manufacturing unfired bricks using coal fly ash, calcium fluoride … · 2020. 5. 4. · figures 2...

International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 20 No: 02 1

202502-7474-IJCEE-IJENS © April 2020 IJENS I J E N S

Manufacturing Unfired Bricks using Coal Fly Ash,

Calcium Fluoride and Calcium Silicate Alpha Ousmane TOURE1, 2*, Falilou Mbacke SAMBE1, Codou Gueye Mar DIOP1, Rita Maria GHANTOUS2

1Cheikh Anta Diop University of Dakar, Ecole Superieure Polytechnique, Department of Chemical Engineering and Applied

Biology, Po Box 5085 Dakar-Fann, Senegal 2Oregon State University, College of Engineering, School of Civil and Construction Engineering, Corvallis, OR 97331, USA

*Corresponding Author: [email protected]; [email protected]

Abstract-- The manufacture of sustainable bricks using waste

materials is a topic of ongoing research in many countries.

Unfired bricks made from industrial waste materials provide

one promising approach. This study examined the potential to

manufacture bricks for use in housing applications that are

made using coal fly ash (CFA), calcium fluoride and calcium

silicate (CFS). Moreover, in practice, the CFS comes from the

reaction between calcium hydroxide and sodium fluosilicate.

The bricks described in this paper were made at atmospheric

pressure and room temperature. The materials were

characterized using X-Ray Fluorescence, X-Ray Diffraction and

Scanning Electron Microscopy. In addition, the bulk density,

the compressive strength and the water absorption were

measured and compared with common building bricks. The

compressive strength of the brick specimen containing 75% of

CFA at 28 day was 6.1 MPa and the water absorption was found

to be about 23.8%.

Index Term-- Calcium fluoride; calcium silicate; coal fly ash;

industrial waste; unfired brick.

1. INTRODUCTION

Cement, calcium silicate and clay have been the primar used

as raw materials that have been used to produce brick for

construction and building [1].

Cementitious bricks are typically made using ordinary Portland cement (OPC). The production of cement

contributes to CO2 emissions due to the combined effects of

combustion, calcination and transportation. It is estimated

that 0.9 tons of CO2 is produced to obtain 1 ton of clinker [2].

Many cement alternatives have historically been

manufactured, while clay bricks manufacturing dates back to

6000 B.C using primarily he soft mud process in which a relatively moist clay is pressed into simple rectangular molds

by hands. Modern bricks are produced using the manufacture

and the process which all require energy to fire the bricks [3].

Clay brick is generally regarded as the most common brick

material due to its high compressive strength and the well-

established manufacturing process that enables it to be taken

from the laboratory to industrial production [3].

Calcium Silicate (CS) bricks were developed over the past

decades and are useful construction and building materials

due to the higher compressive strength they provide. They are

made from high-grade sand and mixed with high calcium

hydroxide in presence of water [3].

Innovative alternatives have also been proposed. For

example, bricks containing Calcium Fluoride have been

manufactured using patented process developed in 1962 [4].

Since, fluorine might be harmful to health, it was concluded

that there is no increase health risk of exposure to bricks containing calcium fluoride as compared with common

bricks [5].

Many recent studies have shown the benefits of using alternatives materials to make bricks from industrial waste

products [6, 7]. Several studies examined the use of CFA in

production of bricks [8]. The preparation and the

characterization of lime activated unfired bricks made of

CFA were performed [9, 10, 11, 12].

It is well known that CFA, from coal power plant is very

useful for the production of building bricks [13]. The long-

term performance of compacted-clay industrial waste

materials by using CFA has been demonstrated [14] as well

as the manufacturing of unfired bricks using CFA, lime and

wood aggregates [15].

There is interest in using CFA in terms of resources

conservation, environmental protection and low-cost

building material [16, 17, 18, 19, 20].

The objective of this study is to manufacture unfired brick at

room temperature and atmospheric pressure by combining CFA and CFS which comes from the reaction between

calcium hydroxide and sodium fluosilicate.

2. MATERIALS AND METHODS This section discusses the physical and chemical properties

of the raw materials used in this study: coal fly ash (CFA),

calcium fluoride and calcium silicate (CFS). In addition, the

main steps of the manufacturing process of the bricks and the

characterization of the manufactured bricks are outlined.



