Short and Long-Term Effect of Sodium Chloride on Strength ... · Draft 1 Short- and Long-Term...

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Short and Long-Term Effect of Sodium Chloride on Strength and Durability of Coal Fly Ash Stabilised with Carbide Lime

Journal: Canadian Geotechnical Journal

Manuscript ID cgj-2018-0696.R2

Manuscript Type: Article

Date Submitted by the Author: 08-Feb-2019

Complete List of Authors: Consoli, Nilo Cesar; Universidade Federal di Rio Grande do Sul, Saldanha, Rodrigo; Universidade Federal di Rio Grande do SulScheuermann Filho, Hugo; Universidade Federal di Rio Grande do Sul

Keyword: environment-friendly product, short and long-term behavior, reuse of industrial wastes, sodium chloride, strength and durability

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Canadian Geotechnical Journal

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Short- and Long-Term Effect of Sodium Chloride on Strength and Durability of Coal Fly Ash Stabilised

with Carbide Lime

Nilo Cesar Consoli, Ph.D.1; Rodrigo Beck Saldanha, Ph.D.2; and Hugo Carlos

Scheuermann Filho, M.Sc.3

ABSTRACT: The present research aims to quantify the influence of the curing period

(t), dry unit weight (γd), lime amount (L), and the addition of sodium chloride (NaCl) on

the short and long-term behaviour of coal fly ash-carbide lime blends. Strength and wet-

dry durability tests were carried out for differing values of η (porosity), L, and t (curing

time) on mixtures that either contained small amounts of NaCl or contained none.

Addition of NaCl to the mixtures resulted in significant increase in early strength gain

when compared to specimens without NaCl, which demand longer curing periods to

reach similar strength values. The addition of NaCl to coal fly ash-carbide lime blends

reduced the accumulated loss of mass (ALM) after 12 wet-dry-brushing cycles for

specimens at early stages of curing (about 50% for 7 days). Equivalence in the

unconfined compressive strength (qu) and ALM(after 12 wet-dry cycles) between the specimens

with and without NaCl is achieved in the long term. Finally, a variance analysis

performed regarding qu and ALM results yielded that the order of importance of the

controllable factors changed from t, NaCl addition, γd and L for strength to γd, t, NaCl

addition, and L for ALM(after 12 wet-dry cycles).

Keywords: short and long-term behavior; coal fly ash; carbide lime; sodium chloride; durability; strength;

environment-friendly product.

1 Professor of Civil Engineering, Graduate Program in Civil Engng., Universidade Federal do Rio Grande do Sul, Brazil. E-mail: [email protected]

2 Postdoc Fellow, Graduate Program in Civil Engng., Universidade Federal do Rio Grande do Sul, Brazil. E-mail: [email protected]

3 Ph.D. candidate, Graduate Program in Civil Engng., Universidade Federal do Rio Grande do Sul, Brazil. E-mail: [email protected]

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1. INTRODUCTION

The construction industry is a significant exploiter of natural resources and contributes

notably to environmental impacts (Spence and Mulligan 1995). In addition, those

resources are finite and, owing to their unrestricted exploitation, the needs of future

generations are being compromised. Hence, there is a prominent necessity to substitute

the standard materials and techniques with environment friendly alternatives that

present satisfactory mechanical performance.

In this sense, fly ash (FA), originated as a residue of coal combustion at thermal power

plants, possesses characteristics (e.g. chemical composition and mineralogical structure)

that enables it to be used in the building industry for distinct purposes (Wang and Wu

2006). In the same context, carbide lime (CL), also termed calcium carbide residue, is a

by-product of acetylene gas production and due to its composition (mainly calcium

hydroxide) can be adequately employed as an alkaline activator in the stabilization of

soils and/or pozzolanic materials. Therefore, carbide lime can alkali-activate the coal fly

ash and generate cementitious hydration compounds that can be useful for civil

engineering like pavement, bricks, soil stabilization and monolithic walls (Beeghly

2003; Consoli et al. 2014a; Saldanha et al. 2017; Jiang et al. 2016; Phummiphan et al.

2017, 2018; Mohammadinia et al. 2017).

