PSA and ISA are industrial waste products resulting from the combustion of waste and by-product materials in a power plant. Physical and chemical properties play a substantial role in EACM preparations and service life stages. The comparative physical properties and chemical compositions of the above waste and by-product materials are summarised below.

ISA has been supplied by the incinerator of the wastewater treatment plant from United Utilities Limited sewage sludge incineration system located at Mersey Valley SPC, Widnes, United Kingdom. The Mersey Valley Sludge Pipeline is used to transport the digested sludge to Shell Green. The sludge is centrifuge-dewatered to a dry solids content of about 26% before being transported to sludge dryers. After that, it is fed into a fluidized bed incinerator, which runs at a temperature of 800°C–900°C. After being extracted from fly ash using an electrostatic precipitator, it is kept in a silo for collection before being dumped in a landfill. The production in the region is about 16–20,000 tonnes of material per annum. The material has around 10% water added as it is very fine and brown-coloured.

PSA samples were collected from a United Kingdom recycled newsprint mill using recycled recovered magazines and newspapers. PSA results from the incineration of paper sludge ash and other input materials throughout the recycling of newspapers and similar products.

The particle size distribution curves of filler materials obtained by the laser analysis method (A Beckman Coulter Laser Diffraction Particle Size Analyser LS 13 320) are shown in Figure 2. It can be observed that the d₁₀, d₅₀, and d₉₀ of ISA were 1.313 μm, 19.55 μm and 99.91 μm, respectively. Most of the particles of PSA have d₁₀, d₅₀, and d₉₀, which are equal to 2.991 µm, 41.24 µm and 119 µm, respectively. It can be stated that TLF is composed of 1.454 μm, 24.22 μm, and 96.48 μm d₁₀, d₅₀, and d₉₀, respectively.

It can be observed, as listed in Table 1, that the ISA has a significant content of SiO₂, Fe₂O₃, CaO, and Al₂O₃, which generates good expectations about using it as an active addition and shows its potential for use as a pozzolanic material. The SO₃ content is determined by semi-quantitative analysis to be .749%. Lam et al. (2016) reported that the main components in ISA are Al, Si, Fe, Ca, P, and Na. In addition, the findings demonstrate that silicon dioxide (SiO₂) and calcium oxide (CaO) make up the majority of the TLF. Furthermore, the quantitative chemical analysis by Energy Dispersive X-ray Fluorescence (EDXRF) (using a Shimadzn EDX 720, energy dispersive, X-ray fluorescence spectrometer) of PSA revealed that the major oxides are Ca, Al, Si, Mg, and Fe, with some trace elements.

The petrographic analysis by scanning electron microscopy (SEM) (using both Inspect S and Quanta 200 scanning electron microscopes) shown in Figure 3 supports the similarity of grain size distribution among three candidate fillers revealed during the particle size analysis displayed in Figure 2. However, the distinctive disparity in agglomeration of particles in the case of ISA & PSA against TLF justifies the industrial incineration process experienced by these fillers. To speed up the expected hydraulic and pozzolanic reactions of the selected fillers in the proposed EACM, the agglomerated particles must be spread out through emulsion interaction.

Figure 4 displays the ISA, PSA, and TLF’s X-ray diffraction (XRD) patterns (using a Rigaku Miniflex diffractometer). The powder XRD of ISA revealed the mineralogy composed of quartz (SiO₂), hematite (Fe₂O₃), magnetite (Fe₃O₄), and whitlockite (Ca₃(PO₄)₂) with some amorphous phases. The mineralogical configuration of PSA in XRD reveals that the powder diffraction pattern is crystalline due to the fact that it has sharp peaks and minimal background noise; the principal crystal peaks were lime (CaO), mayenite (Ca₁₂Al₁₄O₃₃), calcite (CaCO₃), gehlenite (CaAl [Al,SiO₇]), and merwinite (Ca₃Mg [SiO₄]). In addition, calcite (CaCO₃) and quartz (SiO₂) were the two primary crystal peaks that could be seen in the X-ray diffraction pattern of the TLF.

