Mechanical properties of concrete with Portland cement blended with fly ash, silica fume, and a large quantity of pre-treated cattle bone ash

Abstract

Concrete comprises aggregates of various sizes bound by a cementitious paste, with Portland cement (PC) as the primary binder since the 19th century. However, PC production depletes non-renewable natural resources and causes environmental degradation. Meanwhile, approximately 130 billion kilograms of cattle bones (CB) are generated globally each year, posing environmental challenges due to their non-biodegradability. CB is rich in calcium oxide, making it a potential supplementary material in cement production. This study explores the feasibility of using pre-treated cattle bone ash (CBA) as a partial replacement for PC in concrete, combined with 5% silica fume and 10% fly ash. CBA was incorporated at 10%, 25%, 50%, and 75% by weight of cement. The results indicated that mixes containing 10% and 25% CBA achieved high-strength concrete exceeding 60 MPa after 28 d, while mixes with 50% and 75% CBA produced structural-grade concrete with strengths above 25 MPa. The findings demonstrate that pre-treated CBA can effectively replace a portion of PC in concrete when combined with an appropriate pozzolanic material. This substitution reduces environmental pollution and promotes the sustainability of concrete production.

Graphical abstract

Keywords

pre-treated cattle bone ash / fly ash / silica fume / compressive strength / splitting tensile strength / flexural strength / sustainable concrete

1 Introduction

Increasing world and regional populations continue to create demand for concrete for the construction of houses and infrastructure [1–4]. Concrete essentially consists of aggregates of varying sizes bound together by a cementitious paste. Since the 19th century, Portland cement (PC) has been a primary input in concrete production. The PC has largely been produced from naturally occurring limestone, chalk, silica, and alumina-bearing clays with small inclusions of iron ore and gypsum [5–7]. It is estimated that one ton of PC requires 1.65 t of raw ingredients [8,9]. Considering the large quantities of cement consumed annually, concern has been raised over the possible depletion of raw materials for cement production in some areas. On the other hand, moving raw materials, cement clinker, or cement over large distances increase the CO₂ signature of the cement and potentially increases the product cost. This creates the necessity to explore renewable and sustainable inputs for cement production [10–12].

Cattle bones (CB) are produced as waste by slaughterhouses and meat processing and consuming facilities, including households. Bones decay very slowly, posing a tremendous environmental risk if improperly disposed of Ref. [13]. According to the Archeological Survey of India, animal remains (bones) may still be unearthed after hundreds of years constituting massive environmental pollution [14]. Globally, an estimated 130 billion kg of discarded animal bones are produced yearly, a concerning environmental load [15]. In Africa, Ethiopia ranks first in terms of livestock numbers. Every year, around 10% of this population generates animal bones from slain cattle, with cows and oxen weighing more than 300 kg. Bones constitute 20% to 30% of animal weight, leading to an annual generation of around 400.5 million kilograms of animal bone waste [16]. In South-west Nigeria, residents consume over 8000 cows daily, and slaughtered cows yield 900 t of bone daily [14].

Animal bone is an essential source of calcium, mainly in the form of calcium carbonate (CaCO₃), from which calcium oxide (CaO) can be extracted. The skeleton contains around 97% of the body’s total calcium. CaO is a primary component in cement manufacturing, making up almost half of the weight of PC [17]. Therefore, CB can be a renewable and sustainable source of raw material for cement production, with the added advantage of reduced environmental pollution from disposable bone waste.

Essentially, CB must be calcined at temperatures below 1200 °C to convert the CaCO₃ in the bone to CaO. However, the ash cannot be used directly in cement production as it contains copious amounts of phosphorous pentoxide (P₂O₅), which hydrates to phosphoric acid (H₃PO₄), which inhibits the hydration of cement. Therefore, it is imperative that the ash be pre-treated to remove the P₂O₅.

