AASCIT Journal of Materials

2018; 4(1): 6-13

http://www.aascit.org/journal/materials

ISSN: 2472-9736 (Print); ISSN: 2472-9752 (Online)

Keywords Repair Mortar,

Acid Attack,

Pozzolan,

Bond Strength

Received: December 27, 2017

Accepted: January 16, 2018

Published: January 29, 2018

Select a Suitable Repair Mortar for Concrete Segments Damaged by Acidic Attack

Erfan Riahi1, Alireza Joshaghani

2

1Civil Engineering Department, Amirkabir University of Technology, Tehran, Iran 2Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas, USA

Email address joshaghani@tamu.edu (A. Joshaghani)

Citation Erfan Riahi, Alireza Joshaghani. Select a Suitable Repair Mortar for Concrete Segments Damaged

by Acidic Attack. AASCIT Journal of Materials. Vol. 4, No. 1, 2018, pp. 6-13.

Abstract The existence of hydrogen sulfide gas in the natural environment produces sulfuric acid,

which causes rapid destruction in concrete. The aim of this study is to propose potential

resistant repair mortars to protect against the sulfuric acid attack. Three types of cement

(Type 1-Delijan, Sepahan slag and Kurdistan) with three types of pozzolan (micro silica,

trass and pumice) were used to make ten mix designs. Capillary and water absorption,

strength reduction and weight loss tests in an acidic environment were also conducted.

Finally, the most resistant mix was selected based on the parameters of the repair

materials, adhesive shear strength and pull-off tests. The sample that contained micro

silica and trass performed better and was proposed as a repair mortar against sulfuric

acid attack.

1. Introduction

Chemical attacks affect the durability of concrete significantly, which determines the

expected service life of concrete members [1]. Structures degraded by acid attacks have

caused excessive maintenance costs. The existence of a suitable repair mortar can

increase the life of the structure and reduce the amount of acid attack [2]. Hydrogen

sulfide, which is oxidized to sulfuric acid on the concrete surface, is formed under

anaerobic conditions. The sulfuric acid reacts with the alkaline components of the

concrete to form gypsum that has a little structural strength [3]. The increasing

temperature results in increased rates of oxygen consumption, which leads to anaerobic

conditions and enhances the rate of hydrogen sulfide formation [4]. Ying-fang et al. [5]

reported that the acid solution creates a more porous concrete microstructure, and the

average porosity in the concrete specimen increased with the conditioning age linearly.

Bemdt et al. [6] believed that replacing cement with 5-10% micro silica increased

concrete resistance against acidic attack. Idriss et al. [7] subjected mortar specimens to a

H2S solution for a year and concluded that 8% cement replacement by micro silica and

water to cement reduction led to suitable resistance against H2S. However, Kawai et al. [8]

reported that an increase in water to cement ratio improved the resistance against sulfuric

acid. Sersale et al. [9] also reached the same results against acidic rain. Bakharev et al. [10]

investigated the acidic resistance of concrete made by alkali-activated slag. Cylinder

specimens were subjected to acetic acid (pH=4) for a year, and meanwhile compressive

strength and pH variations were monitored. Results indicated the satisfactory performance

of alkali-activated slag specimens against the acidic solution.

AASCIT Journal of Materials 2018; 4(1): 6-13 7

Momayez et al. [11] tested both repair mortars composed

of the different percentages of micro silica and mixes with

10% cement replacement with K100 polymer glue and 20%

SBR polymer. They observed that micro silica improved the

bond strength by 25%; however, this strength was lower

compared to the main sample. The optimum cement

replacements for K100 and SBR were 7%. Parhizgar et al.

