Properties of seashell aggregate concrete: A review

Construction and Building Materials 192 (2018) 287–300

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

Properties of seashell aggregate concrete: A review Uchechi G. Eziefula a,⇑, John C. Ezeh b, Bennett I. Eziefula c a

School of Engineering Technology, Imo State Polytechnic, Umuagwo, Imo State, Nigeria Department of Civil Engineering, Federal University of Technology, Owerri, Imo State, Nigeria c Department of Mechanical Engineering, Federal University of Technology, Owerri, Imo State, Nigeria b

h i g h l i g h t s
Concrete with higher seashell ratio has lower physical and mechanical properties.
Ground seashell aggregate can replace up to 20% natural fine aggregate in concrete.
Up to 50% seashell as a partial coarse aggregate can give normal-weight concrete.
Current data on durability of seashell aggregate concrete indicate varied effects.
Sound absorption and thermal insulation studies of seashell concrete are required.

a r t i c l e

i n f o

Article history: Received 13 July 2018 Received in revised form 15 October 2018 Accepted 15 October 2018

Keywords: Aggregate Concrete Durability Mechanical properties Physical properties Recycled material Seashell Waste management

a b s t r a c t Trends in concrete technology are currently directed towards sourcing alternative sustainable materials for concrete in order to minimise over-reliance on natural resources. Many of the substitute materials used for producing green concrete are recycled materials obtained from industrial wastes and by-products. A promising solution to the challenge of seashell waste management involves utilising seashells as construction materials in concrete. Experimental investigations have been carried out on the use of mollusc seashells such as periwinkle shell, mussel shell, oyster shell, cockle shell, crepidula shell, clam shell and scallop shell as aggregate replacement materials in concrete. The seashells were utilised as partial or total replacement of fine and coarse aggregates in concrete. This paper is a literature review of seashell aggregate concrete. The paper first presents an overview of the physical, mechanical and chemical properties of the seashells. This is followed by a discussion of the physical, mechanical and durability properties of seashell aggregate concrete in fresh and hardened states. Possible applications in the construction industry are also highlighted. Mollusc seashells have similar chemical composition with limestone-type aggregates but characteristically contain traces of chloride and sulphate salts. Although inclusion of seashell aggregate reduces the physico-mechanical properties of concrete, utilising some seashells as partial coarse aggregate at up to 50% substitution level can produce normal-weight concrete for non-structural and low-strength structural functions. The current understanding of seashell aggregate concrete provides a basis for further research on various aspects of its behaviour including the sound absorption and thermal insulation properties. Ó 2018 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods adopted in previous studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Preparation and treatment of seashell aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Other component materials of seashell aggregate concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Mix proportioning and preparation of concrete specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Evaluated properties of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of seashells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (U.G. Eziefula). https://doi.org/10.1016/j.conbuildmat.2018.10.096 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

288 289 289 289 289 291 291

288

4.

5.

6.

7. 8. 9.

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3.1. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Mechanical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Chemical composition and impurities content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical properties of concrete produced with seashell aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Workability of fresh concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Density of fresh and hardened concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical properties of concrete made with seashell aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Splitting tensile strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Flexural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Modulus of elasticity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Durability-related properties of seashell aggregate concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Water transportation properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Air content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Shrinkage and weight loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Freeze-thaw resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Other durability-related properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sound absorption and thermal insulation properties of seashell aggregate concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potential applications and directions for future investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Concrete is the second most consumed material in the world. It is estimated that each human being uses three tonnes of concrete per annum (van Oss, 2007 cited in [21]). The global production of concrete remarkably increased during the last century and it is expected that the demand for concrete will continue to rise in future [29,23,1]. Natural aggregate such as sand, gravel or crushed rock is the major constituent of concrete in terms of both volume and mass. Since huge amount of concrete is produced annually, it logically follows that much quantity of natural aggregate is mined for the production of concrete. A conservative estimate of the world’s consumption of aggregate exceeds 40 billion tonnes a year, and between 64 and 75% of the mined aggregate is used for concrete [48]. Environmental impact of dredging, excavating and processing natural aggregate such as threat to river ecosystems and non-reversible land erosion have already been reported (Table 1). As a result of these environmental concerns, authorities in some parts of the world have imposed restrictions on mining of aggregate through taxation as well as banning of mining on some sites [23,5]. Environmentalism is a basic principle of sustainable development which aims at protecting the environment and conserving the earth’s natural resources. Because of the sustainability issues associated with the production of concrete, current trends are now directed towards finding alternative sustainable materials Table 1 Summary of the major consequences of extraction of aggregates. Source: [48]. Impact on

Description

Biodiversity Land losses Hydrological function Water supply Infrastructures

Impacts on related ecosystems (e.g. fisheries) Both inland and coastal through erosion Change in water flows, flood regulation and marine currents Through lowering of the water table and pollution Damage to bridges, river embankments and coastal infrastructures Directly through transport emissions, indirectly through cement production Coastal erosion, changes in deltaic structures, quarries, pollution of rivers Decline of protection against extreme events (flood, drought, storm surge)

Climate Landscape Extreme events

291 292 292 292 292 293 293 293 295 295 296 296 296 296 297 297 297 298 298 299 299 299

for concrete. Many of the substitute materials used in the production of green concrete are recycled materials obtained from industrial wastes and by-products. The aquaculture industry provides food and employment for humans and therefore plays an important role in the economy of nations [38]. Seashells are protective shells of shellfishes and are by-products of the aquaculture industry. A variety of shellfishes are consumed as food while the inedible shells are discarded. These seashells generally have little or no commercial value and are often dumped in open fields or landfills, thereby creating unsightly appearance and unpleasant smell. Untreated seashell wastes left for a long time can lead to microbial decomposition of salts into gases such as hydrogen sulphide, ammonia and amines [54]. In locations where large quantities of seashell wastes are generated, the seashells can cause serious environmental problems. A promising solution to the challenge of seashell waste management is to use seashells as aggregate in concrete. Because seashells possess desirable properties, attempts have been made in using seashells as a partial or total substitute for natural aggregate in concrete. Local residents particularly in coastal regions have used seashells as alternative aggregates for simple concrete structures including residential houses, septic tanks, soak-away pits, grave slabs, pavement slabs and drainage gutters [6,39,45]. Scientific research on the use of seashells as aggregate replacement materials in concrete have been carried out for at least two decades with the aim of establishing the feasibility of practical applications. Using seashells in construction contributes to the protection of the environment in addition to preservation of natural resources. Costs are also saved when materials are re-used which range from not having to landfill or dispose waste materials and not having to source new materials [45]. Utilising seashell by-products as recycled materials in concrete is more economical in cases where the transportation, storage and processing costs involved are more favourable in comparison with conventional aggregates [15]. Shells of bivalves and gastropods of mollusc shellfishes are the main seashells used as aggregate in concrete. Worldwide production of molluscs (16 million tonnes) represents about 22% of the total global aquaculture production [20]. Bivalve molluscs are very common among marine shellfish species. Around 87% of molluscan aquacultures are bivalve molluscs – 33.0% clams (including arkshells and cockles), 31.3% oysters, 12.1% mussels and 10.9% pectens

U.G. Eziefula et al. / Construction and Building Materials 192 (2018) 287–300

Miscellaneous marine molluscs 8.9%

Freshwater molluscs 1.2% Abalones, winkles, conchs 2.8%

Pectens, scallops 10.9%

Clams, cockles, arkshells 33.0%

Mussels 12.1%

Oysters 31.3%

Fig. 1. Percentage of worldwide aquaculture production of mollusc shellfishes. Source: FAO, 2014 cited in [52].

