Rheological study of the interactions between MHEC and superplasticizers in aqueous solutions for applications in cement industry

5 
 
 
 
 
 
 
 
 
 
 
 
 
Introduction
6 
 
INTRODUCTION 
 
1.1 The cement and the clinker 
 
A cement is a binder, a substance that sets and hardens independently, and can bind other materials 
together. The most important use of cement is the production of mortar and concrete to form strong 
building materials that are durable to normal environmental effects. 
It is uncertain where cement was first discovered, also if there are proofs that Egyptians and 
Assyrians made many buildings using very fine powders. Almost certainly, concrete made from 
such mixtures was first used on a large scale by Roman engineers. From that moment, cement and 
its by-products were extensively used in the realisation of an enormous number of buildings, 
infrastructures and facilities. 
The main component of cement is a product made by heating of some natural materials called 
clinker. The clinker, in combination with other components, gives many kinds of cement. The most 
common cement is the Portland cement (so called because it has a great similarity with the rock of 
Portland, an island in Dorset, UK) and its clinker is obtained by heating, in some predefined 
proportions, lime, silica, alumina, calcium and iron oxides, and in small amount also magnesia, lead 
oxides, phosphates and alkalis. In particular, lime comes from chalky stones; it has alkaline nature 
and it is the main component of a Portland clinker because it is present by about 65% of the total. 
Silica instead comes from shale and clayey sands and it is characterized by an acid chemical nature; 
it is present by about 25%. The rest of the components comes from other inorganic materials. 
The production of clinker begins always with a first milling of raw materials. The so obtained 
powder, technically called “flour”, is introduced in a rotating oven, composed by a lightly inclined 
cylinder which is internally coated with a refractory material, and it moves forward against heat 
flow (produced by combustion of fuels and other). During the advance of the flour, the latter is 
subjected to a series of chemical transformations which consist essentially in a free-water ejection, 
followed by a bind-water ejection which occurs at a temperature between 100°C and 750°C. At a 
higher temperature, the decarbonation of the limestone occurs, followed by a partial fusion of the 
mixture (clinkerization) which is responsible for the formation of calcium silicates. When the mix 
begin to get colder (at about 1350°C) a partial crystallization is visible with the formation of
7 
 
calcium aluminates. The material is then cooled and further milled to obtain particles with size less 
than 100 μm. 
From the clinker of Portland cement is then possible to obtain other cements. Cement kinds 
regulated by the normative systems are for example: 
 
 Sulfoaluminate cement: obtained by mixing calcium sulfoaluminate clinker with calcium 
sulphate and Portland cement; 
 Portland blast furnace cement: it contains up to 70% ground granulated blast furnace 
slag, Portland clinker and chalk. All compositions produce high ultimate strength, but as 
slag content is increased, early strength is reduced, while sulphate resistance increases and 
heat evolution diminishes. Used as an economic alternative to Portland sulphate-resisting 
and low-heat cements; 
 Portland pozzolan cement: it includes fly ash cement, since fly ash is a pozzolan, but also 
includes cements made from other natural or artificial pozzolans. In the countries where 
volcanic ashes are available (e.g. Italy, Chile, Mexico, the Philippines) these cements are 
often the most common form in use; 
 Portland fly ash cement: it contains up to 30% fly ash. The fly ash is pozzolanic, so that 
ultimate strength is maintained. Because fly ash addition allows a lower concrete water 
content, early strength can also be maintained. Where good quality cheap fly ash is 
available, this can be an economic alternative to ordinary Portland cement; 
 Portland silica fume cement: obtained with addition of silica fume which can yield 
exceptionally high strengths. Cements containing 5-20% silica fume are occasionally 
produced; 
 Masonry cements: they are used for preparing bricklaying mortars and stuccos, and must 
not be used in concrete. They are usually complex proprietary formulations containing 
Portland clinker and a number of other ingredients that may include limestone, hydrated 
lime, retarders, waterproofers and colouring agents. They are formulated to yield workable 
mortars that allow rapid and consistent masonry work; 
 White blended cements: may be made using white clinker and white supplementary 
materials such as high-purity metakaolin.
8 
 
In addition to these kinds of cements there are also some other cements which are not obtained from 
Portland cement. 
The main mineralogical constituents of a Portland cement are essentially four: tricalcium silicate 
(C
3
S), bicalcium silicate (C
2
S), tricalcium aluminate (C
3
A) and tetracalcium alumino-ferrite 
(C
4
AF). These constituents never exist in the pure form; for example, C
3
S is essentially a solid 
solution containing Mg and Al called “Alite”. The C
2
S instead contains elements like Al, Mg and 
potassium oxides and it is called “Belite”, while the ferric phase is a solid solution of variable 
composition made up of C
2
F, C
6
A
2
F and other iron-based compounds, generally indicated as C
4
AF. 
A rough illustration of cement composition is reported in the figure 1.1. 
 