The approach established by theAsphalt Institute, (1997) (Marshall Method for Emulsified Asphalt Aggregate Cold Mixture Design (MS-14) (Asphalt Institute 1997)for developing the cold asphalt bituminous emulsion mixtures served as the basis for the design procedure. The incorporation of waste and by-product materials was accomplished by substituting a portion of the typical mineral filler. PSA was the main material that substituted filler percentage, which is 6% by aggregate weight.

To assess the impact of replacing PSA for traditional mineral filler at five different replacement percentages (0, 1.5, 3, 4.5, and 6) % by aggregate weight, the indirect tensile stiffness modulus test (ITSM) was carried out. Such a replacement of the PSA was to examine the performance of the mixes with 25%, 50%, 75%, and 100% of PSA as a replacement of TLF. ISA was utilized to activate the PSA and was added as an extra percentage of the total aggregate weight to explore its full potential. Additionally, the results were contrasted with those obtained using conventional hot asphalt concrete and cold asphalt concrete that included limestone. The specimen was compacted using a Marshall hammer and 50 blows were delivered to each face.

The sample conditioning for ITSM testing was accomplished in two stages. The specimens were left in the mould for 1 day to prevent specimen disintegration, which represents the first stage of the specimen’s condition according to the procedure adopted by the Asphalt Institute. The samples were produced the following day and placed in a vented oven at 40°C for an additional day to reach the second stage. This curing regime, i.e., 24 h@ 20°C plus 24 h@ 40°C, corresponds to 7–14 days in the field, as proven by Jerkins (2000). Following these steps, all the samples were left in the lab at 20°C and tested at different ages, i.e., 2, 7, 14, and 28 days. All the materials were completely mixed at each stage of addition through a Hobart mixer.

An appropriate comparison for analysed waste materials can be derived by the measurement of their ITSM compared to the normal AC 20 hot dense binder course. Throughout the experiment, two types of hot binder course mix, AC 20 dense binder course 100/150 and AC 20 dense binder course 40/60, were used with the same aggregate type and gradation. The ideal binder content by weight of aggregate was 4.6 percent, as per PD 6691 (European Committee for Standardization, 2016b) for the dense binder AC 20 course.

The emulsion mixtures were made and compacted at room temperature, whereas the laboratory mixing temperatures for hot mixtures were (150–160°C) for the 100/150 pen and (160–170°C) for the 40/60 pen. Specimens of EACM were fabricated with different ratios of PSA (0%, 1.5%, 3%, 4.5%, and 6%) as a percentage of dry aggregate instead of mineral filler. ISA was utilized as an addition in varying proportions (.5%, 1%, 2%, and 4% by total aggregate mass) to investigate the influence of PSA added to the EACM on the ITSM. Three samples were made for each test to ensure the accuracy of the results.

Figure 5 displays the EACM specimen testing methods. First, an indirect tensile stiffness modulus (ITSM) test was used to evaluate the mixes’ early-age strength. Then, tests for rutting resistance and moisture susceptibility were used to evaluate the engineering performance. Finally, the SEM technique was used to characterize the microstructural characteristics of EACM.

Figure 6 displays the outcomes of the test for the indirect tensile stiffness modulus. It is evident that when the PSA increased, the stiffness rate of the EACM mixes at early ages improved. With curing time, the ITSM dramatically increased for all EACM combinations with PSA, especially for the higher concentrations. This compares to conventional HMA with TLF, which indicates imperceptible variations in ITSM with time. The goal ITSM of 2157 MPa for the 131 pen HMA may be surpassed in 2 days for the EACM containing 6% PSA. The ITSM of EACM containing 6% PSA increased by approximately 10 times in comparison with conventional EACM -TLF at just 2 days. The EACM containing 6% PSA had a 30% improvement in the ITSM value in comparison to the 131 pen HMA at 2 days of age, while its ITSM value is close to that of the 49 pen HMA after nearly 28 days, indicating the positive effect of this kind of filler replacement. In general, the addition of PSA to EACM mixes increased the ITSM significantly. These findings are consistent with those obtained by Al-Busaltan et al. (2012).

