Combining Plasticizers/Retarders And Accelerators - Departement

134 Pages·2006·8.44 MB·English

by  Dirk Reynders

Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen
Departement Burgerlijke Bouwkunde

Norwegian University of Science and Technology
Faculty of Natural Sciences and Technology
Department of Materials Science and Engineering

Combining Plasticizers/Retarders
And Accelerators

E2006
Promotor: prof. dr. H. Justnes Klaartje De Weerdt
prof. dr. ir. D. Van Gemert Dirk Reynders
Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen

Academiejaar: 2005-2006

Departement: Burgerlijke Bouwkunde
Adres en telefoon: Kasteelpark Arenberg 40 – 3001 Heverlee – 016/32 16 54

Naam en voornaam studenten: De Weerdt Klaartje
Reynders Dirk

Titel eindwerk: Combineren van plastificeerders/vertragers en versnellers

Korte inhoud eindwerk:

De combinatie van plastificeerders/vertragers en versnellers werd bestudeerd met drie
mogelijke toepassingen in het achterhoofd: 1) het tegengaan van het vertragend effect van
plastificeerders zonder de reologie sterk te wijzigen, 2) de activatie van vertraagd beton op de
werf na veilig transport in warme streken of steden met onvoorspelbaar verkeer en 3) het
oververtragen van overschotten aan vers beton gevolgd door activatie na één of meerdere
dagen.
De experimenten werden grotendeels uitgevoerd op cementpasta. Een Paar-Physica MCR 300
rheometer werd gebruikt ter bepaling van de reologie en een TAM Air isotherme calorimeter
ter bepaling van de hydratiecurves.
Er werd vastgesteld voor toepassing 1) dat calciumnitraat het vertragend effect van natrium en
calcium lignosulfonaat sterk terugschroeft en in het geval van polyacrylaat zelfs volledig
wegneemt terwijl de combinaties werken als plastificeerders, voor toepassing 2) dat de
combinatie natriumgluconaat/calciumnitraat een mogelijk werkend systeem is en voor
toepassing 3) dat de combinatie citroenzuur/calciumnitraat het hergebruik van overschotten
aan vers beton op een later tijdstip mogelijk maakt.

Promotor: prof. dr. ir. D. Van Gemert – prof. dr. H. Justnes
Assessoren: prof. dr. ir. L. Vandewalle – ir. G. Heirman
Katholieke Universiteit Leuven
Faculteit Ingenieurswetenschappen

Year: 2005-2006

Department: Burgerlijke Bouwkunde
Address en tel.: Kasteelpark Arenberg 40 – 3001 Heverlee – 016/32 16 54

Name and surname students: De Weerdt Klaartje
Reynders Dirk

Title of thesis: Combining plasticizers/retarders and accelerators

Summary of thesis:

The combination of plasticizers/retarders with accelerators has been studied in view of three
potential concrete applications: 1) counteracting retardation of plasticizers without negatively
affecting rheology too much, 2) activating retarded concrete at site after safe transport in hot
climate or cities with unpredictable traffic and 3) over-retarding residual fresh concrete one
day and activating it next day or after several days.
The experimental work is largely carried out on cement paste using a Paar-Physica MCR 300
rheometer to determine flow curves and gel strength and a TAM Air isothermal calorimeter
for determination of heat of hydration curves.
It has been found for application 1) that calcium nitrate strongly reduces retardation of sodium
and calcium lignosulphonates and even cancels retardation of polyacrylates, whereas the
blend also has plasticizing effects, for 2) that sodium gluconate/calcium nitrate is a potentially
effective system and for 3) that citric acid/calcium nitrate may facilitate later use of residual
fresh concrete.

Promotor: prof. dr. ir. D. Van Gemert – prof. dr. H. Justnes
Assessors: prof. dr. ir. L. Vandewalle – ir. G. Heirman
Table of Contents

1 Introduction 1

2 Background on cement, cement hydration, rheology and admixtures 4

2.1 Cement.................................................................................................................. 4
2.2 Cement hydration.................................................................................................. 5
2.3 Rheology............................................................................................................... 9
2.4 Plasticizers/retarders............................................................................................. 13
2.5 Calcium nitrate...................................................................................................... 22

3 Materials and apparatus 24

3.1 Materials................................................................................................................ 24
3.2 Apparatus.............................................................................................................. 27

4 Counteracting plasticizer retardation 34

4.1 Introduction........................................................................................................... 34
4.2 Calorimetric and rheological measurements......................................................... 35
4.3 Mortar measurements............................................................................................ 75
4.4 General conclusion................................................................................................ 80

