Figure 5-5 shows the influence of SCM types and replacement rates on particle packing
characteristics of pastes using wet density approach. Except in the case of 15% SF replacement, all of the investigated binders necessitated lower optimum water demand compared to the plain OPC paste.
In view of OWD results, all blended binder systems developed higher solid concentration than that of the plain OPC system. For example, the binary blends of 40% FA or 40% SL were found to have 7% higher solid concentration than that of the pure OPC system. The enhanced solid concentration due to the SCM replacements becomes more dominant for ternary blends of either FA or SL with SF. For instance, the ternary combination of 10% SF with 40% FA resulted in greater solid concentration of around 25% compared to the pure OPC system. This is attributable to both the filling (since they have finer SSA value and broader PSD) and shape features (e.g., spherical shape and smooth surface) of SCMs which can effectively fill the voids among cement grains and produce more efficient arrangement of solid particles. It is noted that in the presence of SF, particle packing improves for both OPC-FA and OPC-SL pastes, wherein OPC-SL systems develop slightly larger solid concentration values compared to their corresponding OPC-FA systems. This is likely due to the higher SSA of SL compared to FA. From Figure 5-5(b), it can be seen that the improvement in solid concentration due to the SF replacement does not carry over to the 15% SF containing mixture. In spite of having the largest SSA for the 40FA15SF system, this mixture was found to have 11% lower solid concentration than that of the similar ternary system prepared with 10% SF replacement. This reduced solid concentration is attributed to the loosening effect and agglomeration formation with increasing inter-particle interactions. The loosening effect associated with the presence of high content of fine particles in paste can push the larger particles apart, and thus hinders the dense arrangement of solid particles (Kwan and Wong 2008; Yu et al. 1997). In addition to the loosening effect, the agglomeration formation of fine particles acts to trap water between flocs which can de-water the paste
and reduce the packing density of cement paste (Roussel et al. 2010). Some studies have demonstrated that in order to elicit the full benefits of the higher SSA in blended system, it is necessary to secure dispersion of the particles in the cement paste, which in turn requires the incorporation of dispersing admixtures, such as PCE, at the optimum dosage rate (Bentz et al. 2012b; Mehdipour et al. 2017b; Mehdipour and Khayat 2016b; Wong and Kwan 2008a). Although the results in this study were reported for a constant PCE dispersant of 0.12% for all binders, the 40FA15SF system was found to have consistently lower solid concentration values across further addition of PCE dispersant dosages compared to the similar ternary mixture containing 10% SF. Similar results were observed by Mehdipour and Khayat (Mehdipour and Khayat 2016b) who noted that there is a threshold content of SF replacement to improve packing density, beyond which further addition of SF does not result in higher solid concentration even with increasing dispersing admixture. This implies that there exists a saturation surface area effect to enhance packing density, beyond which the increased SSA does not necessarily lead to an increment of the particle packing in the cement paste.
(a) (b)
Figure 5-5 Particle packing characteristics of the investigated OPC-SCM systems using
wet density approach: (a) OWD to achieve maximum solid concentration and (b) maximum solid concentration. For all systems, the PCE dosage was remained constant at
0.12%, by volume of binder.
Figure 5-6 presents the flow characteristics (i.e., MWD and RWD) of the
systems exhibited higher water demand to initiate flow (i.e., MWD) compared to those needed to fill the voids (i.e., OWD). The larger extents for MWD is primarily because of further separation needed between neighboring particles to reduce inter-particle contacts and interactions, which can facilitate the movement of solid particles. While the values
are different, a good linear relationship (R2 = 0.97) is observed between MWD and OWD
for the evaluated OPC-SCM systems, as shown in Figure 5-6(c).
(a) (b)
(c)
Figure 5-6 Flow characteristics of the investigated OPC-SCM systems using mini-slump
flow cone test: (a) MWD to initiate flow and (b) RWD to increase fluidity. (c) Showing correlation between OWD and MWD of the investigated OPC-SCM systems. For all
systems, the PCE dosage was remained constant at 0.12%, by volume of binder.
The underlying mechanism of the role of excess water (i.e., free water) in fluidity enhancement can be explained by the electric double layer (EDL) which forms on the surface of solids (e.g., cement grains) upon contact with water. The EDL constitutes: (i)
an inner stern layer of tightly held ions that carries a charge opposite to that on the particle surface and (ii) the more weakly held ions in the diffuse outer layer having the same charge as the particle surface (Hodne and Saasen 2000; Verwey and Overbeek 1955). As water content increases, the thickness of water film around the particles increases and a diffusion layer gradually forms. The diffusion layer cannot pass the hydrostatic pressure and provide fluidity in paste. As water continues to increase, the water molecules can exceed the scope of the gravitational influence of the electric field and become free water. This excess water can further lubricate the particles and act to elevate the separation among solid particles, thus providing fluidity to paste (Zhao et al. 2016).
