Influence of acids and slurries on the properties of recycled concrete aggregates

ABSTRACT

This research investigates to remove and strengthen the weak smeared mortar and enhance its quality through sustainable and eco-friendly treatment techniques. The impregnation of recycled coarse aggregate (RCA) in acids (ATRCA) in different molarities was proposed to eradicate the weak smeared cement particle on the RCA and impregnation of RCA in slurries (STRCA) at various dosages was proposed to strengthen the weak smeared mortar on the RCA. The properties of the RCA were assessed prior and after treatment techniques. The micro-structure of the treated RCA was examined through SEM to assess the impact of treatment techniques on the RCA properties. Results infer that both treatments tend to improve the quality of RCA, however slurry treatment strengthens the weak mortar rather than its removal through acid treatment and thus resulting in better properties to RCA. The optimized molarity was observed at 0.3 M for 3 days for acid treatment and optimized slurry dosage was observed at 0.8w/c ratio for 24 hours. The optimized ATRCA and STRCA show 21.70% and 39.07% lesser water absorption than RCA. Similarly, other physical and mechanical properties of ATRCA and STRCA were enhanced compared to RCA. Correlation was established between physical and mechanical properties of the RCA, ATRCA and STRCA. Life cycle assessment of the aggregates was performed with OpenLCA software.

Keywords:
Recycled concrete aggregate; Acid treatment; Slurry treatment; Aggregate properties; Morphology; Life cycle assessment

1. INTRODUCTION

Increase in the generation of construction wastes has negative effect on the environmental integrity [¹, ²]. So, recycling the construction wastes will be a feasible solution to guard the environment, conserving natural resources and promoting sustainability in the construction. Specifically, for the countries like Qatar, use of natural coarse aggregate (NCA) was increasing consuming 400 million tons of natural resources yearly of which 100 million tons of wastes are generated (QatarMMC). Also, Qatar generates 2.5 million tons of municipal wastes of which construction wastes amounts to 20,000 per day. Many studies have attempted to use the RCA as a substitute to NCA in the production of recycled aggregate concrete (RAC) [³-⁵]. However, the substandard properties of RCA such as higher water absorption (WA), lower density etc. ensuing from the weak mortar on its surface incumbers its probable use in the construction [⁶-⁸]. Researches have implemented two strategies to enhance the quality of RCA through eradication of the weak mortar on its surface by pre-soaking, mechanical, thermal, thermo-mechanical and chemical treatments [⁹-¹³] and through strengthening the weak mortar on its surface by polymer impregnation, microbial, carbonation, silica-fume treatments and modified mixing approaches [¹⁴-¹⁶]. SIVAMANI et al. [¹⁷] reviewed the influence of various treatment methods to RCA and presented a comparative analysis on their efficacy indicating the highest reduction in WA with acids (48%), microbes (68%) and slurries (47%). These treatments as mentioned in the chronological order show augmentation in the strength of the concrete by 25%, 36.5% and 34%. It could be found that acid impregnation eradicates the weak mortar on the RCA while microbes and slurries strengthen the weak mortar through impregnation into voids and micro-cracks on its surface. Microbial treatment reduces the WA on RCA through deposition of CaCO₃ on the micro-cracks on its surface and inside the RCA [¹⁸-²⁰]. However, the practical implementation of microbial treatment on large scale is non-feasible owing to its uneconomical and skilled supervision techniques.