2.1. Coal Fly Ash (CFA)

A Class F coal fly ash was used in this study with the physical

and chemical properties shown in Table 1 and 2, respectively.

Table I

Physical properties of the CFA as measured using ASTM C110

Properties Particle Size (μm) Moisture (wt%) Specific Gravity pH (20% Solids)

Value 6.61 0.27 2.60 12.64

Table II

Chemical composition of the CFA as determined using XRF

Compound CaO SiO2 Al2O3 Fe2O3 MgO K2O SO3 Mn2O3 ZnO Cl TiO2 Na2O P2O5 LOI

Content (%) 16.79 44.31 15.5 6.01 5.05 1.56 0.93 0.10 0.02 0.01 0.95 3.45 0.24 5.08

2.2. Mixture of Calcium Fluoride and Calcium Silicate (CFS)

The CFS was produced from the reaction between Sodium Fluosilicate (SF) 95% and Calcium Hydroxide (CH) 95% as shown in the following chemical reaction [21]:

𝑁𝑎2𝑆𝑖𝐹6 + 4𝐶𝑎(𝑂𝐻)2 → 2𝑁𝑎𝑂𝐻 + 3𝐶𝑎𝐹2 + 𝐶𝑎𝑆𝑖𝑂3 + 3𝐻2𝑂 (Equation 1)

This reaction was previously performed by means of PHREEQC calculations [22]. Experiments were conducted in a

2 L pyrex VWR batch reactor at 50 °C during 2.5h. The homogenization of the reaction mixture was handled by a

magnetic stirrer. The products of the reaction were CFS (𝐶𝑎𝐹2 and 𝐶𝑎𝑆𝑖𝑂3) and an aqueous solution of Sodium

Hydroxide.

The typical physical and chemical properties of the CFS are illustrated in Table 3 and 4, respectively.

Table III

Physical properties of the CFS as measured using ASTM C110

Properties Particle Size (μm) Moisture (wt%) Specific Gravity pH (20% Solids)

Value 6.82 0.26 2.14 13.55

Table IV

Chemical composition of the CFS as determined using XRF

Compound CaO SiO2 Al2O3 Fe2O3 MgO K2O SO3 Mn2O3 ZnO Cl TiO2 Na2O P2O5 LOI

Content (%) 58.04 15.83 0.38 0.08 0.40 0.11 0.11 0.01 0.01 0.01 0.02 6.15 0.03 18.46

2.3. Brick processing

The raw materials CFA and CFS were kept in an oven at 105

°C for 24 h and then crushed with a porcelain mortar and pestle to obtain powder. The two raw materials were mixed

in proportions of 25, 40, 50, 60 and 75 % by weight of CFA

(BM25, BM40, BM50, BM60 and BM75) using a RENFERT

Vacuum Mixer. A paste mixture with a solid-to-liquid ratio

(S/L) of 2 was prepared. The solid contains different mixture

of CFA with CFS. The liquid contains 1.5 M sodium

hydroxide. A total mass of 600 g of solids (CFA+CFS) were

prepared for each sample and 300 mL of the aqueous solution

of Sodium Hydroxide (1.5 M) was then added to the solid

mixture in order to obtain a water-paste (W/P) ratio of about

0.50. A 50×50×50 mm cubic mold was used to cast the

samples. The samples were consolidated using a US-M-

18×18 VISBO VIBRATOR and then cured inside an

environmental chamber at 23 ±1 °C for 7, 14 and 28 days.

2.4. Bricks characterization The bricks were tested to determine: the bulk density, the

compressive strength and the rate of water absorption. The

bulk density was measured using ASTM C29/C29M. The

compressive strengths at 7, 14 and 28 days after curing inside

an environmental chamber at 23 ±1 °C were determined using

the FX 700 testing machine compression (FORNEY LP) in

accordance with the ASTM C109/C109-16a [23]. The water

absorption was determined in accordance with the ASTM

C1585-13 [24]. The characteristics of the bricks were found

by means of XRF performed on EPSILON 3 XLE

(MALVERN PANALYTICAL Manufacturing), XRD using



D8-DISCOVER (BRUKER MANUFACTURER) and SEM

performed on QUANTA 600 F (FEI COMPAGNY).

