Workability is the property of mortar that allows for adequate compaction, placement, and finish without segregation and/or bleeding. The flowability for all samples containing graded and non-graded particles were measured by the flow cone test. Figure 6
illustrates the mortar flow after the flow cone tests. As can be seen, normal and scattered flows were observed. All samples of graded and ungraded TS mortars exhibited normal flow, except ungraded TS100 mortars of w
ratios of 0.5 and 0.55. The flow test results for the ungraded and graded particles are presented in Figure 7
and Figure 8
indicates that the workability of ungraded TS mortar marginally increased up to a 25% replacement, but, thereafter, a downward trend were observed for TS50, TS75, and TS100 mortars.
The workability of the control sample and of TS25 was 170 mm and 171 mm, respectively, at a w
ratio of 0.6. As the percentage of ungraded TS increased, the workability of the TS mortar decreased, provided that the w
ratio remained the same. For example, at a w
ratio of 0.6, the workability increased from 170 mm to 171 mm for TS0 and TS25, and afterwards decreased to 160 mm, 141 mm, and 130 mm for TS50, TS75, and TS100 mortars, respectively. This effect is attributed to particle interference, which increasingly occurred as TS replacement increased. The larger particles (2.36–4.5 mm) were more prominent smaller particles (Figure 2
). Thus, pockets of spaces narrower than the diameter of the smaller particles were created. These spaces trapped water that would have provided better consistency, thereby increasing the water demand [22
]. In addition, due to the lack of smaller particles in the ungraded TS, the ball bearing effect by smaller particles (which is needed to lubricate larger particles) was reduced as TS replacement increased [23
]. This trend was noticed at w
ratios of 0.5 and 0.55, except regarding the TS 100 mortar, for which a scattered flow was observed. The mortar flow diameter does not give an accurate representation of flow. The scattered flow was attributed to inadequate cohesion between TS aggregates within the matrix, which is due to insufficient water for mixing [24
]. Pockets of spaces created by the ungraded TS used up part of the mixing water. Figure 8
shows that the workability of graded TS mortar increased as the quantity of TS increased [25
]. For example, the workability of graded TS mortar increased from 135 to 190, as the quantity of TS increased from 0 to 100% at a w
ratio of 0.5. Cumulative increases of 10 mm, 15 mm, 30 mm, and 35 mm above the control were noticed for TS25, TS50, TS75, and TS100 mortars. The same trend was noticed at a w
ratios of 0.55 and 0.6.
From Figure 7
and Figure 8
, it can be observed that higher w
ratio yielded increased workability, both for graded and ungraded TS samples. For example, when the w
ratio of the ungraded TS was increased from 0.5 to 0.6 at TS50, the workability increased from 132 mm to 160 mm, while an increase from 161 mm to 185 mm was observed for graded TS samples. This was due to an increase in water for the mixing constituents as the water content increased. However, in terms of shape and texture, round and smooth particles have better workability than elongated or angular particles [23
]. Nonetheless, it was observed that graded TS mortars had higher flow diameters than ungraded TS mortars. This was attributed to the low water absorption of TS, which promoted water retention which improved lubrication between particles. Thus, the effect of yield stress and water demand resulting from the angular shape characteristics of TS was insignificant [28
3.3. Compressive Strength
In this section, the 28-day compressive strength of graded TS mortars with different w
ratios are presented and compared to the control sample. In addition, the compressive strengths of the TS cement mortar and control samples obtained at days 3, 7, and 28 are presented to check the early strength of the TS cement mortar samples. Figure 10
shows the 28-day compressive strength of mortar mixes at different w
ratios. In all samples (control and TS), It was observed that, as w
increased, the compressive strength decreased. The compressive strength decreased by as much as 25% due to the increased w