Pre-soaking in acids was considered to be the efficient technique in removing the weak mortar among other removal methods. THAUE et al. [²¹] used acetic acid to enhance the RCA and found that the WA of ATRCA was lowered by 30% and relative density and bulk density was enhanced by 3% and 7.4%. The dissolution of hydration products in the old interfacial transition zone (ITZ) enhances the quality of RCA. PANGHAL and KUMAR [²²] treated RCA with 0.5M HCl acid and found that WA of ATRCA was dropped by 19.06% and the density was enhanced by 3.15%. CHAUHAN and SINGH [²³] pre-saturated RCA in acetic acid and mechanically abrased and observed 53% reduction in the WA of the RCA. The reduction in the WA of RCA through eradication of weak mortar enhances the concrete properties also. KIM and JANG [²⁴] impregnated RCA in 1M HCl acid for 24-hrs and found that WA of ATRCA was lowered in the range of 42% to 83% for different qualities of RCA [²⁴]. The authors also infer that no much variation in the specific gravity was observed in the RCA after acid treatment. RAMALINGAM et al. [¹²] pre-saturated RCA in different molarities of RCA and found that pre-soaking RCA in optimized 0.5 M HCl acid show 17.46% reduction in the WA of RCA. The loosening of weak cementitious material on the RCA reduces its WA and thus exhibits better concrete properties. WANG et al. [²⁵] pre-saturated RCA in crystalline agent for 24-hrs and 7 days and observed that enhancement in the quality of RCA through reduction in the WA of RCA by 24.82% at 24-hrs and 58.49% at 7 days. Similarly, density was augmented by 0.9% at 24-hrs and 9.17% at 7 days and crushing value was reduced by 18.75% at 24-hrs and 24.69% at 7 days. The formation of C-S-H seals the voids in the RCA that reduces the perviousness of the RCA. WANG et al. [¹⁹] impregnated RCA in different percentages of acetic acid for different durations and found higher reduction in the WA (17%) in 1% of acetic acid for 24-hrs and maximum augmentation in the density at equivalent combinations of impregnation. KIM et al. [²⁶] observed that pre-soaking RCA in HCl acid exhibit 39% reduction in the WA while impregnation in Na₂SO₄ show 35% reduction in the WA. SARAVANAKUMAR et al. [²⁷] impregnated RCA in HCl, H₂SO₄ and HNO₃ acids and found 1.52%, 5.63% and 2.50% mass-loss ensuing from the removal of weak mortar. The WA of HCl, H2SO4 and HNO3 treated RCA was 27.14%, 30% and 28.57% lesser than untreated RCA and similarly the acid-treated RCA show enhanced density than untreated RCA. The removal of weak mortar through acid impregnation improves the quality of RCA. REVATHI et al. [²⁸] impregnated RCA in HCl and H₂SO₄ and found 41% and 48% reduction in the WA of the RCA. Furthermore, the bulk density also tends to enhance by 13.83% and 14.72%.

These various methodologies, which are based on various approaches such as analytical, numerical, and experimental, produce varying findings in terms of pile behaviour due to tunneling. Similar to acid treatment, treatment to RCA with slurries tend to be effective in strengthening the adhered mortar. SILETANI et al. [²⁹] observed that optimized pre-soaking of RCA in slurries for 12-hrs show higher reduction in the WA as observed in the case of silica fume and nano-montmorillonite. The study also inferred that slurries that exhibit higher reduction in WA of the RCA does not show better concrete properties. PANGHAL and KUMAR [²²] pre-soaked RCA in cement slurries for 24-hrs and found that WA of STRCA was 12.76% lower than RCA and the density of STRCA was 2.58% higher than RCA. The filling attribute and pozzolanic nature of cement slurry tend to augment the quality of RCA. ALQARNI et al. [³⁰] pre-soaked RCA in 20%, 30%, 40% and 50% of cement-silica fume slurry and observed optimized impregnation in 40% of slurry show 45.33% lesser WA and 21% lesser abrasion than untreated RCA. WANG et al. [³¹] covered RCA with cement and fly ash and with cement, fly ash, accelerator and retarder. It was observed that the former treatment lowered the WA of RCA by 5.88% and the latter treatment lowered the WA by 8.31%. Similarly, the density of the former and latter treated RCA was enhanced by 0.96% and 1.53%. SHABAN et al. [³²] found that pre-soaking RCA with cement and fly ash slurries and 3% nano-silica fume slurries show up to 55% reduction in the WA of the RCA and up to 11% increase in the density of the RCA [³²]. The lowering of Ca/Si ratio and homogenous surface attribute to the augmentation in the characteristics of RCA. NGUYEN et al. [³³] pre-soaked RCA in 20%, 40% and 60% of cement-silica fume slurries for 4, 12, 24 and 48-h and observed optimized pre-soaking in 60% of slurry for 48-h show 46.18% reduction in the WA and 30.55% reduction in the crushing value of the RCA. KUKADIA et al. [³⁴] pre-soaked RCA in cement slurry for 24-h and found that WA of RCA was lowered by 47% and relative-density was enhanced by 1.63%. The impact value, crushing value and abrasion value of STRCA was also lowered compared to RCA. ZHANG et al. [³⁵] modified RCA with nano-silica slurry and found that WA and crushing value was lowered by 23.89% and 13.27% compared to RCA. The filler effect and formation of additional C-S-H densifies the microstructure of RCA and improves its quality.

