An investigation of palm oil fuel ash and limestone powder for use in sustainable high strength concrete construction

2.1. Materials

OPC, POFA and LP are the main materials that have the properties listed in Table 1. In addition, the size distributions of particles of these materials are displayed in Figure 1. The particles of OPC retained on a 45 µm sieve amounted to 20.00 wt%, corresponding to a specific gravity of 3.15. The POFA originated from the disposal area of a palm oil mill in the Thai province of Chumphon, which was produced the burning of palm fiber, palm shells and empty fruit bundles at the temperature of 800-900 ºC. The collected POFA from the disposal area was sun-dried to remove the moisture content and then ground so that the fine particles had a retained weight of 0.66 wt% on a 45 µm sieve, a specific gravity of 2.36 and a d₅₀ of 8.50 µm. The ground POFA used in this study had a strength activity index of over 100% since the age of 7 days. The LP was sourced from a limestone powder company in Thailand. The specific gravity of the LP was 2.70, which corresponds to a d₅₀ value of 2.64 µm.

  

Figure 1 Particle size distribution of materials. 

The chemical constituents of three materials are also listed in Table 1. The principal chemical constituents of ground POFA were SiO₂, CaO and K₂O. The sum of SiO₂, Al₂O₃ and Fe₂O₃ accounted for 51.8%, while CaO accounted for 16.8% (more than 10%). Thus, the ground POFA used in this experiment was categorized as class 2 according to the TIS 2888-18 standard (standard for POFA for use as an admixture in concrete) (23TIS 2888-18. 2018. Palm oil ash for use as an admixture in concrete. Thai Industrial Standards Institute. Bangkok; Thailand.). The principal chemical constituent of LP was CaO (53.9 %). The loss on ignition (LOI) of LP was high (44.8%) because LP was in the form of CaCO₃, which decomposed to CaO and CO₂ at high temperatures (700–850 ºC) (24BernalSA, JuengerMC, KeX, MatthesW, LothenbachB, De BelieN, ProvisJL. 2017. Characterization of supplementary cementitious materials by thermal analysis. Mater. Struct.50(1):1-13. 10.1617/s11527-016-0909-2), using a temperature of 750 ºC in the LOI tests.

2.2. Mixture proportions

The six concrete mixtures investigated are listed in Table 2. The OPC was replaced by POFA in the high volume of 60, 70 and 80 wt%, which were designated as HVPA60, HVPA70 and HVPA80 concretes, respectively. In the HVPA concretes blended with LP, the OPC content was kept constant at 30% wt%. LP was used at 5 wt% and 10 wt% to replace POFA (with POFA+LP maintained at 70 wt%). These concretes were designated HVPA65L5 and HVPA60L10, respectively. All HVPA concretes had a binder volume of 560 kg/m³ and a W/B ratio of 0.25 in order to obtain the expected compressive strength of 55 MPa for high-strength concrete in accordance with ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.). To prevent the compressive strengths of OPC concrete and HVPA concrete from differing greatly when comparing the concrete properties, the OPC concrete (100 wt.% OPC as binder) had an OPC volume of 500 kg/m³ and a W/B ratio of 0.30. All concrete mixture, crushed granite was used as the coarse aggregate, passing through standard sieve No. 3/8 and retained on No. 4, with the water absorption of 0.67% and specific gravity of 2.65. For the fine aggregate, river sand was sieved through standard sieve No. 4 with the water absorption of 0.92% and specific gravity of 2.61. The superplasticizer dosage was trialed to preserve the slump of all fresh concretes at 150–200 mm.

2.3. Experimental methods

2.3.3. Heat evolution

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OPC, HVPA70 and HVPA60L10 concretes were tested for heat evolution. The heat evolution test under semi-adiabatic conditions is illustrated in Figure 2. A 50 mm thick polystyrene insulation was installed on each side of the cube mold (dimensions 450 x 450 x 450 mm). After mixing, the fresh concrete specimens were poured into the molds with insulation for concrete specimens (350 x 350 x 350 mm). A thermocouple was placed in the middle point of each concrete specimen to measure the temperature for 168 hours. All 3 concrete mixtures were tested simultaneously in a laboratory room at an ambient temperature of 25±2 ºC.

  

Figure 2 Setup for heat evolution testing. 