[12] also reported that adding micro silica and SBR polymer

to cement based repair mortars improved the moment

resistance and the modulus of elasticity, and it also reduced

water permeability. Abbasnia et al. [13] used the slant shear

test and reported that SBR polymer admixture reduced

compressive and tensile strength, and it generally reduced the

bond strength. They reported that micro silica led to a little

shrinkage in a repair mortar, but had better performance than

the control and SBR samples. Sadeghi et al. [14] prepared

seventeen mixture designs as repair mortars. They reported

that adding nano silica by 4.5% cement replacement led to

increased compressive strength and bond strength. On the

other hand, adding zeolite by 9% replacement generally

indicated more compressive strength and more bond strength

than the control sample. Moodi et al. [15] investigated the

probability of an acid attack on the inner walls of the water

transfer tunnel. They prepared a sulfate ion profile for cores

excavated and confirmed that an acid attack occurred due to

the conversion of H2S to H2SO4. In this research, mortars

were prepared, and the experiment was carried out to select

the appropriate repair mortar to use in the destructive area,

such as Nosoud tunnel.

2. Materials and Methods

In order to repair damaged segments with the acidic attack,

repair mortars were prepared with three types of cement:

Type I-Delijan cement (D), Sepahan slag cement (S) and

Type II- Kurdistan cement (K). Also, micro silica (MS), trass

(T) and pumice (P) were used as cement replacement

materials. Cement material properties are shown in Table 1.

The physical properties of aggregates were displayed in

Table 2.

Table 1. Material properties.

Delijan Kurdistan Sepahan slag cement Micro silica Trass Pumice

Oxide

SiO2 23.12 22.28 27.32 87.5 67.82 64.9

Al2O3 3.56 4.72 6.08 0.5 14.14 12.1

Fe2O3 3.29 2.75 2.12 1.53 2.96 5.2

CaO 63.07 64.12 55.34 1.27 3.36 7.4

Na2O 0.19 0.28 0.36 0.36 4.3 2.49

K2O 0.66 0.76 0.76 1.14 2.5 1.88

MgO 1.31 1.23 4.21 1.01 1.6 1.98

TiO2 0.146 0.156 0.58 0.02 - 0.79

MnO 0.091 0.112 0.512 0.086 - 0.123

P2O5 0.224 0.263 0.201 0.13 - 0.2

SO3 1.638 1.973 2.355 0.46 - -

L.O.I (%) 2.37 1.12 0.02 5.92 7.18 2.5

Density (gr/cm3) 3.06 3.02 3.07 2.14 2.34 2.54

Fineness (cm2/gr) 2908 3035 3120 650000 4100 5074

Table 2. Aggregates properties.

Fineness modules Density (Kg/m3) Water absorption (%SSD)

3.36 2550 2.67

Ten mix designs were prepared with three types of cement

and three cement replacement materials as shown in Table 3.

The cement was made with Delijan cement as a control

sample with which to compare the other mixture designs’

performance. The replacement content of MS was 5% with a

fixed ratio. However, 10% and 8% were employed as

replacement percentages for trass and pumice, respectively.

The water-to-binder ratio was 0.36, and workability was

fixed between 120-130 mm by adding water reducer

admixture in the flow table test. For evaluation of the bond

strength, designed mortars were implemented in the concrete

with a mix design as shown in Table 4.

Table 3. Mix designs.

Mix designs Sand Cement Water Micro silica Trass Pumice

1 OPC 1781 400 144 0 0 0

2 M.D.5%MS 1774 380 144 20 0 0

3 M.K.5%MS 1774 380 144 20 0 0

4 M.S.5%MS 1774 380 144 20 0 0

5 M.D.5%MS.8%T 1767 348 144 20 32 0

6 M.K.5%MS.8%T 1767 348 144 20 32 0

7 M.S.5%MS.8%T 1767 348 144 20 32 0

8 M.D.5%MS.10%P 1768 340 144 20 0 40

9 M.K.5%MS.10%P 1768 340 144 20 0 40

10 M.S.5%MS.10%P 1768 340 144 20 0 40

8 Erfan Riahi and Alireza Joshaghani: Select a Suitable Repair Mortar for Concrete Segments Damaged by Acidic Attack

Table 4. Mix design of based concrete (kg/m3).