Life Cycle Assessment (LCA) is a systematic analysis of the environmental impacts of a product, process, or service throughout its entire life cycle. Various researches have performed the LCA of concrete, however our research focus on investigating the aggregate properties, discussion of LCA of aggregates needs importance. The production phase of concrete involves mixing cement, aggregates, water, and potentially other admixtures. This process consumes substantial energy, particularly in ready-mix concrete plants. Several LCA studies have focused on optimizing this phase by reducing cement content and incorporating recycled materials. For example, using RCA can significantly reduce the demand for natural resources and lower emissions associated with raw material extraction [³⁶]. ZHANG [³⁷] studied the use of carbon capture in ready-mix plants, which demonstrated the potential to sequester CO₂ in concrete mixtures, thereby reducing the overall carbon footprint [³⁷]. MARINKOVIĆ et al. [³⁸] conducted a comparative LCA of recycled and conventional concrete, concluding that RCA can reduce the overall environmental burden by up to 15%. Studies by SCRIVENER et al. [³⁹] suggest that integrating carbon capture storage technologies in cement and concrete production can dramatically lower CO₂ emissions.

2. RESEARCH SIGNIFICANCE

A series of literature reviews focus on how the mortar that has been smeared on the RCA has direct impact on its inferior properties. It could be observed that traditional methods like mechanical grinding and heat treatment often leave residual adhered mortar, reducing RCA’s quality and impacting concrete performance. Acid treatment offers a targeted approach to dissolve contaminants and enhance RCA surface properties, thereby improving bonding. Similarly, slurry treatment provides thorough cleaning and minimizes surface irregularities, resulting in a more uniform aggregate. These methods were chosen to improve RCA’s mechanical properties and durability in concrete, providing a rationale for their selection in this study. Also, studies diversified in removing the weak mortar and strengthening the weak mortar. However, only few studies discuss on the influence of molarity of acids in removing the weak mortar and influence of cement slurries of various w/c ratios in strengthening the smeared mortar. Furthermore, the RCA was acquired from the regions of Qatar where discussions on effective utilization of untreated and treated RCA was still a gap. This study investigates the behaviour of RCA collected from the construction industries around the regions of Qatar and the influence of acid-treatment and slurry-treatment to enhance the quality of RCA. Though acid treatment and many slurries have been used by researchers to treat the RCA world-wide, being the initial stage of investigation, HCl acid of different molarities and cement slurries of different w/c ratio were used to treat the RCA. The research was performed with acids and basic slurries with an objective to intend a guide for practioners with less skilled operations for its real time field applications. The physical and mechanical properties of RCA prior and later treatment were evaluated. The morphological examinations through SEM were done to assess the influence of the treatments to RCA. Life cycle assessment (LCA) was performed with OpenLCA software to assess the environmental impact of aggregates used in the research.

3. RAW MATERIALS

53-grades ordinary portland cement (OPC) with 3.15 g/cm³ density obtained from the local suppliers was used to prepare slurries to treat the RCA. Table 1 depicts the chemical composition of OPC and Table 2 depicts the physical properties of cement used in the research. Figure 1 depicts the microstructure and oxide composition of the cement used to prepare slurries. The SEM image show a spherical shaped particle with few irregular shaped angular edge particles. The XRD pattern show the peak of C₃S around 2θ = 29.4°, 32.1°, and 47.2°, peaks of C₂S around 2θ = 30.0°, 32.5°, and 50.0°, peaks of C₃A around 2θ = 32.8°, 39.8°, and 43.2°, peaks of C₄AF around 2θ = 28.8°, 32.0°, and 55.4° and peaks of gypsum around 2θ = 11.6°, 20.9°, and 29.4°.

Table 1
Chemical composition of OPC.

COMPOUND	PERCENTAGE (%)
Calcium oxide (CaO)	60-67
Silicon-di-oxide (SiO2)	17-25
Aluminum oxide (Al2O3)	3-8
Ferric oxide (Fe2O3)	0.5-6
Magnesium oxide (MgO)	0.1-4
Sulphur trioxide (SO3)	1-3
Sodium Oxide (Na2O) Potassium oxide (K2O)	0.4-1.3

Table 2
Properties of OPC.

