*3.1. Compressive and Tensile Strength Evolution of Binary (C/WSA) and Ternary (C/WSA/SF) Concretes*

A comparison of compressive strength development between CC and those containing WSA alone (binary mixes) and WSA jointly with SF (ternary mixes) is presented in Figure 2a. As listed in Table 3, a relatively low cement substitution rate in binary concrete mixes was set as 10%, 20%, and 30%, whereas in ternary concrete mixes a slightly higher cement replacement was used as 30%, 40%, and 50% owing to highly reactive SF. The purpose of adding different percentage of SF (5%, 7%, and 10%) in combination with different percentages of WSA (25%, 33%, and 40%) was to explore the optimum cement replacement without affecting the strength and durability properties of concrete as compared to control and binary concrete mixes. In addition to technical benefits in terms of mechanical properties, other aspects of blended concrete having WSA with SF in lowering the GWP were also evaluated and compared to those of control and binary concretes. To avoid the effects of external factors, the specimens of all the concrete mixtures studied were water cured under uniform temperature conditions of 20 ◦C until the age of testing.

As shown in Figure 2a, the binary concrete WSA20 demonstrated highest compressive strength evolution among binary mixes at all testing ages, including the CC. However, a reduction in compressive strength was observed for other binary mixes (WSA10 and WSA30) as compared to CC, regardless of aging. From the current results, it can be seen that the rate and the reduction in compressive strength was higher in WSA30 as compared to that of WSA10. The low strength of WSA10 than that of CC is due to its lower degree of pozzolanic activity, whereas the significantly low compressive strength of WSA30 is due to addition of high amount of WSA, which consequently affected the pozzolanic activity in a significant manner. These results suggested addition of 20% WSA as an optimum amount without compromising the compressive strength of concrete.

To achieve high sustainability in terms of lesser GWP, efforts were made to regain the reduction in the compressive strength of binary mixes containing high percentages of WSA by adding different percentages of SF (5%, 7%, and 10%). The test results show significant improvement in strength of ternary mix with an equal percentage of cement substitution (WSA25SF5) to that of the corresponding binary mix having WSA alone (WSA30) at all ages. Owing to addition of 5% SF, the improvement in strength that occurred remained slightly lower than that of CC. However, an encouraging response was noticed for ternary mix (WSA33SF7) containing a slightly high percentages of SF (7%) in presence of high percentage of WSA (33%), where the compressive strength was higher than that of CC at all ages. These results demonstrated fast early-age hydration and better packing and filling abilities due to slightly increased amount of very fine SF along with the later-age pozzolanic reaction of high-volume WSA. With a further increase in cement substitution with 10% SF in presence of 40% WSA (WSA40SF10), a slight reduction in strength was observed as compared to CC at all ages. This is because a high percentage of cement substitution (50%) affected both the early hydration and later-age pozzolanic reaction due to the production of less calcium hydroxide (C-H). However, the strength performance of this ternary mix (WSA40SF10) is commendable, despite of its high cement substitution, as it either shows higher compressive strength than other binary mixes (WSA10 and WSA30) or comparable to ternary mix WSA25SF5, despite of its low cement substitution. These results are, once again, attributed to the addition of a high percentage of very fine SF (10%) due to its better packing and filling abilities at early ages (7 days). Among all the mixes, the highest compressive strength at 7 days was demonstrated by the binary mix WSA20, while at 28 and 91 days by the ternary mix WSA33SF7.

In contrast to compressive strength, Figure 2b demonstrated a higher splitting tensile strength (STS) development for all concretes tested (binary and ternary) than that of CC. However, among binary and ternary mixes, their trends of STS development with respect to cement substitutions remained similar to that of compressive strength development. Moreover, similar to the compressive strength, the evolution of STS for WSA20 and WSA33SF7

was significantly higher as compared to CC and all other binary and ternary mixes. The only exception was at 7 days when WSA33SF7 exhibited higher STS than only CC. It is worth mentioning that the WSA20 concrete exhibited highest STS among all the concretes tested at all ages.

**Figure 2.** Comparison of strengths of control concrete and concrete having different percentages of WSA alone and blends of WSA with SF: (**a**) compressive strength and (**b**) splitting tensile strength.

To summarize, the current findings demonstrated a decrease in compressive strength owing to a low (WSA10) or high (WSA30) cement replacement with WSA. Contrarily, a significant increase in both compressive and splitting tensile strength was obtained for 20% cement replacement with WSA (WSA20). Similar to binary concretes, the compressive strength of ternary concrete mixes decreased owing to a low (WSA25SF5) or high (WSA40SF10) cement replacements. However, a significant increase in both compressive and splitting strengths was observed for ternary concrete blend having 33% WSA jointly with 7% SF (WSA33SF7).