The pozzolanic reactions are characterized by the interaction between the calcium

hydroxide and the active phase of the pozzolan, which is capable of combining the lime

in a hydrated environment and originate binding compounds (Massazza 1998). In the

FA-CL system, the hydroxyls (OH-) attack the vitreous phase of the FA (mainly

composed of Al2O3 and SiO2) through the breakage of the O-Al and O-Si bonds. This

implies increased concentration of SiO3-2 and AlO2

- ions, which dissolve and react with

lime forming poorly crystalline cementitious hydrating products such as calcium silicate

hydrate (C-S-H), calcium aluminate silicate hydrate (C-A-S-H), among others (Ingles

and Metcalf 1972; Berry et al. 1994; Fu et al. 2002; Malek et al. 2005; Singh and Pani

2014). Nevertheless, the rate of development of pozzolanic reactions in such materials

can be considerably slow, which implies the need of long curing periods in order to

attain adequate mechanical features. This is one of the main reasons for the

underutilization of these kinds of materials in favor of the usage of Portland cement by

the standard construction industry, in spite of the environmental issues attached to its

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use (Cristelo et al. 2015; Fernández-Jiménez 2017). In order to counteract this tendency,

we can attempt to employ stronger alkaline activators (e.g. KOH and NaOH) and/or

combine alkaline activators with chemical additives (e.g. NaCl, Na2SO4 and K2CO2)

capable of catalyzing the pozzolanic reactions and accelerating strength development in

the mixtures. Consequently, the performance requirements would be attained sooner

when compared with regular blends (without additives). For the analysis of the

mechanical behavior of cemented materials Du et al. (2014) determines some

microstructural investigations to characterize the product of cementation, such as: X-ray

diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion

porosimetry (MIP). Other tests such as thermogravimetry aid in the identification of

hydrated compounds, as well as in the verification of the influence of addition of

additives/accelerators on cementation processes (Kishar et al. 2013).

Regarding the usage of chemical additions, Davidson et al. (1960) attested that the

addition of small amounts of sodium chloride (NaCl) resulted in strength gain in a soil

stabilized with lime and fly ash. In this sense, Mateos (1961) and Narendra et al. (2003)

found similar trends, respectively, in soils from Iowa and India, both stabilized with fly

ash and lime. Recently, Saldanha et al. (2017) assessed the effect of the addition of

NaCl in the durability of a fly ash – carbide lime blends and attested a significant

improvement in performance due to this addition for 7 and 28 days. Nevertheless, little

is known about the long-term performance concerning fly ash-carbide lime blends with

NaCl addition. The great advantage of using sodium chloride instead of sodium silicate

or sodium hydroxide is its great availability, reduced market price and reduced amount

necessary to accelerate pozzolanic reactions (about 1%) (Saldanha et al. 2017).

Otherwise, this accelerator has elements that corrode some metals, so its application

should be restricted to structures without steel (e.g. Apostolopoulos et al. 2013).

Regardless of the influence caused by the employment of additions such as NaCl, it is

essential to assess other parameters that control the mechanical response of stabilized

materials such as lime content (L), porosity (η) and curing time (t). Hence, it is possible

to correlate them with the mechanical response of the material, which can be expressed

in terms of unconfined compressive strength (UCS) and durability (Haque et al. 2014;

Dash and Hussain 2015; Consoli et al. 2018a). Those tests are commonly engaged in

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order to quantify and qualify the influence of the key-parameters that control the

response of the stabilized specimens due to its simplicity and effectiveness.

Accordingly, the present research intends to assess the mechanical behavior (unconfined

compressive strength and durability) of fly ash stabilized with carbide lime and sodium

chloride through wet-dry cycles considering a long curing period range. The evaluated

controllable factors are the curing period, the porosity, the amount of carbide lime and

the sodium chloride content. Furthermore, the formation of binding compounds and the

calcium hydroxide consumption are evaluated along the curing periods.

2. MATERIALS AND METHODS

2.1 Materials

The coal fly ash studied herein comes from southern Brazil, which is the main region of

mineral coal exploitation and thermal electricity production in Brazil, generating an

amount of 3 to 4 million tons/year of coal ash. The coal encountered in this zone is sub-

bituminous with a calorific value between 4200 to 2600 kcal/kg and its ash content

ranges between 42% and 59%. Thus, this coal possesses a moderate heating value and

generates a large amount of ash as a residue which is classified as type F according to

ASTM C618 (ASTM 2017). X-ray fluorescence (XRF) tests revealed that the studied

ash is mainly composed of silicon (64.83%), aluminum (20.41%) and iron (4.83%),

which are important elements in the development of the pozzolanic reactions. The

vitreous fraction of coal fly ash encountered in southern Brazil varies between 50% and

70% (Gobbo et al. 2007) with mineral phases corresponding to hematite, quartz and

mullite (Consoli et al. 2017). Table 1 presents more details regarding the physical

characteristics of the FA.

The calcium carbide lime [Ca(OH)2] is a by-product of the acetylene gas production and

is generated nearby the fly ash generation site (~ 30 km). In Brazil, the carbide lime

production is about 12 million tons/year. As stated by Saldanha et al. (2018), this

residue presents favorable characteristics in order to be used in soil and/or residues

stabilization processes, and CL has no environmental impact for heavy metals. The

XRF tests attested that this lime is mainly composed by calcium oxide (70.0%) and

silicon oxide (3.1%). The CL is generated as aqueous slurry, and was previously dried

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in an oven at 60°C for 48 hours until the moisture content reached 0.50%. This lime was

utilized in all of the tests.