PROPERTIES	ATTAINED VALUES	SPECIFIED VALUES IS4031 (Part I): 1996
Initial set time (mins)	28	30
Final set time (mins)	540	600
CS (MPa)	53.2	53
Specific surface area (m2/kg)	370	300 to 400
Density (kg/m3)	1120	830 to 1650

Figure 1
Micrographs of OPC. (a) SEM (b) XRD.

The natural gravel obtained from the local vendors with particle size ranging from 10 mm to 20 mm was employed as NCA in the research. The RCA used in the study was obtained from recycling the concrete proportions of the construction wastes generated through retrofitting of a 50-year-old complex building at Qatar. The large sized boulders obtained from the demolition site was crushed with local crusher plant and sieved to a size equivalent to NCA, washed to remove the loose particles adhered on it, immersed in water for one-day and open-dried for 5-h before its treatment. Figure 2 depicts the image of demolished building, jaw crusher and RCA. Figure 3 depicts the gradation curves of NCA and RCA used in the research. HCl acid with molecular weight of 36.45 g/mol obtained from the local suppliers was used to treat the RCA. Potable water, colorless and odorless and free from impurities with basic pH value was used in the process of concrete manufacture.

Figure 2
Preparation of RCA. (a) Demolished building (b) Jaw crusher (c) RCA.

Figure 3
Gradation curves of NCA and RCA.

4. TREATMENTS TO RCA

4.1. Acid treatment

The acid treatment to RCA was done to augment the characteristics of the RCA through removal of old cement paste, reducing impurities, and enhancing its properties for use in concrete. The prepared RCA was immersed in water for 24-h and open-dried for 5-h to attain SSD. To prepare 1 M of HCl acid, 83.33 ml of conc. HCl acid (calculated based on its molecular weight) was added to 500 ml of distilled water in a 1-litre volumetric container and the container was swirled gently to mix the solution thoroughly and the addition of water continued till 1-litre and thoroughly mixed. To prepare HCl acid, 5% of HCl acid concentration in water was added. The prepared RCA was submerged with its entire surface covered in the HCl acid solution of 0.1 M, 0.3 M, 0.5 M and 0.7 M for 24-h, 3 days and 7 days. Figure 4 depicts the acid treatment to sample RCA (small scale). During the soaking, the acid solution was stirred at regular intervals and after the desired immersion period, the RCA was taken out from the container and rinsed with clean water to eliminate the enduring acids and feeble cement particles on its surface. The RCA was allowed to dry and used as ATRCA in the study. Figure 4 depicts the sample of acid treatment to the RCA at various molarities.

Figure 4
Sample of acid treatment to the RCA at various molarities

4.2. Cement slurry treatment

Cement slurry treatment was performed to augment the characteristics of RCA by coating it with a slim film of cement slurries. The cement slurry was prepared by mixing cement with water at various w/c ratios of 0.5, 0.6, 0.7, 0.8 and 1.0. The cement and water at the specified ratio were added in a container and mixed thoroughly to prepare the consistent mixture with no lumps. The prepared RCA was submerged in the cement slurry prepared at different w/c ratios for one-day and open-dried for 3-h. The dried RCA was sieved to remove the feeble particles on its surface and used as STRCA in the study. Figure 5 depicts the image of untreated RCA and STRCA. Figure 6 depicts the flowchart for the acid and slurry treatment to RCA.

Figure 5
Visual appearance of RCA and STRCA.

Figure 6
Flowchart for RCA treatment (a) acid treatment (b) slurry treatment.

5. TESTING OF AGGREGATES

Four samples of aggregates such as NCA, RCA, ATRCA and STRCA was prepared to assess its physical, mechanical and micro-structural characteristics and the behaviour of ATRCA and STRCA was compared with RCA and NCA. The physical and mechanical properties of NCA, RCA, ATRCA and STRCA were evaluated as per ASTM and BS standards (Figure 7). The specific gravity and WA of aggregates was evaluated as per ASTM C127 [⁴⁰]. To evaluate the WA, the prepared aggregates was oven-dried at 110 ± 5°C, cooled and weighed (W_d) which was then immersed in water for 24-h. The submerged aggregates were removed after 24-h, dried with cloth to achieve SSD and weighed (W_ssd). The WA of the aggregates were computed as per the Equation 1.

Figure 7
Experimental set up for testing of aggregates.