3.1. Superplasticizer requirement

The superplasticizer requirement of concrete to keep the slump at 150-200 mm is presented in Table 2. OPC concrete required the superplasticizer of 1.5 kg/m³ which was less than HVPA concretes. The superplasticizer requirements of HVPA60, HVPA70 and HVPA80 concretes were 6.2, 6.7 and 7.3 kg/m³, respectively. This result indicated that the ground POFA content had a significant influence on the superplasticizer requirement. In other words, the more ground POFA the mixture contained, the higher the need for superplasticizer. Sripan et al. (34SripanT, HaruehansapongS, KroehongW, SenawangW, NamarakC, JaturapitakkulC, TangchirapatW. 2024. Assessment of bonding strength of steel bar in recycled aggregate concrete containing ground palm oil fuel ash. Innov. Infrastruct. Solut.9(3):59. 10.1007/s41062-023-01360-x) found that POFA that was ground had less porosity than original POFA from the disposal area, but the particles of ground POFA still had irregular shapes. The irregular shapes of ground POFA particles affected the friction between themselves, between the particles of ground POFA and OPC, and between the particles of ground POFA and aggregates. Therefore, HVPA concretes required more superplasticizer than OPC concrete.

When the effect of LP on the superplasticizer requirement of HVPA concrete was considered. The HVPA70 concrete mixture was used to compare HVPA65L5 and HVPA60L10 concretes as the OPC replacement was the same. The superplasticizer requirement of HVPA65L5 and HVPA60L10 concretes (7.8 kg/m³) was higher than HVPA70 concrete (6.7 kg/m³). This was due to the fact that the LP had ultrafine particles (d₅₀= 2.64) compared to ground POFA (d₅₀= 8.50), which led to an increase in surface area. For this reason, the concrete containing LP resulted in higher water absorption which meant that more superplasticizer was required than for concrete without LP.

3.2. Compressive strength

3.2.1. Effect of POFA on compressive strength of concrete

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At 7, 14, 28 and 90 days, the compressive strengths of the OPC concretes were 57.1, 61.7, 65.9 and 78.0 MPa, respectively, as displayed in Figure 3. The compressive strength of HVPA concrete at 7 days was 34.7, 29.4 and 26.4 MPa for HVPA60, HVPA70 and HVPA80 concretes, respectively. The HVPA-containing concretes exhibited significantly lower early-age compressive strength than the OPC concrete, even though the W/B ratio of all HVPA concretes was less than that of OPC concrete. This demonstrated that the lightening of OPC content in the concrete resulted in reduced hydration products, which affected the decrease in early-age compressive strength in the HVPA concretes. It is generally known that the OPC reaction is primarily a reaction to generate the early strength of pozzolanic concrete, which depends on the W/OPC ratio. Considering the W/OPC ratio (without POFA), HVPA60, HVPA70 and HVPA80 concretes, it may be that these HVPA concrete mixtures still had the high W/OPC ratios compared to OPC concrete, even though POFA could absorb the water in the mixture. Therefore, the compressive strength of HVPA concrete decreased continuously when the ground POFA content escalated from 60 to 80 wt%. In other words, the early-age compressive strength of HVPA concrete correlated with the proportion of OPC in the mixture. The result agrees with the study by Herath et al. (35HerathC, GunasekaraC, LawDW, SetungeS. 2020. Performance of high volume fly ash concrete incorporating additives: A systematic literature review. Constr. Build. Mater.258:120606. 10.1016/j.conbuildmat.2020.120606) who reported that the high-volume fly ash concrete (with 50–80 wt% fly ash) negatively affects concrete early-age compressive strength.

  

Figure 3 Compressive strength of concretes containing high-volume palm oil fuel ash. 

