Abstract

Alcohol amines have been
used as grinding aids in cement grinding processes for many years. Milling aids
are predominantly developed to increase the performance or grinding capacity of
the cement product.  The setting points
of cement produced with grinding aids are mostly changing. In this study,  the effect of boron compounds to setting
points of the cements which are produced with addition of grinding aids of alkanolamines
and boron compounds in different mixing amounts is searched. Triethanolamine (TEA)
and triisopropanolamine   (TIPA) are used
as alkanoamines and boric acid (BA) and anhydrous borax (AHB) are used as boron
compounds. It is seen that BA with TEA has a small effect on cement setting
points and BA decreases the retarding effect of TEA but has no effect with TIPA
on setting times. AHB decreased the retarding effect of TEA but has no effect with TIPA on setting times. When the
boron compounds added to TEA, they accelerate the initial setting of cement 15
to 20 minutes and the final setting of cement 25 to 30 minutes according to TEA
used itself.  

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Keywords: Grinding Aid, Alkanoamines, Boric Acid, Anhydrous Borax, Initial Setting,
Final Setting

 

1. Introduction

Hydration reaction starts directly when
water is mixed with cement to form a paste. The paste first begins to lose its
plasticity and stiffens, and then hardens 1. Main phases, C3A, C3S,
C2S, and C4AF reacts quickly with water to produce a
jelly-like compound which starts solidifying. The action of changing from a
fluid state to a solid state is called ‘setting’. Basically, a setting is a process of transformation from an
initial state, a scattered concentrated suspension, to a final state, a
connected and strengthened system of particles. This transformation in the
practice of cement and concrete is obtained by chemical reactions between
cement particles and water (i.e., cement hydration) 2. Normal setting of
cement is associated with the hydration of Alite (C3S) and formation
of the calcium silicate hydrate (CSH) phase 3.

Cement
as a slurry has fluidity just after mixed with water and can easily flow into a
mold. The sulfoaluminates create interlocking crystals and the C-S-H starts to
form, the created network of solids causes the cement to set. The cement
paste  has no strength at this hydration stage
and also it is very friable. The  required time length for initial setting can
be determined by the amount of gypsum present in the cement. Hardening of the
cement paste ;in other words the beginning of strength gain  occurs after the formation of C-S-H, which
starts some hours after mixing.

Accelerating
chemical aids causes cement set acceleration according to one or more of
following mechanisms:

(1)   Ability to accelerate compound to flocculate
hydrophilic colloids, resulting in a more permeable C-S-H surface layer,
through which water and ions can diffuse faster 4.

(2)   Accelerating the participation of compound ions in aluminates
reactions, forming some aluminate ion phases mostly when there is not enough sulfate
to react with C3A. Monosulfate conversion of ettringite will not ocur
 if free ions are available 5.

(3)   An increased calcium concentration cause a quicker
super-saturation of the liquid with respect to calcium hydroxide, Ca(OH)2,
meantime a lower sulfate concentration will cause slower formation of
ettringite which will shorten the initiation of aluminate, C3A,
hydration 6.

Retarding admixture is an
admixture that retards the setting of concrete 7. A retarding admixture
causes cement set retardation by one or more of following mechanisms:

(1) Adsorption of the
retarding compound on the surface of cement particles, forming a protective thin
layer which slows down hydration;

(2) Adsorption of the
retarding admixture on to nuclei of calcium hydroxide causes of their growth poisoning,
which is essential for continued hydration of cement after the end of induction
period;

(3) Formation of
complexes with calcium ions in solution, increasing their solubility and
discouraging the formation of the nuclei of calcium hydroxide referred to in
(2) above; and

(4) Precipitation around
cement particles of insoluble derivatives of the retarding compounds formed by
reaction with the highly alkaline aqueous solution, forming a protective skin 8.

Alkanolamines
are amines containing a single, double or triple hydroxy alkyl group. Alkanolamines
which are frequently used as concrete admixtures is used as grinding aids. The
action of TEA in the hydration of cement is not described clearly in the
literature. It is not still concluded whether it is an accelerator or a
retarder. Dodson 1 claimed that “the most popular addition rate of TEA is in
the range of 0.010 % to 0.25 % on the weight of cement. The
conclusions show that at a dosage of 0.02 %, TEA acts as an accelerator, at
0.25 % as a retarder, at 0.5 % as a severe retarder and at 1.0 % as a flash-set
accelerator. Similar behavior was found by Ramachandran 2 when measuring the
initial and final setting characteristics of Portland cement mortar mixed with
0 – 0.5 % TEA. Other studies showed that the most prominent feature of TEA is
its ability to act as both a set retarder and a set accelerator depending on
its dosage. TEA accelerates C3A hydration as well as delays C3S
hydration 2,3. Jolicoeur et al. (2007) used TEA amounts ranging from 0.02% to
0.15% in their study. When TEA 0.1% was used, a strong accelerator effect was
observed due to rapid hydration of C3A (false setting). Heren and
Olmez (1996) investigated the hydration and mechanical effects of ethanolamine
oligomers which they added to the consistency water in varying amounts of 0.1
to 1%. They compared MEA, DEA, and TEA at dosages 0.1 to 1.0 % in white
Portland cement and found no significant effect on the initial setting times at
low dosages, but significant retardation at higher dosages for all three types.
TEA is a weak base and in an aqueous phase, it is mostly in the molecular state.
TEA has the ability to chelate with certain metallicions such as Fe3+
in highly alkaline media 11.