The sodium chloride is manufactured by a specialized company, its analytic pureness is

of 99 % and its specific gravity is 2.17. Distilled water was used in all the steps of the

present research.

2.2 Methods

In order to determine the minimum amount of lime, the Initial Consumption of Lime

method (ICL), proposed by Rogers et al. (1997), was employed. This one consists in

measuring the pH variation of the mixture due to the addition of lime and check the

minimum content responsible for the pH stabilization. Through this procedure it was

established that a minimum content equal to 5 % of carbide lime was needed to create a

favorable environment to the occurrence of the alkali-activation processes. Thus, the CL

contents used herein were 5%, 8% and 11%.

The dry unit weights (γd) of the coal fly ash-lime blends were defined based on standard

energy Proctor tests (ASTM 2012a) and on modified energy Proctor tests (ASTM

2012b) (see Figure 1). Through the results (maximum dry unit weight and optimum

moisture content) of three tests the line of optimum (see Figure 1) for distinct energies

was defined and three maximum distinct dry unit weights and their respective optimum

moisture contents were defined as P1: 11.2 kN/m³ (29.8%), P2: 12.2 kN/m³ (25.7 %)

and P3: 13.2 kN/m³ (21.6 %).

The definition of the amount of NaCl in the mixtures were based on pilot UCS tests

with FA-CL blends presenting 12.2 kN/m³ (25.7% of moisture) and 8% of CL which

were cured along 28 days at 23°C. The specimens were molded in triplicates and three

distinct NaCl contents were tested (0.5, 1.0 and 1.5%). The pilot tests revealed that a

content of sodium chloride equal to 1.0% was responsible for the maximum strength

gain, which is in accordance to previous researches (Saldanha et al. 2016; Ramesh et al.

1999). Hence, the specimens were tested without NaCl (0%) and with 1.0% of NaCl in

respect to the sum of fly ash and carbide lime dry masses.

Intending to assess the mechanical behavior of the studied blend for long curing

periods, unconfined compressive strength tests (ASTM 2009) and durability through

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wet-dry tests (ASTM 2015) were carried out. This standard (ASTM D559) determines

the loss of mass (brushing process), volume and moisture variations of soil cement

specimens during 12 cycles of wetting and drying. For the former (UCS), curing periods

equal to 7, 14, 28, 84, 168 and 360 days were selected, while for the latter 7, 14, 28, 84

and 168 days were established as curing times.

X-Ray diffraction tests (0.05° step size, 1s for scan step time and 5°<2θ<75° range),

thermogravimetry tests (TGA) (10 °C/min from room temperature up to 1000°C) and

scanning electron microscopy tests (SEM) (Microscope Model: XL 30 – Philips) were

performed in the cured blends aiming to investigate the mineralogical phases created

during the cementing reactions for samples with intermediate parameters: 12.2 kN/m³,

8% CL, and with/without NaCl. The samples used for the mineralogy tests were

obtained from the UCS test specimens after being taken to failure, considering all the

studied curing times.

A complete factorial design was accomplished with the chosen factors (and their levels),

which allowed the usage of a statistical analysis of variance (ANOVA). Therefore, it

was possible to determine the statistical significance of the main factors (and their

interactions) in altering the response concerning the unconfined compressive strength

and the durability of the studied blends. The analytical tool used was Minitab® which

allowed the analysis independent variables studied to a level of significance of 95%.

Table 2 summarizes the investigated treatments performed in each one of the tests.

2.2.1 Unconfined Compressive Strength

For the UCS tests, the procedure followed the recommendations stated by ASTM D

5102 (ASTM 2009). The molding processes included (i) the weighting, (ii) the mixture

and (iii) the static compaction of each specimen. When NaCl was used, it was

previously diluted in the water used for the mixture. The molding was statically done in

a cylindrical split mold and, after it was accomplished, each specimen was weighed,

measured and sealed in a plastic bag to be stored in a humid room with controlled

moisture (95%) and temperature (23°C). After the curing period was achieved, the

specimens were immersed underwater for 24 hours in order to minimize suction effects

(Consoli et al. 2011). The range of acceptance of the degree of compaction varied

between 99% and 101% and triplicates were done for each treatment. The UCS results

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were correlated to the adjusted porosity/volumetric lime content index (η/Lva). This

correlation enables the strength prediction of soils and residues stabilized with lime

and/or cement trough a single unique relationship (Consoli et al. 2009).

