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TECHNICAL PAPER

4.2 Alternative rheological models and testing distinct shear rate protocol, which can be applied to study the
rheological behaviour of cement paste in a fully broken-down
methodology
state. The protocol removes reversible structuration and studies
A new test protocol has been designed to understand the the irreversible changes. The study presents the basis towards
irreversible changes by bringing the cement paste to a fully a better understanding of cement rheology by separately
[31]
broken-down state, termed ‘distinct shear rate protocol . In accounting for reversible and irreversible changes.
this protocol, Figure 10, a constant high shear rate is applied to Limitations: The results observed in the proposed distinct
[31]
ensure complete structural breakdown . The applied shear rate shear rate protocol can yield a higher shear resistance than
[31]
is lowered to a specified value γ i , at different time intervals t j . the actual shear resistance of the fully broken-down state due
The test is repeated for different values of shear rate at the same to four factors, which can not be isolated due to experimental
time intervals . The shear stress vs shear rate at any given time limitations. Among them, shear lag (dominates at the start of
[31]
generates the rheological behaviour in a fully destroyed state, the observation period) and structuration at reduced shear
[31]
Figure 11 . rate (dominates at the end of the observation period) provide
slightly higher shear resistance for the observation period. While
The distinct shear rate protocol was applied to understand the boundary effect (dominates at lower w/c ratio) and segregation
rheological behaviour of cement pastes at w/c ratios of 0.40, (dominates at higher w/c ratio) provide incremental changes in
–1
–1
–1
0.45, 0.50 and 0.55 at shear rates of 0.001 s , 0.01 s , 0.1 s , shear resistance over the test duration. The previous work shows
–1
1 s and 10 s –1[31] . The results have demonstrated that the that the effects of these four factors provide a small contribution
testing protocol can differentiate between the reversible and to the overall shear resistance; hence, they are excluded during
[31]
irreversible changes . Further investigations are underway for the mathematical modeling of the fully broken-down state.
the cement hydration duration of 10 to 40 min, w/c ratios of
0.40-0.55, and temperature 15-45 °C, to develop a generalised 4.3 Active rheological control using thermal
mathematical model based on the reversible and irreversible regulation
rheological changes.
In a separate study on heat-cured geopolymers, a constant shear
Key contributions: The study proposed and demonstrated the rate of 1 s was applied over different cement and geopolymeric
–1
pastes, at a constant temperature of 20 °C . It was observed
[32]
that the shear stress in GGBS-based geopolymers showed small
[32]
changes over a long period of time (0.014 Pa/s) (Figure 12) .
On application of heat, at a constant temperature of 60 °C,
the shear stress suddenly increases at a high buildup rate
32]
of 54.6 Pa/s . Furthermore, a similar build-up is observed
for delayed heat application, demonstrating a temperature-
[32]
dependent and time-independent phenomenon (Figure 12) .
The results were used to demonstrate a laser-based 3D printing,
[32]
as shown in Figure 13b .
Key contributions: The study presents a first proof of concept for
a laser-based 3D printing system.

Limitations: The process is at a very early stage and needs
Figure 10: Study of the fully destroyed state using the distinct shear
rate test optimisation in terms of heat delivery and layer stacking. The

Figure 12: Structural buildup in geopolymer due to a change in
Figure 11: Rheological behaviour in the fully destroyed state temperature

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