*3.1. Material Properties*

The chemical composition of the materials, as shown in Table 1, shows that all the binders used were rich in calcium. This is expected to influence the kinetics of hydration in a different way compared to traditional silica-rich SCMs (siliceous fly ash, silica fume, ground granulated blastfurnace slag), especially due to the presence of free-CaO in HCFA and LFS; HCFA has both hydraulic and pozzolanic properties, while LFS can be described as a weak pozzolan with latent hydraulic properties and is often regarded as filler [40,41]. HCFA shows a relatively high sulphate ion content, which is not expected to affect cement hydration negatively [41].

Although SCM use for increasing particle density often relies on materials finer than cement, the particle size distribution of the SCMs used shows that all of them were coarser than cement. According to the literature, however, coarser SCMs may still contribute to cement hydration through the filler effect [13,22,42]. HCFA is the coarsest material, considering both median particle size diameter d50 and specific surface area. LFS and LF have similar specific surface areas but different particle size diameters. This occurrence can be explained by their surface characteristics, as LF is ground natural stone, resulting in more spherical shaped particles compared to LFS, which is molten and then waterquenched, resulting in more irregularly shaped particles. According to Sakai et al. [43], spherical-shaped particles are expected to increase packing density and fluidity of cementbound mixtures.

## *3.2. Effect on Wet Packing Density*

Figures 2–4 show the relationship between packing density and w/cm ratio for various cement replacement rates with SCM. As can be seen in all cases, reducing w/cm ratio increases packing up to the point where the water is not sufficient to fill the voids between the particles. From the curves of the figures, it is easy to estimate the w/cm ratio for the maximum packing, but it is also possible to estimate the effect of each SCM on water demand and its impact on packing density of the cement paste. The use of HCFA, as shown in Figure 2, increases water demand considerably, even at 10% OPC replacement. It also reduces the solid concentration in the paste in all cases.

Cement replacement with LFS, on the other hand, seems to have a different effect. Although water demand was increased (to a lesser extent compared to HCFA), particle packing was equal or even slightly increased compared to that of the reference paste. As with HCFA, the rate of replacement, ranging from 10% to 30%, had little effect. Cement replacement with LF had a similar effect on packing to that of LFS, but more pronounced. At 10% and 20% OPC replacement rates with LF, packing density increased, while it decreased at the 30% replacement rate. Water demand increased, but only slightly.

**Figure 2.** Packing density vs. w/cm in pastes with OPC and HCFA.

**Figure 3.** Packing density vs. w/cm in pastes with OPC and LFS.

**Figure 4.** Packing density vs. w/cm in pastes with OPC and LF.

The effect of cement replacement with the various SCMs on packing density seems in all cases to be linked with their fineness. Indeed, an analysis of variance shows that the influence of fineness on packing density is statistically significant (*p* < 0.05) for all the materials. The finer materials (LFS and LF) contribute to the increase of solid concentration, while the coarser material (HCFA) decreases packing. OPC substitution with 10% and 20% LF results in the highest packing densities, while the lowest are observed with 30% HCFA substitution. HCFA, and LFS to a lesser extent, require increased w/cm ratios in order to reach maximum packing. An analysis of variance shows that the influence of w/cm ratio on packing density is statistically significant (*p* < 0.05), which means that increased water demand, mostly for HCFA, but also for LFS, results in lower packing density. This increase in water demand, however, does not seem to be linked with fineness, but can be associated with chemical composition, and more specifically, with free lime content [44]. LF shows a slight increase in water demand, which can be attributed to the water absorption of limestone particles.