However, as time progressed after 14 days, there was a growth in compressive strength. The compressive strengths of the HVPA60, HVPA70 and HVPA80 concretes increased to 46.5, 39.3 and 35.6 MPa respectively at 14 days and to 59.9, 46.1 and 45.4 MPa respectively at 28 days. Although all HVPA concretes still had lower compressive strengths than the OPC concrete, the compressive strengths of all HVPA concretes grew rapidly from 7 to 28 days compared to the OPC concrete. The percentage increment in compressive strength of the HVPA60, HVPA70 and HVPA80 concretes compared to the OPC concrete was 61, 51 and 46% respectively at 7 days and enhanced to 91, 70 and 69% respectively at 28 days. This was attributed to the ground POFA reacting with Ca(OH)₂ from the hydration products, resulting in increased compressive strength over time owing to the pozzolanic reaction. Additionally, the voids in the paste matrix and the small gaps of paste-aggregates could be filled with ground POFA. As a result, secondary C-S-H compounds that formed by the pozzolanic reaction were able to fill the voids in the concrete structure more completely (8JaturapitakkulC, TangpagasitJ, SongmueS, KiattikomolK. 2011. Filler effect and pozzolanic reaction of ground palm oil fuel ash. Constr. Build. Mater.25(11):4287-4293. 10.1016/j.conbuildmat.2011.04.073). The concrete compressive strength with a higher POFA content still tended to decrease at 90 days. The higher POFA replacement rates possibly resulted in the presence of many non-reactive POFA particles, which led to a reduction in compressive strength owing to pozzolanic reaction. It could be seen that HVPA60, HVPA70 and HVPA80 concretes were found to have a compressive strength of 70.9, 57.2 and 55.2 MPa at 90 days, corresponding to 91, 73 and 71 % compared to OPC concretes, respectively, whose values are close to their percentages at 28 days. It was a fact that in high-volume pozzolanic concrete, the compressive strength from the pozzolanic reaction was increased to a limited extent owing to the lack of Ca(OH)₂ from cement hydration (36ChindaprasirtP, KroehongW, DamrongwiriyanupapN, SuriyoW, JaturapitakkulC. (2020) Mechanical properties, chloride resistance and microstructure of Portland fly ash cement concrete containing high volume bagasse ash. J. Build. Eng.31:101415. 10.1016/j.jobe.2020.101415).

According to ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.), the compressive strength should be 55 MPa or more for high-strength concrete. The HVPA60 concrete exhibited a compressive strength of 59.9 MPa at 28 days, exceeding the 55 MPa. Although the compressive strength of HVPA70 and HVPA80 concrete did not reach the required 55 MPa at 28 days, the compressive strengths of HVPA70 and HVPA80 concretes were 57.2 and 55.2 MPa at 90 days, respectively. It should be noted that the HVPA80 concrete only contained 112 kg/m³ of OPC in the mixture.

3.2.2. Effect of limestone powder on compressive strength of HVPA concrete

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The above results demonstrated that the compressive strength of HVPA60 concrete was better than that of the other HVPA concretes. In addition, the HVPA60 concrete met the requirement according to ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.) for high-strength concrete, while the HVPA70 concrete almost reached a compressive strength of 55 MPa at 28 days. For this reason, LP was admixed to the HVPA70 concrete mixture (constant OPC of 30 wt%) in an amount of 5 and 10 wt% to expect increase the early-age compressive strength while maintaining the lowest cement content. The compressive strengths of HVPA+LP concretes are exhibited in Figure 4. The compressive strengths for HVPA65L5 were 27.4, 35.5, 44.4 and 57.4 MPa at 7, 14, 28 and 90 days, respectively, while the compressive strengths for HVPA60L10 were 30.8, 39.5, 53.4 and 58.3 MPa at 7, 14, 28 and 90 days, respectively. In general, ground POFA acts as a pozzolanic material that produces extra strength when it reacts with portlandite, whereas LP is an inert material. However, the use of LP in appropriate dosage could enhance the compressive strength of concrete owing to filling and nucleation effects (14WangD, ShiC, FarzadniaN, ShiZ, JiaH. 2018. A review on effects of limestone powder on the properties of concrete. Constr. Build. Mater.192:153-166. 10.1016/j.conbuildmat.2018.10.119, 16WangD, ShiC, FarzadniaN, ShiZ, JiaH, OuZ. 2018. A review on use of limestone powder in cement-based materials: Mechanism, hydration and microstructures. Constr. Build. Mater.181:659-672. 10.1016/j.conbuildmat.2018.06.075). Compared to HVPA70 concrete, the compressive strength of HVPA60L10 increased slightly (by approximately 5%) at 7 days, as the LP consisted of solid, very small particles and had a higher specific gravity than the ground POFA, thus acting as a better filler material. On the other hand, HVPA65L5 concrete exhibited a slightly reduced compressive strength (by approximately 7%) at 7 days compared to the compressive strength of the HVPA70 concrete. This information suggested that the replacement of HVPA with 10 wt% LP (HVPA60L10 concrete) may have influenced the mechanism of nucleation and filler effects better than LP 5 wt% (HVPA65L5 concrete). Therefore, the compressive strength of HVPA60L10 concrete was better than that of HVPA65L5 concrete for all test ages. Ramezanianpour et al. (18RamezanianpourAA, GhiasvandE, NicksereshtI, MahdikhaniM. MoodiF. 2009. Influence of various amounts of limestone powder on performance of Portland limestone cement concretes. Cem. Concr. Compos.31(10):715-720. 10.1016/j.cemconcomp.2009.08.003) informed that in low W/B ratio concrete, the use of LP as a substitute for OPC at 10 wt% exhibited better compressive strength than concrete with 5 wt% LP at 3 days and the higher strength was maintained until 180 days. Similar to the results of Klathae et al. (37KlathaeT, TranTNH, MenS, JaturapitakkulC, TangchirapatW. 2021. Strength, chloride resistance, and water permeability of high volume sugarcane bagasse ash high strength concrete incorporating limestone powder. Constr. Build. Mater.311:125326. 10.1016/j.conbuildmat.2021.125326), who concluded that LP at 10 wt% is suitable for use in bagasse-ash concrete.

