Influence of Nano-Silicon Dioxide in the Enhancement of Surface Structure of Public Filler and Properties of Recycled Mortar

: This paper proposes a method of enhancing public filler (PF) with nano-SiO 2 (NS) to prepare modified recycled aggregate mortar (RAM). The improvement effect of NS solution at different concentrations and immersion times on the macroscopic physical properties of recycled public fine aggregates (PFA) was investigated. Moreover, the effect of NS on the basic physical properties and durability of recycled mortar (RM) and the reinforcement mechanism of NS on recycled mortar was analyzed through various techniques. Results indicated that the modification effect of NS could remove loose cement mortar from the surface of PFA. It reacted with calcium hydroxide and calcite to generate nano-particles that could fill pores in PFA. The water absorption rate of PFA decreased to 9.3% when immersed in 2% NS solution for 72 h. There was no significant improvement in the mechanical properties of RM when the solution concentration and immersion time were increased. However, the compressive strength of RM prepared by modifying PFA with 2% NS was increased by about 21.9%, and the capillary water absorption and electric flux were reduced by 56.3% and 15.1%, respectively. Micro-analysis results showed that the volcanic ash effect of NS enabled it to react with Ca(OH) 2 adhered to the surface of PFA, generating C-S-H and improving the interfacial bonding of PFA. Moreover, NS adsorbed on the surface of PFA dispersed into the freshly mixed cement slurry, which further enhanced the internal structure of PFA.


Introduction
With the rapid development of urbanization in recent years, more and more old buildings have been demolished, resulting in large amounts of construction waste [1,2].The amount of construction waste generated annually in China has increased to over 4.5 billion tons with only 5% to 10% being recycled until 2023 [3].Over ninety percent of construction waste consists of inert materials, which are also known as public filler (PF) [4].Moreover, recycled mortar produced by PF can effectively address the issues among construction resources, energy, and the environment.It also meets the urgent demand for a green and sustainable development strategy for building materials, with broad development prospects [5][6][7][8].
However, recycled aggregate mortar often cannot meet requirements for engineering applications due to its numerous internal defects, which affect its mechanical performance and durability [9][10][11].Recycled aggregates may contain residual impurities such as wood, plastics, or metal fragments that have not been completely removed, which can adversely affect the strength and durability of mortar [12,13].Furthermore, the particle shape and surface characteristics of recycled aggregates often differ from those of natural aggregates, potentially influencing the workability and flowability of concrete, thereby affecting its density and strength [14].Moreover, recycled aggregates tend to have higher water absorption rates, which can lead to difficulties in controlling the water-to-cement ratio of mortar, thus impacting both workability and durability [15,16].The old mortar adhered to the surface of recycled aggregates (RA) is generally considered the primary cause of their defects.This is due to the presence of a barrier effect, where multiple interface transition zones (ITZ) form between the aggregates and cement mortar, preventing cement particles from filling the space near the surface of the aggregates [17][18][19][20].Additionally, the old mortar adhered to the surface of PF exacerbates the barrier effect, resulting in more defects near the aggregates and widening the ITZ [21].This phenomenon affects the mechanical performance and durability of PF, so selecting appropriate pre-treatment methods is crucial for improving their properties [22,23].Moreover, the utilization of recycled aggregates in concrete production provides notable environmental and resource benefits, accompanied by several challenges and limitations [24].These include variability in the quality and performance of recycled aggregates, additional processing and preparation costs, uncertainties in technical performance, dosage limitations, and adherence to standards and specifications [25].
Currently, there are mainly two methods for pretreatment of PF: one involves the physical or chemical removal of old mortar adhering to the PF surface, such as mechanical grinding, ultrasonic treatment, and soaking in acidic solutions [26,27].However, this method has the disadvantages of high energy consumption and high pollution, and the immersion of acid solutions also introduces corrosive substances.The other method is to enhance the surface structure of PF.In recent years, researchers have proposed using high-activity nanomaterials to enhance aggregate performance by leveraging advantages in surface, size, and interface properties [28].Previous studies have confirmed that the presence of nanomaterials can accelerate hydration, refine hydration products, increase gel compaction, absorb hydration products, and reduce primary cracks, thereby altering the mechanical properties and durability of concrete [29][30][31].Wang et al. [20] investigated the effects and mechanisms of eight representative nanofillers on the bond strength and interfacial microstructures between aggregates and cement mortars.The results indicated that all types of nanofillers can transfer with water migration toward aggregates and enrich in ITZ, thus improving the bond strength and interfacial microstructures between aggregates and cement mortars through the nano-core effect.Nano-SiO 2 (NS) is a typical modification that possesses strong pozzolanic activity and filling effects.It can react with the hydration products of cement to generate C-S-H through secondary hydration, which effectively improves the pore structure within the ITZ [32,33].Moreover, the surface tension and agglomeration effect of NS accelerates the cement hydration process in concrete through nucleation effects [34,35].
In this paper, the method of "pre-wetting + air-drying" was applied to prepare public fine aggregates (PFA) under different NS concentrations and immersion times.The reinforcement mechanism of NS on PFA was analyzed according to the changes in microscopic morphology, phase composition, and surface groups of the ITZ.Finally, NS-modified PFA was used to prepare recycled mortar, and the improvement effect on the basic physical properties of recycled mortar was investigated, which offers theoretical and technical support for the enhancement of recycled mortar performance with nano-SiO 2 .