Material content (Kg) Coarse aggregates Fine aggregates Cement Water Water Reducer

Based concrete 693 1073 450 137.4 2.7

In preparation of samples for the bond strength test, a

piece of polystyrene was embedded into the molds, and then

based concrete was cast. In order to have better bonding, the

surface of the based concrete was ridged until aggregates

appeared. In this research, durability tests were carried out on

repair mortars to determine what the optimal mix designs

were (phase 1). Then, bond tests were conducted on selected

samples to distinguish the mixtures that performed better

than others (phase 2). In the first section of the experimental

plan, the compressive strength, the water absorption, the

capillary absorption, the shrinkage, the weight loss and the

strength reduction were conducted. In the next phase, pull-

off, slant shear and bi-surface shear tests were carried out.

A capillary absorption test was performed based on ASTM

C1585. Cubic specimens were kept in the oven with 50°C

heat for 14 days. After that, specimens were weighed and

then were put into 5 mm of water. After 3, 6, 24 and 72

hours, the weight of specimens was measured again to

determine the water content. 10cm cubic water absorption

specimens were also kept in the oven with 105°C heat for 72

hours. After that, specimens were weighted and then

submerged in water for 30 minutes. Weight loss in an acidic

solution was used to determine the resistance of 10 cm cubic

specimens against sulfuric acid (pH=1). The acid solution

was circulated due to the pH being constant all around the

tank.

The 10 cm cubic specimens for the strength reduction test

were submerged in an acidic solution (pH=1). In order to

evaluate the strength reduction in the presence of an acid

attack, the compressive strength was measured and compared

with results before subjecting to an acidic solution (28 days

results). According to BS6319, In 10cm×10cm×20cm

specimens for the slant shear test, a failure surface (at an

angle of 30 degrees) between old and new material was

subjected to both compressive and shear stress (see Figure 1

(a)). For the bi-surface shear test, initially,

15cm×15cm×10cm specimens were made as old concrete.

Then 15cm×15cm×5cm a repair mortar was inserted near the

old concrete. Compressive loading was carried out as Figure

1 (b) shows.

Figure 1. (a) The Slant shear test specimen, (b) Bi-surface test specimen.

On the Pull-off test, a core excavated from a 15 cm cubic

sample consisted of repair mortar and old concrete. As shown

in Figure 2, the bond strength was assessed based on the level

of the failed surface.

Figure 2. (a) Schematic of the Pull-off test, (b) Cut from boundary surface, (c) Cut from based concrete.

The difference between shrinkage of old concrete and

repair mortar led to a shrinkage strain on the boundary

surface. Therefore, 2.5cm×2.5cm×28.5cm specimens were

used for measurement of repair mortar shrinkage.

AASCIT Journal of Materials 2018; 4(1): 6-13 9

3. Results and Discussion

3.1. Compressive Strength

Results were obtained for an average of three samples after

7, 28, 90, 180 and 210 days of curing in lime water, as shown

in Figure 3. According to Figure 3, using MS increased the

compressive strength. Regarding trass and pumice, these two

pozzolans reduced the compressive strength at the earlier

time; however, the later time reduction was negligible. The

slag cement (S) indicated weaker results in comparison to

other mixes due to pozzolanic reactions.

Figure 3. Compressive strength test results.

3.2. Capillary Absorption

Results for an average of three samples at the ages of 28, 90,

180 and 210 days are shown in Figure 4. Specimens that

contained pozzolan had less capillary absorption in comparison

to the control sample, and this might be due to pozzolanic

reactions and making secondary silicate gels that converted little

spaces to capillary voids and created a discontinuity between

them. Results showed that MS individually reduced capillary

absorption in earlier times. However, the rate of this reduction in

Delijan/Kurdistan cement mixes reduced gradually. The

composition of trass/pumice and MS in slag cement mixes

performed better in later times. This performance was for a

higher reaction rate of MS in earlier times, which decreased

significantly with time and in the following was continued by

the next pozzolan (trass/pumice) in later times and finally

reduced capillary absorption.