It is interesting to note that in spite of having larger SSA, the lowest MWD was observed for ternary systems containing 5% SF. This can be explained by the competition of two simultaneous effects, while increasing the SF content increases the total SSA of solid particles, the amount of excess water will also increase (since the packing density of the solid particles is increased). The latter effect can offset the effect of increased SSA of solid particles due to the SF addition. However, opposite trend was observed for ternary system containing 15% SF. On account of its very high SSA and reduced solid concentration, the 40FA15SF paste is found to require the highest water demand. Figure 5-6(b) shows the effect of binary and ternary blended cements on RWD to increase fluidity extracted from mini-slump flow cone test. On account of higher SSA, SF containing systems develop higher RWD compared to the plain OPC system. This feature is more dominant for ternary systems of SL and SF than those observed for ternary blends of FA and SF. This is attributed to the higher overall SSA, as well as a relatively more angular geometry of the SL particles, which can elevate the inter-particle friction, thus lowering the sensitivity of cement paste to water addition.
To further investigate the effect of excess water on material behavior of pastes over transition from “concentrated state” to “flow state”, the particle packing and WFT covering solid particles at MWD were evaluated. Figure 5-7(a) shows relative solid
concentration (ϕMWD/ϕOWD) of the investigated binder systems. This parameter for a
given system was calculated as the ratio of solid concentration at MWD to its corresponding maximum solid concentration as identified at OWD. As expected, all
binder systems exhibit lower solid concentration at MWD compared to that of OWD.
Figure 5-7(b) shows a good linear correlation between relative solid concentration and
total SSA of solid particles present in system. While the highest reduction in solid concentration was observed for the plain OPC system, the OPC-SCM systems were found to have lower drop over transition from concentrated to flow onset state. The negative impact of excess water available at MWD on solid concentration is less pronounced for ternary systems containing SF. For example, the 40SL10SF exhibited merely 2% reduction in solid concentration at MWD compared to its particle packing at OWD state.
(a) (b)
(c)
Figure 5-7 Variations in: (a) relative solid concentration in relation to SCM replacements, (b) relative solid concentration in relation to total SSA of solid particles present in system, and (c) WFT covering solid particles calculated at MWD state. In (a) and (b), the
relative solid concertation for a given system was obtained by dividing the solid concentration at MWD by its corresponding maximum solid concentration.
This is hypothesized to be due to the reduced WFT needed to initiate flow (i.e., less separation between particles) in OPC-SCM systems as well as ball-bearing effect of
SF and/or FA particles. This is corroborated in Figure 5-7(c), which shows the WFT
covering solid particles at MWD state for the investigated binder systems. On account of their lower MWD, all OPC-SCM systems were found to have lower WFT around solid particles compared to the plain OPC system. The reduced WFT needed to initiate flow reflects that lower lubrication layer is required for transition from “concentrated state” to “flow state” in OPC-SCM systems. In addition to water demand, the presence of spherical particles in cement paste can reduce inter-particle friction, thus facilitating the movement of particles. This is an important finding as it suggests that the OPC-SCM systems with higher packing density and containing higher spherical geometry of solid particles require lower thickness of water film around solid particles to impart fluidity to paste compared to that of the pure OPC system. The scatter seen in Figure 5-7 for ternary system containing 15% SF is likely due to the effects of agglomeration and inter-particle interactions which can have a significant influence on packing density, free water availability, and inter-particle spacing in this system, thus demonstrating poor relationship.
To further elaborate on the effect of excess water on particle packing of the investigated blended systems, the average inter-particle spacing was calculated using
microstructural simulations. As can be seen in Figure 5-8, at both OWD and MWD
states, the average spacing between solid particles decreases marginally when OPC is partially replaced by SL or FA, and reduces dramatically when a small content of SF is incorporated in system. The effect of SF incorporation on reducing inter-particle spacing is more significant for up to 10% replacement. Upon further increase in SF content, the inter-particle spacing does not change appreciably. For example, in the ternary systems of FA and SF at OWD state, the increase of SF content from 0% to 5% and 10% resulted in reduction in inter-particle spacing of around 91% and 50%, respectively. However, with further addition of SF from 10% to 15%, the average of spacing between particles was only altered by 7%. On account of its higher SSA, the SL containing systems exhibits smaller inter-particle spacing in both binary and ternary systems as compared to FA containing mixtures. Irrespective of the SCM types and contents, all systems in
concentrated state (i.e., OWD) had smaller inter-particle spacing than corresponding mixtures at flow state (i.e., MWD). Since smaller inter-particle spacing is an indication of improved particle packing, a corresponding enhancement in the solid concentration is also noted. From Figure 5-8, it can be noticed that the effect of increasing inter-particle spacing due to the free water available at MWD state (i.e., flow onset) becomes less dominant with increasing SF content. The correlation between simulations and experimental measurements proves that in the OPC-SCM systems, binders with higher total SSA results in smaller reduction in particle packing during transition from “concentrated state” to “flow state”.
(a) (b)
Figure 5-8 Variations in average inter-particle spacing extracted from microstructural generation for the investigated OPC-SCM systems at two different states: (a) (w/cm)v =
OWD and (b) (w/cm)v = MWD. Based on six replicate simulations, the uncertainty was
quantified to be less than 5%.
5.3.3. Compressive Strength Development of OPC-SCM Systems. Figure 5-9