Fly ash (FA) and silica fume (SF) are sources of silica (SiO₂) and alumina (Al₂O₃), which, when added to a PC paste, reacts with calcium hydroxide (Ca(OH)₂) produced by hydrating calcium silicates in PC [18–20]. The reactions are pozzolanic and result in additional cementitious products as shown in Eqs. (1)–(3) [21].

(1){\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H})}_2{+\mathrm{S}\mathrm{i}\mathrm{O}}_2{+\mathrm{H}}_2\mathrm{O}=\mathrm{C}-\mathrm{S}-\mathrm{H}

(2){\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H})}_2{+\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3{+\mathrm{H}}_2\mathrm{O}=\mathrm{C}-\mathrm{A}-\mathrm{H}

(3){\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H})}_2{+\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3{+\mathrm{S}\mathrm{i}\mathrm{O}}_2{+\mathrm{H}}_2\mathrm{O}=\mathrm{C}-\mathrm{A}-\mathrm{S}-\mathrm{H}

When PC is blended with pre-treated cattle bone ash (CBA), the CaO in the ash hydrates to Ca(OH)₂ and therefore provides additional Ca(OH)₂ that reacts with SiO₂ and Al₂O₃ in FA and SF to produce cementitious compounds. The hydration of PC provides early strength while the subsequent pozzolanic reactions increase the latter age strength and improve the densification and durability of the concrete.

CBA has been used to replace PC in concrete production partially. According to studies cited by Akinyele et al. [22] and Awoyemi et al. [23], replacing 10% of cement with CBA significantly boosts the compressive strength of concrete. This has been confirmed by other studies [24]. It has been suggested that CBA improves the natural consistency of cement paste, and this increases with the amount of CBA in the paste. This is attributed to the ability of CBA to absorb more water than PC. Based on the average 28-d strength, 10% is found to be the ideal proportion of cement substitution by CBA without appreciably compromising the concrete’s compressive strength [25]. Olutaiwo et al. [26] studied the substitution of cement with cow bone ash in rigid pavement construction and determined that a 20 wt.% replacement rate of cement with bone ash was suitable for pavement construction, resulting in a cost savings of around 10% when compared to conventional concrete.

According to available literature, P₂O₅ in untreated CBA converts the alite (C₃S) in the PC paste to belite (C₂S) and consequently, the early strength of the paste is reduced. This reduction in strength increases with the amount of P₂O₅ in the cement paste increase [27].

The literature review shows that CBA can be used to partially replace PC in concrete production. CBA was used without treatment, and it has been documented that the presence of P₂O₅ in CBA is detrimental to the performance of concrete. The advantage obtained by incorporating CBA in the concrete is chemical, as CBA contains CaO, which needs SiO₂ and Al₂O₃ to react chemically to produce cementitious products. The purpose of this study is to pre-treat CBA to reduce the content of P₂O₅ before using it to replace PC with FA and SF partially. It is opined that the reaction between the CaO in CBA and SiO₂ and Al₂O₃ in FA and SF will create more cementitious products to enhance the strength of concrete. High-strength concrete with 28-d cube crushing strength exceeding 60 MPa is targeted. The quantity of CBA ranges from 10% to 75% relative to the weight of cement, while the quantities of FA and SF remain constant. Incorporating CBA in concrete for construction offers environmental advantages, including a practical and inventive method of disposing of harmful waste materials for improved environmental health, conserving natural resources, transforming waste into a valuable resource through concrete production, and potentially leading to economic benefits.

2 Materials and methods

2.1 Materials

PC, CEM I/42.5N, in accordance with EN 197, was used as the primary binder. CB were obtained randomly from local slaughterhouses, butcheries, and restaurants. Type F FA conforming to ASTM C-618 and SF conforming to ASTM C1240. Fig.1 shows the physical appearance of FA and SF. The superplasticizer (SP) was Viscocrete 20HE, in a 5 L plastic container. Coarse aggregates were crushed stone with a maximum size of 12.5 mm, and fine aggregate was river sand; both were obtained locally from a supplier. Potable water from the laboratory mains was used throughout the experiment.