  

Figure 4 Effect of limestone powder on compressive strength of HVPA concrete. 

3.3. Elastic modulus of concrete

The elastic modulus of all concretes is illustrated in Figure 5. The elastic modulus of the OPC concrete were 35.1 and 38.7 GPa at 28 and 90 days, respectively. For the HVPA concretes, which contained ground POFA in proportions of 60–80 wt%, the elastic modulus values were in the range of 27.5–30.7 GPa and 30.6–35.1 GPa at 28 and 90 days, respectively. This meant that the HVPA concretes had slightly fewer elastic modulus than the OPC concrete. The result was in agreement with Herath et al. (35HerathC, GunasekaraC, LawDW, SetungeS. 2020. Performance of high volume fly ash concrete incorporating additives: A systematic literature review. Constr. Build. Mater.258:120606. 10.1016/j.conbuildmat.2020.120606) and Siddique (38SiddiqueR. 2004Performance characteristics of high-volume Class F fly ash concrete. Cem. Concr. Res.34(3):487-493. 10.1016/j.cemconres.2003.09.002), who summarized that the concrete with high fly ash content tended to extenuate the elastic modulus of concrete compared to conventional concrete, since the elastic modulus depended on the concrete compressive strength. Additionally, the OPC concrete mixture in the current study had a greater proportion of coarse aggregate (990 kg/m³) than the HVPA concrete mixes (900–935 kg/m³), which affected the increased elastic modulus (39ChenH-J, ShihN-H, WuC-H, LinS-K. 2019. Effects of the loss on ignition of fly ash on the properties of high-volume fly ash concrete. Sustain.11(9):2704. 10.3390/su11092704).

  

Figure 5 Elastic modulus of concretes. 

For HVPA concretes with LP, the HVPA65L5 and HVPA60L10 concretes had the elastic modulus at 28 and 90 days in the range of 30.5–33.7 GPa and 30.7–34.0 GPa, respectively. Meddah et al. (40MeddahMS, LmbachiyaMC, DhirRK. 2014. Potential use of binary and composite limestone cements in concrete production. Constr. Build. Mater.58:193-205. 10.1016/j.conbuildmat.2013.12.012) found that the elastic modulus of OPC concrete was hardly affected at an LP content of 15 wt%. However, the elastic modulus of the concretes decreased by about 15 and 33% when LP was replaced by 25 and 45 wt%, respectively. From the results of the current study, the use of LP at 5 or 10 wt% in the HVPA concrete mixture had no significant effect on the elastic modulus of the concrete. This was due to the fact that the elastic modulus of HVPA concrete with or without LP was highly dependent on its compressive strength.