 
 
Figure 1.1: Main cement compounds 
 
It is important to note that in the cement chemistry symbols have different meaning from classical 
chemistry: C is for CaO, S is SiO
2
 A is Al
2
O
3
 and F is for Fe
2
O
3
.  
The C
3
S is probably the most important constituent because reacting with water it gives high 
mechanical resistance; for this reason, generally high concentrations of C
3
S are preferred in the 
cement fabrication. An high dosage of lime is so used but it is not enough to have high C
3
S content 
in the cement. A rapid formation of this compound is obtained by keeping the temperature between 
1350-1500°C, when the liquid phase is present in the oven. C
3
S is stable only above a temperature 
of 1250°C and for this reason, after cooking, a quench is required.
9 
 
The C
2
S is present in the clinker with the β crystalline form. It can hydrate in a long time, so the 
development of mechanical resistance due to this material is visible after very long maturing times. 
The C
3
A is the compound with the highest hydration velocity but it does not contribute to the 
development of the mechanical resistance. From this point of view, it should be preferred to avoid 
the presence of this compound in the clinker, but practically it is very useful to keep a low melting 
temperature thus reducing the energetic cost. 
As described above, the formula C
4
AF normally indicates a ternary solid solution which is able to 
give a modest contribution to the mechanical resistance after hydration, especially after 15 days 
annealing. As the C
3
A, C
4
AF has also an important rule on the melting temperature drop. 
In addition to the main compounds, some other impurities can be found in the clinker. One of these 
is certainly the magnesium oxide (MgO), which must be present in a maximum amount of 2-2.5% 
due to its expansive behaviour.  
Other impurities are alkaline oxides, phosphates and fluorides which can also contribute to keep 
low melting point
[1]
.  
 
 1.2 The most common uses of the cement 
 
Cement mixed with water is virtually a plastic stone, and it can be used for many purposes in place 
of stone with economy in shaping to the form required, and advantage in securing a hard, fire-proof 
material. It may be used for shop floors, buildings, foundations for heavy machinery, bridge piers, 
walks, waterworks dams, reservoirs, walls, dry-docks, culverts, etc. A concrete casing will protect 
iron or timber structures from corrosion in air or in water, and will protect exposed iron work of 
structures from effects of conflagration. Concrete is a composite construction material, composed of 
cement (commonly Portland cement) and other cementitious materials such as fly ash and slag 
cement, aggregates (generally a coarse aggregate made of gravels or crushed rocks such as 
limestone, or granite, plus a fine aggregate such as sand), water, and chemical admixtures. 
The word concrete comes from the Latin word "concretus" (meaning compact or condensed); it was 
in fact extensively used by Roman engineers, who used a mix of quicklime, pozzolana and an 
aggregate of pumice.
10 
 
Strengthened with iron bars, or meshed wire, placed in it when it is being moulded to shape, cement 
is known as reinforced concrete, and will thus form bridge floors, bridge spans, and the upper floors 
of buildings which must support great weight. In fact, typical concrete mixes have high resistance to 
compressive stresses (about 4,000 psi (28MPa)). However, any appreciable tension (e.g., due to 
bending) will break the microscopic rigid lattice, resulting in cracking and separation of the 
concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the 
development of tension. If a material with high strength in tension, such as steel, is placed in 
concrete, then the composite material resists to both compression and bending and other direct 
tensile actions. A reinforced concrete section where the concrete resists the compression and steel 
resists the tension can be made into almost any shape and size for the construction industry. 
In marine use, concrete is limited because of its weight. It may be used as permanent ballast in the 
bilges of steel ships, and is an effective protection from corrosion when applied to absolutely clean 
iron or to iron surfaces covered with closely adhering red rust. When so used, cement may be mixed 
with water and applied with a brush, or it may be mixed in the proportion of about two parts sand 
and one part cement and applied wet, with a trowel, in a layer varying from 1/4 inch to any desired 
thickness. In this way ship tanks, bunkers, and bilges are protected, as the mixture forms a close 
bond with iron. In no case will this bond form if the iron is oil coated
[2,3]
. 
 