2.2 Material preparation

2.2.1 Cattle bones

The bones were manually cleaned using boiling water, a scalpel, and a wire brush to remove muscle residues and bone marrow. They were then dried, crushed, and burned at 900 °C for three hours. After burning, the bones were ground in a ball mill and sieved through a 75 µm sieve. Fig.2 and Fig.3 illustrates the preparation process. The resulting CBA was stored in plastic bags for use in the research.

Pre-treatment of CBA using sulfuric acid and sodium carbonate:

To remove P₂O₅ from the ash, the ash was soaked in sulfuric acid for 12 h. The P₂O₅ and CaO in the ash reacted with sulfuric acid, as shown in Eqs. (4) and (5). The products formed are sulfur trioxide (SO₃), H₃PO₄, and calcium sulfate (CaSO₄). All the products except CaSO₄ can be removed by washing the ash with water. CaSO₄ is only sparingly soluble in water and remains behind as solid particles. After soaking the ash in sulfuric acid, it was washed several times with tap water until a pH close to 7 was reached.

(4){\mathrm{P}}_2{\mathrm{O}}_5{+3\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4\to {3\mathrm{S}\mathrm{O}}_3{+2\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4,

(5){\mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4\to {\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4{+\mathrm{H}}_2\mathrm{O}.

To convert the CaSO₄ to CaCO₃, the ash was soaked in a sodium carbonate solution for 2 h and then rinsed with water. The CaSO₄ reacted with sodium carbonate to produce CaCO₃ and sodium sulfate as shown in Eq. (6).

(6){\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4{+\mathrm{N}\mathrm{a}}_2{\mathrm{C}\mathrm{O}}_3\to {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3\downarrow {+\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{O}}_4.

CaCO₃ is only slightly soluble in water and precipitates out of solution, while sodium sulfate is highly soluble. Again, the ash was washed repeatedly with tap water to remove sodium sulfate. The ash was then calcined again at 900 °C for 2 h to convert the CaCO₃ to CaO. The calcination reaction is shown in Eq. (7).

(7){\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3\stackrel{900{}^{\circ }\mathrm{C}}{\to }\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{C}\mathrm{O}}_2\uparrow .

2.2.2 Aggregates

Coarse aggregate was washed through a 5 mm sieve to remove dust and clay particles and then sun-dried. It was then sorted into two groups: those passing the 12.5 mm sieve and retained on the 10 mm sieve and those passing the 10 mm sieve and retained on the 5 mm sieve. The prepared coarse aggregates were stored in plastic bins for use.

Fine aggregate was washed through a No. 200 sieve to remove silt and dust particles. It was then sun-dried and stored in plastic bins for use.

2.3 Material characterization

The materials used in this study include CBA, cement, FA, SF, Coarse, fine aggregates, and a SP. CBA was analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to determine its composition and morphology. Chemical and physical properties were assessed through X-ray fluorescence. The chemical and physical properties of cement, FA, SF, and SP were provided by their respective manufacturers.

Coarse aggregates were tested for specific gravity, water absorption, density, voids ratio, aggregate crushing value (ACV), aggregate impact value (AIV), and particle size distribution. Similarly, fine aggregate underwent evaluations for specific gravity, water absorption, density, voids ratio, and particle size distribution. All testing was conducted in accordance with applicable ASTM or BSI standards.

2.4 Concrete mix design

The proportioning of the concrete mixture was conducted in accordance with the American Concrete Institute specification ACI 211.4R-2008[28]. The design was created for M60 concrete with PC and a water/binder ratio of 0.35. Additional mixtures were generated from the control mixture by partially replacing PC with FA and cow bones. SF was included in addition to the mix amounts. To ascertain the ideal proportion of FA replacing PC, cement was replaced with FA at increments of 10% ranging from 10% to 50%, and the proportion yielding the maximum 28-d strength was selected. To investigate the impact of partially replacing PC with CBA, the cement was first mixed with 10% FA and 5% SF, while the CBA content was varied to 10%, 25%, 50%, and 75% by weight of the cement.

