3.2. Compressive Strength
Figure 4 illustrates the variation in the strength of UHPC over time across three distinct curing environments. Generally, early strength development in UHPC is rapid, whereas the late-stage strength increment occurs more gradually. For instance, considering the compressive strength of samples cured in tap water, without the addition of fibers, the compressive strengths at 28 days, 120 days, and 720 days are 104.7 MPa, 120.0 MPa, and 148 MPa, respectively. Compared to the compressive strength at 1 day, which is 99.7 MPa, these values reflect increases of 5.0 MPa (5.0%), 20.3 MPa (20.4%), and 48.3 MPa (48.4%), respectively. With a fiber content of 1%, the compressive strengths of UHPC at 28 days, 120 days, and 720 days are 109.3 MPa, 129.0 MPa, and 153 MPa, respectively. These values represent increases of 5.9 MPa (5.7%), 25.6 MPa (24.8%), and 49.6 MPa (48.0%), respectively, compared to the 1 d compressive strength of 103.4 MPa. For a fiber content of 2%, the compressive strengths at 28 days, 120 days, and 720 days are 138.0 MPa, 148.0 MPa, and 163.0 MPa, respectively. In comparison to the 1 d compressive strength of 129.0 MPa, these values indicate increases of 9.0 MPa (7.0%), 19.0 MPa (14.7%), and 34.0 MPa (26.4%), respectively. With a fiber content of 3%, the compressive strengths at 28 days, 120 days, and 720 days are 143.6 MPa, 157.4 MPa, and 166.6 MPa, respectively. These values show increases of 10.7 MPa (8.1%), 24.5 MPa (18.4%), and 33.7 MPa (25.4%), respectively, compared to the 1-day compressive strength of 132.9 MPa. Across all fiber content levels, the most significant improvements in compressive strength at 28 days, 120 days, and 720 days are observed with a 1% fiber content, while the enhancements are relatively minor with 2% and 3% fiber content.
Furthermore, at the same age, the inclusion of steel fibers from 0% to 3% exhibits an upward trajectory in the compressive strength of UHPC. For instance, considering the strength within tap water curing conditions, in the absence of fibers, the compressive strengths at 1 day, 28 days, and 720 days are 99.7 MPa, 104.7 MPa, and 148 MPa, respectively. However, with a 3% fiber content, the compressive strengths of UHPC are 132.9 MPa, 143.6 MPa, and 166.6 MPa at the same age. These figures represent increases of 33.2 MPa, 38.9 MPa, and 18.6 MPa, respectively, with corresponding percentage increments of 33.3%, 37.2%, and 12.6%. Consequently, the incorporation of steel fibers has significantly enhanced the strength development of UHPC.
Furthermore, upon scrutinizing the evolution of UHPC strength over time across three distinct environments—outdoor, tap water, and seawater—a discernible pattern emerges. Notably, the strength of UHPC within tap water environments demonstrates continuous growth in the later stages. Irrespective of the quantity of steel fiber incorporated, its strength development consistently surpasses that of UHPC in outdoor and seawater environments. This phenomenon primarily stems from the presence of a limited number of interconnected pores within UHPC, enabling water ingress and subsequent hydration of unreacted cement particles to form C-S-H gel, thus enhancing matrix density and strength.
While the strength of UHPC cured outdoors exhibits an incremental trend, it progresses relatively sluggishly compared to specimens cured in water. Conversely, the early strength development of UHPC in seawater environments is gradual, with later-stage strength showing a slow and eventual stabilization at significantly lower levels than those observed in outdoor and tap water environments. This stagnation in strength evolution during later stages can be primarily attributed to two factors. Firstly, the exceedingly low water-to-cement ratio impedes the availability of free water necessary for continued cement hydration and C-S-H gel formation [
33]. Secondly, the strength of concrete in later stages is influenced by the reaction between volcanic ash and the hydration product calcium hydroxide (CH). In standard curing conditions, the degree of this reaction between SF and CH is less pronounced than in thermal curing environments.
3.3. Dimensional Stability
Figure 5 depicts the length change of UHPC. Notably, UHPC in outdoor and tap water environments exhibit a shrinkage tendency, whereas those exposed to seawater display an expansion trend. Generally, shrinkage stabilizes beyond 150 days of aging. At nearly equivalent ages, the specimens reach their peak strengths, indicating a direct correlation between shrinkage and strength development [
34]. Across all conditions, the beneficial effect of steel fiber in stabilizing dimensions is evident [
35].