ratio. For example, at TS50, the compressive strength decreased from 45.42 MPa to 34.22 MPa when the w
ratio increased from 0.5 to 0.6. This result is consistent with previous findings [32
]. However, to achieve good workability, mortars must not exhibit segregation or bleeding. Samples with a w
ratio of 0.5 had the highest compressive strength but were too stiff; thus, their use is not practicable. Additionally, mortars with a w
ratio of 0.6 were most workable, but segregation and bleeding were noticed, as was low compressive strengths (Figure 6
). Contrarily, bleeding and segregation did not occur in mixes with a w
ratio of 0.55 Thus, mortar mixes containing a water to binder ratio of 0.55 were selected based on their workability and strength. Compressive strength development is presented in Figure 11
. However, both ungraded and graded TS mortar samples were tested for compressive strength to evaluate the influence of grading, as workability results showed differing trends (as shown in Figure 7
and Figure 8
shows the compressive strength development of mortar samples with a w
ratio of 0.55 after 28-days of curing. All fine aggregate replacements with graded TS increased the compressive strength of cement mortar on all curing days (3, 7, and 28). For example, the 3, 7, and 28 days compressive strengths of the control sample were 25.43 MPa, 33.87 MPa, and 38.75 MPa, respectively, whereas the TS samples’ lowest values were 26.35 MPa, 34.5 MPa, and 40.2 MPa, respectively. The increase in strength was due to the improved bonding provided by the TS’s aggregate angularity and surface roughness. Furthermore, as TS increased in the cement mortar, the compressive strength also increased. This compressive strength increase peaked at 50% replacement (TS50), while further increase in TS quantity decreased the compressive strength of the TS mortar. For example, at 28 days, when TS increased from 25% (TS25) to 50% (TS50), the compressive strength increased from 40.2 MPa to 43.15 MPa. Thereafter, this value decreased to 42.99 MPa and 41.12 MPa as TS increased to 75% (TS75) and 100% (TS100), respectively. The decrease in compressive strength is attributed to the reduction of aggregate hardness as TS% increased to 75% and 100% [34
Compressive strength was improved because of the densification of TS mortars, which is linked to TS’s rough surface structure and the well-bonded interfacial transition zone (ITZ) of TS mortars. Better adherence was expected for rough-surfaced materials compared to smooth-surfaced fine aggregate. The elongated and angular shape of the TS aggregate also contributed to the filling pores between the TS mortar a greater extent than the control mortars. The elongation of TS aggregates allowed a greater depth of embedment while their angularity improved the bond within the matrix. Similar results were reported by Ouda and Abdel-Gawwad [35
] and Guo et al. [36
]. Ouda and Abdel-Gawwad observed that the rough surface and angularity of oxygen furnace slag aggregate are attributed to the progressive contribution of strength up to 100% replacement. Likewise, Guo et al. indicated that the rough surface and angularity of steel slag resulted in a higher bonding strength. On the other hand, the contribution of natural sand was attributed to the hardness of the natural sand aggregate as compared to the TS. Thus, the combined effect of the fine aggregate strength of natural sand and the good adherence and bonding characteristics of the TS aggregate provided the optimum strength recorded at 50% replacement. Conversely, when the TS content increased above 50%, the contribution of natural sand to compressive strength in terms of hardness reduced. This is because the contribution of natural aggregate reduces while that of TS increases as the amount of TS increases beyond 50%. However, the effects of aggregate hardness and bonding characteristics were optimal at 50% replacement. Thus, the optimum strength was obtained at 50% replacement.
A different trend was observed with the ungraded TS (Figure 12
). A progressive reduction in the compressive strength was noticed, even at 50% replacement, which had the highest compressive strength for graded TS.