(1) $$\text{Water Absorption}\left(\%\right)=\left({\text{W}}_{\text{ssd}}-{\text{W}}_{\text{d}}\right)\text{/}\left({\text{W}}_{\text{d}}\right)\times \text{1}00$$

The bulk density of aggregates was evaluated as per ASTM C127 [⁴⁰]. The procedure is equivalent to WA wherein the submerged weight of the aggregates (W_sub) in water were computed in addition to W_ssd and W_d. The density of aggregates were computed as per the Equation 2.

(2) $$\text{Density}={\text{W}}_{\text{ssd}}/\left({\text{W}}_{\text{ssd}}–{\text{W}}_{\text{sub}}\right)\times \text{Density of Water}$$

The impact value (IV) of aggregates were computed as per BS 812-112 [⁴¹]. The prepared aggregate samples were sieved through 12.5 mm and retained on 10 mm and placed in three layers in the cylindrical steel cup of the impact testing machine with each layer compacted with 25-strokes and balanced. The hammer was allowed to free fall at a height of 380 mm over the cylinder for 15 blows and sieved through 2.36 mm sieve and balanced. The IV of aggregates were computed as per the Equation 3.

(3) $$\text{Impact Value}\left(\%\right)=\left(\text{Weight of fractions passing through 2}.\text{36 mm sieve}\right)/\text{Total weight of the sample}\times \text{1}00$$

The crushing value (CV) of aggregates were computed as IS 2386 [⁴²]. The empty cylinder was balanced as W and the prepared aggregate samples were sieved through 12.5 mm and retained on 10 mm placed in three layers in the cylindrical steel cup with each layer compacted with 25-strokes and balanced (W₁). The set up is subjected to a uniform load of 40 tons over a time span of 10 minutes and sieved 2.36 mm sieve and balanced (W₂). The CV of aggregates were computed as per the Equation 4.

(4) $$\text{Crushing Value}\left(\%\right)={\text{W}}_{\text{2}}\text{/}\left({\text{W}}_{\text{1}}–\text{W}\right)\times \text{1}00$$

The abrasion value (AV) of aggregates were computed as IS 2386 [⁴²]. The prepared aggregate samples were balanced (W₁) and placed in loas Angeles abrasion machine and rotated at 30 rpm with 12 steel charges. The abraded samples were passed through 1.70 mm sieve and retained aggregates were balanced (W₂). The AV of aggregates were computed as per the Equation 5.

(5) $$\text{Abrasion Value}\left(\%\right)={\text{W}}_{\text{1}}–{\text{W}}_{\text{2}}{\text{/W}}_{\text{1}}\times \text{1}00$$

The micro-structural investigation through SEM was performed for all aggregate samples. The aggregates samples were approximated to 5 mm and coated with thin layer of conductive material. The samples were examined at certain magnification factor. LCA was performed with Open LCA software and the goal is to compare the environmental impacts of NCA, RCA, ATRCA and STRCA. The research will cover cradle-to-gate assessment including raw material extraction, transportation, processing (e.g., treatments), and end-of-life options. The NCA was sourced from natural gravel, which involves mining and transportation, the RCA was sourced from demolished building waste, processed via crushing, washing, and drying, ATRCA and STRCA was prepared with the sourced RCA with acids and slurries wherein the latter involves the use of cement and water. The key indicators considered in the research include global warming potential (GWP), water use, energy use and waste management.

6. RESULTS AND DISCUSSIONS

6.1. Optimization of acid molarity

Figure 8 depicts the variation in the WA of the RCA at different molarities and different soaking periods. It is well known that the WA is the key indicator to evaluate the RCA properties as mortar smeared on the RCA absorbs more water resulting in the deprived concrete properties. The WA of RCA was 86% more than NCA owing to the higher perviousness of the RCA. The RCA possess NCA with weak mortar smeared on its surface with micro-voids and pores. The micro-voids on the mortar widens upon crushing during recycling stages which increases its perviousness and thus resulting in the higher WA [³, ⁴³]. Upon acid treatment, the WA of the RCA tend to reduce. The optimized acid treatment was observed at pre-soaking RCA in HCl acid of 0.3 M at 3 days. The WA of ATRCA at optimized dosage was 21.70% lesser than RCA. The acid impregnation of RCA eradicates the weak mortar on it and thus resulting in the reduction in the WA of the RCA [¹⁹, ²⁶, ⁴⁴]. The impregnation of RCA in the acids involves interaction of acids with the loose mortar that disintegrates the weak mortar on its surface.