In
Figure 5a, the impact of steel fiber content on dimensional changes of UHPC in outdoor conditions is elucidated. The shrinkage rate of each sample group gradually increases with curing age, eventually reaching a stable state. Notably, UHPC without steel fibers exhibits the highest shrinkage rate. Conversely, as steel fiber content rises, the UHPC shrinkage significantly diminishes. For instance, while the shrinkage rate of UHPC without steel fibers after 720 days reaches 381.5 × 10
−6, the addition of 1%, 2%, and 3% fibers reduces this rate to 276 × 10
−6, 207.2 × 10
−6, and 191 × 10
−6, respectively. Consequently, the reductions are 105.5 × 10
−6, 174.3 × 10
−6, and 190.5 × 10
−6, corresponding to reduction rates of 27.7%, 45.7%, and 49.9%, respectively.
Figure 5b illustrates the impact of steel fiber content on the dimensional variations of UHPC in a tap water environment. The change in shrinkage rate trend closely resembles that observed under outdoor conditions, albeit with markedly lower shrinkage rates than those of specimens exposed to outdoor conditions. For instance, after 720 days, the shrinkage rates for UHPC samples with 0%, 1%, 2%, and 3% fiber content in a tap water environment were 181 × 10
−6, 167 × 10
−6, 135 × 10
−6, and 128 × 10
−6, respectively. These figures denote reductions of 52.6%, 39.5%, 34.8%, and 33.0% compared to outdoor conditions, respectively. Hence, UHPC specimens in tap water exhibit superior dimensional stability than outdoor specimens. Furthermore, it is evident from the figure that steel fibers mitigate the shrinkage of UHPC. With an increase in steel fiber content, the shrinkage rate of UHPC experiences a significant decrease. For instance, compared to UHPC without steel fibers, the addition of 1%, 2%, and 3% fiber reduces the shrinkage rate by 14 × 10
−6, 46 × 10
−6, and 53 × 10
−6, respectively, after 720 days. This corresponds to reduction rates of 7.7%, 25.4%, and 29.3%, respectively.
The inhibition of shrinkage changes in UHPC samples by steel fibers can be attributed to several factors. Firstly, this phenomenon arises due to the disparity in relative humidity between the external environment and the UHPC system. The incorporation of steel fibers acts akin to providing numerous small supports within the cement paste, preemptively applying to prestress to resist compression. Secondly, steel fibers influence the pore structure, consequently impacting the sample’s length variation. In UHPC specimens lacking steel fibers, more capillary pores form compared to those with steel fibers, resulting in a more refined pore structure. This refinement of pore structure yields both positive and negative effects on length changes [
34]. On one hand, it accelerates the rate of reduction of the critical radius, leading to faster shrinkage. On the other hand, it decelerates the migration speed of the water, causing the shrinkage rate controlled by moisture migration to gradually decrease [
36]. Consequently, the shrinkage rate is faster at an early age, followed by a gradual slowdown.
Figure 5c illustrates the UHPC dimensional changes in the seawater. Generally, UHPC length in each group exhibits gradual increases with curing age, ultimately stabilizing. Unlike the preceding exposure conditions, UHPC specimens in seawater manifest a tendency to expand. This phenomenon can be attributed to the prolonged soaking period, during which certain substances from seawater gradually permeate into the concrete, generating hydration products such as ettringite and brucite, consequently resulting in lengthening. Santhanam et al. [
37] reported a 1.25% expansion in seawater conditions for 32 weeks, associating this expansion with the formation of ettringite. The quantity of brucite fluctuates between 2 and 3%, while the amount of CH decreases steadily. Nagataki et al. [
38] observed expansion in concrete attributed to the formation of ettringite and Friedel’s salt resulting from the ingress of chloride and sulfates into the matrix.
Moreover, the inclusion of steel fibers serves to diminish the expansion rate of UHPC. As the quantity of steel fiber increases, the expansion rate of UHPC gradually diminishes. For instance, the expansion rate of specimens without steel fiber after 720 days reached 525 × 10−6. Conversely, the expansion rates after adding 1%, 2%, and 3% fiber were 509 × 10−6, 420 × 10−6, and 345 × 10−6, respectively. This corresponds to reductions in expansion rates of 16 × 10−6, 105 × 10−6, and 180 × 10−6, with reduction rates of 3.0%, 20.0%, and 34.3%, respectively.
3.4. Mass Change Rate
Figure 6 displays the mass change observed in specimens subjected to three distinct environmental conditions. The findings indicate a correlation between the variations in length and mass of UHPC specimens over exposure time attributable to progressive hydration of the cementitious binder [
39]. Notably, a trend of mass reduction is evident in outdoor conditions, while specimens exposed to seawater and tap water environments exhibit a trend of mass increase. Particularly, the mass of specimens immersed in seawater demonstrates a greater increase compared to those in tap water environments.
Figure 6a illustrates the mass change rate of UHPC with varying steel fiber content under outdoor exposure conditions. It was observed that with increasing exposure time, the mass change rate of UHPC gradually escalated, particularly displaying rapid increments within the initial 150 days. Eventually, the final mass change rate stabilized within the range of 0.5% to 0.63%. Notably, the inclusion of steel fibers notably mitigates the early mass change rate of UHPC. However, there is no evident direct correlation between the later mass change rate and the fiber content.
