CHEMICAL REAGENTS TO PREVENT THE THAUMASITE FORM OF SULFATE ATTACK

CHEMICAL REAGENTS TO PREVENT THE
THAUMASITE FORM OF SULFATE ATTACK
Ghorab, H.Y. ; M. Rizk ; Y. Abdel Tawab Osman
and Ibrahim Mohamed Ali
Faculty of Science, Helwan University
ABSTRACT
Two reagents were used to prevent the thaumasite formation in Portland
limestone cement mortar; disodium hydrogen phosphate chelating agent and
barium hydroxide.3 and 5% of each reagent were added to the cement mortar
with 1:2 cement to sand and a w/c ratio of 0.46. The specimens were immersed
in 5% magnesium sulfate solution at 7oC up for 90 days. The compressive
strength of the cubic samples were measured at different time intervals. After 90
days exposure, the appearance of the cubic samples were inspected visually and
the phases formed were monitored by means of X-ray diffraction.
The results revealed that barium hydroxide is capable to prevent the
formation of thaumasite whilst the samples with chelating agent were
significantly damaged. The discussion is based on previous work in this field.
1. INTRODUCTION
As early as the end of the nineties, and the beginning of the
millennium, different mechanisms were reported for the thaumasite
formation (1-9): The most important condition for its formation was
reported to be the presence of lime (10). Pozzolanic admixtures were
therefore used to reduce the concentration of lime and prevent the
thaumasite form of sulfate attack (11-13).In order to understand the
mechanism of the thaumasite formation more deeply, different routes
were used to obtain this salt in pure systems and Portland limestone
cement pastes and mortars (14-17). It was confirmed that the
requirements for its existence are the availability of carbonate, reactive
silica, sulfate and enough calcium.
In reference 14, studies were carried out on the addition of small
concentrations of 0.3% from disodium hydrogen phosphate chelating
agent as well as from barium hydroxide to prevent the thaumasite form of
sulfate attack in Portland limestone cement pastes. With these additions,
both reagents showed positive results. The present work is an extension
of this work and deals with the effect of higher concentrations of 3 and
5%, from these reagents on the durability of Poland limestone cement
mortars and its susceptibility to form the destructive thaumasite salt (17).
2. EXPERIMENTAL
Portland limestone cement was prepared by mixing 70% of CEM I
42.5 with 30% high purity commercial limestone. Commercial sand was
used for mortars. Reagent grade sodium hydrogen phosphate (NaH2PO4)
cheating agent and barium hydroxide (Ba(OH)2) were provided..
Egypt. J. of Appl. Sci., 35 (5) 2020 96-102
The effect of chelating agent and barium hydroxide on the pHvalue
of the cement was performed by measuring the pH-of a cement
slurry made of 20 g cement in water (W/C ratio= 20) as well as in mixes
of the cement with 3 and 5% from each reagent. The pH-values were
recorded as soon as the water was added as well as after 30, 60, 90 and
120 minutes. The effects of the reagents on the water consistency and
setting times of the cement was carried out according to EN196-
3:2016(18).
The cement was mixed with sand at a ratio of 1:2, and water was
added with a w/c ratio of 0.46. Mixing was performed in compliance to
ASTM C 305-20(19). The mix was cast in 5x5x5 cm stainless steel
molds, stored 24 h in humidity chamber, demolded then cured in 5%
MgSO4 at 7ºC for 3 months .The effect of the reagents was performed by
adding 3%, and 5% by weight chelating agent or barium hydroxide to
Portland limestone cement then proceeding as described.
The appearance of the mortars cubes was examined after 3 months
curing in sulfate solution and were recorded photographically. The
compressive strength of the hardened mortar samples were measured
during the 90 days curing according to B.S. EN 196-1:2016(21). The 90-
day samples were analyzed by means of X-ray diffraction.
3. RESULTS
Table 1 (a, b) illustrates the oxide and phase composition of CEM I 42.5 N.
Table (1, a): Oxide composition of CEM I 42.5N (m. %)
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 Cl LOI
% 18.98 4.81 3.72 62.49 1.75 0.27 0.50 3.13 0.081 3.2
Table (1, b): Phase composition of CEM I 42.5N (m. %)
C3S C2S C3A C4AF
% 59.5 16.48 6.47 11.31
The pH-values of the cement slurry was not affected by the
addition of chelating agent and barium hydroxide and remained alkaline
with a value of ~ 13.
Table 2 presents the effect of chelating agent and barium hydroxide
on the water consistency and setting time of Portland limestone cement.
The results indicate a clear increase in the water consistency and the
retardation of the cement with the addition of the chelating agent, whilst
the variation in the original values are changing only slightly in presence
of barium hydroxide.
Table (2): Effect of chelating agent and barium hydroxide on the
water consistency and setting time of Portland limestone
cement paste
Chelating agent Barium hydroxide
Consistency I.S. F.S. Consistency I.S. F.S.
0% 28 135 325 28 135 325
3% 29 188 510 28.5 155 324
5% 30 195 535 28.9 157 327
97 Egypt. J. of Appl. Sci., 35 (5) 2020
The effect of both reagents on the compressive strength of Portland
limestone cement paste shown in Figure 1 (a, ab) indicate a clear drop of
strength with the addition of 3 and 5% sodium hydrogen phosphate
(NaH2PO4) cheating agent, the decrease in strength in presence of barium
hydroxide is not significant.
A
B
Figure 1: Effect of a) chelating agent, b) barium hydroxide on the
compressive strength of Portland limestone cement mortar
Picture 1 (a, b) illustrates the visual appearance of Portland limestone
cement mortar in presence of 3 and 5% disodium hydrogen phosphate
chelating agent and barium hydroxide and cured 3 months in 5% MgSO4
solution at 7oC. The appearance of the samples in the two concentrations
was similar. The results show that, the cement cubes treated with
disodium hydrogen phosphate are strongly deteriorated, that with barium
hydroxide are sound and show no deterioration.