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therefore be explored and assessed in order to reduce the energy consumption associated with oven-drying [49]. The sizes of seashells used as fine aggregate and coarse aggregate in concrete were also different. Generally, seashells utilised as fine aggregate were crushed and sieved to sizes below 5 mm. Yusof et al. [55] used crushed clam seashells passing through 500 lm as partial replacement of fine aggregate. Different types of crushing device, like, jaw crusher [51,50,33], drum compactor [10] and hammer [49] were used to grind the seashells. Some seashells utilised as coarse aggregate were either uncrushed or the particle sizes of the seashell aggregate were not mentioned [18,19,12,14]. The maximum size of the uncrushed seashell aggregates was usually 25 mm or less [18,4,19,47] while crushed seashell used as coarse aggregate ranged from 10 to 20 mm [32,10]. Nguyen et al. [36,37] used seashell crushed to 2/4 mm size as partial substitute of 2/6.3 mm coarse aggregate in pervious concrete. Cuadrado-Rica et al. [10] adopted a method of replacing aggregate with queen scallop shells on the basis of particle packing density rather than by equivalent size fraction. Hence, they replaced a combined mixture of 0/4 mm, 4/10 mm and 10/20 mm natural aggregates with shells crushed to 10 mm size at different substitution levels. 2.2. Other component materials of seashell aggregate concrete

and scallops (Fig. 1). Abalones, winkles and conchs make up around 2.8% of mollusc shellfish production (Fig. 1). Relative abundance of molluscan seashells in coastal regions around the world indicates that the seashells are available as aggregate replacement materials in concrete. Seashells thus have the prospect of becoming important secondary aggregate sources for concrete around localities where seashells are found. This paper is a literature review of seashell aggregate concrete. The concrete specimens were manufactured with the following seashells: periwinkle shell, mussel shell, oyster shell, cockle shell, crepidula shell, clam shell and scallop shell. The seashells were utilised as partial and total replacement of fine and coarse aggregates in concrete. The paper first presents an overview of the physical, mechanical and chemical properties of the seashells which is followed by a discussion of the physical, mechanical and durability properties of seashell aggregate concrete in fresh and hardened states. Practical implications of the study and directions for future investigations are provided.

2. Methods adopted in previous studies 2.1. Preparation and treatment of seashell aggregates Seashell aggregates used in the production of concrete specimens were generally cleaned in order to remove organic matter and salts (primarily chlorides and sulphates). Nguyen et al. [37] however noted that they did not follow any cleaning steps to limit the organic matter and chloride ions content of scallop seashell. Some authors removed impurities from the seashells without specifying the method of shell cleaning [4,10]. The most common cleaning operation involved washing the seashells with water and air-drying or sun-drying the washed shells. In addition to washing, handpicking of impurities [47] or removal of dirt and other organic matter with domestic brushes [49] were carried out. Some studies subjected the seashells to elevated temperatures at varying durations in order to dehydrate and disinfect the shells. Such seashells were oven-dried at 50 °C for 24 h [55], 105 °C for 4 h [6], 110 °C for 24 h [51,50,9] or 135 °C for 30 min [31]. It is useful to note that although the heating process kills bacteria in seashells, energy is expended when oven-drying the shells especially for longer heating durations. Simpler treatment methods should

The binder used in majority of the investigations was ordinary Portland cement (OPC) which is normally denoted as ‘Type I’ by ASTM C 150 classification and ‘CEM I’ by European Standards classification. OPC is the most common cement utilised in general concrete construction. Other cement types such as moderate sulphate resistant Portland cement blended with blast-furnace slag [49] and Portland cement mixed with 6–20% siliceous fly ash and limestone (CEM II/A) [31] were also used. These modified Portland cements are more suitable for concrete structures susceptible to moderate sulphate attack such as concrete exposed to sulphates in the soil or in groundwater [34]. The cement grades mentioned in the literature were 42.5R [31] and 52.5R [36,37,10]. The normal fine aggregates include alluvial quartz sand [36,37], alluvial silica sand [49], feldspathic crushed sand [10] and river bed sand [19,44]. The natural coarse aggregates were river stone [6,49], crushed granite [4,44]; crushed quartzite [10] and crushed limestone. Most studies did not add any chemical additives. Nevertheless, additives such as naphthalene air-entraining water-reducing admixture [51] and naphthalene sulphonate condensate superplasticiser [31] were added in concrete mixes to compensate for loss of workability. Yang et al. [50] used water reducers, highrange water reducers, air-entraining agents or various combinations of these admixtures in their study. 2.3. Mix proportioning and preparation of concrete specimens In the reviewed papers, the constituent materials were batched by weight although Adewuyi and Adegoke [4] batched by volume. The fine and coarse natural aggregates were also substituted with seashell aggregate by weight. The percentage substitution levels of conventional aggregate with seashell by-products ranged from 5 to 100%. The physical, mechanical and durability properties of concrete containing partially or completely replaced aggregate were compared with those of control concrete (i.e. 0% replacement). The mix proportions given in Table 2 represent the ratio of cement: fine aggregate: coarse aggregate by weight. In replacing normal aggregate with seashell, it is essential to have good values for seashell specific gravity and water absorption in order to accurately calculate the mix proportions per cubic metre of concrete. If the actual values of specific gravity and water absorption are

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Table 2 Methods of preparation of seashell aggregate concrete reported in the literature. Type of seashell

Type of replacement

Percentage of replacement

Type of cement

w/c ratio

Mix proportiona

Duration of curing (days)

Soneye et al. [47]

Periwinkle

10, 30, 50, 100

–

–

1:2:4

3, 7, 28, 56

Ekop et al. [12] Ettu et al. [14]

Periwinkle Periwinkle

Coarse and fine aggregates Coarse aggregate Coarse aggregate

OPC OPC

0.55 0.65

1:2:4 1:1.5.3, 1:2:3, 1:2.5:3

7, 14, 28 7, 28

Falade et al. [19] Agbede and Manasseh [6] Adewuyi and Adegoke [4] Falade [18] Khankhaje et al. [25–26] Muthusamy et al. [33] Ponnada et al. [44] Muthusamy and Sabri [32] Varhen et al. [49]

Periwinkle Periwinkle Periwinkle Periwinkle Cockle Cockle Cockle Cockle Scallop

Coarse aggregate Coarse aggregate Coarse aggregate Coarse aggregate Coarse aggregate Fine aggregate Coarse aggregate Coarse aggregate Fine aggregate

25, 50, 75, 100 25, 35, 45, 50, 55, 65, 75 100 25, 50, 75, 100 25, 50, 75, 100 10, 20, 30, 40, 50, 100 25, 50, 75 5, 10, 15, 20, 25 5, 10, 15, 20, 25 5, 10, 15, 20, 25, 30 5, 20, 40, 60

0.6, 0.8 0.5 0.5, 0.6 0.5, 0.55 0.32 0.54 – 0.5 0.41, 0.45, 0.55

1:2:2, 1:2.5:2 1:1.5:3 1:2:4, 1:3:6 1:1.5:3, 1:2:4, 1:3:6, 1:3.5:1, 1:4.5:1 1:0.41:3.93, 1:0.41:3.93, 1:0.41:4.05 1:1.61:2.41 – – 1:0.82:2.18, 1:1.01:2.20, 1:1.61:2.81

7, 21, 90 7, 14, 28 3, 7, 14, 21, 28 7, 14, 21, 28 7, 28 7, 28 28 28 7, 28, 90

Cuadrado-Rica et al. [10]

Scallop

Mixed aggregate

20, 40, 60

OPC OPC OPC OPC OPC Type I OPC OPC OPC Type I Moderate sulphate resisting Portland cement + blast-furnace slag OPC CEM I 52.5R

Scallop Oyster

Coarse aggregate Fine aggregate

20, 40, 60 10, 20

OPC CEM I 52.5R OPC Type I

1:2.21:2.65, 1:1.44:2.16, 1:0.74:1.73, 1:0.16:1.40 1:0.37:5.23, 1:0.30:4.34 1:1.86:2.62, 1:1.85:2.62, 1:1.72:2.76

28, 91

Nguyen et al. [37] Yang et al. [50]

0.65, 0.67, 0.68, 0.70 0.37 0.45

Yang et al. [51] Martinez-Garcia et al. [31]

Oyster Mussel

5, 10, 20 25, 50, 65b

OPC Type I CEM II/A-M (V-L) 42.5R

0.45 0.50, 0.75

1:1.81:2.51, 1:1.81:2.50, 1:1.81:2.49 1:3.23:2.14, 1:5.88:3.37

Nguyen et al. [36] Yusof et al. [55] Richardson and Fuller [45]

Crepidula Clam –

Fine aggregate Coarse and fine aggregates Coarse aggregate Fine aggregate Coarse and fine aggregates

28 7, 14, 28, 56, 91, 180, 365 3, 7, 28 3, 7, 28, 90

20, 40 10, 20, 30 10, 50

OPC CEM I 52.5R Portland cement OPC CEM I

0.3 0.47 0.55

1:0.23:4.7 1:1.33:2.70 1:1.82:2.72

7, 28 28 28

a The mix proportion represents the ratio of cement: fine aggregate: coarse aggregate by weight without including any admixtures. Yang et al. [51] applied naphthalene air-entraining water-reducing admixture and MartínezGarcía et al. [31] used naphthalene sulphonate condensate superplasticiser. b 5% and 12.5% replacement rates were used when both fine and coarse aggregate fractions were replaced together.