 1.3 The hydration of cement 
 
The most important mineralogical constituents of a cement are certainly the silicates, which are 
directly responsible for the mechanical properties of hardened cement. 
The hydration reaction for C
3
S is: 
 
 
 
   
 
        
 
    
(     )
   
(   )
                                     (   ) 
 
where H is for H
2
O. The stoichiometry of this reaction is approximate because the coefficients 
depend on the temperature and the presence of additives and they change during the reaction. In the 
same way the reaction for hydration of C
2
S is the following:
11 
 
 
   
 
   
 
        
 
  
 
  
 
                                                      (   ) 
 
Also for the reaction 1.2 the stoichiometry is not very strict but depends on several factors. As 
visible, the hydration reaction of C
2
S is different from the reaction of C
3
S especially for the minor 
content of calcium hydroxide produced after hydration. 
The C
3
A, although present in the clinker in a very small amount, strongly influences the hydration 
kinetics of the other compounds, as it suddenly reacts with water to give: 
 
   
 
              
 
  
  
   
 
   
 
                                                 (   ) 
 
These phases are so thermodynamically unstable that they suddenly transform in other compounds 
as shown in the reaction 1.4: 
 
 
 
  
  
  
 
  
 
          
 
  
 
                                                       (   ) 
 
The fast hydration of C
3
A is generally not very convenient in the production of mortars and 
concrete, because it increases the temperature of the mix too fast and it greatly accelerates the 
hydration of the other phases, giving a material which hardens and loses workability too rapidly. 
For this reason, generally a certain amount of chalk is added to the clinker to give the reaction 1.5: 
 
   
 
   (    
 
   )              
 
       
 
  
  
                                (   ) 
 
The formation of the quaternary salt (called ettringite) is responsible for the reaction time 
prolongation.
12 
 
The hydration of C
4
AF, present in the clinker with an amount of about 8-13%, was always not very 
studied just because it is assumed as very similar to the hydration process of C
3
A. The main 
difference from this latter is in the reaction rate which is certainly slower and becomes even slower 
with the addition of chalk. 
After mixing with water, the fine powder of cement becomes a plastic dough where the calcium 
hydroxide generally represents the 18-25% of the total. The rest of the material is essentially 
composed of variable percentages of other materials such as the hydrated phase CSH (characterized 
by needle-like crystals) and the ettringitic phases (also called Aft phase, to indicate that they are 
composed of aluminium and ferric compounds). The monosulphate is formed after the 
disappearance of the ettringitic phases; it normally constitutes the 10% of the solid mass and it is 
characterized by hexagonal crystals of submicron size. 
The hydration reactions are accompanied by a considerable heat production; the representation of 
the heat production vs. time in figure 1.2 gives an idea of the kinetic of cement hydration. 
 
 
Figure 1.2: Hydration kinetic of cement phases
13 
 
The first stage is characterised by a heat production due to the hydration of C
3
A which causes the 
formation of ettringitic phases that cover the anhydrous C
3
A preventing its hydration. This results in 
a sharp decrease of the reaction rate which is in turn responsible of a heat production decrease. This 
phenomenon is then followed by a so-called “dormant period”, where the ettringitic phases 
thickening almost completely stops the hydration of anhydrous compounds. This stage can last few 
hours, then a third period occurs which is characterised by a strong heat development. This is 
essentially due to two phenomena: one is the break of ettringite film and continuation of aluminates 
hydration, the other is the conversion of the CSH in hydrated phase which becomes more permeable 
to the ionic species in solution. This phase coincides with the so-called “initial setting time” and 
with the subsequent loss of plasticity. Then the setting process continue for some hours and it 
concludes with the so-called “final setting time” which coincides with the beginning of the 
hardening process. 
The third peak in the graph is finally due to the transformation of the ettringitic phase in 
monosolfoaluminate. 
 
 
1.4 Modified cement and the water/cement ratio 
 
The technological content of the cement and of its derivatives has been considerably increased in 
the last century because of the growing interest for these materials and for their incredible potential 
development. This interest have led to a large number of studies having the purpose to increase the 
performances and quality especially of concrete and mortars. 
The production of “High Performance Concrete” (HPC) is now a current practice in nations like 
Japan, France, Norway  and others. The HPC is characterised by a compression resistance in the 
range of 60-100 N/mm
2
 thanks to: 
 
1. a minor water/cement ratio (0.40-0.30) due to the use of  additives; 
2. addition of minerals with elevated surface area; 
3. high-quality chipping aggregates (granite, basalt, etc.…).
14 
 
Recently, a further evolution of these materials has led to the so-called “Reactive Powder 
Concrete” (RPC) with incredibly high performance (very elevated with respect to the HPC), 
generally obtained with the employment of polymeric fibres. The RPC are still in the project stage 
and so they are not already extensively used, but some applications have been already proposed, 
such as the achievement of tunnels or building in areas where can be very much stressed (i.e. 
seismic areas). 
On the other side of the HPC and RPC, which together are classified as DSP (Densified with Small 
Particles), there is another class of high performance cement-based materials; they are classified as 
MDF (Macro Defect Free) and are different from DSP because the increase of performance is 
generally reached by adding some particular additives. They are generally some water-soluble 
polymers which modify the water/cement ratio and the mix rheology allowing to use some peculiar 
manufacturing processes such as calendering or extrusion. 
One of the most common solutions to increase the compression resistance (R
c
) is to reduce the 
capillary porosity of the cement (V
p
). To achieve this target, two parameters can be modified: 
 
- the cement hydration rate α; 
- the water/cement ratio a/c. 
 
