|The Eddystone lighthouse, now re-built on the Plymouth waterfront
The term 'hydraulicity'
is derived from the French word 'hydraulique' which, at its simplest
is defined as relating to water. It was adopted into construction
usage to describe waterproof structures either to convey water
or to withstand the ingress of water.
In the early 18th century,
French engineer Bernard Forest de Belidor used the term to describe
construction techniques to resist the action of seawater. Early
in the 19th century the eminent French engineer, Jean Louis Vicat,
adapted the term to describe limes which would harden under water.
They were thereby distinguished from the pure calcium limes, or
'air limes', which hardened by a different means resulting from
the action of carbon dioxide when exposed to air. It was some
time before the term came into common usage in Britain where an
equivalent term 'water lime' had developed over a similar timescale.
This initially arose with the work of Smeaton in developing his
lime for seawater-resistant mortar for the Eddystone lighthouse
(left) and continued through to the Portland
cement precursor developed by Frost.
The term 'hydraulic'
is now used internationally to describe cements and other binders
which set and harden as a result of chemical reactions with water
and continue to harden even if subsequently placed under water.
There are a few exceptions but, in general, these chemical reactions
involve calcium, silica and aluminium constituents which react
with water to form a whole family of calcium silicate and calcium
aluminate hydrates. Table 1 (below) sets out the chemical composition
of typical hydraulic cements and binders.
1 Main element oxide composition of hydraulic cements and
1 Comparative strength contribution of calcium silicates and
composition in itself is only a limited guide to the hydraulic
properties of the lime or cement. The key is in the way the components
are combined into the various silicates and aluminates. Figure
1 (above) shows the relative strength obtained by the hydration of the
common silicates and aluminates. In common with cement chemistry
notation the four main compounds are described as:
- C3S - three parts calcium oxide combined
with one part silicon oxide, also known as 'alite'
- two parts calcium oxide combined with one part silicon oxide,
also known as 'belite'
- three parts calcium oxide combined with one part aluminium
- four parts calcium oxide combined with one part aluminium
oxide and one part iron oxide, these latter two being generally
termed 'aluminate phases'.
2 Changes in the properties of Portland cement
(* MPa = megapascals: the unit of resistance to compression
which is equal to one Newton per square millimetre)
overall bulk chemistry can be similar for different products,
the burning process has a major bearing on their hydraulicity.
Limes are burnt at temperatures below 1,000°c where the main reaction
products are belite and free lime together with small amounts
of low reactivity aluminates. Portland cements are burnt at temperatures
up to 1,500°c where almost all lime is combined, forming alite,
which is the main product, and reactive aluminates. The development
of hydraulicity as measured by compressive strength resulting
from changes in technology over the past two centuries is illustrated
in Figure 2 (right).
It can be
seen that the calcium silicates are the main strength giving components
in hydraulic limes and cements. The aluminate and ferrite compounds
contribute little strength.
lack of either a detailed knowledge of chemistry or the technology
to burn at high temperatures, Greek and Roman engineers succeeded
in producing 'hydraulic' constructions. They achieved this by
combining a calcium-bearing constituent (that is to say lime)
with products providing the silica and/or aluminium constituents.
Such products are referred to as 'latent hydraulic' materials,
that is: they are not hydraulic of themselves but become so when
exposed to calcium-rich solutions. A number of materials fall
into this category, both naturally occurring and manufactured.
The best known traditional, naturally occurring example is the
volcanic dust from the Mount Vesuvius region of Italy to the south
of Naples. Termed pozzolana after the town of Pozzuoli around
which the dust deposits are centred, the material was known by
Roman builders to be capable of reacting with lime to produce
'pozzolanic' materials include the volcanic trass of the Rhine
valley and various zeolites. Manufactured pozzolans are also commonly
used today. These include metakaolin, a thermally treated china
clay, and the waste by-products of several major industries such
as blastfurnace slag, power-station fly ash and silica fume from
the ferro-silicon industry. Thus the definition of a pozzolan
could be a material that will react hydraulically with lime solution.
Details of the composition of these pozzolans are set out in Table
2 Main element oxide composition of hydraulic pozzolans
to which such latent hydraulic materials react with lime is often
termed their 'hydraulicity' or 'pozzolanicity' and a number of
tests have been developed in order to measure this. The chemical
test set out in BS EN 196-5 determines the quantity of lime absorbed
from a standard lime solution by the pozzolan. In order to qualify
for use in the standard pozzolanic cement of BS EN 197-1 a pozzolan
must be capable of absorbing a specified amount of lime. Other
more pragmatic tests have been developed to assess the strength-bearing
properties in mortars and concretes. In the recent BS EN 206-1
concrete specification the term 'k factor' is used for latent
hydraulic binders and is calculated as the amount of strength
attributed to the pozzolan in concrete in comparison with a standard
Portland cement. In the recent UK Foresight research into hydraulic
limes a similar technique was used to determine the hydraulicity
of pozzolans in mortar in comparison with an equal mass of the
standard natural hydraulic lime used for the project.
3 Hydraulicity of cements and pozzolana compared with NHL
3.5 hydraulic lime
Table 3 (above) sets
out the hydraulicity of these pozzolans in relation to the natural
hydraulic lime and cement. The pozzolans vary in their speed of
reaction so that a measure of hydraulicity depends, to some extent,
on the age at which it is determined. In this experiment small
amounts of the pozzolans were added to a 1:3 moderately hydraulic
lime mortar (1:3 NHL3.5:sand). The proportion of pozzolan to lime
did not exceed 30 per cent by volume.
||Concrete caisson breakwater at Brighton Marina
to supplementing the strength of mortars and concretes, while
utilising otherwise waste materials, latent hydraulic materials
modify the hydration products formed and as a result modify the
properties of the mortar or concrete. The values in Table 3 illustrate
the various reaction rates of different silicates, different proportions
of the silicates and decay of strength gain as either the reactive
silicate or the available lime becomes depleted. The initial low
hydraulicity of some pozzolans is ascribed to the slow diffusion
mechanism whereby the reactive silicates are released for reaction
with the lime-rich solution.
the term 'hydraulicity' is the property of limes and cements to
set and harden under water whether derived from a naturally hydraulic
lime, cement or a pozzolan. 'Cements' in this context can be single
products or combinations of calcium bearing cement or lime mixed
with materials that contribute reactive silica and alumina. The
hydraulic characteristic of such materials is derived from their
reactive calcium and silicon phases which combine with water to
form calcium silicate hydrates of various densities. The density
of the hydrate phases provides the binding power of the cement
or lime and determines the strength of the mortar or concrete
produced from it. The relative strength of mortar or concrete
is used to quantify the 'hydraulicity' of the material. Products
of moderate hydraulicity are capable of being used for mortar
in exposed situations such as that shown in the title illustration. Products of high
hydraulicity are required as the main construction material in
extreme exposure situations such as that illustrated to the right.
- PC Hewlett
(ed), Lea's Chemistry of Cement and Concrete,
Arnold, London, 1998
- GC Bye, Portland
Cement, Thomas Telford, London, 1999
- S Holmes and
M Wingate, Building with Lime, Intermediate Technology Publications,
article is reproduced from The
Building Conservation Directory, 2003
LIVESEY, technical manager at Castle Cement Limited, is a chemist
with 40 years' experience in the cement industry. He is lead UK expert to the British Standards
and European Standards committees drafting the new generation
of standards for cement, lime and methods of testing them,
a director of Concrete Information Limited, Chairman of the
British Cement Association Standards and Technical Committee
and a member of the Building Limes Forum national committee.
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