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Reference

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unknown, accurate values of concrete density cannot be obtained which might lead to increase in cement consumption when substituting normal aggregate with seashell. The lowest water-cement (w/c) ratio recorded in the literature was 0.3[36] whereas the highest recorded w/c ratio was 0.8 [19]. The common w/c ratios reported ranged from 0.3 to 0.4 for pervious concrete and 0.45 to 0.7 for normal concrete (Table 2). Some researchers included additional water to the effective w/c ratio [36,10,31]. The amount of extra water, which varied according to the shell content, were added to take account of the moisture content and water absorption of the seashell aggregates. The design, preparation and casting of concrete mix containing seashell aggregate were similar to those of normal concrete mix design and were achieved according to the specifications of various standards. Methods of mix design such as the absolute weight method [6] and Bolomey method [31] were adopted. After casting, the specimens were generally left for 24 ± ½ h to set before being removed from the moulds. The demoulded concrete samples were mostly wet cured for periods ranging from 3 to 365 days. The most popular durations of curing of seashell aggregate concrete were 7 and 28 days (Table 2). The temperature of curing reported in the literature varied from 18 to 27 °C. 2.4. Evaluated properties of concrete Slump of fresh concrete, density of both fresh and hardened concrete, and different strength properties of hardened seashell concrete such as compressive strength, splitting tensile strength, flexural strength and modulus of elasticity were investigated (Table 3). These physical and mechanical properties were studied along with some durability properties such as water absorption, water permeability, air content, freeze-thaw resistance, elevated temperature resistance, shrinkage, chloride migration, chemical attack, carbonation, microstructure, abrasion resistance and slip resistance (Table 4). Various standards for evaluating conventional

concrete especially BSI, Eurocodes and ACI were used to test the properties of seashell aggregate concrete. 3. Properties of seashells 3.1. Physical properties Some of the physical properties of seashell aggregate given in Table 5 depend on the size of the seashells. Finer sizes of seashells tend to have higher water absorption and specific gravity (particle density) and lower fineness modulus than the coarser-sized grains [36,25–26,31]. When compared with normal aggregates, most seashells have similar or slightly lower bulk density and specific gravity. The bulk densities of the cockle shell and some of the values for periwinkle shell fell within the range for normal weight aggregate (1280–1920 kg/m3) given in ACI [2]. The bulk density of scallop shell was slightly below the normal aggregate range, but nevertheless, exceeded 1100 kg/m3, the maximum bulk density of most lightweight aggregates [34,46]. Falade [18] and Agbede and Manasseh [6] reported bulk density of 694 and 515 kg/m3 for periwinkle shell, respectively. Mussel shell and periwinkle shell had the highest and lowest values of specific gravity, respectively. The values of specific gravity of all the seashells were greater than 2, except for oyster shell where 1.85 was reported by a study. The specific gravity of scallop shell and mussel shell were within the ACI [2] limit for normal weight aggregates used in concrete (2.30–2.90). Some of the values of specific gravity for oyster shell and cockle shell were less than the expected values for normal weight aggregate. Nevertheless, the specific gravities of all the seashells were higher than the ACI (2003) recommended values for lightweight aggregates i.e. ⅓–⅔ of that of normal weight aggregates. Therefore, the seashells cannot be classified as lightweight aggregates on the basis of specific gravity, unlike some agro by-products such as oil palm kernel shell and coconut shell.

Table 3 Physical and strength properties of seashell aggregate concrete reported in the literature. Reference Khankhaje et al. [25] Martinez-Garcia et al. [31] Varhen et al. [49] Cuadrado-Rica et al. [10] Soneye et al. [47] Ekop et al. [12] Ettu et al. [14] Nguyen et al. [36] Nguyen et al. [37] Muthusamy and Sabri [32] Falade et al. [19] Yang et al. [50] Yang et al. [51] Falade [18]

Slump

p p p p p p p

Fresh density p p p p

Dry density p p p p

p p

p p p p

p p

p

Compressive strength p p p p p p p p p p p p p p

Split tensile strength

Flexural strength

p p p

Elasticity modulus p

p p p

p p

p p

Table 4 Durability-related properties of seashell aggregate concrete reported in the literature. Reference

Shrinkage

Khankhaje et al. [25] Martinez-Garcia et al. [31] Varhen et al. [49] Cuadrado-Rica et al. [10] Nguyen et al. [36] Nguyen et al. [37] Falade et al. [19] Yang et al. [50] Yang et al. [51]

p

Water permeability p p p p p

Air/void content p

Carbonation

Chloride migration

Freezethaw

Chemical attack

Elevated temperature

Abrasion

Slip

p

p

p p p

p p p

p

Microstructure

p

p p

p

p

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Table 5 Physical and mechanical properties of mollusc shells. Sources for periwinkle shell: [18,4,6,19,8,16,43]. Sources for cockle shell: [44,25,26]. Sources for oyster shell: [51,50,27,13]. Source for mussel shell: [31]. Sources for scallop shell: [10,49]. Property

Periwinkle shell

Cockle shell

Oyster shell

Mussel shell

Scallop shell

Shell thickness (mm) Uniformity coefficient Fineness modulus Specific gravity Loose bulk density (kg/m3) Compacted bulk density (kg/m3) Moisture content (%) Water absorption (%) Aggregate impact value (%) Aggregate crushing value (%) Aggregate abrasion value (%) Los Angeles coefficient (%)