3.4. Comparison of elastic modulus of high-volume pozzolan concretes with suggested ACI

The relation between elastic modulus and compressive strengths of the HVPA concretes in this study was investigated together with concretes containing a high proportion of other pozzolans (≥50 wt%), such as high-volume fly ash (HVFA), high-volume bagasse ash (HVBA) and high-volume coal bottom ash (HVCBA) (38SiddiqueR. 2004Performance characteristics of high-volume Class F fly ash concrete. Cem. Concr. Res.34(3):487-493. 10.1016/j.cemconres.2003.09.002, 41-44), as shown in Figure 6. The elastic modulus from ACI 318M (43RerkpiboonA, TangchirapatW, JaturapitakkulC. 2015. Strength, chloride resistance, and expansion of concretes containing ground bagasse ash. Constr. Build. Mater.101(1):983-989. 10.1016/j.conbuildmat.2015.10.140) can be determined as Equation [1] for normal weight concrete. For ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.), the elastic modulus for normal weight concrete is represented by Equation [2], which contains boundary conditions for compressive strengths between 21 MPa and 83 MPa. Both equations are used to compare the elastic modulus of high-volume pozzolan concrete (HV-pozzolan) from the current study with other research results. Based on the data of HV-pozzolan concrete, the equation for the elastic modulus of concrete (E_c) is presented based on Equation [3]. It was interesting to note that most of the data of HV-pozzolan concrete were close to the recommended line of ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.). When calculating the compressive strength at 21 MPa using Equation [3], the elastic modulus value of HV-pozzolan concrete was 16 and 17% lower than the elastic modulus value calculated from the Equation [1] of ACI 318M (45ACI 318M. 2014. Building code requirements for structural concrete (ACI 318M-14) and commentary: ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.) and Equation [2] of ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.), respectively. However, for the compressive strength at 83 MPa, the elastic modulus of HV-pozzolan concrete was about 6% higher than the value suggested by ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.), while the elastic modulus value of HV-pozzolan was still about 8% less than the elastic modulus value calculated by the equation of ACI 318M (45ACI 318M. 2014. Building code requirements for structural concrete (ACI 318M-14) and commentary: ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.). Further information from Figure 6 for the lower compressive strength of about 50 MPa or the square root of the compressive strength of 7 MPa shows that the trend line of the elastic modulus of HV-pozzolan concrete is underestimated based on ACI 363R (25ACI 363R. 2010. Report on High-Strength Concrete, ACI Committee. American Concrete Institute & International Organization for Standardization, Farmington Hills; Michigan.).

From ACI 318M (45): $E_c=4.73\sqrt{f_c}$ [1]

From ACI 363R (25): $E_c=3.32\sqrt{f_c}+6.9$ [2]

From HV-pozzolan: $E_c=4.81\sqrt{f_c}-3.5$, $R^2=0.71$ [3]

where E_c is the elastic modulus of concrete (GPa) and f_c is the compressive strength of concrete (MPa).

  

Figure 6 Relationship between elastic modulus and square root of compressive strength. 

3.5. Shrinkage strain of concrete

The shrinkage strain of concrete is displayed in Figure 7. The shrinkage strain of the OPC concrete enhanced from 161 to 874 micro strains from 1 to 28 days. Although the W/B ratio of the OPC concrete at 0.3 was higher than that of the HVPA concretes (W/B ratio = 0.25), the shrinkage strains of the HVPA60, HVPA70 and HVPA80 concretes decreased by approximately 35, 45 and 58% of the values for the OPC concrete at day 1 and 14, 23 and 32% of the values for the OPC concrete at 3 days of age. It should be noted that self-drying occurred in high-strength concrete without external water supply and that autogenous shrinkage occurred in the early stages owing to the hydration reaction in the cement (46AıtcinP. 2003. The durability characteristics of high performance concrete: a review. Cem Concr Compos. 25(4-5):409-420. 10.1016/S0958-9465(02)00081-1). Therefore, the use of high-volume POFA could reduce the shrinkage strain owing to the lower OPC content, which led to a reduction in the hydration rate of the cement. The shrinkage strain of all concretes increased continuously with increasing concrete age and the shrinkage strain was lower with a higher POFA content. At 28 days, the shrinkage strains of the HVPA60, HVPA70 and HVPA80 concretes were also about 10, 14 and 19 % lower than those of the OPC concrete, respectively. These results were consistent with another study (47KristiawanSA, AdityaMTM. 2015. Effect of high volume fly ash on shrinkage of self-compacting concrete. Procedia. Eng.125:705-712. 10.1016/j.proeng.2015.11.110), which found that the total shrinkage of the concrete decreased by approximately 30% and 40% after 28 days when the concrete samples contained 55% and 65 wt% of fly ash in the binder, respectively. This result was due to the fact that the concrete with a high POFA or pozzolanic content in the mixture resulted in a reducing cement paste (OPC + water), while the remaining POFA from the pozzolanic reaction acted as an aggregate (48KumarB, TikeG, NandaP. 2007. Evaluation of properties of high-volume fly-ash concrete for pavements. J. Mater. Civ. Eng.19(10):906-911. 10.1061/(ASCE)0899-1561(2007)19:10(906)). Consequently, the shrinkage strains of these HVPA concretes decreased as time progressed.

  

Figure 7 Shrinkage strain of concretes. 