The control mortar had the highest compressive strength of 35.78 MPa, whereas TS25, TS50, TS75, and TS100 had compressive strengths of 32.51 MPa, 28.67 MPa, 28.33 MPa, and 22.54 MPa, respectively. This shows that the effect of the TS aggregate in terms of bond improvement in graded TS was not seen in the ungraded TS. This was due to inadequate distribution of aggregates within the matrix because larger aggregates dominate as TS replacement increases. Consequently, more pores are formed within the matrix and the progressive compressive strength reduces.
shows the strength development in graded and ungraded samples. An upward trend was observed with graded TS and a downward trend for ungraded TS samples. The increase in strength noticed in the graded TS mortar was attributed to improved packing density, as smaller aggregates fill pore spaces between larger aggregates. This was affirmed in Li et al. [37
]. It was observed that packing density is a major factor that influences the compressive strength behavior of cement composites. The compactness of composites produced from mixed aggregates slows down crack propagation because of improved packing density as pore spaces are filled by smaller aggregates. This results in a smaller number of cracks and crack diameters, as compared to uniform or ungraded aggregates. On the other hand, in ungraded samples, a progressive reduction in compressive strength was noticed as TS replacement increased. This was credited to the progressive increase in particle interference due to the increasing number of larger particles. Thus, packing density was not optimized, so a larger number of pores and, consequently, a low compressive strength resulted.
3.4. Splitting Tensile Strength
Cement composites such as mortar are generally weak while in tension. Under tension, the ITZ bears the tensile stresses for the matrix to be together. The strength of the ITZ is much weaker than that of the aggregates; thus, disintegration occurs as tensile stresses increases beyond the ITZ strength. Figure 14
shows the tensile strength of graded TS mortar samples with a w
ratio of 0.55 at 3, 7, and 28 days. The tensile strength increased as number of curing days increased. For example, for TS25, when number of curing days increased from 3 days to 28 days, the tensile strength increased from 3.08 MPa to 4.1 MPa. The tensile strengths obtained at 28 days were similar for all the TS and control samples. The control sample C, TS25, and TS50 achieved tensile strengths of 4.08 MPa, 4.1 MPa, and 4.15 MPa, respectively. Similar findings were observed after 7 days of curing in all considered samples. However, after 3 days of curing, the tensile strength increased for TS25 and TS50, but decreased to values close to the control sample C. The 3-day tensile strength increased from 2.63 MPa for TS0 up to 3.08 MPa for TS50, and afterwards there was a decline to 2.62 and 2.57 for the TS75 and TS 100 mortars, respectively. The higher tensile strength observed in the TS50 mix was due to the greater contribution of the hardness effect by natural sand and the bonding effect in TS when compared to other mixes. However, Waheed [38
] observed a significant 35% increase in concrete strength over the control, when 20% of TS was used for sand replacement. Therefore, the gains in strength in relation to aggregates might slightly differ from mortar because of the involvement of coarse aggregates in concrete.
3.5. Flexural Strength
Flexural strength is sensitive to the aggregate type and little variations in specimen preparation and testing. Figure 15
shows the results of the flexural strength test of the graded TS mortar samples. The test was conducted on the TS and control samples after 3, 7, and 28 days of water curing. It was observed that, as the curing days increased, the flexural strength increased for the control and for all TS samples. Flexural strength gain of 11% to 15% was recorded from 7 to 28 days curing periods across all replacement levels. In addition, it was observed that the flexural strength increased as TS levels increased up to 75%, but peaked at 50%. However, a further increase in TS (TS100) resulted in decreased flexural strength. At 28 days of curing, the flexural strengths of TS25, TS50, TS75 and TS100 were 12.00 MPa, 12.35 MPa, 11.79 MPa and 11.48 respectively, whereas the control sample was 11.54 MPa. This shows that compressive strength at all replacement levels up to 100% were comparable to the control. The bond properties of the TS mortar matrix are influenced by the irregular and elongated aggregate shape.
The angularity and distribution of aggregates in the cement paste played a significant role in the process of load transfer [39
]. In normal-strength concrete, cracks are mostly propagated around the aggregate particles [40
]. Thus, a stronger ITZ and aggregate particle interlocking in TS mortar reduces the impact on the failure plane as the stresses induced are distributed.