3. RESULTS AND DISCUSSIONS

The measurements of physical properties are reported in

terms of bulk density, compressive strength and water

absorption. The chemical composition and phases

identification of the materials were measured.

3.1. Bulk density

The bulk density of the obtained bricks at 28 days is presented

in Table 5.

Table V

Bulk density of bricks (23 ±1 °C and 1 atm) as measured using ASTM C29/C29M

BM25 BM40 BM50 BM60 BM75

1730 1756 1766 1868 1876

Bulk density at 28 days (kg/m3)

The CFA+CFS bricks have a medium weight because the bulk densities are between 1680 – 2000 kg/m3 [25, 26].

3.2. Compressive strength

The compressive strength of the brick as a function of CFA content after varying curing period is shown in Figure 1.

Fig. 1. Compressive strength of the brick samples as function of CFA content

The experimental results showed an increase of the compressive strength as the proportion of CFA increased to reach maximum

values with BM75 (75% of CFA). The CFA is then the primarily factor leading to the compressive strength of the bricks. There was also a reaction between CFA and NaOH leading to geo-polymer formation as illustrated by Equation 2 and 3 [27]:

(𝑆𝑖2𝑂5, 𝐴𝑙2𝑂2)𝑛 + 3𝑛𝐻2𝑂𝑁𝑎𝑂𝐻→ (𝑛(𝑂𝐻)3 − 𝑆𝑖 − 𝑂 − 𝐴𝑙 − (𝑂𝐻3))

− (Equation 2)

(𝑛(𝑂𝐻)3 − 𝑆𝑖 − 𝑂 − 𝐴𝑙 − (𝑂𝐻3))−

𝑁𝑎𝑂𝐻→ (𝑁𝑎)(−𝑆𝑖𝑂 − 𝑂 − 𝐴𝑙𝑂 − 𝑂 −)𝑛 + 3𝑛𝐻2𝑂

(Equation 3)

The addition of CFS trended to lower the compressive strength due to the content of fluorine. At 28 days, compressive strength of BM75 (6.1 MPa), shown in figure 1, was consistent with the strength of third class of brick [28]. However, BM50 was found

interesting as optimum proportion regarding its compressive strength at 28 days (4.8 MPa) and the requirement for some facing

bricks (4 MPa) in Senegal. The obtained compressive strengths were lower to common bricks made of coal fly ash [29].

However, the unfired CFA+CFS bricks were carried out at the minimum conditions (23 ±1 °C and 1 atm) which demands no

energy consumption from extend source and no pressure during the bricks manufacturing. In fact, pressing and firing the bricks

could improve the compressive strength and the dimensional stability.

20 30 40 50 60 70 80

0

1

2

3

4

5

6

7

Co

mp

res

siv

e S

tren

gth

(M

Pa

)

CFA content (% wt)

7 days

14 days

28 days

w/b=0.50



3.3. Water absorption

Figures 2 and 3 illustrate the rate of water absorption of the bricks specimens in accordance to ASTM C1585-13.

Fig. 2. Water absorption (%) of the brick samples

Fig. 3. Rate of water absorption of bricks samples

The addition of CFA decreased the water absorption to a minimum of 23.8% for BM75 and 27.8% for BM50 at 28 days cured

samples, while the water absorption for solid brick is less than 25% [30].

3.4. Chemical composition

The chemical composition of the bricks specimens BM50 and BM75 was determined by XRF analysis and results are shown

in Table 6.

20 30 40 50 60 70 80

20

25

30

35

40

45

Wat

er a

bs

orp

tio

n (

%)

CFA content (% wt)

7 days

14 days

28 days

w/b=0.50

0 100 200 300 400 500 600 700 800

0.0 0.1 0.5 1.0 1.9 2.9 4.2 5.7 7.4

0

1

2

3

Time (days)

BM40

BM50

BM60

BM75

Ab

so

rpti

on

, i (m

m3/m

m2)

Time (s)

14 d cured

0 100 200 300 400 500 600 700 800

0.0 0.1 0.5 1.0 1.9 2.9 4.2 5.7 7.4

0

1

2

3

Time (days)

BM40

BM50

BM60

BM75

Ab

so

rpti

on

, i (m

m3/m

m2)

Time (s)

28 d cured

0 50 100 150 200

0.0 0.0 0.1 0.3 0.5

0

1

2

3

Time (days)