– 1.14–1.23 – 2.05–2.07 514 515–1353 1.1–8.32 9.03–12.99 32.5 59.6 – 45.73

– – – 2.09–2.64 – 1408–1420 – 0.1–2.5 52.8 48.7 15.8 –

– – 2.0–6.5 1.85–2.48 – – – 2.9–9.2 – – – –

– – 1.9–5.38 2.62–2.73 – – – 2.17–4.12 – – – 20

2–3 – 4.4–4.57 2.5–2.64 1015 1224 0.3 1.88–3.65 – – – –

The water absorption values of crepidula shell were 2.02% and 2.25% for 4/6.3 mm and 2/4 mm sizes, respectively [36]. The water absorption of crepidula shell and cockle shell are close to those of most normal weight aggregates i.e. less than 2% [34]. Among the reviewed seashells, periwinkle shell had the highest water absorption. Periwinkle and oyster shells exceeded the 8% maximum absorption value recommended by ACI [2]. From the results of the moisture content tests, periwinkle shell and scallop shell possessed the typical values for surface moisture content recommended by ACI [2], although the 8.32% obtained by Falade [18] was above the normal limit. 3.2. Mechanical properties The mechanical properties of seashells presented in Table 5 are the aggregate impact value, aggregate crushing value, and aggregate abrasion value which measure the toughness, strength, and hardness, respectively. From Table 5, cockle shell did not meet up with the BS requirements for aggregate impact value of aggregates for concrete pavement wearing surface (maximum value of 30%) and other types of concrete (maximum value of 45%). The aggregate impact value of periwinkle shell met the requirement for normal non-wearing surface concrete. The crushing values of cockle shell and periwinkle shell were approximately more than the recommended value of ACI [3] for normal concrete by 19% and 30%, respectively. Mussel shell achieved the specification for Los Angeles coefficient, and shows potential for use in concrete having good resistance to wear and abrasion (maximum value of 30%), whereas periwinkle shell only satisfied the hardness requirement for non-wearing surface concrete (maximum of 50%). The range of values given by Shetty [46] for some natural aggregates – aggregate crushing value (12–33%) and aggregate impact value (10–34%) – signify that natural aggregates are generally stronger and more shock-resistant than periwinkle shell and cockle shell. 3.3. Chemical composition and impurities content Molluscan shells are fundamentally formed by biomineralisation and they contain calcium carbonate with a small amount of organic matter; most seashells contain 95–97% calcium carbonate [53,41]. Seashells also contain chloride ions [36,37,31,49] and sulphates [31,49]. Organic matter in aggregate interferes with the cement hydration process, prevents effective aggregate-matrix bonding and sometimes reduces aggregate durability [34,46]. Salts in aggregate creates unsightly appearance on the concrete surface (due to efflorescence) and also affects the setting properties and ultimate strength of concrete [34,46]. High chloride content in concrete is known to accelerate the corrosion of steel reinforcement while excessive sulphate content triggers expansion of hardened concrete. Organic matter and chloride ion percentages in untreated

seashell aggregates are small but usually exceed the maximum values permitted for conventional concrete [31,35,49]. The organic matter and chloride content of mussel shell increases with fineness of shell crushing [31]. Cleaning and heat treatment of seashells help to minimise the amount of chlorides, sulphates and organic impurities content in the shells [37,31]. Washing removes the dirt and salt content of seashell whereas heating removes the water and kills germs. Scallop shell washed with water, brushed and sun-dried by Varhen et al. [49] satisfied the ACI 222R chloride ion and sulphate content requirements for conventional concrete but exceeded the maximum acceptable limit for total salt content. Heat-treated mussel shell used by Martinez-Garcia et al. [31] complied with the Spanish EHE-08 standards for aggregate quality needed in concrete, except for the visual organic matter, chlorides and total sulphates contents. Although washing and heating improves the quality of seashells, present research findings imply that the probable inclusion of cleaned seashell aggregates in high performance concrete such as reinforced or prestressed concrete is restricted unless higher levels of cleaning and treatment are applied to the seashells. The chemical composition of mollusc shells is comparable with those of natural aggregate such as limestone (Table 6). Just like most natural aggregates, the major chemical compound in mollusc shell is calcium oxide. The high calcium oxide content of seashell suggests that seashell could be an inert material in concrete similar to limestone. Seashell aggregate only acts as filler in a concrete matrix and its influence on the cement hydrate is insignificant [51]. Traces of other oxides are present in mollusc shells including magnesium oxide, aluminum oxide, iron oxide and oxides of a few other elements. The loss on ignition (LOI) of mollusc seashells generally ranges from 41–46% by weight, although Ez-zaki et al. [17] reported 51% for oyster shell. The LOI of seashell is similar to the numerical values mentioned for limestone [11] and slightly higher than those of marble [42] and recycled concrete aggregate [7]. Garcia et al. [22] reported the chemical composition of an unidentified seashell to contain 51.35% calcium oxide and 43.37% LOI. 4. Physical properties of concrete produced with seashell aggregate 4.1. Workability of fresh concrete In the research investigations, slump was the major property employed to assess the workability of seashell aggregate fresh concrete. Generally, workability of fresh concrete decreases as the percentage replacement of the seashells increases (Table 7). High replacement levels resulted to very low values of slump. The reduction in workability was attributed to increase in the specific surface area [18,4], rough texture and irregular, flaky, elongated or angular shape of the seashells [6,31] and higher water absorp-

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Table 6 Chemical composition of mollusc shells. Source for periwinkle shell: Malu and Bassey cited in [39]. Sources for cockle shell: [28,40,41]. Sources for oyster shell: [24,51,28,56,27,30,17]. Sources for mussel shell: [24,28]. Source for scallop shell: [49]. Sources for clam shell: [24,28,41]. Chemical compound

Calcium oxide Silicon dioxide Aluminum oxide Magnesium oxide Iron oxide Sodium oxide Potassium oxide Sulphur trioxide Phosphorus pentoxide Titanium oxide Sulphate Chloride Loss on ignition

Weight (%) Periwinkle shell

Cockle shell

Oyster shell

Mussel shell

Scallop shell

Clam shell

38.4 0.014 0.211 18.7 0.019 – – – – – – – –

51.56–54.24 0.38–1.60 0.17–0.92 0.02–1.43 0.05–0.06 0.08–0.37 0.03–0.06 0.13 – – 0.07 0.01 41.84–42.87

48.0–77.81 0.64–13.28 0.05–0.64 0.01–0.94 0.03–0.20 0.23–0.93 0.01–0.51 0.60–1.09 0.01–0.18 0.02–0.11 0.43 0.01–2.92 42.83–51

53.38–53.70 0.20–0.73 0.13 0.03–0.33 0.03–0.05 0.44 0.02 0.34 – – 0.11 0.02 42.22–45.61

53.70 0.10 0.10 0.18 0.03 0.50 0.01 0.32 – – 0.01 0.01 44.40

53.92–67.70 0.39–0.84 0.14–0.28 0.08–0.22 0.02–0.06 0.39 0.03 0.16 – – 0.06 0.02 42.73–45.16

tion of the seashells in comparison with natural aggregate [51]. The irregular shapes increase the frictional resistance between the seashell grains, while the water absorption occurs due to existence of internal voids within the surface of the seashells. Workability is also influenced by the seashell grain size. Workability of seashell aggregate concrete tends to reduce if the seashell grains are finer than natural sand and if seashell particle sizes that are less than 1 mm are included [49]. Using seashell aggregate that is coarser than natural sand improves the workability of concrete [10,49]. Workability of oyster shell aggregate concrete decreases with fineness modulus of oyster shell [51]. Mussel shell used as coarse aggregate substitution had greater influence on workability than when used as fine aggregate substitution [31]. When fine aggregate is partially replaced with crushed seashell in concrete at low substitution levels (5–25%), workability in some mixtures slightly increase [51,31,49]. This is mainly due to the shape and particle size of the seashell; the water absorption property of the seashells has no effect on the slump if the shell particle sizes are similar to sand [49]. Increase in the cement content of the mix ratio increased the workability of periwinkle shell aggregate concrete [4]. CuadradoRica et al. [10] found that mixes containing scallop shells had higher cement/aggregate ratio which decreases the workability but higher air content which increases the fluidity of concrete. The presence of organic matter in mussel seashell could also reduce the slump [31]. Pervious concrete made with cockle shell or scallop shell as a partial replacement of coarse aggregate does not show much significant reduction in density [37,25]. The use of admixtures to improve concrete workability was not emphasized in the reviewed papers. Certain admixtures such as water-reducers and superplasticisers help to improve the workability of concrete for a given water/cement ratio. It should be noted that these admixtures are required for producing high performance concrete exposed to aggressive environment such as seawater and aggressive soil. Slump of concrete containing oyster shell as a partial fine aggregate substitute improves with addition of naphthalene air-entraining water-reducing admixture, but the effect of the admixture on slump reduces as the shell substitution rate increases [51]. 4.2. Density of fresh and hardened concrete Important features of seashell aggregate which generally affect concrete density are water absorption and specific gravity (or particle density). Water absorbed by aggregate particles causes the specific gravity of the aggregate particles to be higher than the apparent specific gravity of oven-dry particles; this higher specific gravity is relevant to the density of concrete containing lighter-