2.5 Concrete mixing

The concrete was prepared using a rotating drum mixer; water and a portion of the SP were added to the mixer, and the mixer was turned on to mix the water and SP into a homogenous mixture. To create a homogenous paste, the powders of cement, FA, SF, and where applicable, pre-treated CBA were combined with water. After that, one third of the fine aggregate was combined with paste, and each addition was blended to produce a homogeneous mortar before the next third was added. Finally, coarse aggregate was added to the paste in one third portions and mixed to create concrete of uniform consistency. The remaining SP was added to the mix as necessary to create the desired workability without bleeding.

2.6 Test on fresh concrete

The workability of the fresh concrete was determined using the slump test in accordance with ASTM C143/143M-10. A steel cone with a top diameter of 100 mm, a bottom diameter of 200 mm, and a height of 300 mm was utilized for the test. The cone was filled with three equal depths of concrete, which was compacted by 25 uniformly distributed tampings using a standard steel tamping rod. Following the final addition, the upper surface was leveled and smoothed using a steel trowel. The cone was subsequently elevated, resulting in the unsupported concrete collapsing under its own weight. The slump was quantified by the height differential between the slumped concrete and the steel cone. A more workable concrete exhibited greater slump compared to a stiffer concrete. Fig.4 illustrates the test.

2.7 Preparation and curing of test samples

Eighteen 100 mm × 100 mm × 100 mm cubes were cast and tested at 28 d for optimum FA replacement of PC. To find out how CBA affected the development of concrete’s strength, further 60 cubes with varying CBA contents were cast and tested at 3, 7, 14, and 28 d. Fifteen beams with a 100 mm square cross-section and 350 mm length were also cast and tested for the flexural strength at 28 d. Fifteen 100 mm diameter, 200 mm long cylinders were then cast and subjected to a 28-d splitting cylinder tensile test. All specimens were demolded after 24 h, and they were then placed in a curing tank with room temperature water to cure until testing time.

2.8 Tests on hardened concrete

2.8.1 Compressive strength

Compressive strength tests were conducted on 100 mm cubes following water curing periods of 3, 7, 14, and 28 d. The test samples were extracted from the water, cleaned with a soft absorbent cloth, and air-dried for one hour. The sample was subsequently weighed and positioned centrally between the battens of a universal compression testing machine, as illustrated in Fig.5. The load was applied at a constant rate until the sample failed, at which point the maximum load was recorded. Three samples were analyzed to derive an average result.

2.8.2 Splitting tensile strength

The test sample was removed from the curing tank after 28 d of water curing, wiped dry with a soft absorbent fabric, air-dried for 1 h, and subsequently weighed. The test specimen was then placed horizontally between the battens of the universal compression testing machine and tested in accordance with ASTM C496/C496M-04. Three samples were tested and the average used for each record. Fig.6 illustrates the test.

2.8.3 Flexural strength

The test sample was removed from the curing tank after 28 d of water curing, wiped dry with a soft absorbent cloth, air-dried for 1 h, then weighed. The test specimen was then placed horizontally between the battens of the universal compression testing machine and subjected to 3-point loading in accordance with ASTM C78-02 over a supported span of 300 mm. Three samples were tested and the average used for each record. Fig.7 illustrates the test.

3 Results and discussions

3.1 Material characterization

3.1.1 Cattle bones

1) Chemical and physical analysis

Results of the chemical and physical analyses of the powders used in concrete production are shown in Tab.1. It is observed that raw cattle bone powder has more than 50% CaO. It is also observed that it has 26.89% P₂O_5, and a loss on ignition (LOI) of 32%. Upon calcination, the amounts of CaO and LOI reduce to 50.37% and 1.51%, respectively, while the amount of P₂O₅ increases slightly to 27.51%. These changes are occasioned by losses of combustible tissues and decomposition of some compound which release gases which escape to the atmosphere. Upon pre-treatment with sulphuric acid and sodium carbonate, with attendant washing, the amounts of CaO, P₂O₅, and LOI become 56.01%, 4.88%, and 1.50% respectively. The increase in CaO and reduction in P₂O₅ are due to soluble compounds that are formed during pre-treatment and are washed away. The physical analysis shows that CBA is 23% lighter than PC and therefore has the potential to reduce the unit weight of concrete.