In HVPA concretes with the same cement content (HVPA70, HVPA65L5 and HVPA60L10 concretes), the shrinkage strain increased with increasing LP content in the mixture (replacement of POFA at 5 and 10 wt%) at an early age of 1-3 days. This can be attributed to the fact that the nucleation effect of the LP content accelerated the hydration reaction (15ZhaoL, HeT, NiuM, ChangX, WangL, WangY. 2024. Effect of Limestone Powder Mixing Methods on the Performance of Mass Concrete.Mater.17(3):617. 10.3390/ma17030617, 17HanK, ShuX, RanQ, ShiJ, ZhangZ. 2023. Understanding the mechanisms behind the effects of limestone powder on microstructure evolution of cement paste. J. Sustain. Cem. Based Mater.12(8):995-1008. 10.1080/21650373.2022.2144534.). Consequently, the higher autogenous shrinkage (at an early age) of the concrete containing LP occurred due to water loss during the reaction and intensified the hydration degree. De weerdt et al. (49De WeerdtK, HahaMB, Le SaoutG, KjellsenKO, JustnesH, LothenbachB. 2011. Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash. Cem. Concr. Res.41(3):279-291. 10.1016/j.cemconres.2010.11.014) observed that OPC mortar containing 5 wt% LP had higher early-age shrinkage than OPC mortar without LP. However, as time progressed, the shrinkage strains in the HVPA65L5 and HVPA60L10 concretes showed similar values to the HVPA70 concrete. This result also demonstrated that the HVPA70, HVPA65L5 and HVPA60L10 concretes exhibited lower shrinkage than OPC concrete, which could be owing to the reduction in autogenous shrinkage and drying shrinkage. It should be noted that the HVPA70, HVPA65L5 and HVPA60L10 concretes had the same OPC replacement of 70 wt%; therefore, the less cement paste resulted in a reduction in autogenous shrinkage. In addition, the pozzolanic product of POFA could refine the pores in the concrete and also serve as a filler together with POFA and LP. Therefore, the pores of the concrete were expected to be filled by both POFA and LP. Consequently, the slowing of water evaporation led to a lessening in the drying shrinkage of the concrete.

3.6. Heat evolution of concrete

OPC, HVPA70 and HVPA60L10 concretes were used to investigate the effect of POFA and PA on the heat generation of the concrete (see Figure 8). OPC concrete got a maximum temperature rise of 40.9 ºC per 500 kg/m³ OPC content, which corresponds to a time of 15 hours at maximum temperature. This result is close to that reported by Amnadnua et al (50AmnadnuaK, TangchirapatW, JaturapitakkulC. 2013. Strength, water permeability, and heat evolution of high strength concrete made from the mixture of calcium carbide residue and fly ash. Mater. Des.51:894-901. 10.1016/j.matdes.2013.04.099), who reported a temperature rise of 42 ºC per 500 kg/m³ OPC content 18 hours after mixing for OPC concretes with a W/B ratio of 0.25. For the HVPA70 and HVPA60L10 concretes, the maximum temperature rises were 19.4 and 19.7 ºC at the times of 41 and 40 hours, respectively. Although the appropriate amount of LP replacement in the concrete could accelerate the hydration reaction, the heat evolution for the HVPA60L10 concrete was similar to that for the HVPA70 concrete. This finding indicated that the use of LP at 10 wt% binder had little effect on the heat evolution of HVPA concrete. In other words, reducing the OPC in the mixture was the main factor influencing the reduction of heat evolution of concrete. This reduction in OPC content resulted in a decreased tricalcium silicate (C₃S) content, leading to a lower heat temperature rise during the hydration reaction. The use of supplementary material up to 70 wt% in the concrete mixture not only reduced the peak temperature rise by about 50%, but also delayed it by about 25 hours compared to the OPC concrete. In addition, both the HVPA70 and HVPA60L10 concretes required high superplasticizer dosages, which resulted in a delay in heat hydration (51AtişCD. 2002. Heat evolution of high-volume fly ash concrete. Cem. Concr. Res.32(5):751-756. 10.1016/S0008-8846(01)00755-4.). The temperature rises of HVPA70 and HVPA60L10 concretes was started 20–40 hours after mixing, the temperature change of these HVPA concretes was equal to that of OPC concrete about 60 hours.

  

Figure 8 Heat evolution curves of different concrete types. 

3.7. Microstructure of pastesa

3.7.1. SEM/EDS

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Figure 9 shows the microstructure characterization of high-volume pastes containing 70 % POFA and 60 wt% POFA blended with 10 wt% LP in comparison the microstructure of OPC paste. Figure 9a shows the microstructure of OPC paste. It was clear that the hydration products of OPC paste had a fibrous network structure that intertwines in an across-linked manner, connecting and appearing compact. The EDS spectra of OPC paste were Si and Ca, indicating the formation of C-S-H, which occurred from the hydration reaction of cement.