2.2.2 Durability Through Wet-Dry Cycles

The durability tests followed the recommendations described by the standard ASTM

D559 (ASTM 2015). Duplicates were molded for each treatment, one to be brushed

along the cycles and other in which the volumetric and moisture variations were

verified. After the curing period, the specimens were submitted to the cycles that

initiated by immersion underwater for 5 h, followed by oven drying at 71°C for 42 h

and then brushed with a force equal to 13.3 N (20 brushings along the circumference of

the specimen covering it twice and 4 brushings in the top and in the bottom). Those

procedures were repeated 12 times. As done for the UCS results, the durability results

(presented as accumulated loss of mass for each cycle) were correlated to the η/Lva

index. This allows the loss of mass prediction of the stabilized specimens through a

single unique relationship (Consoli et al. 2018b).

3. RESULTS AND DISCUSSIONS

3.1 Unconfined Compressive Strength

The unconfined compressive strength results were correlated to the adjusted

porosity/volumetric lime content index through a power equation in accordance to the

form presented below [Eq. (1)]. An internal exponent equal to 0.11 was employed in

order to adjust the Lv, which resulted in an external exponent of -3.0.

Eq. (1)𝑞𝑢(𝑀𝑃𝑎) = 𝐴𝑠[ 𝜂(𝐿𝑣)0.11] ―3.0

In this equation, qu corresponds to the unconfined compressive strength of the cemented

material (MPa), As is the scalar, Lv is the volumetric lime content which is expressed by

the ratio between the volume of lime and the total volume of the specimen (%); and η is

the porosity of the mixture (%).

The internal exponent (0.11) of Eq. (1) was chosen as it provides the best fit (higher

coefficient of determination) for the coal fly ash – carbide lime blends, considering its

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range varies from 0.01 to 1.0 (Consoli et al. 2014a 2016; Saldanha et al. 2017). The

external exponent (3.0) of Eq. (1) has seen to be around 3.0 for all previous studies of

distinct soil and/or coal fly ashes stabilized with different limes (Consoli et al. 2014b;

Saldanha et al. 2017). In the present study, it also showed excellent responses.

However, further studies still have to be carried out to develop a theoretical derivation

explaining the internal and external exponents of Eq. (1) for lime treated materials, as

the development described by Diambra et al. (2017) for soil-Portland cement blends.

Figure 2 shows the UCS results for distinct curing periods and also for the mixtures

with and without the addition of NaCl. Accordingly, within the experimental limits

tested herein, it is possible to predict the unconfined compressive strength of the

mixtures through the ratio between porosity and volumetric lime content for distinct

curing periods and with (or without) the addition of sodium chloride.

Table 3 presents the functions (Eqs. 2 to 13) for the specimens without and with the

addition of NaCl for the curing periods (7, 14, 28, 84, 168, and 365 days). For all curing

times studied, without NaCl and with 1.0% of NaCl addition, the unconfined

compressive strength related to [η/Lv0.11]-3.0 with satisfactory coefficients of

determination (R² > 0.93). Thus, it is observable that the strength behavior follows

similar trends for the mixtures with and without the addition of NaCl. It allows the

establishment of single relationships that correlate porosity and volumetric lime content

with the UCS (with 1.0 NaCl and without NaCl) by using distinct scalars depending on

the curing periods.

Correlations with time of curing are presented in Figure 3 and evidence the significant

early strength gain proportioned by the addition of sodium chloride in comparison with

the specimens without NaCl, which demand longer curing periods to reach similar

strength values. Moreover, it is possible to add the curing period (t) as a variable

through the normalization of the previous equations by dividing qu per [η/Lv0.11]-3.0.

These results are shown in the Eqs. (14) and (15), that can predict the strength of fly ash

– carbide lime blends without and with 1.0% NaCl, respectively. The former presents

significant strength gains until 84 days of curing, while in the latter strength increase is

observed until 168 days. After this period of 168 days the mechanical behavior is

similar for the mixtures with and without NaCl. As most chemical additives/accelerators

ionize in water by adding them to the cementation system, it is possible to change the

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type and concentration of the ionic constituents of the aqueous solution, thus

influencing the dissolution of the cementing compounds. In the present case, the

addition of Na+ and Cl- ions (in certain amounts) promotes influence on the

solubilization of calcium, silicates and aluminates (Joisel 1973). Consequently, favors

the formation of hydrated compounds (binders) at an early stage.

(R²=0.97) Eq. (14)𝑞𝑢 = [ ―5.2𝑥10 ―4(𝑡2) + 0.33(𝑡) ― 2.7]𝑥104 [ 𝜂(𝐿𝑣)0.11] ―3.0

(R²=0.93) Eq. (15)𝑞𝑢 = [11.21𝑥𝐿𝑛(𝑡𝑁𝑎𝐶𝑙) ― 11.33]𝑥104 [ 𝜂(𝐿𝑣)0.11] ―3.0

The variance analysis (Table 4) performed in the strength results yielded that the four

controllable factors were significant in altering the unconfined compressive strength.

Curing period (B), sodium chloride addition (D), dry unit weight (A) and amount of

carbide lime (C) were, in this order, the most significant in altering this response (F-

Value). Second-order interactions were also statistically significant, especially the

interactions between dry unit weight and curing time (AB), dry unit weight and NaCl

(AD) and the one between curing time and usage of NaCl (BD).

