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L. Biolzi et al.
amount of ettringite in this mix that was produced by hydration of ye’elimite, which is
the main component of CSA cement, is the main reason for this improvement. It was
also observed that the flexural strength of CSA-blend mixes was lower compared
to that of the OPC mix. For instance, the flexural strength of OPC50-CSA50 and
OPC25-CSA50-SL25 mixes were 44 and 35% lower than that of the reference OPC
mix. As can be seen, the replacement of OPC with 25% of slag caused an improvement in the flexural strength compared to that of the OPC50-CSA50 concrete. This
improvement can be attributed to the formation of additional C–S–H gel which is the
main strength-contributing compound as a result of the reaction between slag and
calcium hydroxide. Moreover, slag may fill in the capillary pores and improve the
features of transition zones and microstructures of the cement matrix.
On the other hand, the results of fiber-reinforced concretes illustrate that the
addition of fibers remarkably improved the post-cracking behavior of FRC with an
extensive cracking process between first crack load and peak load. It was noticed
that the addition of 1% DHE steel fibers changed the behavior of concrete and a
deflection-hardening performance was observed in all mixes reinforced with steel
fibers. In these concrete mixes, once the first crack occurred, the fibers bridging
the crack resisted the load and prevented further crack propagation. The excellent
performance of these mixes can be attributed to the ability of DHE steel fibers to
carry the load after matrix cracks until further cracks form. Figure 4 shows that
the best performance was observed with the mix where OPC was fully replaced
with CSA cement and reinforced with 1% steel fiber (i.e., CSA-DHE). The flexural
strength of this mix increased by 87% and 55% as compared to that of the OPC and
CSA concrete, respectively. The expansive behavior of CSA cement can lead to a
better bond between the cement matrix and steel fibers, which subsequently led to an
increase in the flexural strength of concrete. The results further show that the flexural
strength of OPC, CSA, OPC50-CSA50, and OPC25-CSA50-SL25 mixes containing
1% DHE steel fibers was increased by 60%, 55%, 120%, and 113%, respectively, as
compared to that of their corresponding mixes without fibers. As it can be observed in
the graph, the inclusion of steel fibers had the most influence on the flexural strength
of concrete where CSA cement was used in blend mixes. As previously mentioned,
the expansive behavior of CSA-blend mixes may lead to a better bond between the
cement matrix and steel fibers as a result of self-stressing, which subsequently leads
to an increase in the flexural strength of concrete.
3.6 SEM Observation
To study the microstructural properties of concretes fabricated with different binders,
an SEM method was used and images of the fracture surface are shown in Fig. 5.
As one can observe, the hydration products of OPC concrete consist of a featureless
gel of C–S–H, ettringite crystals with a needle-like shape, and calcium hydroxide
(CH) crystals with a plate-like shape. The results indicate that the content of calcium hydroxide is relatively higher than that of the ettringite. Additionally, it can
L. Biolzi et al.
amount of ettringite in this mix that was produced by hydration of ye’elimite, which is
the main component of CSA cement, is the main reason for this improvement. It was
also observed that the flexural strength of CSA-blend mixes was lower compared
to that of the OPC mix. For instance, the flexural strength of OPC50-CSA50 and
OPC25-CSA50-SL25 mixes were 44 and 35% lower than that of the reference OPC
mix. As can be seen, the replacement of OPC with 25% of slag caused an improvement in the flexural strength compared to that of the OPC50-CSA50 concrete. This
improvement can be attributed to the formation of additional C–S–H gel which is the
main strength-contributing compound as a result of the reaction between slag and
calcium hydroxide. Moreover, slag may fill in the capillary pores and improve the
features of transition zones and microstructures of the cement matrix.
On the other hand, the results of fiber-reinforced concretes illustrate that the
addition of fibers remarkably improved the post-cracking behavior of FRC with an
extensive cracking process between first crack load and peak load. It was noticed
that the addition of 1% DHE steel fibers changed the behavior of concrete and a
deflection-hardening performance was observed in all mixes reinforced with steel
fibers. In these concrete mixes, once the first crack occurred, the fibers bridging
the crack resisted the load and prevented further crack propagation. The excellent
performance of these mixes can be attributed to the ability of DHE steel fibers to
carry the load after matrix cracks until further cracks form. Figure 4 shows that
the best performance was observed with the mix where OPC was fully replaced
with CSA cement and reinforced with 1% steel fiber (i.e., CSA-DHE). The flexural
strength of this mix increased by 87% and 55% as compared to that of the OPC and
CSA concrete, respectively. The expansive behavior of CSA cement can lead to a
better bond between the cement matrix and steel fibers, which subsequently led to an
increase in the flexural strength of concrete. The results further show that the flexural
strength of OPC, CSA, OPC50-CSA50, and OPC25-CSA50-SL25 mixes containing
1% DHE steel fibers was increased by 60%, 55%, 120%, and 113%, respectively, as
compared to that of their corresponding mixes without fibers. As it can be observed in
the graph, the inclusion of steel fibers had the most influence on the flexural strength
of concrete where CSA cement was used in blend mixes. As previously mentioned,
the expansive behavior of CSA-blend mixes may lead to a better bond between the
cement matrix and steel fibers as a result of self-stressing, which subsequently leads
to an increase in the flexural strength of concrete.
3.6 SEM Observation
To study the microstructural properties of concretes fabricated with different binders,
an SEM method was used and images of the fracture surface are shown in Fig. 5.
As one can observe, the hydration products of OPC concrete consist of a featureless
gel of C–S–H, ettringite crystals with a needle-like shape, and calcium hydroxide
(CH) crystals with a plate-like shape. The results indicate that the content of calcium hydroxide is relatively higher than that of the ettringite. Additionally, it can