weight aggregate [34]. The effect of high absorption of water by a seashell aggregate can be resolved by adding calculated amount of extra water to the mixing water (based on the effective w/c ratio) but such a procedure will result to rise in the density of concrete [34]. The density of concrete slightly decreases as the aggregate replacement with seashell aggregate increases (Table 7). This reduction in density is applicable to concrete in both fresh and hardened states (Fig. 2). Although the specific gravities of seashells are similar or slightly lower than those of natural aggregates, the angular or irregular shape of seashell and the presence of organic substances creates more entrapped air in the concrete which reduces the density [10,25,31,49]. As shown in Fig. 2, the density of 28-day concrete containing up to 50% incorporation of some coarse seashell aggregates is greater than 2100 kg/m3 and thus can still be regarded as normal weight concrete [18,4,14]. Martínez-García et al. [31] found that the density of hardened concrete exceeded 2000 kg/m3 when fine aggregate is replaced with crushed mussel shell for up to 100% replacement level. The least bulk density was observed for 50% fine aggregate substitution which gave a 10% reduction in comparison with the reference density. Decreasing the grain size of seashell increases the density of pervious concrete [25]. This is probably because finer seashell grains cause reduced void content in concrete and also have comparatively higher specific gravity than coarse shell aggregates. 5. Mechanical properties of concrete made with seashell aggregate 5.1. Compressive strength Compressive strength was the main mechanical behaviour studied in researches on seashell aggregate concrete. The incorporation of coarse seashell aggregates generally reduces the compressive strength of concrete (Table 7). The influence of seashell replacement is more pronounced in concrete mixes having high aggregate/cement ratios. Fig. 2 shows the variation of the 28-day compressive strength of concrete containing seashell aggregates with percentage of aggregate replacement. The strength reduction of concrete is largely attributed to the higher water absorption of seashell aggregates, elongated or flaky shape of seashells and presence of organic matter. Also, higher surface area of the seashells results to less available cement paste for coating, thereby causing reduction in bond strength. Flaky and elongated shaped aggregates have poor bond with cement paste and create larger volume of voids within the concrete matrix which contribute to reduction of compressive strength.

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Table 7 Effects of seashell aggregates on selected physico-mechanical properties of control concrete. Reference

Type of seashell

Type of replacement

Percentage of replacement

Changes in slumpa

Changes in 28-day densitya

Changes in 28-day compressive strengtha

Changes in 28-day splitting tensile strengtha

Ekop et al. [12]

Periwinkle

Coarse aggregate

25% 50% 100%

28% 52% 99%

– – –

19% 37% 67%

– – –

Ettu et al. [14]

Periwinkle

Coarse aggregate

25% 50% 75%

– – –

0% to 32% 5% to 36% 14% to 41%

5% to 14% 12% to 27% 21 to 28%

– – –

Agbede and Manasseh [6]

Periwinkle

Coarse aggregate

25% 50% 100%

13% 23% 57%

8% 14% 22%

– – –

– – –

Adewuyi and Adegoke [4]

Periwinkle

Coarse aggregate

25% 50% 100%

17% to 27% 44% to 47% 72% to 87%

11% to 15% 13% to 16% 35% to 40%

14% to 16% 32% to 38% 65% to 72%

– – –

Falade [18]

Periwinkle

Coarse aggregate

10% 20% 30% 50%

20% to 40% 50% to 90% 67% to 85% 96%

4% to 6% 12% to 16% 17% to 18% 35%

9% to 27% 25% to 60% 32% to 48% 63%

– – – –

Soneye et al. [47]

Periwinkle

Coarse aggregate

10% 30% 50% 10% 30% 50%

– – – – – –

– – – – – –

5% 14% 32% 5% 23% 27%

– – – – – –

Fine aggregate

Khankhaje et al. [25]

Cockle

Coarse aggregate

25% 50% 75%

– – –

+3% to +2% 0% to 2% 0% to 3%

14% to 20% 18% to 25% 30% to 38%

– – –

Muthusamy and Sabri [32]

Cockle

Coarse aggregate

10% 20% 30%

20% 67% 84%

– – –

7% +17% 23%

– – –

Muthusamy et al. [33]

Cockle

Fine aggregate

10% 20% 25%

– – –

– – –

+18% 7% 29%

– – –

Nguyen et al. [37]

Scallop

Coarse aggregate

20% 40% 60%

– – –

+1% to 1% 3% to 4% 6% to 7%

+4% to 1% 13% to 15% 21% to 29%

7% to 9% 18% to 21% 26% to 28%

Cuadrado-Rica et al. [10]

Scallop

Mixed aggregate

20% 40% 60%

27% 36% 45%

– – –

10% 20% 27%

– 10% –

Varhen et al. [49]

Scallop

Fine aggregate

20% 40% 60%

3% to +8% +10% to +42% 4% to +29%

– – –

0% to 7% 7% to 8% 3% to 10%

+7% to 8% 8% to 11% 0% to 15%

Yang et al. [51]

Oyster

Fine aggregate

5% 10% 20%

+8% to 45% 29% to 78% 90% to 100%

– – –

+13% to 8% +5% to 15% +2% to +1%

3% to 5% 7% +10%

Martinez-Garcia et al. [31]

Mussel

Coarse aggregate

25% 50% 25% 50% 5% 12.5%

20% to 92% – +10% to 8% 10% to 15% +5% to 8% +10% to 46%

+1% to 0% 1% 4% to 5% 6% to 10% 1% to 3% 3% to 5%

17% to 23% 46% 32% to 37% 36% to 68% 8% to 18% 27% to 35%

4% to 5% 18% 4% to 28% 4% to 42% +2% to 3% 9% to 20%

Fine aggregate Fine + coarse aggregates Nguyen et al. [36]

Crepidula

Coarse aggregate

20% 40%

– –

1% to 3% 1% to 5%

11% to 15% 8% to 27%

16% to 17% 7% to 32%

Yusof et al. [55]

Clam

Fine aggregate

10% 20% 30%

– – –

– – –

+7% +3% +16%

– – –

Richardson and Fuller [45]

–

Coarse aggregate

10% 50% 10% 50%

– – – –

– – – –

4% 17% 6% 29%

– – – –

Fine aggregate a

Changes in properties with reference to control concrete: () signifies reduction, (+) signifies increment.

Concrete mixes which contain smaller-sized seashell grains generally show higher compressive strength [55,25]. This can be attributed to lower void content and better aggregate-paste bond. Clam shell powder acting as partial replacement of natural sand in concrete at 10–30% replacement percentage gives higher compres-

sive strength than the control strength [55]. As shown in Fig. 3, using crushed fine seashell as a partial replacement of fine aggregate (sand) at 5–20% substitution levels gives higher compressive strength than conventional concrete in most cases [51,33,49]. Varhen et al. [49] attributed the increase in strength of scallop shell

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Martinez-Garcia et al. (2017)

Density (kg/m3)

Varhen et al. (2017)

2300

2200

2100

2000

40

Compressive strength (N/mm2)

2400

Mussel shell

30

Periwinkle shell

25

Scallop shell

20 15 10 5 0 0

0 10 20 30 40 50 60 70 80 90 100

Ettu et al. (2013) Agbede and Manasseh (2009)

2500

Adewuyi and Adegoke (2008) Falade (1995)

2250 2000 1750 1500

Compressive strength (N/mm2)

2750

Density (kg/m3)

(a)

Replacement of fine aggregate (%)

(a)

Cockle shell

35

20

30

40

50

60

70

80

90 100

Replacement of coarse aggregate (%) 45

Clam shell

40

Cockle shell

35

Mussel shell

30

Oyster shell

25

Perwinkle shell

20

Scallop shell

15 10 5 0

0 10 20 30 40 50 60 70 80 90 100

0

(b)

Replacement of coarse aggregate (%)

10

10

20

30

40

50

60

70

80

90 100

Replacement of fine aggregate (%)

(b) Khankhaje et al. (2017a) Nguyen et al. (2013a)

1900

Nguyen et al. (2013b)

1850 1800 1750 1700 0

(c)

10

20

30

40

50

60

70

80

Replacement of coarse aggregate (%)

Fig. 2. Density of concrete containing seashell aggregate at various levels of replacement. (a) Fine aggregate replacement for fresh concrete. (b) Coarse aggregate replacement for 28-day concrete. (c) Coarse aggregate replacement for 28-day pervious concrete.