Raw Materials
The PF was collected from a demolished residential area in Guangzhou and primarily consisted of brick aggregate.These aggregates have undergone crushing and sieving processes: the first step involved crushing the PF using a jaw crusher (Henan, China) to break down larger particles into smaller sizes suitable for further processing.After crushing, the PF underwent sieving to separate particles into different sizes, resulting in PFA ranging from 0.15 mm to 4.75 mm.By controlling both the crushing and sieving processes, a consistent particle size distribution in the PFA was obtained.The microstructure and mineral composition of PFA are shown in Figure 1.The PF exhibited a melted state after high-temperature calcination and showed larger internal pores than natural aggregates (NA), most of which were interconnected and had strong water absorption capabilities.The particle size distribution curve of PFA is shown in Figure 1e.The cement used in this experiment was P.O 42.5 ordinary Portland cement (Guangzhou, China), and the nano-silica sol had a particle size of 5-10nm with a silica dioxide content of 30 ± 1%.Tap water (Guangzhou, China) was used and the water reducer was a polycarboxylate superplasticizer (Shanghai, China) with a water reduction rate of 22.5%.PFA ranging from 0.15 mm to 4.75 mm.By controlling both the crushing and sieving processes, a consistent particle size distribution in the PFA was obtained.The microstructure and mineral composition of PFA are shown in Figure 1.The PF exhibited a melted state after high-temperature calcination and showed larger internal pores than natural aggregates (NA), most of which were interconnected and had strong water absorption capabilities.The particle size distribution curve of PFA is shown in Figure 1e.The cement used in this experiment was P.O 42.5 ordinary Portland cement (Guangzhou, China), and the nano-silica sol had a particle size of 5-10nm with a silica dioxide content of 30 ± 1%.Tap water (Guangzhou, China) was used and the water reducer was a polycarboxylate superplasticizer (Shanghai, China) with a water reduction rate of 22.5%.

Modification Steps of PFA
The method of "pre-wetting + air-drying" was applied to enhance PFA with NS.The specific process consisted of three steps: (1) pre-treatment of PFA, (2) adsorption of NS, and (3) air-drying.Firstly, the PFA was dried in a 105 °C blast oven until constant weight, and the excess water was removed and cooled at 23 ± 2 °C for 24 h.Then, the PFA was immersed in NS solutions with different concentrations (1%, 2%, and 3%) for 24 h, 48 h, and 72 h, and a control group was set up with the concentration of the immersed solution and the immersion time of 0. The immersion solid-liquid ratio was 1:10, and the prepared mixtures were rinsed and immersed for 2 h and dried in an oven at 70 °C for 6 h.