5 Long transport of fresh concrete 81

5.1 Introduction........................................................................................................... 81
5.2 Sodium lignosulphonate........................................................................................ 81
5.3 Citric acid.............................................................................................................. 95
5.4 Lead nitrate............................................................................................................ 98
5.5 Sodium gluconate.................................................................................................. 101
5.6 General conclusion................................................................................................ 110

6 Reutilizing residual fresh concrete 111

6.1 Introduction........................................................................................................... 111
6.2 Phase I – Screening of retarders............................................................................ 111
6.3 Phase II – Determination of required retarder dosage.......................................... 114
6.4 Phase III – Activation using calcium nitrate......................................................... 116
6.5 Phase IV – Strength measurements....................................................................... 120
6.6 General conclusion................................................................................................ 126

7 Conclusions 127
Chapter 1

Introduction

This thesis continues a long tradition of Erasmus exchanges between the “Katholieke
Universiteit Leuven” (Belgium) and the “Norges Teknisk-Naturvitenskapelige Universitet i
Trondheim” (Norway). For many years students have been studying advanced aspects of
cementitious materials. Thys, A. and Vanparijs, F. ([1]) studied the longterm performance of
concrete with calcium nitrate, Ardoullie, B. and Hendrix, E. ([2]) focused on the chemical
shrinkage of cementitious pastes and mortars, Clemmens, F. and Depuydt, P. ([3])
investigated early hydration of Portland cements, the thesis of Van Dooren, M. ([4])
concerned the factors influencing the workability of fresh concrete, and Brouwers, K. ([5])
studied a number of cold weather accelerators.

In this thesis the combination of plasticizers/retarders and accelerators has been investigated
in view of three different potential concrete applications.

The first application, which made up the major part of this study, focused on the fact that
plasticizers that are used to increase flow for cementitious materials at equal water-to-cement
ratio also to a variable extent retard setting as a side effect. The objective was to find an
accelerator that at least partially would counteract this retardation without negatively affecting
the rheology too much. Whereas earlier studies on this topic focused on plastic viscosity at
high shear rate (i.e. relevant for mixing) and relatively low dosages of plasticizer, the study
reported here focused on the lower shear rate range (i.e. relevant for pouring concrete) and
higher dosages of plasticizer. The results of this study are presented in Chapter 4. These
results are valuable elements in evaluating the combined use of plasticizers and accelerators,
as it was e.g. applied during construction of Statoil’s Troll platform (Figure 1.1), a huge gas
platform located 80 km north-west of Bergen (Norway) that reaches 303 m below the surface
of the sea. During the construction of its 350 m tall base an accelerator has been used to speed
1
Chapter 1: Introduction 2
up the slip forming process of the plasticized concrete as construction works were behind
schedule.

The second application concerns long transport of fresh concrete. The preliminary study was
largely carried out on paste. It was investigated if a concrete mix from a ready mix plant after
being deliberately over-retarded for long transport in for instance hot climate or cities with
unpredictable traffic (e.g. traffic jam) could be activated by adding an accelerator in the
revolving drum close to the construction site before pumping the concrete in place. Results
are discussed in Chapter 5.

The third potential application, presented in Chapter 6, concerns the search for a system to
preserve residual fresh concrete for a few days (e.g. over a weekend) followed by activation
before use. However, it might also be used as an overnight concept. Whereas recently a
freezing preservation technique has been proposed as method for reutilizing left-over
concrete, this study concentrated on a technique consisting of over-retardation of residual
fresh concrete followed by later activation using an accelerator.

Figure 1.1 Troll gas platform (1996)
Chapter 1: Introduction 3
The necessary background on cement, cement hydration, rheology and admixtures is given in
Chapter 2. Chapter 3 introduces and describes the materials and the apparatus that have been
used throughout this work.
Chapter 2

Background on cement, cement
hydration, rheology and admixtures

2.1 Cement

Cement chemists use in general a short hand notation, C = CaO, S = SiO , A =Al O ,
2 2 3
F = Fe O and S= SO , for the main elements in the chemical analyses of cement, in
2 3 3
addition to H = H O to describe hydration processes. The elements are determined by
2
X-ray fluorescence or analytical chemistry and given as the corresponding oxides.
Assuming that the only minerals in the cement are alite (C S), belite (C S), aluminate
3 2
phase (C A), ferrite phase (C AF) and anhydrite (CS) the content of these minerals
3 4
may be calculated through mass balances. The first four minerals are formed during
equilibrium conditions in the burning of the cement clinker, while the latter mineral
(or gypsum,CSH ) is added to the mill when clinker is ground to cement. In
2
specification sheets, the content of other oxides is also given: N (Na O), K (K O) and
2 2
M (MgO). “Free lime” is the content of free CaO due to insufficient burning or due to
the decomposition of C S into C S and “free lime” if the cooling rate is too low.
3 2
The specific surface area (m /kg) of cement is commonly determined directly by an
2
air permeability method called the Blaine method. In addition to the specific area, the
particle size is of importance for the hydration rate of cement, since the hydration
takes place at the interface between the cement grain and the water phase. However, it
is important to realise that the surface of a cement grain is inhomogeneous. The
distribution of C S/C S- and C A/C AF-domains are determined by the milling
3 2 3 4
process and the difference in resistance against fracture. Since cement grains are
composite grains with possibly all 4 major phases in one grain, efforts to simulate
4
Chapter 2: Background 5
cement by adding corresponding amounts of individual minerals will therefore fail.
(Justnes, H., [6], p.10)