2) Microstructure analysis

Fig.8 shows the results of SEM images of the powders used to partially replace cement in the production of test specimens. The images show the morphology of raw cattle bone powder and CBA. Calcining the CB and milling produces a much finer powder than raw cattle bone powder. This is due to the increased brittleness of bone particles when subjected to heat, which makes milling more efficient.

Fig.9 shows the result of XRD analysis of CBA. The analysis revealed the presence of hydroxylapatite (Hap) and dellaite in the CBA. Hap is a mineral composed mainly of phosphorus and calcium and is a naturally occurring mineral form of calcium apatite with the formula Ca₅(PO₄)₃(OH), often written Ca₁₀(PO₄)₆(OH)₂. Dellaite, with the chemical formula Ca₆Si₃O₁₁(OH)₂, is a compound of calcium, silicon, hydrogen, and oxygen. The presence of calcium in dellaite and Hap could contribute to the formation of calcium silicate hydrates (C-S-H) when mixed with pozzalanic materials, enhancing the strength and durability of concrete. These findings highlight the complex mineral composition of CBA.

3.1.2 Cement

The chemical and physical properties of cement are given in Tab.1. The results were given by the manufacturers and conform to the European standard EN 197 [29].

3.1.3 Fly ash and silica fume

The chemical and physical properties of FA and SF are given in Tab.1. The results were given by the supplier, and the FA conforms to Type F FA to ASTM C-618 [30] and the SF conforms to ASTM C1240 [31].

3.1.4 Superplasticizer

The properties of the SP supplied by the manufacturer are given in Tab.2. The SP was a clear liquid with a specific gravity close to that of water, with a solid content of 34%.

3.1.5 Coarse and fine aggregates

The physical properties of coarse and fine aggregates are given in Tab.3, while the particle size distribution of the aggregates is illustrated in Fig.10. According to ASTM C136, the values obtained are within the recommended range. Medium coarse fine aggregate is indicated by the fineness modulus of 2.92. Both coarse and fine aggregates exhibited the prescribed upper and lower limits for particle size distribution, indicating that the aggregates were well-graded.

3.2 Concrete mix design

According to ACI 211.4R-2008 [28], the concrete mix proportions with a water/binder ratio of 0.35 are displayed in Tab.4 and Tab.5. The amounts of FA that can be used in CBA mixtures are determined by the proportions shown in Tab.4. Tab.5 gives the proportions used to investigate the effect of CBA addition to partially replace cement. In Tab.5, the amounts of pozzolans were maintained constant at 10% for FA and 5% for SF as CBA was increased from 0% to 75%. The water/binder ratio was gently adjusted, and a SP was added when needed to make the mixtures more workable.

3.3 Tests on fresh concrete

Fig.11 shows the effect of FA on the workability of fresh concrete. As the amount of FA replacing the cement increased, the workability of concrete increased at a constant water/binder ratio and SP addition. A substantial slump increase of 31% occurred with a modest FA addition of 10% and increased to 46% at a FA content of 50%. FA consists of round particles of the same size and order as cement and acts as a bearing in the cement paste, thus increasing the flowability and slump of the paste.

Fig.12 gives the effect of CBA on the workability of fresh concrete. As the amount of CBA replacing cement increases, the workability of the mix drops from 15% at 10% CBA content to 24% at 75% CBA content. These outcomes are consistent with the findings of Getahun1 and Bewket [25]. Tab.5 shows that as CBA content increased beyond 10%, more SPs had to be added to maintain good flowability of the mix. CBA particles absorb moisture from the paste and increase its stiffness, as revealed through SEM, hence the reduction in the slump.