Preparation of RM with Modified PFA
The mix proportion for all specimens in this experiment was 472 kg/m 3 for cement, 1451 kg/m 3 for fine aggregate, 261 kg/m 3 for water, and 1.62 kg/m 3 for water reducer.Natural sand (NS) was used instead of PFA as a control group and denoted as NA.The unmodified PFA group was labeled as Control (0% PEI), and the PFA modified with 1%, 2%, and 3% concentrations of SiO2 solution were labeled as Si-PFA-1, Si-PFA-2, and Si-PFA-3, respectively.The measured cement and PFA were firstly mixed in a forced planetary mixer for 1 min, followed by the addition of water mixed thoroughly with the The method of "pre-wetting + air-drying" was applied to enhance PFA with NS.The specific process consisted of three steps: (1) pre-treatment of PFA, (2) adsorption of NS, and (3) air-drying.Firstly, the PFA was dried in a 105 • C blast oven until constant weight, and the excess water was removed and cooled at 23 ± 2 • C for 24 h.Then, the PFA was immersed in NS solutions with different concentrations (1%, 2%, and 3%) for 24 h, 48 h, and 72 h, and a control group was set up with the concentration of the immersed solution and the immersion time of 0. The immersion solid-liquid ratio was 1:10, and the prepared mixtures were rinsed and immersed for 2 h and dried in an oven at 70 • C for 6 h.

Preparation of RM with Modified PFA
The mix proportion for all specimens in this experiment was 472 kg/m 3 for cement, 1451 kg/m 3 for fine aggregate, 261 kg/m 3 for water, and 1.62 kg/m 3 for water reducer.Natural sand (NS) was used instead of PFA as a control group and denoted as NA.The unmodified PFA group was labeled as Control (0% PEI), and the PFA modified with 1%, 2%, and 3% concentrations of SiO 2 solution were labeled as Si-PFA-1, Si-PFA-2, and Si-PFA-3, respectively.The measured cement and PFA were firstly mixed in a forced planetary mixer for 1 min, followed by the addition of water mixed thoroughly with the water reducer and stirred for 3 min.Compared to other mixing methods, the use of a forced planetary mixer can enhance the quality and consistency of recycled mortar by ensuring thorough mixing and precise control over mixing parameters, making it a preferred choice for many applications in the construction and building materials industries.After thoroughly mixing all materials, the mixture was poured into 70.7 mm × 70.7 mm × 70.7 mm molds.The molds were vibrated to remove air bubbles and the surface was leveled and they were demolded after curing for 24 h.The specimens were then placed in a curing box with a humidity of 95% and a temperature of 23 ± 2 • C for 28 days before testing their strength.

Microstructure Test
The pore volume and pore size distribution of the samples were determined through the nitrogen adsorption-desorption test.Each sample weighed approximately 5 g and was degassed at 60 • C for 12 h before testing.Scanning electron microscopy (SEM) was used to examine the surface morphology of the samples before and after carbonization, while energy-dispersive X-ray spectroscopy (EDS) was used to determine their elemental composition.The samples were soaked in isopropanol for 48 h to eliminate any residual water and dried in a vacuum dryer at 50 • C for 48 h before testing.

Mineral Compositions
X-ray diffraction (XRD) was performed to analyze the phase composition of untreated PFA and pre-treated PFA.The samples were measured within the range of 0-100 • with intervals of 10 • , and the results were analyzed by the International Crystal Structure Database (ICSD).A Fourier transform infrared spectrometer (FTIR) was utilized to validate the attenuated total reflectance (ATR) FTIR spectra of the PF samples.

Physical Properties
The water absorption, crushing value, and apparent density of PFA were tested according to GB/T14685-2011 [36].A rapid chloride migration test (RCM) was conducted to characterize the migration characteristics of RA, and the migration depth of chloride ions was described according to GB/T 50082-2009 [37].Moreover, the procedure of capillary water test referred to the method of Oltulu and Şahin [38].The flow of raw material processing and specimen preparation is shown in Figure 2.
were demolded after curing for 24 h.The specimens were then placed in a curi humidity of 95% and a temperature of 23 ± 2 °C for 28 days before testing their

Microstructure Test
The pore volume and pore size distribution of the samples were determ the nitrogen adsorption-desorption test.Each sample weighed approximatel degassed at 60 °C for 12 h before testing.Scanning electron microscopy (SE to examine the surface morphology of the samples before and after carboni energy-dispersive X-ray spectroscopy (EDS) was used to determine th composition.The samples were soaked in isopropanol for 48 h to eliminate water and dried in a vacuum dryer at 50 °C for 48 h before testing.