The microstructure image of the high volume POFA pastes matrix (Figure 9b and Figure 9c) shows the good quality of adhesion at the interfaces of the structural matrix. The Ca, Si and Al elements detected in the high volume POFA paste could induce the formation of C-S-H and C-A-H compounds such as the hydration and pozzolanic products. For example, at point 2 in Figure 9b, the compact and homogeneous paste matrix has a high content of Ca, Si and small amounts of K, which could be a pozzolanic product of POFA. However, the un-hydrated or unreacted POFA particles can be seen in the structural matrix of the HVPA70 paste. The surface of POFA was still smooth, and no hydrate had formed on their surface (point 1, Figure 9b). It is worth noting that the EDS results of HVPA70 at the unreacted position of the POFA particle at point 1 in Figure 9b indicated the elements Si and K, which are related to the chemical composition of the raw materials of POFA. For the paste with the addition of LP (Figure 9c), the SEM image of HVPA60L10 paste displays an altered structure, there is an improvement in the microstructure of the samples. The HVPA60L10 paste exhibits a more compact matrix compared to the HVPA70 paste and the particle of POFA (point 1 in Figure 9c) appeared to be coated with a thin layer of hydration products. When comparing the EDS results of HVPA70 and HVPA60L10, it was found that the Ca element increased with the replacement of LP in the mixture. The Ca content in the HVPA60L10 paste was not only due to the hydration and pozzolanic products, but also due to the raw LP that was replaced in the mixture. This was because the raw LP consisted of a configuration of Ca and O atoms in calcite that was similar to the CaO layers in the formation of C-S-H (19BentzDP, ArdaniA, BarrettT, JonesSZ, LootensD, PeltzMA, SatoT, StutzmanPE, TanesiJ, WeissWJ. 2015. Multi-scale investigation of the performance of limestone in concrete. Constr. Build. Mater.75:1-10. 10.1016/j.conbuildmat.2014.10.042). Therefore, more elemental Ca was detected in the EDS results for the HVPA60L10 paste than for HVPA70 paste. In addition, the EDS results at point 2 in Figure 9c show high Ca, O and C elements, suggesting that these are LP particles. The LP had bonding on interface of POFA particles, resulting in the formation of more compactness and less permeable voids. This result was related to the findings of Liu and Yan (13LiuS, YanP, 2010. Effect of limestone powder on microstructure of concreteJ. Wuhan. Univ. Technol. Mater. Sci. Ed.25(2):328-331. 10.1007/s11595-010-2328-5), who found that the LP could act as a filler and improved the microstructural matrix of the cement paste by reducing porosity. These results and the SEM images confirm the effect of limestone powder on the compressive strength in section 3.2.2, according to which the compressive strength of the concrete could be increased by adding LP.

Figure 9

  

Figure 9a OPC paste. 

  

Figure 9b HVPA70 paste. 

  

Figure 9c HVPA60L10 paste. 

3.7.2. TGA

 ⌅

The hydration products such as C-S-H, C-A-H and Ca(OH)₂, including the CaCO₃ compound of the OPC, HVPA70 and HVPA60L10 pastes were analyzed by mass loss at specific temperature boundaries as shown in Figure 10. The loss of bound water, i.e., the decomposition of C-S-H and C-A-H (dehydration), occurs in the temperature range of 105-430 °C. The dehydroxylation of Ca(OH)₂ and the decarbonation of CaCO₃ occur in the temperature ranges of 430 -550 °C and 550-900 °C, respectively (52DjenaoucineL, PicazoÁ, de la RubiaMÁ, MoraguesA, GálvezJC. 2024. Influence of graphene oxide on mechanical properties and durability of cement mortar. Mater.17(6):1445. 10.3390/ma17061445.).

The mass losses during the dehydration phase of OPC, HVPA70 and HVPA60L10 pastes were 11.1 %, 9.9 % and 10.0 %, respectively, which corresponds directly to their compressive strength. In other words, a higher mass loss in the dehydration phase of the OPC paste resulted in a higher compressive strength of the OPC concrete. It is noteworthy that the mass loss of OPC paste in the temperature range of 430-550 °C was the highest at 3.0 % due to the decomposition of Ca(OH)₂. In contrast, the mass losses of HVPA70 and HVPA60L10 pastes in the dehydroxylation phase were lower at 0.7% and 1.5%, respectively. The observed values of this result imply that the HVPA70 and HVPA60L10 pastes had lower Ca(OH)₂ content, which could be due to their lower OPC content and the potential consumption of Ca(OH)₂ by pozzolanic reactions with POFA. In the decarbonation phase, the OPC, HVPA70 and HVPA60L10 pastes exhibited mass losses of 3.5, 5.4 and 8.1%, respectively. In particular, the HVPA60L10 paste, which contained a considerable amount of CaCO₃ due to the presence of LP, showed the highest mass loss during decarbonation. Furthermore, it was observed that the HVPA70 paste had a greater mass loss compared to the OPC paste. This can be attributed to the high amount of unburned carbon in the POFA, as shown by its LOI value.