3.2 Mineralogy

The X-Ray patterns for the mixtures with and without NaCl are presented in Figure 4.

The main mineral phases identified herein are hematite, mullite and quartz, which are

present in the fly ash; portlandite and calcite, arising from the carbide lime; and a

mineral form of calcium silicate hydrate that forms due to the pozzolanic reactions in

both mixtures (with and without NaCl). In addition, for the mixtures with NaCl, peaks

relative to the calcium aluminum chlorohydrate (Ca2Al(OH)6Cl•2H2O) (e.g. ~11.3°,

22.5° and 23.5° 2Theta) were observed (Ma et al. 2015; Ogirigbo and Black 2017; Shao

et al. 2013). This mineral is formed due to the reaction of alumina (Al2O3r- reactive) in

conjunction with calcium and chlorine (Talero et al. 2011). Through the

thermogravimetric results (DTG and TGA) of the NaCl containing specimens (Figure 5)

it was possible to identify the degradation peak of calcium aluminum chlorohydrate

near 290 – 320°C and other peaks between 600 – 800°C relative to the volatilization of

the CO2 that occurs due to the degradation of the carbonates, decomposition of

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anhydrous groups and degradation of remaining hydroxyl groups (Shi et al. 2017;

Grishchenko et al. 2013; Lannegrand et al. 2001).

Regarding the cementitious compounds, it is observable in the XRD patterns that the

peak relative to the calcium silicate hydrate (29.4° 2Theta) grows as the curing period

increases for both mixtures (with and without NaCl). This is an indicative of the

formation of such compounds along the curing time. The same trend is also observable

through the DTG and TGA results for the temperature range between 30°C and 300°C

that corresponds to the degradation of aluminates and silicates hydrate binding

compounds (Rojas 2002).

For the mixtures containing NaCl, it is possible to attest a more remarkable presence of

hydrate cementitious binders at the early stages of curing. Analogously, the

consumption of calcium hydroxide, used in the pozzolanic reactions, is more

pronounced at this early phase when sodium chloride is added. This can be noticed by

the decreasing of the Ca(OH)2 peak in the DTG curve (between 350°C and 450°C)

which is relative to the dehydration of lime (Saldanha et al. 2017). The same Ca(OH)2

peak decay tendency is observed as the curing period increase, indicating the

consumption of lime in the pozzolanic reactions.

For the NaCl containing specimens this decrement stops at 84 days, while this happens

after 168 days for the ones without NaCl. This corroborates the stagnation in the

strength gains presented in Figure 3. Thus, the sodium chloride performs as a catalyzer

of the pozzolanic reactions and enables a faster consumption of carbide lime for the

formation of cementitious compounds. The same trend regarding the Ca(OH)2

consumption is also shown in the XRD patterns through the decay of the portlandite

peaks (e.g. ~ 18°, 29° and 34° 2Theta).

Figures 6(a) and 6(b) present the morphology (obtained through SEM tests) of the

mixtures cured along 28 days with and without the addition of NaCl, respectively. In

both images it is possible to observe spheres that corresponds to the fly ash. Besides,

precipitated binding compounds (C-S-H) are visible and form a net-like structure like

characterized in literature by Mehta and Monteiro (2006) and Hilal (2016). In the

present SEM test is possible to attests the net-like structure is more pronounced and

denser in the specimen containing NaCl [Fig. 6(b)].

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3.3 Durability

As previously showed for the strength tests, the results regarding the accumulated loss

of mass (ALM) after 12 wet-dry-brushing cycles were correlated to the

porosity/volumetric lime content index. This approach was already done by Consoli et

al. (2017 2018b) in the prediction of the ALM per cycle for specimens cured along 7

days. Nevertheless, this method is applied herein for distinct curing period and for

mixtures with and without NaCl.

Figure 7 presents the accumulated loss of mass results as a function of the adjusted

η/Lv0.11 (best fit) index for the mixtures with and without NaCl and cured along 7, 14,

28, 84 and 168 days. In general, it is observable that the addition of NaCl implied in

smaller mass losses, especially for short curing periods. However, both mixtures (with

and without NaCl) presented the same trend regarding the mass loss along the cycles,

which allowed the same model of equation [Eq. (16)] to represent their behavior along

the different curing periods.

Eq. (16)𝐴𝐿𝑀(%) = 𝐴𝑠[ 𝜂(𝐿𝑣)0.11]7.0

The Equation (16) is of the power type and is the general representation of the durability

tests results, being obtained as the best fit for those results at each curing period and two

sodium chloride contents (zero and 1%). The external exponent was adjusted as 7.0, the

internal was maintained equal to 0.11 (equal for the UCS equation) and the scalar (As) is

a function of the curing period and the presence of sodium chloride in the mixes. The

coefficients of determination (R2) obtained herein were greater than 0.94, which

demonstrates a satisfactory adjustment between this kind of approach and the obtained

results.