Splitting tensile strength (N/mm2)

Density (kg/m3)

1950

Fig. 3. 28-day compressive strength of concrete containing seashell aggregate at different levels of replacement. (a) Coarse aggregate replacement. (Sources: Cockle shell – [32]; Mussel shell – [31]; Periwinkle shell – [4]; Scallop shell – [10]). (b) Fine aggregate replacement. (Sources: Clam shell – [55]; Cockle shell – [33]; Mussel shell – [31]; Oyster shell – [50]; Periwinkle shell – [47]; Scallop shell – [49].)

3.5

Crepidula shell (coarse) Mussel shell (coarse)

3

Scallop shell (coarse)

2.5

Mussel shell (fine) Oyster shell (fine)

2

Scallop shell (fine)

1.5 1 0.5 0

10 20 30 40 50 60 70 80 90 100

Replacement of aggregate (%) concrete at 5% fine aggregate substitution to the absence of particles smaller than 1 mm size; the coarser and angular fine aggregates helped to improve the interlocking bond of the concrete matrix. Compressive strength of seashell aggregate concrete is also influenced by curing age, cement content and addition of admixtures. Compressive strength of seashell aggregate concrete generally increases with curing age. Increase in the amount of cement was found to increase the compressive strength of seashell concrete [4,37]. Yang et al. [51] noticed that concrete containing naphthalene air-entraining water-reducing admixture and up to 20% crushed oyster shell as partial replacement of sand gave lower compressive strength than mixes where admixtures were not added.

Fig. 4. 28-day splitting tensile strength of concrete containing seashell aggregate at different levels of replacement. Sources: Crepidula shell (coarse) – [36]; Mussel shell (coarse) – [31]; Scallop shell (coarse) – [37]; Mussel shell (fine) – [31]; Oyster shell (fine) – [51]; Scallop shell – [49].

5.2. Splitting tensile strength

5.3. Flexural strength

The inclusion of seashell aggregate decreases the splitting tensile strength of concrete, similar to compressive strength (Table 7).

The effect of adding seashell aggregate on concrete flexural strength is analogous to those of compressive strength and split-

The reasons for the decrease in the splitting tensile strength are similar to those reported for the reduction in compressive strength of seashell aggregate concrete. From Fig. 4, however, a slight increase in 28-day splitting tensile strength can be noticed for concrete containing oyster shell and scallop shell as partial fine aggregate at up to 20% substitution. Using coarser size fractions of crepidula shell aggregate reduces the splitting tensile strength of pervious concrete [36].

U.G. Eziefula et al. / Construction and Building Materials 192 (2018) 287–300

Modulus of elasticity, Ec (N/mm2)

296

Flexural strength, ff (N/mm2)

1.8

35000

1.6 1.4 1.2

Coarse aggregate

30000

Fine aggregate

25000

Fine + coarse aggregates

20000

1

Coarse: Ec = 904.8fc0.985 R² = 0.843 Fine: Ec = 4825fc0.536 R² = 0.903 Fine + coarse: Ec = 4668fc0.523 R² = 0.613

15000

0.8

ff = 0.087fc1.027 R² = 0.904

0.6 0.4

10000

0.2 0 0

5

10

15

20

Compressive strength, fc (N/mm2) Fig. 5. Relationship between flexural strength and compressive strength of periwinkle shell aggregate concrete. Source: [12].

5000 0 0

10

20

30

40

50

Compressive strength, fc (N/mm2) Fig. 6. Relationship between modulus of elasticity and compressive strength of mussel shell aggregate concrete. Source: [31].

6. Durability-related properties of seashell aggregate concrete ting tensile strength; higher ratio of seashell leads to lower flexural strength. Falade [18] obtained 28-days results ranging from values less than 1.0 N/mm2 to values slightly above 3.0 N/mm2 using various mix and replacement ratios. Addition of periwinkle shell has more influence on the flexural strength of mixtures with higher aggregate/cement ratios [18]. The 28-day flexural strength of Pachymelania Aurita periwinkle shell concrete obtained by Ekop et al. [12] ranged from 2.55 to 0.79 N/mm2 for 0 to 100% replacements, respectively. The lowest flexural/compressive strength ratio was 7% for 50% replacement, whereas the successive highest flexural/compressive strength ratios were 11% for control concrete and 10% for the wholly-replaced sample. The relationship between the flexural strength and compressive strength of periwinkle shell concrete is illustrated in Fig. 5. Fig. 5 is a model in the form, ff = a (fc)b where fc is the compressive strength and ff is the flexural strength. This model is often adopted in the prediction and evaluation of concrete properties from its corresponding compressive strength.

5.4. Modulus of elasticity Yang et al. [51,50] and Martinez-Garcia et al. [31] observed a reduction in the modulus of elasticity of concrete when conventional aggregate is replaced with seashell. Using oyster shell as partial replacement of fine aggregate, the elastic modulus obtained using the compressive strength suggested by ACI Code is more conservative than experimental results [51]. Yang et al. [50] attributed the reduction of elastic modulus of concrete to the fact that elastic modulus of oyster shell is lower than that of fine aggregate. The elastic modulus of concrete containing oyster shell aggregate at 20% substitution rate decreased approximately by 10–15%. For curing periods lasting up to twelve months, they discovered that the elastic modulus of concrete increased with curing age, reaching its peak at six months after which the modulus decreased at one year. Martinez-Garcia et al. [31] reported that replacement levels up to 25% aggregate limited the reduction of the elastic modulus to 25% or less; with replacement rate of 50% or more, the reduction reached up to 50%. From the results of their study, reduction in elastic modulus of mussel shell aggregate concrete was more profound for coarse aggregate replacement, both for structural and non-structural concrete. Fig. 6 shows the relationship between the modulus of elasticity and compressive strength of concrete containing different proportions of fine and coarse mussel shell aggregate.

6.1. Water transportation properties By increasing the content of coarse seashell aggregates in pervious concrete, the water permeability of the concrete increases due to higher porosity of the seashells [36,37,25]. The water permeability coefficient rises by increasing the aggregate substitution by scallop shell [10]. Martínez-García et al. [31] found out that water permeability decreases with the content of mussel shell aggregate, especially when coarse mussel shell is used. Yang et al. [50] also obtained a reduction in the water permeability ratio of concrete when produced with oyster shell as partial fine aggregate replacement owing to the small particle size of crushed oyster shell. The water penetration depth of concrete reduces with inclusion of seashell aggregates [45]. Similar trend was observed by Martínez-García et al. [31] for water absorption of mussel shell aggregate concrete. The decreased penetration depth and absorption was due to the horizontal orientation of the flat-shaped seashells which acts a barrier to water penetration. Conversely, the substitution of natural aggregates with scallop shells increases the water penetration depth of concrete [10]. Cuadrado-Rica et al. [10] noted that the fixed effective water/cement ratio, rather than the seashell aggregate substitution, was a more significant influence on the higher water penetration depth. Nguyen et al. [37] and Cuadrado-Rica et al. [10] noted that water accessible porosity of concrete containing scallop shells increases with percentage replacement of aggregate. This was attributed to higher porosity of scallop seashell in comparison with natural aggregate and increase in packing degree of mixes containing seashells. 6.2. Air content Yang et al. [51] observed an insignificant change in the air content for oyster shell as partial replacement of fine aggregate up to 20% replacement limit. Addition of naphthalene water-reducing admixture at 0.3% of unit cement amount significantly increased the air content at 5% replacement level although the 20% replacement level remained almost the same. Varhen et al. [49] also reported that inclusion of fine seashell aggregate (scallop shell) had an unnoticeable effect on air content of concrete, even up to 60% replacement level. They mentioned elimination of organic matter and removal of finer particle size fractions (less than 1 mm) as the reasons. On the other hand, some researchers reported that air content increases with inclusion of seashell aggregate [10,13,25–26].

