Civil Engineering Reference
In-Depth Information
i.e. lower setting time, and to the formation of a less dense paste lowering
the early-age compressive strengths (Marchi and Costa, 2011) but higher 28
day compressive strengths (Winnefeld and Barlag, 2009). on the other hand,
CSA has also been used to improve the water resistance of plaster of Paris
as it counteracts the high solubility of bassanite (Kuryatnyk et al., 2010).
18.4.2 Role of W/C ratio
The W/C ratio affects the microstructure of pastes/mortars/concretes, providing
water to hydrate phases and the porosity (space) where hydration products
can precipitate. The ratio between porosity and hydration degree strongly
affects final concrete performance. At low W/C ratio, CSA or BCSAF will
develop a denser pore structure, as space available for hydration products
formed is smaller. Moreover, pastes with low W/C ratio can undergo self-
desiccation, as ettringite formation requires huge amounts of water. This
effect can be critical for expansion properties as large amounts of cement
particles remain unhydrated after setting. This can cause expansion if cement
is later exposed to external water from the environment, by the formation
of secondary ettringite from the reactivity of anhydrous phases (Beretka et
al., 1996). However, very dense frameworks make water diffusion to the
interior of the mortars and concretes difficult. On the other hand, the use
of a high W/C ratio makes cements dimensionally stable, even with high
amounts of yeelimite because the water in the system is enough for C
4
A
3
S to
fully react at early ages, but the microstructure of the paste/mortars is more
porous.
Porosity of pastes is directly affected by W/C ratio. low W/C ratios lead
to low porosities, enhancing mechanical strengths, although in the first hours
of hydration there is a loss of plasticity. High W/C ratios lead to high porous
microstructures which may result in lower mechanical strengths (Berteka
et al., 1996; Bernardo et al., 2006; Marchi and Costa, 2011). It should be
underlined that very different W/C ratios have been used in the hydration
reactions of CSA. reaction [18.1] theoretically requires a water-to-binder
mass ratio of 0.64. However, this water-to-binder mass ratio also depends
on the minor phases as well as the final performances of the paste/mortar/
concrete to be produced.
It must be noted that due to the large compositional variability of CSA
cements, one general theoretical water demand for full hydration cannot be
established. However, for a given CSA clinker phase assemblage and sulfate
content, a water-to-binder mass ratio for full hydration can be calculated.
This value may not be very far from 0.60. From the experimental point of
view quite different water-to-binder mass ratios have been employed to study
the hydration of CSA cements. These values range from quite low ratios
0.35-0.45 (Zivica, 2000, 2001; Fu et al., 2003; Canonico et al., 2007) to