2.2. Cement hydration

In the discussion of rheology of cement paste and the interaction with plasticizing
admixtures and retarders, it is of importance to know something about the hydration
until setting. It is sometimes believed that no hydration takes place in the so-called
“dormant” period between water addition and initial setting, while actually a
substantial growth of hydration products takes place on the surface of the cement
grains. (Justnes, H., [6], p.10)

2.2.1 The interstitial phases C A/C AF
3 4

In the absence of calcium sulphates the first hydration product of C A which appears
3
to grow at the C A surface is gel-like. Later this material transforms into hexagonal
3
crystals corresponding to the phases C AH and C AH . The formation of the
2 8 4 19
hexagonal phases slows down further hydration of C A as they function as a hydration
3
barrier. Finally the hexagonal phases convert to the thermodynamically stable cubic
phase C AH disrupting the diffusion barrier, after which the hydration proceeds with
3 6
a fairly high speed. The overall hydration process may thus be written as

2 C A + 27 H → C AH + C AH → 2 C AH + 15 H
3 2 8 4 19 3 6
(hexagonal phases) (cubic phase)

In the presence of calcium sulphate (as in a Portland cement) the amount of hydration
of C A in the initial state of hydration is distinctly reduced when compared to that
3
consumed in the absence ofCS. Needle-shaped crystals of ettringite are formed as the
main hydration product:

C A + 3 CSH + 26 H → C AS H
3 2 6 3 32

Minor amounts of the monosulphate C ASH or even C AH may also be formed
4 12 4 19
if an imbalance exists between the reactivity of C A and the dissolution rate of
3
calcium sulphate, resulting in an insufficient supply of SO2-- ions.
4
Then ettringite formation is accompanied by a significant liberation of heat. After a
rapid initial reaction, the hydration rate is slowed down significantly. The length of
this dormant period may vary and increases with increasing amounts of calcium
sulphate in the original paste.
Chapter 2: Background 6
A faster hydration, associated with a second heat release maximum, gets under way
after all the available amount of calcium sulphate has been consumed. Under these
conditions the ettringite, formed initially, reacts with additional amounts of tricalcium
aluminate, resulting in the formation of calcium aluminate monosulphate hydrate
(monosulphate):

C AS H + 2 C A + 4 H → 3 C ASH
6 3 32 3 4 12

As ettringite is gradually consumed, hexagonal calcium aluminate hydrate (C AH )
4 19
also starts to form. It may be present in the form of a solid solution with C ASH or
4 12
as separate crystals.
The origin of the dormant period, characterised by a distinctly reduced hydration rate,
is not obvious and several theories have been forwarded to explain it. The theory most
widely accepted assumes the build-up of a layer of ettringite at the surface of C A that
3
acts as a barrier responsible for slowing down the hydration. Ettringite is formed in a
through-solution reaction and precipitates at the surface of C A due to its limited
3
solubility in the presence of sulphates. The validity of this theory has been questioned
arguing that the deposited ettringite crystals are not dense enough to account for the
retardation of hydration. The four proceeding alternative theories have been proposed:

i) The impervious layer consists of water-deficient hexagonal hydrate
stabilised by incorporation of SO2-. It is formed on the surface of C A and
4 3
becomes covered by ettringite.
ii) C A dissolves incongruently in the liquid phase, leaving an aluminate rich
3
layer on the surface. Ca2+ - ions are adsorbed on it, thus reducing the
number of active dissolution sites and thereby the rate of C A dissolution.
3
A subsequent adsorption of sulphate ions results in a further reduction of
the dissolution rate.
iii) SO2-- ions are adsorbed on the surface of C A forming a barrier. Contrary
4 3
to this theory it has been found that C A is not slowed down if the calcium
3
sulphate is replaced by sodium sulphate.
iv) Formation of an amorphous layer at the C A surface that acts as an
3
osmotic membrane and slows down the hydration of C A.
3

The termination of the dormant period appears to be due to a breakdown of the
protective layer, as the added calcium sulphate becomes consumed and ettringite is
converted to monosulphate. In this through-solution reaction both C A and ettringite
3
dissolve and monosulphate is precipitated from the liquid phase in the matrix.