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Cuadrado-Rica et al. [10] observed that the entrapped air of concrete was due to flat and elongated shape of the scallop seashell and the possible presence of organic matter trapped within the shells which can have air bubbles. Eo and Yi [13] attributed the increased air content to the porous nature of the oyster shell aggregate. Khankhaje et al. [26] stated that angular shape of cockle shell decreases the compactness of the concrete mixture and disturbs the granular arrangement of compact pervious concrete. Khankhaje et al. [25] noted that using smaller sized scallop shell decreases the void content of pervious concrete in comparison with coarser shell sizes. 6.3. Shrinkage and weight loss The increase in drying shrinkage of concrete made with oyster shell as partial replacement of fine aggregate based on regression analyses was 7% and 28% at replacement levels of 10% and 20%, respectively, compared with control concrete [50]. The increased shrinkage resulted from lower rigidity of oyster shells and influence of size of fine seashell aggregate. According to MartínezGarcía et al. [31], the use of mussel shell as fine aggregate in concrete, even in small percentages, increases the weight loss of concrete. However, using coarse mussel shell aggregate did not affect this property; mussel shell gravel concrete had similar weight loss to control concrete. The increased weight loss is related to higher water absorption of mussel shell aggregate in comparison with natural sand [31]. The high water absorption led to addition of extra water during preparation of concrete, thereby causing evaporation of water retained in the pores of the shells over a period of time. 6.4. Freeze-thaw resistance Freeze-thaw resistance of seashell aggregate concrete was studied using number of freezing and thawing cycles [50,37,35]). ASTM C666 [50] and NF EN 1338 [37] were the standards used in assessing the freeze-thaw durability. Yang et al. [50] noted that even though the freezing and thawing cycles increase within the 300 cycle limit for oyster shell concrete, the variance rates of the dynamic elastic modulus and weight of oyster shell concrete respectively showed smaller and more satisfactory values than control concrete values. The durability factors (DF) of the oyster shell concrete specimens (93.3 and 85.4) exceeded that of the control concrete (82.7). Oyster shell generally improves the freezethaw resistance of concrete because the finer particles of the shells filled the void spaces in concrete [50]. Conversely, Nguyen et al. [37,35] reported that the presence of scallop seashell weakens the resistance of pervious concrete to freezing and thawing. Presence of organic matter, high content of chloride ions, poor aggregate-shell interconnection and weaker tensile strength of concrete were the possible reasons for the poor freeze-thaw resistance of scallop shell concrete [37,35]. 6.5. Other durability-related properties Yang et al. [50] analysed the chemical attack resistance and carbonation resistance of oyster shell aggregate concrete for up to 20% substitution rate of fine aggregate. Concrete specimens were deposited in 5% sulphuric acid and 2% hydrochloric acid, respectively, to determine the chemical attack resistance. The weight variance of concrete with age was measured for up to six weeks and it was seen that the attack on concrete gradually increased with age. Sulphuric acid attack on concrete surface continuously occurred with age, whereas hydrochloric acid attack did not show continuous variation with age because the weight variation stopped after three weeks. The influence of fine oyster shell aggre-

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gate on the chemical (acid) attack resistance of concrete is insignificant [50]. For the concrete carbonation characteristics measured within a five-week period, carbonation depth was found to increase with time. The oyster shell aggregate concrete gave comparable resistance to carbonation attack and the aggregate replacement rate had an insignificant effect on carbonation velocity [50]. Even though oyster shells generally contain chlorides and sulphates, the oyster shell aggregate concrete showed remarkable resistance to chemical and carbonation attacks probably due to the low replacement level and heat-treatment of the oyster shell aggregate. Presence of sulphates and chlorides in concrete are known to negatively affect the chemical durability properties of hardened concrete by causing efflorescence, expansion and disruption [34]. Falade et al. [19] investigated the behaviour of periwinkle shell aggregate concrete at elevated temperatures. The concrete specimens were heated between 50 and 800 °C/h. They reported that the surface of the periwinkle shell aggregate concrete showed signs of minor cracking and spalling between 300 and 400 °C. Major cracks were noticed at 400–600 °C, and smoke was emitted from the concrete samples between 700 and 800 °C. At 800 °C, heavy smoke was emitted from the samples and the colour of the periwinkle shells changed from black to brownish. The bond characteristics of the concrete matrix decreased as the temperature increased due to the near-smooth surface texture of the periwinkle shells and weakening of shell strength at high temperature. Periwinkle shell concrete should not be exposed to temperatures higher than 300 °C. Cuadrado-Rica et al. [10] measured the chloride migration of 91-day concrete prepared by partially substituting conventional fine aggregate with scallop shell and the samples were very permeable to chloride ions. The chloride diffusion coefficient increased with replacement level of crushed scallop seashell. Their results also indicated that the variation of the permeability of chloride ions along the height of the concrete cylinders was larger in samples containing scallop shell. The microstructure of mussel shell concrete observed by Martínez-García et al. [31] revealed that the coarse mussel shell aggregates had a horizontal orientation in the concrete matrix. The mussel shell concrete also possessed high porosity throughout the concrete mass with apparently uniform distribution. There was reduced bonding between the mussel shell aggregate and the cement paste, and this phenomenon was enhanced by higher content of coarse mussel shell aggregate. This reduced bonding predominantly occurred in the inner part of the shell (nacre layer) and was attributed to the presence of chitin which decreases the tensile strength. In addition, the interfacial transition zone at the outer part of shell (periostracum layer) showed some cracks and high presence of pores. The reduced aggregatepaste bond and high porosity of mussel shell aggregate concrete affect the properties of concrete especially the mechanical properties [31]. Nguyen et al. [37] assessed the abrasion resistance and slip resistance of pervious concrete comprising of scallop shell as a partial coarse aggregate. The highest values of abrasion length were obtained for the control concrete; pervious concrete for the various paver types containing scallop shell aggregate at different substitution rates were more resistant to abrasion than the control pervious concrete. However, the abrasion length of scallop shell pervious concrete ranged from 25.41 to 28.93 mm which failed to achieve the EN 1338 abrasion resistance requirement (i.e. the measured value should be less than 23 mm). The measured slip resistance values within the range of 86.27–89.85° indicated that there is no risk of slip in wet weather; the pervious concrete for all the replacement percentages therefore possessed good skid resistance [37].

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7. Sound absorption and thermal insulation properties of seashell aggregate concrete Khankhaje et al. [26] showed that pervious concrete made with cockle shell aggregate generally has better sound absorption properties (sound absorption coefficient, noise reduction coefficient, transmission loss coefficient and sound transmission class) than control pervious concrete. Pervious concrete made with cockle shell aggregate had higher noise coefficient (16%) than control pervious concrete (9%). Improved sound absorption of cockle shell concrete was attributed to higher air content as a result of the heterogeneous structure and angular shape of cockle shell. There is dearth of literature on thermal insulation properties of seashell aggregate concrete. Lertwattanaruk et al. [28] observed that the incorporation of crushed powder seashells (oyster shell, mussel shell, scallop shell and clam shell) as a partial replacement material in cement mortar (5–20% by weight of binder) decreases the thermal conductivity of mortar. High porosity was given as a reason for the reduced thermal conductivity. Addition of seashell reduced the thermal conductivity of mortar by 1–45%. The authors concluded that seashell mortar can provide better thermal insulation in building construction. 8. Potential applications and directions for future investigations Using seashell by-products as substitute aggregate in concrete can improve various properties of concrete. The incorporation of seashell by-products in concrete will provide environmental benefits in different ways – effective waste management of seashells, conservation of natural aggregate resources and production of greener concrete. Utilisation of seashell aggregates will likely provide economic advantages in construction since seashell byproducts are generated in large quantities and have comparatively lower commercial value than conventional aggregates. Some of the practical implications of seashell aggregate concrete in construction are highlighted below:
Utilising some types of seashells as partial coarse aggregate replacement at lower substitution rates (below 50%) can produce normal-weight concrete having acceptable strength properties. Such seashell concretes generally have compressive strength values that are greater than 50% of the control strength and thus can be applied as non-structural plain concrete or lowstrength structural concrete. Up to 20% of natural sand can be substituted by weight with ground seashells to obtain concrete possessing satisfactory density and strength.
Reduction in density of concrete owing to the incorporation of seashell aggregates will be beneficial in special applications where lower self-weight of concrete is needed and great strength is not required. The reduced self-weight will invariably decrease the dead load which is desirable for structural design of concrete members.
A study on cockle shell aggregate concrete indicates that seashell aggregate concrete has better sound absorption than control concrete. Improved sound absorption is valuable in buildings where reduction in noise and echo effects is required. Possible application of seashell aggregate concrete in road pavements subjected to light traffic volume will depend on the mechanical properties (especially hardness) of the seashells, as well as the skid resistance and abrasion resistance of seashell aggregate concrete.
The utilisation of seashell aggregates in concrete may probably improve the thermal insulation as a result of decreased thermal conductivity. This is because seashell aggregate concretes gen-