TIPA
is a tertiary amine and TIPA is used in the cement industry as a grinding aid,
and it is also used in concrete formulas. The addition of small amounts of TIPA
can result in a signi?cant increase in the strength of cement pastes at early
and late ages 12. Aggoun (2008) investigated the early strength and setting
effects of TIPA/TEA/ calcium nitrate mixture combinations in normal portland
cement and obtained important and promising results for both the setting and
curing accelerator effects of the amine/calcium nitrate composition. Aggoun has
found that TIPA’s curing accelerator performance is higher than TEA regardless
of cement type 13.

Katsiotia
and others make a research work for the evaluation of six commercial grinding
additives, which were used for the production of Portland cement (ground in a
ball mill at a laboratory stage). They tested all cement mixtures for initial
and ?nal setting times, consistency of standard paste, ?ow of normal mortar and
compressive strengths after 2, 7 and 28 days. They found the slightly
decreasing of the initial and final setting times in case of triethanolamine
containing grinding aids that was based on the C3A reaction acceleration.
However, they also found that the presence of triisopropanolamine as a grinding
aid in the cement mixture increases the initial and ?nal setting times about
15%, acting as a retarder of hydration at early stages 14. The aim of another
research work made by Allahverdi and Babasafari is to interpret the
effectiveness of TEA on grindability, set and strength behavior of Portland
cement in laboratory ball and vibrating disk mills when added 0.06 % amount of by
the weight of the cement. It is concluded that an addition of TEA increases
both initial and final setting times and decreases compressive strengths at 3,
7 and 28day curing ages in both ball and vibrating disk mills 15.

Water-soluble
sugars. sugar acids and salts. borax and boric acid are known as set retarders
16. But
in the study of Koyuncu (2004) and others 12 sodium borax pentahydrate (Na2B407.5H20)
was used in concrete as replacement materials in a certain ratio instead of
Portland Cement. In the concrete specimens it was observed from the experiments
that while the amount of borax ratio increases the compressive strength,
flexural strength, setting time, unit weight, and workability decreased. Additionally,
it was observed in the studies that the cement paste made with boron containing
cement had an increased  setting point
both beginning and final 18,19.  Davraz
20 used boron modified active belite cement, calcium aluminate cement and Portland
cement as a binder material in the mortar mixtures. Their aim was to research the
effects of the boron compounds into hydration reactions of the different cement
types and  these effects controllability.
Boric acid (BA) was used in the studies at the ratios of 0.25–1.00% of the weight
of cement in mortar mixtures with Portland cement. It was found that by
increasing the usage portion of B2O3 the redarding effect
was increased, this could be controlled by adding accelerator aids to the
mixture. The best results were taken from sodium aluminate and 1 % Boric acid
mixture. The formation of calcium diborate (CBH6)
barrier layers on particle surfaces should be delayed to reach to optimum
setting times and the strength in convenient B2O3
concentration 20.

Sonoda
and friends 21 used NMR spectroscopy to study the complex formation behaviors
between  TEA and boric acid in aqueous
solutions and to determine the stoichiometric compositions, stability, and
structures of boron complexes. They concluded formation of two kinds of 1:1
complexes in aqueous solutions. They were TEA-B and its hydrolysis product with
a bicyclo 3,3,0 structure. Both of them had tetrahedral structure around
boron atom, having a boron-nitrogen bond 21.

Generally,
the concentration range of grinding aids added is from 50 to 500 ppm. After the
grinding process, the additives might not be any longer in their original
chemical form. Moreover, grinding aid composition might not only consist of
mixtures of pure compounds but also rather more complex raw materials 22.

Our
aim in this study is to determine the contribution of the boron compounds with alkanolamines type
grinding aids to the cement settings. Also making a command about way of
setting mechanism can be helpful for further studies. Most of the boron
compound studies with cement were focusing on a higher concentration of boron
compound in past. It will be helpful for further studies to see the result of
lower boron compound containing cement experiments. 

 

2. Experimental

2.1. Materials

Portland
cement clinker and gypsum are used as the main components of the cement in the
study. All cement shown in Table 1 are produced from the same batch of clinker
and gypsum in the study. As a grinding aid, triethanolamine (TEA) and
triisopropanolamine (TIPA) are used. In addition, BA and AHBA were used as
boron compounds in which the effects on the performance of grinding aid were
searched. For experiments, precision scales Precisa XB6200D. 5-6200 gr. ±0.1 g
ve Kern BJ-220-4NM. 0-220 g ±0.0001 g, laboratory crusher, laboratory mill,
mortar mixer, Vicat test equipment and mold manual and automatic Atom Teknik ,
deiosined water, lithium tetraborat (66%) for fuse beads of XRF analysis,Fluxana
fusion machine, platinum crucible, mould, and XRF Panalytical Zetium were
used.

