of the fibers, it is obvious that increasing the fiber dosage and aspect ratio (AR) has a negative effect on workability. This result is in accordance with several studies [
52,
53,
54,
55,
56,
57,
58].
All values found for both the soft and rigid fiber formulations had acceptable flow rates with no segregation. The slump values remained stable until 1.2 kg/m3, when the slump started to decrease drastically for the soft fibers, while for the rigid fibers the slump remained almost the same.
The main parameter that can explain this behavior is the aspect ratio AR (length/diameter) of the fibers (parameter set in
Table 6). For the soft fibers embedded in the mortar, this ratio was higher than for the rigid fibers, making them slenderer, which created more blending problems with the consequent loss of fluidity and the lower slump value.
The dosage of 1.2 kg/m3 was the upper limit dosage beyond which the results did not improve with the addition of extra soft fibers, tending to fiber balling. Through that statement, it can be observed below in the mechanical strength section that the greatest results at 90 days of cure with soft fibers were obtained with a dosage of 1.2 kg/m3, which could be justified by the negative effect that the soft fibers had on the mortar’s mechanical strength when agglutinated in higher dosages.
As the results up to a dosage of 0.8 kg/m3 were very close at 0 min, tests were carried out over time (at 60, 120 and 180 min) to see how the time factor influences the decrease in the slump value for the mortars with 0.4, 0.6, and 0.8 kg/m3 fiber dosages. A range between 380 and 420 mm in diameter was considered a satisfactory slump result taking into account the values proposed by DTA N° 13/18-1387-V1 of CSTB.
Several reports [
32,
59,
60] have verified the samples reinforced fibers’ low workability, but, in this work, we could not confirm this in the case of the rigid fibers, which presented good behavior even in high dosages.
The aspect ratio, geometry, volume fraction, matrix proportions and fiber–matrix interfacial bond qualities of FRS in its freshly mixed form all affect its attributes [
61]. Good workability should be ensured for conventionally placed FRS applications to allow placement, consolidation, and finishing with the least amount of effort while providing uniform fiber distribution and preventing segregation and bleeding. As with ordinary concrete, the degree of consolidation affects the strength and other qualities of the hardened material for a particular composition. The addition of fibers may lessen the composite’s observed slump as compared to a non-fibrous mixture in the normal volume fraction ranges utilized for FRS, which is between 25 and 102 mm [
40], as can be evidenced by the results obtained for every slump (
Figure 7).
A mixture with a low slump can have excellent consolidation capabilities, according to studies [
62]. FRS and non-fibrous mortar exhibit similar time-dependent slump reduction properties [
63]. In addition to the aforementioned factors, it is important to prevent fiber balling. When shaken together, soft fibers such as hemp and flax, which have an aspect ratio greater than 100, will tend to interlock to produce a mat or ball that is very problematic to separate. On the other hand, rigid fibers such as miscanthus and bamboo, which have an aspect ratio of less than 50, are unable to interlock and are easily scattered [
39]; therefore, the slump of soft fibers is much less fluid than that of rigid fibers for all lengths and dosages (
Figure 7).
The maximum size and overall gradation of the aggregates used in the mix, the aspect ratio of the fibers, the volume fraction, the fiber shape, and the method of introducing the fibers into the mix are all factors that affect an FRS mix’s propensity to clump or produce fiber balling in the freshly mixed state. The lowerst volume percentage of fibers that can be added without a tendency to clump or ball depends on the maximum aggregate size and aspect ratio.
Figure 7 represents slump test values for the soft and rigid fibers at 0 min, while
Figure 8a–d illustrates slump tests over time (at 0, 60, 120, and 180 min) for the two soft and two rigid fibers separately. It can be seen from the charts that higher dosages of the soft and rigid fibers decreased the workability of the formulation [
52,
53,
54,
55,
56,
57,
58] because the addition of fibers led to an increase in the surface area, affecting the workability.
The big decrease in flow at a soft fiber dosage of 4 kg/m
3 (
Figure 7) was due to the phenomenon of fiber agglomeration, which is difficult to avoid at a high reinforcement ratio. The presence of small fiber pellets in the mortar slurry resulted in a lower flowability of the mortar compared to normal mortar, which, combined with a high AR, resulted in a lower flow result.
Workability increases with an increase in moisture content [
41]. During the first and final setting times, the rheological characteristics of fresh mortar change constantly, which leads to a reduction in workability and increased energy consumption during subsequent consolidation [
42].
A higher AR ratio resulted in a smaller flow, so it was logical that the use of shorter fiber lengths should enhance the slump; this was the case for most fibers in all formulations, although the minor differences are not significant. Instead, the higher dosage of fibers did lead to a loss of workability in all formulations.
In
Figure 7, the flow at 0 min is represented and it is observed that, for all dosages, the flow was lower than the sample control and higher than the formulation with a dosage of 0.6 kg/m
3 of synthetic fibers, the exception being the soft FRS with the highest dosage of fibers, 4 kg/m
3, which decreased up to 325 mm in the case of F15.
In
Figure 8a–d, flow over time is represented, showing that all formulations were less fluid than the control sample. On the other hand, the formulation with synthetic fibers performed very similarly to those with the vegetal fibers. Considering that a good slump range is between 380 and 420 mm (depicted by the two black horizontal lines in
Figure 8), the slumps from 60 to 120 min fit better in this range for all the formulations. A more fluid slump was observed for all formulations with the rigid fibers compared to those with the soft fibers; this was due to their lower AR ratio, which makes rigid fibers less crimped with the mix and more workable.
The increment in fiber content increases water absorption, reducing the water-cement ratio with a resultant decrease in the workability. The water absorption of a mix is a vital property because it influences its water–cement content and other properties.
The volumetric content of the fiber is one of the most critical parameters that adversely lowers the workability of the screed mortar [
52]. The higher the W/C (water/cement) ratio, the higher the concrete’s workability.
Looking at the trend in the workability (
Figure 8a,b) values from H5, H15 and F5, F15 at 120 min, there was a loss of workability as the fiber dosage increased. Hence, these findings strongly agree with those in the literature that workability decreases with an increase in the fiber dosage.
From
Figure 8c,d, it can be seen that the bamboo had higher slump values than the miscanthus, giving it the highest slump of all the formulations. However, B15-0.8 with a higher dosage had more fluid behavior; therefore, the fiber type had a greater impact on the slump result than the fiber dosage in the case of the rigid fibers below a dosage of 1.2 kg/m
3.