The equations for all results are shown in Table 5 [Eqs. (17) to (26)], and were obtained

for the blends without and with NaCl and cured for 7, 14, 28, 84 and 168 days. For all

curing time studied and without NaCl and with 1.0% of NaCl addition, the accumulated

loss of mass (ALM) related to [η/Lv0.11]-3.0 with high coefficients of determination (R² >

0.94).

In order to insert the curing period (t) as a variable in the prediction of the durability

performance (accumulated loss of mass) of the studied blends, the same normalization

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procedure as previously done for the strength tests was performed herein (Figure 8).

Thus, Equations (27) and (28) were obtained, respectively, for the mixtures without

NaCl and with NaCl. It is noticeable that the addition of sodium chloride improves the

performance in the early stages of curing. The equality in the performance between the

specimens (with and without sodium chloride) is attained at 84 days of curing. This

parity between the mixtures is achieved earlier than the observed for the strength results

due to the catalyzer effect generated by the temperature (71°C ± 2°C) at which the

specimens are submitted during the drying periods, which accelerates the pozzolanic

reactions.

(R²=0.99) Eq. (27)𝐴𝐿𝑀 (%) = [9.22 (𝑡) ―0.55][ 𝜂(𝐿𝑣)0.11]7.0

𝑥10 ―11

(R²=0.96) Eq. (28) 𝐴𝐿𝑀𝑁𝑎𝐶𝑙 (%) = [2.69 (𝑡𝑁𝑎𝐶𝑙) ―0.27][ 𝜂(𝐿𝑣)0.11]7.0

𝑥10 ―11

Table 6 shows the results of the statistical analysis (ANOVA) performed for the total

loss of mass results for all variables and their levels studied. In this case, all variables

are significant in the alteration of the response variable (ALM), being: density, time,

addition of NaCl, and carbide lime content the factors (from highest to lowest) (F-

Value). In this case, the sequence of the factors with capacity to change the durability

diverge from the RCS results and is attributed to the 12 wetting and drying cycles

required to simulate an accelerated degradation of the cemented material that influence

the pozzolanic reactions (Rao and Asha 2012).

4. CONCLUDING REMARKS

Based on the findings portrayed in this research, the following conclusions can be

drawn:

● Significant early strength gain (up to approximately 5 times for specimens

in the early stages of curing - 7 days) is proportioned to coal fly ash-

carbide lime blends by the addition of sodium chloride (NaCl) in

comparison with the specimens without NaCl, which demand longer

curing periods to reach similar strength values. The former presents

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significant strength gains until 84 days of curing, while in the latter

strength increase is observed until 168 days. After this period of 168 days

the mechanical behavior is similar for the mixtures with and without NaCl,

meaning that NaCl acts uniquely as a catalyzer;

● The addition of NaCl to coal fly ash-carbide lime blends reduces the

accumulated loss of mass (ALM) after 12 wet-dry-brushing cycles (about

50% for 7 days of curing) in the early stages of curing. Equality in the

performance between the specimens with and without sodium chloride is

achieved at 84 days of curing. This parity between the mixtures is reached

earlier than the observed for strength results due to the catalyzer effect

generated by the temperature (71°C ± 2°C) at which the specimens are

submitted during the drying periods, which accelerates the pozzolanic

reactions;

● Individual relationships qu, η and Lv versus t (see Fig. 2), as well as

ALM(after 12 wet-dry cycles), η and Lv versus t (see Fig. 7) were obtained for fly

ash-carbide lime and fly ash-carbide lime-NaCl;

● A variance analysis performed in the strength results yielded that the order

of importance of the controllable factors was (1) curing period, (2) sodium

chloride addition, (3) dry unit weight and (4) amount of carbide lime,

while the same analysis regarding loss of mass results yielded distinct

results: (1) dry unit weight, (2) curing period, (3) sodium chloride

addition, and (4) carbide lime content.

ACKNOWLEDGEMENTS

The authors wish to express their appreciation to Edital 12/2014 FAPERGS/CNPq –

PRONEX (project # 16/2551-0000469-2), CNPq (Edital Universal, INCT-REAGEO

and Produtividade em Pesquisa) and PROEX-CAPES for funding the research group.

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REFERENCES

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Table Captions

Table 1. Physical properties of coal fly ash and carbide lime.

Table 2. Investigated treatments.

Table 3. Equations for UCS without and with NaCl according to the curing

period.

Table 4. ANOVA for the unconfined compressive strength.

Table 5. Equations for ALM without and with NaCl according to the

curing period.

Table 6. ANOVA for the durability (wet and dry cycles).

