Comparison between blended cement and cement with added fly ash

Till now, it can be observed that the rheological properties of cement with added fly ash in presence of superplasticizers was dependent on both the fly ash amount and also type of superplasticizer present. Often, it is insufficient to simply adjust the amount of superplasticizers added to the cement clinker content as interaction has been shown to occur between fly ash and superplasticizer. In such cases, particular attention was needed when polycarboxylate based superplasticizers were utilized due to their sensitivity.

Table 9. W/c ratios and densities of cement slurries possessing 20 % fly ash after Table 10. W/c ratios and densities of cement slurries possessing 40 % fly ash after consolidation centrifugation Table 11. W/c ratios and densities of cement slurries possessing 60 % fly ash after consolidation centrifugation In this section, another factor which can influence the effectiveness of the superplasticizers, and thus the rheological properties of the resulting cement pastes is highlighted. This factor involved the point of blending together the fly ash and cement clinker. Industrially, the fly ash materials are premixed with the cement clinker prior to grinding, which is different from

the preparation method for the cements used earlier (fixed mass of fly ash was manually mixed with the ordinary Portland cement in the desired proportion). This section thus deals with this difference. As a fly ash cement, a cement containing ~18.2 wt.% fly ash was analyzed. The findings were then compared to that of an ordinary Portland cement (Table 3) and the cement containing 20 wt.% fly ash (Table 6).

Table 12 displays the rheological properties of the fly ash cement slurries in presence or absence of superplasticizers. The FR₂ of the neat fly ash cement slurry was 36,200 Nm/m³s,

~ 60 % higher than the FR₂ of a neat cement paste containing 20 wt.% fly ash (22,800 Nm/m³s) but similar to that of an ordinary Portland cement (35,400 Nm/m³s). Comparing the efficiency of the dispersing agents at the dosage of 0.2 %bwob, the plasticizing effectiveness of the superplasticizers was as follow: SX > NRG > SRN > NAPH > LS. Whereas the slump flow retention is SRN > SX > NRG > LS ≈ NAPh. In general, the plasticizing effectiveness of these polymers was more similar to that of the ordinary Portland cement than the cement containing 20 wt.% fly ash.

This stark difference in rheological properties of a neat fly ash cement and a neat cement containing 20 wt.% fly ash (despite the similarity in fly ash content, 18.2 versus 20 wt.%) can be attributed to the nature of the packing behavior of the different materials. In the cement-fly ash mix, the cement clinker phases and fly ash particles were separated as individual particles. Therefore, the availability of fly ash to interact with water/

superplasticizers in solution will be dependent on the actual surface area on the fly ash particles in relation to the clinker phases. In our investigation, the surface area of fly ash and the ordinary Portland cement possessed similar surface areas. At onset of hydration, superplasticizers interacted rapidly with the positively charged clinker phases, which may be embedded in the cement particles. This in turn generated an overall advantage for the fly ash particles, which are fully exposed and ready for interaction. In such a situation, dilution of cement by addition of fly ash will be proportional to the amount of fly ash added (Table 5).

In the flyash blended cement (faC), the surface is generally higher as it is finer ground and some of the coarser hollow fly as particles are crushed as well giving an even higher surface measureable with BET (outer and inner surface) but perhaps not reflected by the Blaine method (only "outer" surface).

The effectiveness of superplasticizers in greater details showed that the performance of 0.2

%bwob of NAPh and LS in the fly ash cement was worse than both that in the ordinary Portland cement and cement containing 20 wt.% fly ash. This is a puzzling trend and no real explanation could be given at this point of time. However, when the dosage was doubled, the performance of such dispersing agent normalized to give rheological data average of that between that of an ordinary Portland cement and the cement containing 20 wt.% fly ash. The slump loss retention effectiveness of NAPh/LS, regardless of dosage was similar to that in the case of polycarboxylates and can be attributed to the competitive adsorption of NAPh and LS for both the surfaces of fly ash and clinker phases. This signified that addition of fly ash has a direct impact on the time dependence performance of the superplasticizers, greater than the direct plasticizing effectiveness.

In the case of polycarboxylate superplasticizers, the performance of these polymers in the fly ash cement lies intermediate between that in an ordinary Portland cement and the cement containing 20 wt.% fly ash. This, coupled with the findings from NAPh and LS, potentially indicated that the affinity of polycarboxylate based superplasticizers for fly ash was higher than that of NAPh/LS, or alternatively, the degree of adsorbed NAPh/LS for the clinker phase outweighed for fly ash. In such a situation, the mechanism driving the process must be electrostatic in nature, where potentially, the fly ash surfaces could be more cationic charged in specific points, thus can act as an ideal anchor site for both incoming polycarboxylates and NAPh/LS, regardless of their anionicity. On the other hand, on the less positively charged surfaces of the clinker phases, the more anionic charge NAPh/LS can adsorbed better than the polycarboxylates. The adsorption of these polymers can also be governed by secondary adsorption mediated by Ca²⁺ present in the pore solution, similar to that occurring for other SCMs such as microsilica, etc. These two explanations could justify the higher

impact of increasing fly ash content on the effectiveness of the different superplasticizers (Table 6 to 8).

Table 12. Rheological properties of the fly ash cement slurries in the presence or absence of superplasticizers at a w/c ratio of 0.36 as a function of time

SP Amount μ1 μ2 μ1/μ2 τd τs10s τs10m Δτs FR2 Hysteresis

Due to the high viscosity and general rheological properties of fly ash cement with added NAPh/LS, an additional experiment utilizing a dosage of 0.8 %bwob of NAPh was conducted (Table 13). It can be observed that ideal fluidity of the fly ash cement could be achieved with increased NAPh dosage, without any segregation of the cement pastes. A potential explanation could be the presence of sufficient polymers to cover both the clinker phases and also accommodate the fly ash, thus rendering an overall decrease in flow resistance of the system.

Table 13. Rheological properties of the fly ash cement slurry with 0.8 %bwob of NAPh as a function of time, w/c = 0.36 For better understanding of the rheological state of the fly ash cement, packing density was calculated and presented in Table 14. The change in packing density of this fly ash cement was lower than that of the other two systems as a result of increase in their fineness. In presence of superplasticizers, the same trend remained with the exception of the fly ash cement with 0.2 %bwob of NRG added. This signified that when cement particles and fly ash were in close proximity, an addition mechanism involving the ability of the cement particles in retaining water molecules was occurring. However, this effect observed could potentially be an outcome of prehydration or the presence of grinding agents. Clarification can be done by performing further thermal gravimetric analysis on the cement samples.

Table 14. W/c ratios and densities of fly ash cement slurries after consolidation centrifugation