*3.4. Effect on Strength Development in Binary and Ternary Pastes*

Table 3 shows the cement pastes tested for compressive strength development. The reference and binary mixtures were prepared either with w/cm ratios for maximum packing (optimum water demand) obtained from Figures 2–4 or with w/cm ratios for standard consistence, obtained from Figures 5–7. Two ternary mixtures were prepared with w/cm for maximum packing, determined following the same procedure as for the binary mixtures.

Table 3 shows that 20% replacement with either HCFA, LFS, or LF at optimum water demand gave 92–94% of the 28-day compressive strength of the reference paste, despite lower packing density and increased w/cm. Figure 8 shows that the same pattern continued at 90 days, while HCFA had some accelerating effect at 7 days, which may be attributed to the presence of free lime. Strength development was similar for all of the SCMs in the binary systems, despite the fact that pastes with HCFA had higher w/cm ratios and reduced packing densities. For normal consistence, as shown in Figure 9, 20% OPC replacement with HCFA showed slightly better strength development compared to LFS and LF, again despite higher w/cm ratios and lower packing densities. The results show that the reactivity of HCFA had a greater effect on strength development than fineness, water demand, or packing. LFS and LF, on the other hand, seemed to be contribute to strength development mostly through the filler effect.



**Figure 8.** Compressive strength development at optimum water demand for binary pastes.

**Figure 9.** Compressive strength development at normal consistence for binary pastes.

Based on these results, it was decided to test ternary binders with 20% HCFA and 10% LFS or LF cement substitution. The results shown in Table 3 showed that the ternary binders reached a packing density close to that of the reference paste at a higher w/cm ratio; 28-day compressive strength, however, was higher when LF rather than LFS was used as the third constituent. The rate of strength development, as shown in Figure 10, shows that the accelerating effect at 7 days observed with 20% HCFA use was enhanced with an extra 10% LF replacement, while the 90-day compressive strength was equal to that of the reference.

The results show a good synergy between HCFA and LF, also identified by other researchers. De Weerdt et al. [47] suggest that limestone interacts with the hydration products of OPC–fly ash systems and increases compressive strength. Thongsanitgarn et al. [48] have shown that the increase in strength development of OPC–fly ash systems is higher when finer limestone is added. Other researchers point out that different SCMs may co-operate in the cement paste matrix, despite showing different chemical activity and physical characteristics [49]. The same synergy, however, did not take place when HCFA was combined with LFS, resulting in reduced strength development (88% of the reference at 28 and 90 days). A possible explanation for this is that the calcium in LF was in the form of CaCO3, which is known to promote cement hydration [50], while in LFS, it was mostly in the form of CaO or Ca(OH)2, as shown from the values of loss on ignition in Table 1.

**Figure 10.** Compressive strength development at optimum water demand for ternary pastes.

SEM photos taken from samples at 28 days were used to explore the microstructure of the reference and ternary pastes (Figures 11–13). The pores observed were of similar size, while micro cracks were visible in all the cement pastes. Portlandite (Ca(OH)2) crystals were identified in the pores of the reference paste, as a result of cement hydration (Figure 11). Ettringite needle-shaped crystals, on the other hand, were visible in the pores of the ternary paste with 70% OPC + 20% HCFA + 10% LF, confirming enhanced reactivity in the pore solution.

**Figure 11.** Portlandite crystal into a pore of 100% OPC paste at 28 days.

**Figure 12.** Microstructure of 70% OPC + 20% HCFA + 10% LFS paste at 28 days.

**Figure 13.** Effect of OPC replacement with LF on fresh paste consistency.