The properties of PSA are responsible for this improvement, i.e., the cementitious property with high water absorption as well as high pH, which play a substantial role by producing a high bond within the internal microstructure. So, the reasons for this enhancement in ITSM are the generation of a new binder, in addition to the residue binder, as a result of the hydration process of the hydraulic reaction of PSA. In addition, the trapped water, which considerably forms the mixture’s weakness, was consumed by the hydration process. Furthermore, the presence of PSA in the EACM increases the breaking of bitumen emulsion as it increases the pH due to the alkaline nature of the PSA paste. This allows the bitumen emulsion to flocculate and coalesce rapidly due to an increase in the dissociation rate of the emulsifier on the bitumen droplets (Wang et al., 2013). It was reported that the presence of cementitious materials speeds the demulsification of bitumen emulsion, which could increase the strength of EACM (Dolzycki et al., 2017).

ISA was incorporated as an addition in varying amounts (.5%, 1%, 2%, and 4% by total aggregate mass) as shown in Figure 7 to assess the impact of including PSA in the EACM on the ITSM. Furthermore, a comprehensive comparison has been considered with the conventional hot AC 20 and EACM. The experimental results indicate that PSA and ISA enhance the stiffness of EACM mixes.

The ITSM test was conducted following BS EN 12697-26 (European Committee for Standardization, 2012) at various ages, i.e., 2, 7, 14, and 28 days, to determine the degree of ITSM growth in the EACM containing ISA with PSA. From these figures, it might be said that adding calcium hydroxide-rich PSA to pozzolanic material improves the performance of the mixture PSA. It contains a high concentration of silica oxide, which influences the pozzolanic reaction when it reacts with calcium hydroxide, Ca(OH)₂, formed during the hydration process. Thus, generating more calcium silicate hydrate (C-S-H) gels has helped with the strength development of the PSA with ISA mixtures. It can be stated that a stiffness of more than 2152 MPa, comparable to the soft asphalt hot mix’s stiffness, can be achieved easily with 6% of PSA and .5% ISA after only 2 days. In addition, stiffness comparable to the hard asphalt hot mix’s stiffness can be achieved with 6% of PSA with 4% ISA after 2 days, which proves the efficiency of the ISA in the improvement of stiffness. There was an increase in hydration degree due to the addition of ISA, so the main aim of the ISA was to activate the PSA hydration process. In more detail, the increase in the ITSM of the mixes with 6%PSA +.5%ISA, 6%PSA +1%ISA, 6%PSA +2%ISA ,and 6%PSA +4%ISA after 2 days of curing were around 13%, 31%, 50%, and 64%, respectively, compared to reference 6%PSA.

Waste materials like ISA could be added to EACM to improve its characteristics as well as address the issue of disposal. The packing of the mixture is physically improved by these components acting as micro fillers. Therefore, a denser microstructure will be formed. They combine chemically with calcium hydroxide (CH) in the cementitious system to undergo the pozzolanic process, which speeds up the hydration of PSA. In addition, the medium’s pH value rises as a result of Ca(OH)₂ dissociating from PSA. Similar to the reaction between a weak acid and a strong base, high alkalinity dissolves the silica in ISA and breaks the glassy phase of the pozzolanic material (Shi and Day, 2000). The hydrated calcium silicate (CSH), which is crucial for the development of strength, is produced when the hydrous silica reacts with calcium ions that are released during the hydrolysis reaction of PSA.

Permanent deformation, which can be characterized as an accumulation of the little irrecoverable portion of the deformation induced by each wheel load, is one of the main distresses encountered in bound and unbound layers in asphalt pavements. The test was conducted at 45°C following BS EN 12697-22 (European Committee for Standardization, 2003). The rutting depth increased with the number of cycles for TLF and both hot mixes. The gap between the rutting depth curves of the TLF mix and other EACM mixes is obvious. This is related to the effect of both PSA and ISA used in this study. Figure 8 shows that mixes of 6%PSA +.5%ISA, 6%PSA +1%ISA, 6%PSA +2%ISA and 6%PSA +4%ISA have an average rut depth of .7 mm, .63 mm, .51 mm and .36 mm, while a mix with TLF has an average rut depth of 11.8 mm after 10,000 loading cycles.