Figure 8
Variation of WA of RCA with acid molarity.

6.2. Optimization of w/c ratio of slurries

Figure 9 depicts the variation in the WA of the RCA upon variation in the w/c ratio for slurries. The optimum w/c ratio for slurries was observed at 0.8w/c ratio showing 39.07% reduction in the WA of the RCA. The WA of RCA tend to lower 0.8 w/c ratio and then increases. The WA of STRCA at 0.5, 0.6, 0.7 and 0.8 w/c was lowered by 14.70%, 17.08%, 21.70% and 39.07% compared to untreated RCA. However, the WA of STRCA at 1.0 w/c was lowered by only 26.89% compared to untreated RCA. The filler attribute and pozzolanic effect of slurry attributes to the reduction in the WA [³², ³⁵]. The slurry impregnates into the micro-voids on the RCA and pores inside the RCA and showing higher reduction in the WA of RCA than acid impregnation. However, too low or too high slurry coating results in the either lesser resistance towards the porosity or higher perviousness of the cement paste.

Figure 9
Variation in the WA of RCA with slurries.

6.3. Physical properties of aggregates

Figure 10 depicts the specific gravity of the NCA, RCA, ATRCA and STRCA. The specific gravity of RCA was 12.87% lesser than NCA and that was attributed to the residues of weak mortar attached to the RCA with pervious texture [⁴⁵]. The porous texture was attributed to the crushing process that cracks the mortar smeared on the RCA. Upon treatments, the specific gravity of the RCA tends to enhance. The specific gravity of ATRCA was enhanced by 2.06% and STRCA was enhanced by 3.26% than RCA. The acid impregnation eradicates the weak cement particles on the RCA while slurry treatment strengthens the weak mortar by filling the micro-cracks/pores on/inside the RCA [²¹, ²⁴, ²⁹].

Figure 10
Specific gravity of aggregates.

Figure 11 depicts the bulk density of the NCA, RCA, ATRCA and STRCA. Similar trends were observed in bulk density as in case of specific gravity. The density of RCA was lowered by 3.32% compared to NCA and was attributed to the mortar smeared on its surface, while the density of ATRCA and STRCA was improved by 1.56% and 2.11% compared to RCA. The removal of weak mortar on the RCA through acid treatment and strengthening of weak mortar through slurry treatment attributes to the enhancement in the bulk density of RCA.

Figure 11
Bulk density of aggregates.

Figure 12 depicts the WA of NCA, RCA, ATRCA and STRCA. The absorption of water by RCA was 86% more than NCA. The RCA was obtained through recycling the concrete proportions that comprises NCA and cement mortar attached over it. The mortar smeared on the RCA was loose with high porous texture which further increases upon crushing it to reduce its particle sizes [⁴⁶, ⁴⁷]. Such porous texture of RCA increases the WA of the RCA and thus affecting the concrete properties. However, upon treatment, the RCA tend to absorb lesser. The WA of ATRCA and STRCA was lowered by 21.70% and 39.07% than RCA. The acid impregnation of RCA disintegrates the loose particles dispersed on the RCA and lowers its WA. Nevertheless, impregnation of RCA in the cement slurry results in sealing the cracks on the RCA and impregnation of slurries into the voids of the RCA and reduces its WA [²²]. Figure 13 depicts the relationship between WA and density of aggregates. It was observed that converse relationship was observed between density and the WA of the RCA. The mortar on the RCA possesses loose mortar with lower density and micro-cracks on it with higher perviousness and thus resulting in such attribute.

Figure 12
Water absorption of aggregates.

Figure 13
Correlation between water absorption and bulk density.

6.4. Mechanical properties of aggregates

The IV of the aggregate indicates the aggregate toughness and its capacity to withstand fracture. Figure 14 depicts the IV of NCA, RCA, ATRCA and STRCA wherein good quality strong aggregates exhibit lesser impact value. The IV of NCA is 17.14% whereas the IV of RCA was 34.32% more than NCA. The mortar smeared on the RCA reduces the density of RCA as it possesses more loose particles with crack and porous texture that reduces the toughness of RCA and eventually its IV. However, treatments to RCA removes the smeared mortar and strengthens it. The IV of ATRCA was lowered by 10.19% while the IV of STRCA was lowered by 29.38% compared to RCA. The impregnation of RCA in the acids eradicates the pervious mortar whereas slurry treatment strengthens the pervious mortar [⁴⁴, ⁴⁸]. However, significant improvement was observed with slurry treatment as acid treatment does not ensure complete removal of smeared mortar while in slurry treatment, slurry fills the voids inside the RCA and seals the micro-cracks on its surface by forming a thin layer on it. This eventually improves the toughness of RCA and thus its IV.