BM40

BM50

BM60

BM75

Ab

so

rpti

on

, i (m

m3/m

m2)

Time (s)

28 d cured

0 50 100 150 200

0.0 0.0 0.1 0.3 0.5

0

1

2

3

Time (days)

BM40

BM50

BM60

BM75

Ab

so

rpti

on

, i (m

m3/m

m2)

Time (s)

14 d cured



Table VI

Chemical composition of bricks from XRF

Content (%) CaO SiO2 Al2O3 Fe2O3 MgO K2O SO3 Mn2O3 ZnO TiO2 Na2O P2O5 LOI

BM50 35.11 31.49 8.57 3.24 2.86 1.00 1.14 0.05 0.02 0.51 7.03 0.11 8.87

BM75 25.80 36.51 11.62 4.54 3.82 1.38 1.69 0.08 0.02 0.71 6.36 0.15 7.33 The chemical composition highlights interesting cementitious properties of the obtained composites in terms of major oxides

contents.

3.5. Solids phases identification The XRD patterns of the BM50 and BM75 specimens are displayed in Figure 4 and Figure 5

Fig. 4. XRD pattern of BM50 specimen

Fig. 5. XRD pattern of BM75 specimen

-500

0

500

1000

1500

2000

2500

3000

3500

15 20 25 30 35 40 45 50 55 60 65

Co

un

ts

2- Theta -Scale

-500

0

500

1000

1500

2000

2500

3000

3500

4000

15 20 25 30 35 40 45 50 55 60 65

Co

un

ts

2- Theta -Scale

Calcium Fluoride-CaF2

Aluminum Silicate-Al2SiO5

Calcium Carbonate-CaCO3

Calcium Oxide-CaO

Calcium Fluoride-CaF2

Aluminum Silicate-Al2SiO

5

Calcium Carbonate-CaCO3

Calcium Oxide-CaO



The patterns revealed a low degree of crystallization of

compounds such as aluminum oxide from CFA. Indeed, the

microstructure of the CFA activated NaOH is amorphous

instead of crystalline [31]. The main phases were Al2SiO5, CaF2, CaCO3, and CaO characteristically of their peaks.

Calcium silicate, which was in amorphous phase also, could

not be detected. However, aluminum silicate was formed

from the geo-polymerization and the CFS provided

cementitious properties due to the system CaO-SiO2-CaF2.

The SEM images with Fluorine (F) contents are shown in Figure 6 and Figure 7.

Fig. 6. SEM image of BM50 specimen (6.47% of F content)

Fig. 7. SEM image of BM75 specimen (2.96% of F content)

The SEM images of bricks after 28 days of curing showed

unreacted spherical particles of CFA because of the

incomplete geo-polymerization reaction. Heterogeneous

phases with some macrospores were also formed. Based on

these experimental results, it would be better to improve the

performance of the CFA+CFS bricks by changing the S/L.

4. CONCLUSION This paper examined the potential to manufacture bricks

using coal fly ash (CFA), calcium fluoride and calcium

silicate (CFS). Experiments showed the feasibility of making

bricks using this process at room temperature and atmosphere

pressure. The unfired bricks have an average bulk density of

1816.5 kg.m-3. The average compressive strength was found

to be 4.32 MPa and the water absorption about 26.78% after

28 days curing inside an environmental chamber at 23 ±1 °C. The chemical composition seems suitable for construction

and building materials. The results highlighted the potential

use of industrial waste materials from phosphoric acid plants

in unfired bricks manufacturing. Indeed, calcium fluoride and

calcium silicate (CFS) are the solid compounds generated by

working-up fluosilic acid which is the main liquid by-product

from the manufacturing of phosphoric acid.

5. ACKNOWLEDGEMENTS

This study was supported by a grant from the United States Department of State, Bureau of Educational and Cultural

Affairs (ECA) through the Fulbright African Senior Research

Scholar Program 2018 – 2019 administered by the Institute

of International education’s (IIE), Council for International

Exchange of Scholars (CIES). The research project was

conducted in Oregon State University (OSU), College of

Engineering, School of Civil and Construction Engineering

(CCE).



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manufacturing unfired bricks using coal fly ash, calcium fluoride … · 2020. 5. 4. · figures 2...

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