*3.2. Compressive Strength*

Figure 4a presents the preliminary test of the compressive strength of specimens made with non-cement materials at various S/FA ratios without an alkali activator. Compressive strength was shown to increase significantly with curing age between 28 and 56 days. The compressive strength of specimens with 40% FA was significantly higher than that of specimens with only 10% FA, at all stages of curing. The compressive strength of the non-cement specimens reached 70% that of the mortar made using Portland cement. The non-cement blended specimens with the highest compressive strength were those made using an S/FA ratio of 6:4. Results of compressive strength development were indicated that S and FA reacted with water and then with calcium hydroxide via a pozzolanic reaction to form hydration products (i.e., calcium silicate hydrate (C-S-H) and/or calcium aluminium silicate hydrate (C-A-S-H) colloids). These findings are consistent with those reported in previous studies [18,24] and it was produced produces viscous hardening behavior in the cementless composites [23].

**Figure 4.** (**a**) Histogram presenting compressive strength results of the preliminary test; and (**b**) compressive strength development at 28 days for all mixes.

Non-cement blended composite without alkali activator is an innovative material. For concrete design, the minimum compressive strengths of concrete for normal structural components such as walls or slabs were used as 21 MPa. For this reason, the target strength was set as 21 MPa in this study. As shown in Figure 4b, all of the mixtures reached the target strength of 21 MPa, which is suitable for normal use in civil and construction. We observed a positive correlation between the proportion of fibers and compressive strength, due perhaps to suppressed crack formation under axial loading. The inclusion of fibers in the non-cement blended composites significantly increased the compressive strength, particularly in S50-F2 and S40-F2 (30% to 40% higher than the S50) [26]. This also had indirect effects on the toughness and mechanical properties of the composites. Hydration reactions among the fibers can increase the interfacial bonding strength, resulting in higher compressive strength. These findings are also consistent with those reported in previous studies [27]. The decrease in compressive strength observed in Figure 4b when the fiber content is increased from mixes P-F2 to P-F5. This diminution may be attributed to the lump of noncontiguous fibers, which is consistent with previous study [28].

#### *3.3. Tensile Strength*

Figure 5a illustrates the tensile strength of non-cement blended specimens and cement mortar specimens without fibers. Clearly, the increase in tensile strength in the non-cement mortar was not significant at 7 days compared to the P specimens (average 15% lower than P specimens), but quite noticeable at 28 days (the tensile strength of S60 specimen was slight higher than that of P specimens). The tensile strength of S60 was 18% higher than that of S40. We speculate that this can be attributed to the mix ratio, which allowed the S and FA to be gelatinized; however, the slowness of the reaction hindered strength development. On the basis of the previous study [29], FA can be used as a sustainable alkali activator for S to activate the alkali-activated or pozzolanic reaction. The tensile strength of the non-cement blended specimens increased with an increase in the proportion of polypropylene fibers, as shown in Figure 5b. At 28 days, the tensile strength of samples S50-F2 and S60-F2 were respectively 20% and 23% higher than that of the control specimens. This obvious increase in tensile strength can be attributed to the fibers arresting crack propagation at the micro scale. In addition, the decrease in tensile strength when the fiber content is increased from P-F2, PF5, S50-F2 and S50-F5 was also due to the lump of noncontiguous fibers in composites.

**Figure 5.** (**a**) Tensile strength development curves of select samples and (**b**) tensile strength results of all samples at 28 days.

#### *3.4. Drying Shrinkage*

As shown in Figure 6a, the shrinkage development curves of S40 and S50 specimens were similar to those of P specimens. By contrast, the shrinkage development curve of specimen S60 was 4 times higher than that of cement mortar, indicating a high SO3 content following a rapid hydration reaction with a corresponding rapid volume expansion. Previous studies [17,18,24] have also reported on the considerable influence of SO3 on the strength of mortar specimens containing FA.

**Figure 6.** Length change results: (**a**) comparison to P, S40, S50 and S60 specimens and (**b**) comparison to S60, S60-F1, S60-F2 and S60-F5.

Our results revealed that the addition of fibers to specimen S60 (S60-F1/2/5) did not have a significant effect on shrinkage and the effect of fibers in shrinkage development curves of S40 and S50 specimens were similar to those of S60 specimens. We also found that shrinkage was largely independent of the proportion of FA. FA contains large quantities of free lime (f-CaO) and SO3, which are beneficial to strength development of hydrated in non-cement blended composites containing S. Note however that f-CaO and/or excessive SO3 can cause expansion in hardened mortar. Increasing the amount of FA could increase the number of sulphate ions in solution, thereby hindering ettringite formation [29,30]. This can have profound effects on the volume stability and strength of mortar specimens with a high proportion of FA.