Figure 14
Impact value of aggregates.

The CV of the aggregate indicates the resistance of aggregates to crushing load for it intend use in the road applications. Figure 15 depicts the CV of the NCA, RCA, ATRCA and STRCA. Similar to IV, aggregates with low CV indicates its better quality. The CV of NCA was 20.21% while the RCA was 20.21% higher than NCA. The attribute to such variation of CV of RCA is equivalent to that of IV. The CV of ATRCA and STRCA was lowered by 12.43% and 22.54% compared to RCA. The CV was better in STRCA than ATRCA as slurries were efficient in enhancing the surface resistance of RCA. The slurries form a thin coat over the RCA like a film that resists the crushing load on it [³²]. However, in ATRCA only weak mortar was removed partly and no such protective layer to enhance the crushing resistance was formed [⁴⁹, ⁵⁰].

Figure 15
Crushing value of aggregates.

The AV of the aggregate indicates the resistance of aggregates towards abrasion and lower the AV and stronger the aggregates. Figure 16 depicts the IV of the NCA, RCA, ATRCA and STRCA. The trend line of AV is similar to CV and IV. The AV of NCA was 16.84% while the AV of RCA was 29.48% higher than NCA. The presence of weak mortar on its surface reduces its abrasion resistance and thus showing lower AV than NCA. However, the AV of ATRCA and STRCA was lowered by 11.85% and 18.89% compared to RCA. Similar to IV and CV, AV was better in STRCA than ATRCA. The treatment to RCA lowers the stress caused by the smeared mortar and enhances the strength of RCA [⁵¹, ⁵²].

Figure 16
Abrasion value of aggregates.

6.5. Correlation between physical and mechanical properties

Figure 17 depicts the correlation of WA with mechanical properties of RCA. The linear relationship of WA was strong with IV and CV showing a regression factor of 0.9446 and 0.8246, while the relationship of WA with AV is nominal showing a regression of 0.5953. Figure 18 depicts the correlation among mechanical properties of RCA. In this case, relationship was established between IV with AV and CV. It was observed that all mechanical properties of untreated and treated RCA vary equivalently showing incremental value for untreated RCA and decremental value for treated RCA. The linear relationship was established between IV and CV with a regression factor of 0.8878 and also linear relationship was established between IV and AV with a regression factor of 0.9222. Table 3 shows the correlation coefficient among various parameters.

Figure 17
Correlation of WA with mechanical properties.

Figure 18
Correlation among mechanical properties.

Table 3
Correlation among various parameters.

VARIABLE I	VARIABLE II	CORRELATION EQUATION	REGRESSION FACTOR
Water absorption	Impact value	y = 0.752x + 18.449	0.5953
Water absorption	Abrasion value	y = 1.4471x + 14.761	0.8243
Water absorption	Crushing value	y = 1.0884x + 15.383	0.9446
Impact value	Crushing value	y = 0.5762x + 9.5747	0.8878
Impact	Abrasion value	y = 0.6747x + 5.928	0.9222

6.6. Microstructural investigations

Figure 19 depicts the microstructure of RCA, ATRCA and STRCA. Figure 18a depicts the microstructure of RCA wherein the smearance of mortar are visible with porous surface texture. The texture of RCA was rough, pervious with mortar smeared on it. The weak mortar is smeared extensively of various thickness resulting in non-homogenous texture. Such texture attributes to the higher porosity and lower density for the RCA. Microstructural evidences obtained through SEM support the findings of higher water absorption ensued through smeared mortar from laboratory experiments. Figure 18b depicts the microstructure of ATRCA wherein the percentage of smeared mortar was reduced compared to RCA. The mortar smeared on the NCA was removed through the action of acids and even some loose particles were observed on the surface. Figure 18c depicts the microstructure of STRCA. In comparison to RCA, roughness on the RCA was coated with thin layer of slurries. The slurries coat the micro-voids on the surface of RCA and impregnates into the pores of RCA and clogs it. Thus, porosity was arrested on the surface and inside the RCA resulting in better properties compared to ATRCA. Furthermore, lumps are observed on the microstructure of STRCA indicating the uneven dispersion of the slurries on the surface.