Figure 6b presents the mass change rate of UHPC with varying steel fiber content under tap water immersion conditions. As depicted in the figure, an increasing trend in UHPC mass is evident with prolonged soaking time. After surpassing 28 days, the mass growth rate of UHPC stabilizes within the range of 0.15% to 0.20%. Notably, the addition of steel fiber does not exert a significant effect on the mass growth rate of UHPC under these conditions.
The mass changes of UHPC under various steel fiber content levels under seawater immersion conditions are depicted in
Figure 6c. As illustrated, with the duration of immersion increasing, the mass of UHPC exhibits rapid augmentation before 216 days, reaching levels of 0.79% to 0.92%. Eventually, the final mass growth rate stabilizes within the range of 0.843% to 0.962%. This phenomenon primarily arises from the heightened production of ettringite and brucite [
37]. Interestingly, as the steel fiber content increases, the mass growth rate during the later period diminishes compared to samples without steel fiber. It is worth noting that the mass change of UHPC in these conditions is significantly smaller than that reported for conventional concrete earlier (ranging from 2% to 12%) [
40,
41].
3.5. TG Analyses
Figure 7 displays the DTG curve of UHPC exposure in various conditions for 60 and 720 days. From low to high temperatures, the endothermal peaks were observed within three distinct temperature ranges, corresponding to the dehydration of C-S-H, ettringite, and evaporable water, dehydroxylation of calcium hydroxide, and decomposition of calcium carbonate, respectively [
42,
43].
At 60 days, notable differences are observed in the first endotherm among samples subjected to outdoor conditions, indicating a greater mass loss with significantly stronger intensity compared to the other two conditions. This suggests that the UHPC in outdoor conditions contained a higher proportion of hydration product phase, including C-S-H and ettringite. In contrast, it decreased in UHPC immersed in water and seawater. The slightly elevated temperature associated with the water evaporation in samples exposed to outdoor conditions indicates a tighter bond with the phases. Moreover, the prominent peak attributed to the dehydroxylation of calcium hydroxide indicates that UHPC exposed to outdoor conditions experienced fewer phase changes compared to those in the other two conditions, despite some conversion of calcium hydroxide to calcium carbonate due to carbonation effects.
The UHPC immersed in seawater exhibits the smallest peak, indicating a significant loss of C-S-H gel due to exposure to seawater. This loss is likely due to the reaction between C-S-H and various ions in seawater, particularly sulfates. At 720 days, the content of CH in UHPC cured in seawater diminishes more rapidly compared to UHPC cured in other environments. This reduction in CH content over time may be associated with the consumption of volcanic ash reaction products from BFS. The leached calcium hydroxide reacts with Mg2+ and SO42− in seawater, forming brucite and ettringite.
The concentration gradient induced by the reaction further facilitates leaching, perpetuating the formation of brucite over time. Additionally, soluble carbonate ions present in seawater can trigger a reaction with calcium hydroxide to precipitate calcite on the UHPC surface. Consequently, the leaching rate of calcium hydroxide is highest in UHPC exposed to seawater.
The calcium hydroxide content of UHPC exposed to tap water exceeds that of seawater-exposed specimens but falls below that of specimens in outdoor conditions. Additionally, its CaCO3 content is lower than that of specimens in seawater and outdoor conditions. Furthermore, the calcium hydroxide content decreases from 60 to 720 days. This phenomenon arises from the reaction of calcium hydroxide formed during cement hydration with BFS to produce C-S-H and its reaction with soluble carbonate ions in water to form CaCO3.
UHPC in outdoor exhibits the highest levels of calcium hydroxide (CH) and CaCO
3. This is attributed to moisture loss during outdoor exposure, preventing the CH formed during cement hydration from reacting with BFS. The elevated CaCO
3 content results from the reaction of atmospheric carbon dioxide with CH. Notably, the carbonization rate of UHPC is minimal [
44], primarily occurring on the surface.
Given the negligible difference in UHPC composition, it is evident that leaching and carbonization of CH are the primary factors contributing to its consumption within the matrix. According to Carde et al. [
45], when the binder encounters water, ion migration occurs between the soluble matrix and the external water, disrupting the chemical equilibrium of the hydrates and medium. However, once hydrates dissolve or precipitate, equilibrium is restored [
46,
47]. CH is the initial mineral that dissolves, followed by the gradual decalcification of C-S-H. Furthermore, CH formed by cement hydration in UHPC specimens cured in seawater and tap water can react with BFS to produce C-S-H. This reaction improves the pore structure of the specimen, thereby enhancing its mechanical properties and impermeability.