Figure 4. Capillary absorption test results.

3.3. Water Absorption

Results at ages of 28, 90, 180 and 210 days were obtained

as shown in Figure 5. Pozzolanic specimens had less

absorption, like the previous section. Less water absorption

indicated that specimens containing pozzolan would perform

well against corrosive ions in the aspect of durability. Here,

MS reduced water absorption in earlier/later times. Slag

cement mixes like the previous section, devoted less

absorption in later times in comparison to other mixes.

10 Erfan Riahi and Alireza Joshaghani: Select a Suitable Repair Mortar for Concrete Segments Damaged by Acidic Attack

Figure 5. Water absorption test results.

3.4. Weight Loss

Results were obtained at 7, 14, 28, 56, 90, 120, 180, 210

and 240 days of submerging to acid exposure after 28 days

curing. The weights of specimens after submerging were

compared with their weight before subjecting to acid attack

in order to get the weight loss, as shown in Figure 6. Most of

the specimens had the same weight loss until 56 days of

submerging. After that, slag cement specimens had less

weight loss, and the reason was attributed to the high

resistance of slag against loss of cement bonding in the

presence of an acid attack. In slag cement specimens, the

composition of MS- trass/pumice pozzolans performed better

than using MS singly. The slope of the diagram up to a

month exposure was approximately equal to the slope for

more than three months, and the slope between the ages of

one to three months exposure was greater than other ages.

Figure 6. Weight loss test results caused by acid attack.

3.5. Strength Reduction

Results were obtained at 7, 28, 90, 180 and 210 days of

subjecting to acid attack after 28 days curing (see Figure 7).

For better comparison, the compressive strength of

specimens was divided on primary compressive strength

before subjecting to acid attack and strength reduction

measured. The strength reduction in MS samples was more

than the control. After 180 days submerging, pumice

specimens experienced less strength reduction in earlier

times in comparison to other specimens. Slag cement

specimens generally experienced less strength reduction over

time. The composition of MS and pumice in slag cement

performed better than other samples.

AASCIT Journal of Materials 2018; 4(1): 6-13 11

Figure 7. Compressive strength test results caused by acid attack.

3.6. Slant Shear

According to experiments conducted for the evaluation of

mortar performance against acid attack, as slag cement

specimens generally performed better than other specimens,

these mixes made to investigate the phase 2 of experiments.

Table 5 displays the 28 days compressive strength of new

slag cement specimens and concrete. These results were not

much different from the results of the first phase.

Table 5. The 28 days compressive strength of selected specimens.

Selected mixes Compressive strength (MPa)

1 RM.S.5%MS 52

2 RM.S.5%MS.8%T 61

3 RM.S.5%MS.10%P 47

4 Based concrete 67

Results were obtained for an average of three samples at

the age of 7, 28 and 90 days by dividing failure load on a

boundary surface. According to Figure 8, usage of MS and

the composition of micro silica-trass performed better than

micro silica-pumice specimens. Trends showed that more

than 90% of final strength was obtained after 28 days.

Figure 8. Slant shear test results of selected specimens.

3.7. Bi-surface Shear

The bi-surface shear test was conducted at the ages of 7,

28 and 90 days by dividing failure load on a boundary

surface. According to Figure 9, adding trass/pumice to MS

slag cement specimens reduced bond strength in the earlier

ages. This reduction was compensated in the later ages in

samples containing trass.

Figure 9. Bi-surface shear test results of selected specimens.

3.8. Pull-off

Results were obtained at the ages of 7, 28 and 90 days by

dividing failure load on connecting the surface of repair

mortar to based concrete (circle with a diameter of 6.9 cm).

As shown in Figure 10, specimens containing MS

individually and consisted of MS- trass, performed better

than micro silica-pumice specimens in the whole of the time.

Usage of trass leads to increase the tensile bond strength.

Figure 10. Pull-off test results of selected specimens.