Figure 19
Microstructural images of aggregates.

7. LIFE CYCLE ASSESSMENT

Table 4 depicts the details used for the LCA of the aggregates. The cement production, in particular, has significant carbon emissions. The acid treatments involve chemical production, while RCA processing is mainly energy-intensive due to crushing. The RCA and STRCA treatments use water (e.g., rinsing, cement slurry preparation). The Reduced water use in ATRCA due to acid treatment may result in lower overall water demand. The NCA production involves mining, RCA involves crushing, and treatments like slurry preparation are energy-intensive. Cement production for STRCA is notably energy-heavy. The RCA reduces construction waste and prevents further natural resource extraction, while the slurry treatment leaves residues that need disposal. Acid treatments may require neutralizing waste acid.

Table 4
Data for LCA.

AGGREGATES	TRANSPORT DISTANCE (km)	ENERGY SPENT (MJ/kg)	WATER USE (litres/kg)	CO2 EMISSION
NCA	50	0.2	0.05	0.03
RCA	10	0.4	0.1	0.02
ATRCA	–	0.5	0.2	0.1
STRCA	–	4.2	0.8	0.93

Figure 20 shows the energy, water use and carbon emission of aggregates used in the research. The STRCA has the highest emissions due to cement production, which is a major contributor to GWP. The ATRCA has lower emissions compared to NCA due to the reuse of RCA. The STRCA requires the most water due to slurry preparation, followed by RCA and ATRCA. Acid treatment could save water compared to slurry-based methods. The cement production for STRCA is energy-intensive, while ATRCA’s energy demands mostly come from acid preparation and treatment. Furthermore, both ATRCA and STRCA contribute positively to waste reduction by recycling demolition waste. The ATRCA is advantageous in terms of reducing untreated concrete waste, though disposal of acidic residues needs consideration.

Figure 20
Output of LCA.

While STRCA improves mechanical properties, it comes at a higher environmental cost due to cement use. The ATRCA may offer a more balanced solution, with significant water absorption reductions (21.7% less than RCA) and moderate environmental impacts. However, reducing cement in slurry treatments or opting for alternative binders could lower GWP. Alternatively, scaling ATRCA could provide a lower-impact solution if acid waste can be properly managed. While acid and slurry treatments improve RCA quality, they also present limitations that warrant further research. Large-scale acid treatment faces challenges in terms of handling, disposal of acidic waste, and potential impacts on the environment. Similarly, slurry treatments, often using cement-based slurries, may contribute to environmental concerns due to cement production’s carbon footprint.

8. CONCLUSIONS

The experimental data obtained from the research explicate the feasibility of acid and slurry impregnation of RCA to enhance quality and properties of the RCA. The subsequent inferences are obtained as follows:

1. The treatments to RCA through acids and slurries enhances the properties of the RCA. The past treatment eradicates the weak cement particles on the RCA while the later treatment strengthens the weak cement particles through formation of thin layer surrounding the RCA. The acid disintegrates the slack particles on the RCA while slurries coat the micro-voids on/inside the RCA.

2. The optimization dosage of acid treatment was observed at 0.3 M for 3 days and optimized dosage of slurries was observed with 0.8 w/c for 24-h. The higher molarity beyond

3. The physical properties and mechanical properties of the RCA was inferior compared to NCA, while the same of ATRCA and STRCA was better compared to RCA.

4. The performance of STRCA was better compared to ARCA as ATRCA does not completely eradicate the weak mortar, while STRCA coats the micro-voids on/inside the RCA.

5. Significant correlation was established between physical and mechanical properties of RCA and among various mechanical properties of the RCA. Highest correlation was observed between WA and density and WA and IV of the RCA.

6. Microstructure images show the RCA with porous and non-homogeneous texture, ATRCA with minimum traces of weak and loose mortar and STRCA with thin film or slurries covering the RCA.

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Publication Dates

• Publication in this collection
  20 Jan 2025
• Date of issue
  2025

History

• Received
  10 Oct 2024
• Accepted
  21 Nov 2024