A
B
Picture 1: The visual appearance of Portland limestone cement mortar
with a) 3 and 5% chelating agent b) 5% barium hydroxide cured in 5%
magnesium sulfate solution for 3 months at 7oC
Egypt. J. of Appl. Sci., 35 (5) 2020 98
The X-ray diffraction patterns of Portland limestone mortars with
a) 5% chelating agent b) 5% barium hydroxide cured in 5% magnesium
sulfate solution for 3 months at 7oC are depicted in Figure 2 (a. b). The
figures show the formation of thaumasite in the mortar sample with 5%
chelating agent and that with barium hydroxide indicates the absence of
thaumasite patterns, instead the gypsum phase forms.
A
B
Figure 2: The X-ray diffraction patterns of Portland limestone cement
mortar with a) 5% chelating agent b) 5% barium hydroxide cured in
5% magnesium sulfate solution for 3 months at 7oC
4. DISCUSSION
4.1 The thaumasite formation
The thaumasite form of sulfate attack known to damage the cement
system, depends on the availability of carbonate, sulfate and calcium ions
in presence of reactive silica. Recent work has shown that the mechanism
of thaumasite formation follows a sequence expressed as stages which
can be described as follows:
a. Stage 1: The carbonation process
In this stage the pH-value of the cement decreases and reduces its
alkalinity from ~12-13 to < 10 through carbonation. The carbonation
process is initiated by intensive exposure of the cement system to
atmospheric CO2 and humidity, or by the presence of calcium carbonate
as inorganic admixture. Under these conditions the calcium silicate
hydrates decompose to calcium carbonate and silica gel, and the calcium
sulfoaluminate and sulfoferrite to aluminum hydroxide, ferric hydroxide
and gypsum. Calcium carbonate is a common decomposition product
from portlandite as well as from the cement hydrates. Worth to note that
the silica gel freshly precipitated exists in a reactive form.
b. Stage 2: The thaumasite formation
The carbonation process takes place at the surface of the cement
system, the bulk of the cement remains unaffected because of the
99 Egypt. J. of Appl. Sci., 35 (5) 2020
thickness factor. The calcium hydroxide is a strong nucleophilic agent. It
originates from the calcium silicate hydrate of the bulk or from newly
hydrated residual calcium silicate. The splitted calcium hydroxide attacks
the carbonated layer. The reactive silica precipitating from the
carbonation of the CSH phase, dissolves as silicon hexa-coordinated with
OH ions which then combines with the calcium ions to form a positively
charged silicate column similar to the aluminate column of the ettringite.
The column is neutralized by the negatively charged carbonate and
sulfate of the surrounding or provided from the system internally. The
next question is to determine the minimum concentration of sulfate
needed to form the thaumasite.
4.2 Prevention of the thaumasite formation
To prevent the thaumasite formation the carbonation process of the
must be avoided, and the calcium supply must be controlled.
The present work used a chelating agent to encapsulate the calcium
ions and control its supply, and barium hydroxide to precipitate the
carbonate and sulfate ions.
The use of 0.3% of disodium hydrogen phosphate chelating agent
prevent the thaumasite formation successfully (14). But higher
concentrations of 3 and 5% damage the cement system. This is due to the
possible chelation of excess calcium ions beside the precipitation of
calcium phosphate causing a lack of alkalinity upon long exposure.
On the other hand, the addition of 0.3 to 5% barium hydroxide to
Portland limestone cement prevented the thaumasite formation. The
insolubility of barium sulfate and barium carbonate has probably limited
the supply of these ions and the respective concentrations needed for the
thaumasite to be formed.
5. CONCLUSIONS
The prevention of thaumasite formation can be performed by using
chemical reagents suitable for this process. 0.3% disodium hydrogen
phosphate chelating agent can be safely added to Portland limestone
cement but higher concentrations of 3 and 5% lead to the thaumasite
damage of the cement system. On the other hand, a high concentration
range of 0.3 to 5% can be used from barium hydroxide to prevent the
thaumasite form of sulfate attack successfully.
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مواد کیمیائیة لمنع تکون ممح التومازیت الناتج من هجوم الکبریتات
هناء یوسف غ ا رب ، محمد رزق ، .یاسر عبد التواب ، اب ا رهیم محمد عمی
قسم الکیمیاء، کمیة العموم، جامعة حموان.
فی ىذه د ا رسة تم استخدام مادتین لمنع تکوین ممح التامو ا زیت فی اسمنت الحجر الجیری
المادة الاولى ماده مخمبیة )دای صودیوم ىیدروجین فوسفات( والمادة الثانیة باریوم ىیدروکسید،
۰ . تم ،٤ ۲ اسمنت لرممو ونسبة ماء للاسمنت ٦ : تم تحضیر مونة من الاسمنت مکونة من ۱
حفظ العینات المحضرة فی محمول کبریتات المغنسیوم ترکیز ٥ % وتخزینیا عند درجة ح ا ررة ٧
لمدة ۰۰ یوم. اجریت اختبا ا رت التکسیر لمعینات فی ازمنة مختمفة ومعاینة العینات بالفحص
البصری واج ا رء الاختبار عمییا بستخدام الاشعة السینیة.
اثبتت الد ا رسة نجاح الباریوم ىیدروکسید فی منع تکوین ممح التومازیت بینما العینات
التی استخدم فییا المادة المخمبیة تضررت تماما.
Egypt. J. of Appl. Sci., 35 (5) 2020 102