2.2.
Preparation and Experimental Method

The
size of the clinker was reduced (3 mm or less) by passing it through the
laboratory type crusher before the experiments to ensure homogeneity and then
mixed. Portland CEM I type cement was produced in the experiments. In the
experiments, grinding aids were used in all the cement produced except the
reference cement. For each experiment, 3000 g of clinker was weighed and then
the grinding aid + boron compound were added to give a total weight of 0.1% and
the mixture was thoroughly mixed. This mixture was run for 40 min after the
laboratory-type ball mill was loaded. Samples were taken from the stopped mill
and the SO3 level was checked and the gypsum was added to the mill
3.05 ± 0.05% according to the target final level and the mill was run again
for 40 min. Cement samples ground for a total of 80 mins were taken separately
for each experimental set by XRF elemental analysis.

For
the Vicat tests of cement samples, they were subjected to the consistency test
according to TS EN 196-3:2017 23. The cement samples with consistency water
were prepared for the Vicat test according to TS EN 196-3:2017 standard and the
beginning and the final setting times were determined in automatic Vicat
device.

 

3.
Results and Discussion

Setting
time tests with varying grinding aid contents were performed under the constant
laboratory conditions (T:20oC and Relative Humidity>50 %). An
average of three test readings was taken as the final reading. To compare the
changes occurred in setting times by addition of grinding aids, the setting
time of cement paste without grinding aid content was used as a reference. The
setting times were recorded in minutes.

All
of the cement samples produced by using TEA  mixtures are more than 75 minutes which are suitable
for initial setting values as specified in TS EN 197-1: 2012 24. A total of 3
g of material (0.1 wt% cement) is used as grinding aid in each experiment. According
to the cement sample referred in Table 1 and Graph 1, the beginning and final
setting times are higher for the cement sample ground with 100% triethanolamine
according to the reference cement. This demonstrates the a mild retarding
effect of the TEA in hydration reactions which is compatible with Dodson’s 1
and Allahverdi’s studies 15 .

When
the TEA content in the total cement is reduced by the addition of boric acid
instead of 25% TEA, the settings begining and final are very similar to 100%
TEA study (Table 1, Graph 1). By increasing boric acid ratio to 33% where TEA
was used at 67% as grinding aid, initial setting time was faster than
experiments with 100% TEA and 75% TEA + 25% boric acid. The final setting of
this sample was also earlier than the the final setting of samples of 100% TEA
and 75% TEA + boric acid (Table 1, Graph 1). Addition of boric acid showed a
mid accelaration in setting points acording to %100 TEA results which is
contradictory
 to the mentioned results in previous
studies 2,3,9,10,13,15. The mechanism of this
contradictory could be because of a reaction between TEA and boric acid as it is
discussed in Sonoda 21 study. Up to pH level of 10.9 and at low temperatures
these two compound consume each other and make a new complex triethanolamine
borate. So during the pre-induction and dormant period of cement hydration
reactions by adding the boric acid to the system, the TEA-B complexes formed which
reduce the concentration of TEA and boron compounds in the liquid phase. So the
retardation effect of TEA reduced and it is seen in 75% TEA + 25% boric acid
and 66 % TEA + 33% boric acid samples as a reducing retarding effect.

The
typical B2O3 content of boric acid and anhydrous Borax
are 56.5 % and 68.5% respectively. For 25 % boric acid mixture this means
14.13% B2O3 and for 33 % boric acid mixture it means
18.65% B2O3. The B2O3 content of
25% and 33% anhydrous borax are 17.25% and 22.61% respectively. It is
understood from setting points of 75% TEA and 25% anhydrous borax sample that
there occurred a reaction between the TEA and anhydrous borax molecules in an aqueous
solution of cement paste also. Like the mechanism of boric acid the anhydrous
borax molecules and TEA balanced each other and only the remaining grinding aid
mixture which is mostly TEA slightly retard the cement set points (Graph 1).
The initial and final setting points of 66% TEA+ 33% boric acid mixture and 75%
TEA+ 25% anhydrous borax are very similar because the total B2O3
content in the mixtures is also proximate 18.65% and 17.25% respectively. The
mixture of 67% TEA and 33% anhydrous borax has nearly the same setting points
with 25% anhydrous borax containing mixture. 5 minutes retardation may because
of more SO3 content (0,06% more) which can react with C3A.

All
of the cement samples produced using TIPA are compatible with the initial
settings values specified in TS EN 197-1: 2012. A total of 3 g of material (0.1
% weight of cement) was used as grinding aid in each experiment. It is seen in
Table 1 and Graph 2 that the cement sample ground with 100% TIPA has 10 minutes
retarded initial setting and similar final setting according to the reference cement.
Although in literature TIPA behaves as a setting accelerator, it is not seen a
significant change in settings for 100% TIPA used cement sample. For both boric
acid and anhydrous borax mixture experiments no significant change found in
setting points for two different  25% and
33%  mixing ratios. This mainly because
of calcelation of their setting effects.