  

Figure 10 Thermogravimetric analysis of OPC, HVPA70 and HVPA60L10 paste. 

3.8. Estimation of CO₂-equivalent emission of concrete

Figure 11a shows the CO₂-e value of the individual concrete mixtures in this study. The CO₂-e value of OPC concrete was 438 kg∙CO₂/m³. This result was consistent with the data for OPC concrete from another study for the similar strength class of 50–60 MPa, for which about 400-460 kg∙CO₂/m³ was reported (36ChindaprasirtP, KroehongW, DamrongwiriyanupapN, SuriyoW, JaturapitakkulC. (2020) Mechanical properties, chloride resistance and microstructure of Portland fly ash cement concrete containing high volume bagasse ash. J. Build. Eng.31:101415. 10.1016/j.jobe.2020.101415, 31BragaAM, SilvestreJD, de BritoJ. 2017. Compared environmental and economic impact from cradle to gate of concrete with natural and recycled coarse aggregates. J. Clean. Prod.162:529-543. 10.1016/j.jclepro.2017.06.057). The CO₂-e value of OPC concrete was related to the amount of OPC in the mixture (94% of the total CO₂-e of the OPC concrete mixture). HVPA concretes (based on the feedstock only) had CO₂-e values of 222, 178, 134, 182 and 185 kg∙CO₂/m³ for HVPA60, HVPA70, HVPA80, HVPA65L5 and HVPA60L10, respectively. When the addition of POFA transportation at 1,000 km, the CO₂-e values of HVPA60, HVPA70, HVPA80, HVPA65L5 and HVPA60L10 concretes increased to 236, 195, 153, 198 and 200 kg CO₂/m³, respectively. The result indicated that the POFA transportation distance of 1,000 km had little effect on the CO₂-e intensity of the HVPA concretes. Figure 10a also demonstrates that although the HVPA concretes are able to reduce the OPC content in the mixtures to 60–80 wt%, the CO₂-e value still depends on the amount of OPC in the mixtures. For the HVPA60 and HVPA80 concretes, for example, the CO₂-e values owing to OPC in the mixtures (184 and 92 kg∙CO₂/m³ respectively) accounted for around 84 and 70% of the total CO₂-e values of these concretes.

Figure 11b illustrates the relation between CO₂-e and compressive strength at 28 days for the other OPC concretes (37KlathaeT, TranTNH, MenS, JaturapitakkulC, TangchirapatW. 2021. Strength, chloride resistance, and water permeability of high volume sugarcane bagasse ash high strength concrete incorporating limestone powder. Constr. Build. Mater.311:125326. 10.1016/j.conbuildmat.2021.125326, 54-57) in comparison to a similar strength class of HVPA concretes. It is obvious that the CO₂-e value of OPC concrete increased with increasing compressive strength. As already mentioned, the CO₂-e value in concrete is mainly generated by the OPC in the mixture. The increased compressive strength required a higher OPC content to keep the W/C ratio low, resulting in high CO₂-e values. Figure 10b gives more information about the trend line with the CO₂-e values for OPC concretes. When the CO₂-e values of HVPA concretes are compared with those of OPC concretes with the same strength (from the trend line), the CO₂-e values for HVPA60, HVPA70, HVPA80, HVPA65L5 and HVPA60L10 concretes decrease by approximately 44, 49, 62, 47 and 51%, respectively. However, even when including the transportation of POFA (approx. 1,000 km), HVPA60, HVPA70, HVPA80, HVPA65L5 and HVPA60L10 concretes still had lower CO₂-e values than OPC concrete (40, 44, 56, 42 and 47%, respectively). This result indicates that concrete with low CO₂-e emissions can be produced by using high replacement rates of OPC by POFA and LP in the mixture.

Figure 11

  

Figure 11a CO₂-e emission of concrete in this study. 

  

Figure 11b Relationship between CO₂-e emission and compressive strength of concrete.