Figure Captions

Figure 7. Compaction test results.

Figure 8. Variation of UCS with adjusted porosity/volumetric lime content

without NaCl (left) and with 1.0% NaCl (right).

Figure 9. Relationship for the variation of UCS with η, Lv, and time (t)

without NaCl and with 1.0% NaCl.

Figure 10. X-Ray diffraction: evolution of mineralogy at 7, 28, 84 and 168

days of curing without NaCl (left) and with 1.0% NaCl (right).

Figure 11. Thermogravimetric analysis for different curing times without

NaCl (left) and with 1.0% NaCl (right).

Figure 12. Scanning electron microscopy for sample with 28 of curing: (A)

without NaCl and (B) 1.0% NaCl.

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22

Figure 13. Variation of ALM with adjusted porosity/volumetric lime

content without NaCl (left) and with 1.0% NaCl (right).

Figure 14. Relationship for the variation of ALM with η, Lv, and time of

curing (t) without NaCl and with 1.0% NaCl.

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Table 1. Physical properties of coal fly ash and carbide lime.

Properties Method Fly Ash Carbide Lime

Specific gravity of grains ASTM D 854 2.15 2.19

Surface area (m²/g) ASTM D5604 3.20 22.6

Medium sand (0.2 - 0.6 mm) ASTM D 422 0.96% -

Fine sand (0.06 - 0.2 mm) 20.6% 11%

Silt (0.002 - 0.06 mm) 76.3% 82.9%

Clay (<0.002 mm) 2.1% 6.1%

Mean particle size (D50) 0.024 mm 0.021mm

Plastic index ASTM D 4318 Non-plastic Non-plastic

Plastic limit - -

Liquid limit - -

USCS classification ASTM D 2488 ML -

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Table 2. Investigated treatments.

Parameter UCS Durability

Curing period (days) 7, 14, 28, 84, 168 and 360 7, 14, 28, 84 and 168

Dry unit weight (kN/m³) 11.2, 12.2 and 13.2 11.2, 12.2 and 13.2

CL content (%) 5%, 8% and 11% 5%, 8% and 11%

NaCl content (%) 0 and 1 0 and 1

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Table 3. Equations for UCS without and with NaCl according to the curing period.

Days Equations for UCS (qu - MPa)

0% NaCl 1% NaCl

7 1.69𝑥104 [𝜂

(𝐿𝑣)0.11]

−3.0

Eq. (2) 8.10𝑥104 [𝜂

(𝐿𝑣)0.11

]−3.0

Eq. (8)

14 2.51𝑥104 [𝜂

(𝐿𝑣)0.11]

−3.0

Eq. (3) 16.16𝑥104 [𝜂

(𝐿𝑣)0.11

]−3.0

Eq. (9)

28 6.16𝑥104 [𝜂

(𝐿𝑣)0.11]

−3.0

Eq. (4) 29.68𝑥104 [𝜂

(𝐿𝑣)0.11

]−3.0

Eq. (10)

84 15.47𝑥104 [𝜂

(𝐿𝑣)0.11]

−3.0

Eq. (5) 42.25𝑥104 [𝜂

(𝐿𝑣)0.11

]−3.0

Eq. (11)

168 43.04𝑥104 [𝜂

(𝐿𝑣)0.11]

−3.0

Eq. (6) 47.41𝑥104 [𝜂

(𝐿𝑣)0.11

]−3.0

Eq. (12)

365 49.64𝑥104 [𝜂

(𝐿𝑣)0.11]

−3.0

Eq. (7) 50.16𝑥104 [𝜂

(𝐿𝑣)0.11

]−3.0

Eq. (13)

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Table 4. ANOVA for the unconfined compressive strength.

Source Degrees of

freedom

Sum of

squares

Mean

square F-Value P-Value

A 2 660.14 330.069 2164.91 0.000

B 5 4275.38 855.076 5608.40 0.000

C 2 105.46 52.728 345.84 0.000

D 1 685.34 685.340 4495.12 0.000

AB 10 252.05 25.205 165.32 0.000

AC 4 10.31 2.577 16.90 0.000

AD 2 56.37 28.187 184.88 0.000

BC 10 45.44 4.544 29.80 0.000

BD 5 347.69 69.538 456.10 0.000

CD 2 8.80 4.400 28.86 0.000

Error 280 42.69 0.152

Total 323 6489.66

A = Dry unit weight / B = Time / C = Carbide Lime / D = NaCl

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Table 5. Equations for ALM without and with NaCl according to the curing period.