3.4 Tests on hardened concrete

3.4.1 Compressive strength

The result to determine the optimal amount of FA to replace PC is given in Fig.13. The highest 28-d strength is given 10% FA substitution of cement. Beyond that amount, the compressive strength progressively reduces. Strength gain at 10% FA content resulted from the pozzolanic reaction of the SiO₂ and Al₂O₃ in FA with Ca(OH)₂ released by the hydration of the C₃S and C₂S in PC. When more FA replaced cement, the amounts of C₃S and C₂S were reduced; hence, the generated Ca(OH)₂ was reduced. Therefore, part of the SiO₂ and Al₂O₃ from FA had Ca(OH)₂ to react with, and it started to dilute the cement paste, resulting in a loss of strength.

Similarly, the results of the investigation of the effect of CBA on the strength development of concrete is given in Fig.14. CBA was used to replace 10%–75% of cement while keeping the FA content at 10%. 5% SF was added to enhance the early strength. It is observed that for CBA addition of up to 25%, the strengths of the mixes are enhanced over the control beyond 7 d of curing. At 50% and 75% CBA content, the strength of the mixes reduces with increasing CBA content. This is attributed to excess CaO from CBA which has no SiO₂ and Al₂O₃ to react with and therefore become dilutants in the cement paste. It is, therefore, to be expected that for each CBA replacement of cement, an optimal mix of FA and SF should be determined. It is, however, observed that even with the very high CBA content at 75%. Structural grade concrete with 28-d strength beyond 25 MPa was attained.

3.4.2 Splitting tensile strength

Fig.15 illustrates the effect of CBA on the 28-d strength of concrete. In general, as the amount of CBA in the paste increased, the splitting tensile strength of the concrete decreased. The loss of strength ranges from 11% at 25% CBA content to 28% at 50% and 52% at 75%. The reduction in strength with CBA content can possibly be attributed to flaws in the bonding and microstructural integrity of the CBA and the dilution of the binding paste with high CBA content beyond 25%.

3.4.3 Flexural strength

Fig.16 illustrates the impact of CBA on the 28-d flexural strength of concrete. The results show that with CBA content of up to 50%, the loss of flexural strength averages only 10%, whereas at 75% CBA content, the loss of strength reaches 29%. It is therefore observed that flexural strength is less sensitive to CBA replacement of cement compared to the splitting tensile strength.

4 Conclusions and recommendations

4.1 Conclusions

An experimental study was conducted to evaluate the effects of pre-treated CBA on the mechanical properties. The investigation included tests for chemical analyses of untreated and treated CBA, workability, compressive strength, split tensile strength, and flexural strength of concrete incorporating CBA. Based on the findings, the following conclusions are drawn.

1) Pre-treating CBA with sulfuric acid and sodium carbonate increases the CaO content by 11% and reduces P₂O₅ by 82%.

2) Incorporating CBA in concrete reduces its workability, and at a low water/binder ratio, a workability-enhancing admixture is necessary.

3) CBA can partially replace a significant portion of PC if mixed with an appropriate amount of pozzolanic material. This study obtained construction-quality concrete using 50% and 75% CBA, while mixes containing 10% and 25% CBA achieved compressive strengths exceeding 60 MPa. These results were obtained by adding 10% FA and 5% SF.

4) CBA replacement of PC affects the splitting tensile strength of the concrete more than the flexural strength.

4.2 Recommendations

4.2.1 Recommendations from the research

Pre-treated CBA can be used to replace a large proportion of PC in concrete production. This would mitigate the disposal problem of the poorly degradable waste CB, while reducing the use of non-renewable natural raw materials for the production of PC. However, the optimal amount of pozzolanic addition should be determined to optimize the performance.

4.2.2 Recommendations for further research

1) Optimal pozzolanic material must be determined for each CBA replacement of PC in order to enhance pozzolanic activity in the paste.

2) Since pozzolanic reactions are slow, long-term strength tests should be carried out to assess the long-term strength development of CBA concrete.

3) Durability assessments should be performed to determine the long-term impact of CBA on concrete.

4) It would be interesting to investigate the properties of pure CBA-pozzolana concrete.

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