12 Erfan Riahi and Alireza Joshaghani: Select a Suitable Repair Mortar for Concrete Segments Damaged by Acidic Attack

3.9. Shrinkage

Shrinkage results were obtained every 7 days until the age

of 56 days (see Figure 11). Specimens containing micro

silica-pumice had more shrinkage in comparison to other

specimens. Here, the composition of micro silica-trass

performed better than MS.

Figure 11. Shrinkage test results of selected specimens.

4. Discussion

Adding pozzolans to cement paste decreased C3A content

and produced secondary silicate gel with consumption of

Ca(OH)2. Although the rate of hydration development in

trass/pumice pozzolan specimens was generally lower than

the control samples, in the later ages, the development of this

reaction and promoted interfacial transition zone,

compensated strength reduction and other durability

parameters were observed. However, it should be noted that

the performance of specimens is important in both earlier and

later times; failure to meet minimum mechanical and

durability specifications in earlier times causes a bad

performance in the whole service life of repaired concrete. In

this research, almost all specimens’ results were acceptable

in earlier times. MS was used in all mixes except for OPC, so

specimens containing MS were used as a control to

investigate the effect of adding trass or pumice to mixes.

In this investigation, the compressive strength was

important in three aspects; at first, the inherent mechanical

properties of specimens were studied by compressive

strength test, but the performance of specimens against acid

attack would be different. Consequently, a compressive

strength test was carried out on the specimens subjected to

the acid solution. Eventually, the ratio of strength reduction

was selected to evaluate the specimens’ performance. This

parameter could be useful in the evaluation of specimens’

performance with time against the corrosive environment, but

compressive strength content of every specimen shouldn’t be

forgotten. A specimen had high strength reduction would be

excluded in the evaluation, while its primary compressive

strength was high enough that could be compensated a large

percentage of compressive reduction against acid attack. For

example, MS specimens singly devoted high compressive

strength before being subjected to acid attack; these

specimens also had a high strength reduction. Therefore, this

high primary compressive strength could be relatively

enough for the resistance against acid attack, so both primary

compressive strength and strength reduction should be

considered in the evaluation.

In phase 1 tests, pumice in slag cement generally

performed better than the other samples. On the other hand,

trass generally reduced workability of mortar that needed

water reducer to supply workability. Trass also needed time

to react, so results in earlier times usually were slightly lower

than the control sample. In later times, the performance of

specimens containing trass would be better than the control

sample or the same as its performance. Slag cement

specimens performed well in comparison to other cement

types. The C3A reduction in slag cement was more than other

samples. As repair mortars should have sufficient bond

strength, determination of this parameter is important. Slant

shear and bi-surface tests results were very similar to each

other. These two bond tests with pull-off test ensured that

composition of trass and MS in slag cement would be

appropriate as a repair mortar.

5. Conclusion

From the results of this investigation, the following

conclusions can be drawn:

MS led to promote the performance of mortar generally in

all aspects of durability and mechanical properties. Since MS

generally had a high rate of reaction in earlier times,

pozzolanic reaction continued at a lower rate over time.

Adding another pozzolan to MS samples could promote the

performance in later times. Trass and pumice were generally

effective in this respect.

Usage of pozzolans reduced mortar permeability and

gradually improved mortar performance against acid attack.

Producing secondary silicate gel and making discontinuity

between capillary voids might be the causes.

AASCIT Journal of Materials 2018; 4(1): 6-13 13

Slag cement mixes generally performed better than other

cement types, especially in later times. High capability of

slag pozzolan could be beneficial to filling the pores and

making discontinuity between them. Acceptable compressive

strength, low absorption, low strength reduction and low

weight loss led to select these mixes as alternatives to repair

mortar.

Phase 2 experiments indicated that composition of slag

cement with MS and trass performed better than other slag

cement mixes. More bond strength in slant shear, bi-surface,

and pull-off tests and also less shrinkage show that this

composition will be an appropriate repair mortar against

acidic environments.

References

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