Days Equations for ALM (%)

0% NaCl 1% NaCl

7 3.14𝑥10−11 [𝜂

(𝐿𝑣)0.11]

7.0

Eq. (17) 1.66𝑥10−11 [𝜂

(𝐿𝑣)0.11]

7.0

Eq. (22)

14 2.21x10−11 [η

(Lv)0.11]

7.0

Eq. (18) 1.28x10−11 [η

(Lv)0.11]

7.0

Eq. (23)

28 1.41x10−11 [η

(Lv)0.11]

7.0

Eq. (19) 1.01x10−11 [η

(Lv)0.11]

7.0

Eq. (24)

84 1.41x10−11 [η

(Lv)0.11]

7.0

Eq. (20) 0.84x10−11 [η

(Lv)0.11]

7.0

Eq. (25)

168 0.57𝑥10−11 [𝜂

(𝐿𝑣)0.11]

7.0

Eq. (21) 0.45x10−11 [η

(Lv)0.11]

7.0

Eq. (26)

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Table 6. ANOVA for the durability (wet and dry cycles).

Source DF Adj SS Adj MS F-Value P-Value

A 2 82.220 41.1099 249.80 0.000

B 4 59.277 14.8192 90.05 0.000

C 2 13.710 6.8551 41.65 0.000

D 1 14.777 14.7765 89.79 0.000

AB 8 20.791 2.5989 15.79 0.000

AC 4 4.748 1.1869 7.21 0.000

AD 2 6.940 3.4699 21.08 0.000

BC 8 3.993 0.4991 3.033 0.000

BD 4 11.986 2.9965 18.21 0.000

CD 2 1.290 0.6450 3.92 0.027

Error 280 42.69 0.152

Total 323 6489.66

A = Dry unit weight / B = Time / C = Carbide Lime / D = NaCl

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Figure 1 - Compaction test results.

18 20 22 24 26 28 30 32

Moisture (%)

11.0

11.2

11.4

11.6

11.8

12.0

12.2

12.4

12.6

12.8

13.0

13.2

13.4

P1

P2

P3

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Figure 2. Variation of UCS with adjusted porosity/volumetric content of CL without (left) and with 1.0% of NaCl.

q u(M

Pa)

q u(M

Pa)

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Figure 3. Relationship for the variation of UCS with η, Lv, and time (t) without and with 1% of NaCl.

0% NaCl - 23°C

1% NaCl - 23°C

0 50 100 150 200 250 300 350 40025 75 125 175 225 275 325 375

Time of curing - days (t)

1

10

100

2

3

4

5

6

789

20

30

40

50

60

708090

qu / 104( Lv0.11)-3.00= -5.2 x 10-4(t)2 + 0.33(t) - 2.7 (R²=0.97)

qu / 104( Lv0.11)-3.00= 11.21 x Ln(t) - 11.33 (R²=0.96)

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Figure 4. X-Ray diffraction: Evolution of mineralogy to 7, 28, 84 and 168 days of curing without (left) and with 1.0% of NaCl.

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75Position [°2 ]

Ca=Calcium aluminium chlorohydrate - M=Mullite - P=Calcium hydroxideQ=Quartz - C=Calcite - S=Calcium silicate hydrate - H=Hematite

7 days

28 days

84 days

168 days

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75Position [°2 ]

7 days

28 days

84 days

168 days

A=(Mono)Calcium aluminate hydrated - M=Mullite - P=Calcium hydroxideQ=Quartz - C=Calcite - S=Calcium silicate hydrate - H=Hematite

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Figure 5. Thermogravimetric analysis for different curing times with (left) and without NaCl.

0 100 200 300 400 500 600 700 800 900 1000Temperature (°C)

0

0.01

0.02

0.03

0.04

0.05

90

91

92

93

94

95

96

97

98

99

100

DTG - 168 days - 0% NaClDTG - 84 days - 0% NaClDTG - 28 days - 0% NaClDTG - 7 days - 0% NaCl

TG - 168 days - 0% NaClTG - 84 days - 0% NaClTG - 28 days - 0% NaClTG - 7 days - 0% NaCl

0 100 200 300 400 500 600 700 800 900 1000Temperature (°C)

0

0.01

0.02

0.03

0.04

0.05

90

91

92

93

94

95

96

97

98

99

100

DTG - 168 days - 1% NaClDTG - 84 days - 1% NaClDTG - 28 days - 1% NaClDTG - 7 days - 1% NaCl

TG - 168 days - 1% NaClTG - 84 days - 1% NaClTG - 28 days - 1% NaClTG - 7 days - 1% NaCl

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Figure 6. Scanning electron microscopy for sample with 28 of curing: (A) 0% NaCl and (B) 1% NaCl.

Binder

A B

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Figure 7. Variation of ALM with adjusted porosity/volumetric content of CL without (left) and with 1% of NaCl.

Acc

umul

ated

Los

s of M

ass (

%)

Acc

umul

ated

Los

s of M

ass (

%)

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Figure 8. Relationship for the variation of ALM with η, Lv, and time of curing (t) without NaCl and with 1.0% of NaCl.

ALM

(%)

10-1

1 (L v

0.11

)7.0

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