This outcome can be largely attributed to the fact that the mixture of PSA and ISA sped up the demulsification of bitumen emulsion. In the meantime, PSA hydration could be promoted by the water present in the bitumen emulsion. As a result, a network structure might be formed by connecting asphalt films and hydration products to offer superior deformation resistance. Overall, the production of hydration products caused by the substitution of PSA and ISA for conventional fillers significantly increased the permanent deformation resistance of EACM. These results are consistent with the results of Lu et al. (2021).

By comparing the stiffness modulus ratio (SMR) between the mixes with conditioning and the mixes without conditioning, the effects of moisture on the EACM mixes were examined. In order to make comparisons, the HMA mixes’ moisture susceptibility was also examined. The findings of the moisture susceptibility tests are displayed in Figure 9. The EACM mixes with PSA exhibited slightly higher ITSM than the HMA mixture after conditioning. The conditioned ITSM of the PSA, 131 pen HMA and 49 pen HMA were 3,733, 2152, and 4,946 MPa, respectively. Following the water conditioning, the ITSM for the PSA mix clearly increased, while there was a decrease for both the 131-pen and 49-pen mixes. The wet ITSM of the PSA, 131 pen HMA and 49 pen HMA were 3,842, 1,495, and 4,371 MPa, respectively. The highest ITSM was found in the PSA mix both before and after conditioning. The SMR for PSA, 131 pen HMA, and 49 pen HMA were 102.9%, 69.5%, and 88.4%, respectively.

The SMR for .5% ISA +6%PSA, 1% ISA +6%PSA, 2% ISA +6% PSA, and 4% ISA +6%PSA were 103.5%, 104.7%, 108.4%, and 109.7%, respectively. Therefore, the findings indicate that the water susceptibility of PSA + ISA mixes made with 6 percent PSA is not particularly of concern. One of the key contributors to the mix’s good moisture damage resistance may be the use of PSA and ISA, which are rich in hydration products. These results are in agreement with the findings of Lu et al. (2021). It was reported that the water damage resistance increased due to the enhanced bond strength between the mortar, the EACM mixture and the aggregate with the addition of cementitious fillers (Dong et al., 2018; Xiao et al., 2019).

The paste of PSA and ISA was prepared using the similar concentration used in the optimum mixture (4%ISA+6%PSA) to understand their hydration products after 28 days of curing in a noise-free environment without interference from emulsion and aggregate. The microstructural properties of the hydration products were investigated using petrographic analysis through the SEM technique and shown in Figure 10. A successful hydration reaction in the proposed binary filler was achieved that was conducive to the demulsification of the emulsion. A dense microstructure with hydration products is evident in Figure 9 which justifies the fillers’ contribution to the CBEM. The significant appearance of ettringite crystals as well as hexagonal platy portlandite and dense C-S-H in the binary mixes not only justifies the improvement in early (after 2 days) but also in long-term (after 28 days) strength. The sustained progressive improvement relating to the sample’s ITSM performance for inclusion of ISA within the PSA filler also counteracts the possibility of delayed ettringite formation within the microstructure. A similar progressive dominant role of C-S-H over ettringite in a mixture containing PSA was reported by Sadique et al. (2019) where PSA was activated with alkali-reach minerals. This hydration kinetics within the proposed filler where a successful activation of PSA was achieved by reactive ISA particles and created a uniform and dense structure also justifies its advanced performance during the water sensitivity test. Moreover, the creation of dense microstructure within the CBEM also ensures an improved interface between aggregate and matrix that contributes to the mixture’s mechanical strength. In this case, the advanced hydration products generated from the proposed binary blended filler created a composite structure of an interlinking network with an emulsion film and showed advanced engineering properties. Similar microstructural kinetics were reported by Lu et al. (2021) where a ternary blended cementitious filler was used. This will be a breakthrough in CBEM technology where a balanced oxide composition within the proposed binary blended filler not only accomplished a successful hydration reaction but also contributed to exhibiting a progressive strength generation that will ensure a future resilient infrastructure with durable road pavement. These results are consistent with those obtained by Dulaimi et al. (2017).