erally have more porosity and slightly lower bulk density than normal concrete. Concrete with enhanced thermal insulation properties is advantageous for buildings in very cold and hot climates. Improved thermal insulation of the seashell concrete buildings will control heat gains during hot seasons and heat losses during cold seasons.
Current research indicates that seashell aggregate has restricted potential in high performance applications such as prestressed concrete and concrete exposed to aggressive environment. This is due to the presence of salts and organic impurities in seashells as well as reduced workability and lower strength of seashell aggregate concrete. Any future application of seashell aggregates in high performance concrete will likely require addition of water-reducing or superplasticiser admixtures in addition to proper cleaning and treatment of the seashells. Even though seashell aggregate concrete has potential applications in the construction industry, several important aspects of its properties need to be further evaluated before it should be practically utilised. The literature review presented herein points out a number of gaps in the state of the art. The following recommendations are hereby made for future investigations:
More research is needed on the different types of seashell aggregate concrete especially for concrete manufactured with clam shell, crepidula shell and mussel shell aggregates in order to firmly characterise and understand their behaviour.
Since seashells are biomaterials whose properties may be influenced by environmental conditions, future research should specify the specie, storage period and exposure condition of the seashells. The effects of different cleaning and heat treatment procedures in addition to the influence of specie, storage age and exposure condition of seashells on various concrete properties also need to be studied in detail.
Studies should also be conducted to ascertain the types of bacteria present in seashells and to also observe if the synergy of bacteria and cement can produce concrete with specific desirable properties. Certain bacteria may be found in seashells which can help to improve the properties of concrete; also, ground-based bacteria can be present in stored shell dumps given a nutrient and oxygen supply [45].
The mechanical properties of various seashell aggregates such as toughness (aggregate impact value), strength (aggregate crushing value) and hardness (aggregate abrasion, Deval abrasion and Los Angeles abrasion values, etc) should be firmly identified and classified.
Long-term hardened properties of seashell aggregate concrete lasting up to 12 months should be investigated. Most of the existing studies on the mechanical behaviour restricted the results to 90-day old concrete. There are very few reports on the flexural strength and modulus of elasticity of seashell aggregate concrete. To the best of the authors’ knowledge, there is no available study on the ultrasonic pulse velocity (UPV) of seashell aggregate concrete.
The influence of supplementary cementitious materials (SCMs) on the properties of seashell aggregate concrete should be researched. Some examples of such SCMs include silica fume, fly ash, rice husk ash and ground granulated blast furnace slag.
The possibility of using seashell aggregate in reinforced concrete should be explored. The structural behaviour of reinforced concrete slabs, beams and columns manufactured with treated seashells as partial replacement aggregate at different substitution levels and mix ratios should be studied. The ultimate and serviceability limit states need to be checked and compared with conventional reinforced concrete.

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Although some results are available in the literature on the durability of seashell aggregate concrete, further investigations are needed to be carried out on alkali-aggregate reaction, colour changes, resistance to high temperature, behaviour when subjected to fire, abrasion resistance, slip resistance, carbonation, chloride ingress, freeze-thaw resistance, cement pasteaggregate bond, resistance to sea water attack, resistance to chemical and acid attacks, shrinkage and swelling, weight loss, water transportation properties and air content. The study of durability-related behaviour of seashell aggregate concrete is very important because seashells are generally porous, contain significant proportions of salts and organic matter and can possibly contain small amounts of decomposing shellfish flesh.
The sound absorption and thermal insulation properties of concrete containing various fine and coarse seashell aggregates should be investigated.
Research on the odour of seashell aggregate concrete should be initiated to discover if any resulting odour is healthy and comfortable for humans. This is because seashells give off foul smell mostly due to the presence of organic matter. Any unpleasant smell from seashell aggregate concrete construction, especially buildings, can cause discomfort for users.
It is necessary to determine the cost analysis of seashell aggregate concrete for various types of seashell, types of replacement, percentages of replacement and mix proportions. The obtained cost analysis should be compared with the cost of producing conventional concrete.
















9. Conclusions
This review paper has discussed the properties of the seashells and the properties of seashell aggregate concrete in fresh and hardened states. Based on the state-of-the-art review, the following conclusions can be drawn:
The physical properties of seashell aggregates depend on aggregate size and influence most of the properties of seashell aggregate concrete. Finer sizes of seashell aggregates tend to have higher water absorption, higher specific gravity, higher particle density and lower fineness modulus than the coarser-sized grains. When compared with normal aggregates, most seashells have similar or slightly lower specific gravity and bulk density. Seashells generally have higher water absorption than normal aggregates due to the porous nature and irregular surface and shape of seashells.
The chemical composition of seashell aggregate is similar to that of natural limestone-type aggregate. The loss on ignition of seashells ranges from 41 to 51% and the calcium oxide content varies from 38 to 77%. The high calcium oxide content of seashell suggests that seashell could be an inert material in concrete.
Seashell aggregates generally contain significant amounts of organic matter, chlorides and sulphates which lower the strength and durability of seashell aggregate concrete. Cleaning and heat treatment of seashells help to reduce the quantity of salts and organic impurities which improve the quality of the seashells. From the literature review, possible inclusion of cleaned seashell aggregates in reinforced or prestressed concrete is limited unless higher levels of cleaning and treatment are applied to the seashells.
The workability of seashell aggregate concrete generally decreases as the percentage of seashells increases. The use of coarse-sized seashell aggregate improves the workability of concrete. Most relevant studies did not lay emphasis on the use of water-reducing admixtures for improving workability

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which is a prerequisite for producing high performance concrete exposed to aggressive environment. The density of hardened seashell aggregate concrete reduces with increasing proportion of seashells. For most mix proportions, density of concrete containing less than 50% seashell aggregate as partial replacement of fine or coarse aggregate is generally greater than 2100 kg/m3. The strength properties of concrete containing seashell aggregate as a partial replacement of coarse aggregate generally decrease with increasing content of seashells, especially at higher substitution percentages. The reduction of physical and mechanical properties of seashell aggregate concrete in comparison with conventional concrete is generally attributed to the peculiar characteristics of seashell aggregates. The principal seashell characteristics include the elongated or flaky shape, higher specific surface area, higher water absorption and presence of organic impurities. Up to 20% of natural fine aggregate by weight can be replaced with ground seashells to obtain normal concrete having adequate density and strength. Research findings on the durability-related characteristics of seashell aggregate concrete indicate varied effects. Chloride migration and shrinkage properties of concrete generally increase with the addition of seashell aggregates. There are opposing views on the effect of seashell aggregate on the water transportation properties, air content and freeze-thaw resistance of concrete. Using oyster shell as partial replacement of fine aggregate at 20% replacement rate has an insignificant influence on the carbonation and acid attack resistance of concrete. Pervious concrete manufactured with cockle shell aggregate has better sound absorption properties than conventional pervious concrete.

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