The theory developed by Powell
[4]
 allows to correlate these two parameters with R
c
 and V
p
. This 
theory, validated by a great number of experimental evidences, is based on the equations 1.6 and 
1.7. 
 
 
 
       ⁄                                                                 (   ) 
 
 
 
     
       
          ⁄
  
 
                                                       (   )
15 
 
It is easily deducible from equations that an increase of α and a decrease of a/c leads to a reduction 
of porosity V
p
 and so finally to an increase of R
c
. To modify α and the ratio a/c, the addition of 
some particular additives is necessary. 
 
1.5 The additives of the cement 
 
As seen in the previous section, in recent years the composition of cement formulations have 
radically changed and this is essentially due to the introduction of a large variety of additives. 
Today, in the cement field there are practically additives for every use: 
 
 water reducers: they increase the workability of the cement formulations allowing a partial 
water reduction; 
 setting accelerators: substances able to change the initial and final setting time; 
 setting retardants: they delay the initial reaction between water and cement compounds; 
 hardening accelerators: they improve the hydration rate of the concrete to favour a faster 
development of compression resistance; 
 aerants: chemicals able to increase the air content in the formulation improving the 
workability and reducing the freezing effect on the concrete; 
 water proofers; they provide resistance to water; 
 water retention additives: they prevent a rapid evaporation of the water in the mix; 
 rheology modifiers: able to modify the flow properties of the slurry. 
 
These additives are generally chemicals of organic nature (commonly used in liquid form) added to 
the cement mix in a variable percentage between 0.1% and 3%, producing a relevant effect on the 
features of the formulation, both in fresh stage and after hardening.  
Each additive usually also presents some secondary effects (sometimes also slightly negative) on 
the slurry. For example, some water reducers are also able to delay the initial reaction between 
water and cement compounds, practically acting also like a setting retardant.
16 
 
The use of these substances is so common that about 80% of concrete is generally made using 
additives. Only in Italy, their consumption is estimated to be about 250000 ton/year and expected to 
increase in the next years. 
The world of additives is so wide that there are dedicated books just for this subject; here we will 
analyse in particular two categories of additives: the rheology modifiers and the fluidificants. 
 
 
 1.6 The rheology modifiers 
 
Rheology modifiers (RMs) are a class of chemical substances extensively employed in many fields, 
from food to cosmetics, in the realisation of some inks, paints and also explosives and many other 
chemical products. They are generally substances that, when added to an aqueous mixture, increase 
its viscosity without substantially modifying its other properties. They provide body, increase 
stability, and improve suspension of added ingredients. Thickening agents are often used as food 
additives and in personal hygiene products. Some thickening agents are gelling agents, forming a 
gel. The agents are materials used to thicken and stabilize liquid solutions, emulsions, and 
suspensions. RMs dissolve in the liquid phase as a colloid mixture that forms a weakly cohesive 
internal structure. 
In the cement field, RMs are more correctly called “water retention and rheology modifier” 
(WRRM), because their very hydrophilic structure allows for an high hydration of molecules. This 
effect is of primary importance when WRRMs are added to the cement, because they control the 
hydration process of cement constituents. This is the reason why the WRRMs, even if they are 
present in small amounts, significantly influence the rheological properties of the solutions.  
It is possible to find commercial WRRMs with molecular weight on between 10000 and more than 
10
6
 Dalton and of different kind. Consequently, the properties of the solution can be tuned by 
several parameters such as concentration, molecular weight, degree of substitutions and so on
[5,6]
. 
Most commercial WRRMs are cellulose derivates obtained by alchilation and alcossilation of the 
cellulose chain. One of the principal cellulose derivatives used in cement industry is 
methylhydroxyiethylcellulose (MHEC).
17 
 
The cellulose is a linear homopolymeric polysaccharide, made up of glucose with β-1.4 bonds 
(figure 1.3). 
 
 
Figure 1.3: Molecular structure of cellulose 
 
 
As strong intermolecular interactions due to H-bonds are established, the pure cellulose is insoluble 
in water. The substitution of functional groups –OH by C2, C3 and C4 of monomeric unit make the 
cellulose soluble in water. These substitutions can be operated by etherification. The most frequent 
substitutions are made with (OCH
3
), hydroxypropylic (POOH) and hydroxyethylic groups (EOOH). 
MHEC is obtained through etossilation and metilation of cellulose (figure 1.4). 
 
 
 
Figure 1.4: Molecular structure of MHEC