Mineral Compositions
X-ray diffraction (XRD) was performed to analyze the phase co untreated PFA and pre-treated PFA.The samples were measured within th 100° with intervals of 10°, and the results were analyzed by the Internat Structure Database (ICSD).A Fourier transform infrared spectrometer (FTIR to validate the attenuated total reflectance (ATR) FTIR spectra of the PF sam

Physical Properties
The water absorption, crushing value, and apparent density of PFA according to GB/T14685-2011 [36].A rapid chloride migration test (RCM) w to characterize the migration characteristics of RA, and the migration dept ions was described according to GB/T 50082-2009 [37].Moreover, the capillary water test referred to the method of Oltulu and Şahin [38].The material processing and specimen preparation is shown in Figure 2.

. Water Absorption Capacity
Figure 3 shows the improvement in water absorption of public fine aggregates (PFA) with the addition of nano-SiO 2 (NS).The water absorption of PFA decreased to below 10% after modifying them with NS, indicating its beneficial role in reducing water absorption.Moreover, the water absorption further decreased when PFA was pre-treated with a higher concentration of NS solution.Specifically, after 72 h of pre-treatment, the water absorption rates of Si-PFA-1, Si-PFA-2, and Si-PFA-3 were 11.9%, 9.3%, and 8.6%, respectively.This demonstrated the advantage of selecting NS as a modification to enhance the performance of PFA.

Water Absorption Capacity
Figure 3 shows the improvement in water absorption of public fine aggregates (PFA) with the addition of nano-SiO2 (NS).The water absorption of PFA decreased to below 10% after modifying them with NS, indicating its beneficial role in reducing water absorption.Moreover, the water absorption further decreased when PFA was pre-treated with a higher concentration of NS solution.Specifically, after 72 h of pre-treatment, the water absorption rates of Si-PFA-1, Si-PFA-2, and Si-PFA-3 were 11.9%, 9.3%, and 8.6%, respectively.This demonstrated the advantage of selecting NS as a modification to enhance the performance of PFA.Under the same concentration, when the immersion time increased from 24 h to 48 h, the decrease in water absorption rate of NS-modified PFA was significantly greater than that when the immersion time increased from 48 h to 72 h.Under the same immersion time, the increase in solution concentration from 0% to 2% showed a more significant effect on water absorption compared to the increase from 2% to 3%.According to the decreasing trend of water absorption, the most significant improvement in water absorption of recycled coarse aggregates (RCA) occurred when immersed in a 2% NS solution for 72 h, decreasing from 13.5% to 9.3%.The water absorption reduction rate of 2% NS-modified PFA (4.2%) was higher than that of 2% nano-Fe2O3 (3.09%) and 2% nano-TiO2 (3.75%) [39,40].However, the improvement in water absorption tended to level off as the solution concentration and immersion time continued to increase.
When the solution concentration and immersion time reach a certain level, the effectiveness of improving water absorption will diminish.This could be attributed to the incorporation of nano-SiO2 reacting with Ca (OH)2 in PFA to produce C-S-H, thereby reducing the porosity of RA [41,42].However, the content of Ca (OH)2 in PFA was limited and gradually depleted with the increase in solution concentration and immersion time.In the later stages, the main way of reducing water absorption of PFA was filling PFA pores with nano-SiO2.Moreover, the potential correlation between surface wettability measurements and water absorption rates in construction materials has been confirmed in recent research [43,44], which should be further investigated to better analyze changes in properties.

Apparent Density
The surface of PF is covered with a large amount of irregular and aged cement mortar.The higher the apparent density, the lower the proportion of attached mortar and the lower the porosity, typically resulting in higher quality [45].As shown in Figure 4a, the apparent density of RA increased with the increase in the concentration of NS solution and immersion Under the same concentration, when the immersion time increased from 24 h to 48 h, the decrease in water absorption rate of NS-modified PFA was significantly greater than that when the immersion time increased from 48 h to 72 h.Under the same immersion time, the increase in solution concentration from 0% to 2% showed a more significant effect on water absorption compared to the increase from 2% to 3%.According to the decreasing trend of water absorption, the most significant improvement in water absorption of recycled coarse aggregates (RCA) occurred when immersed in a 2% NS solution for 72 h, decreasing from 13.5% to 9.3%.The water absorption reduction rate of 2% NS-modified PFA (4.2%) was higher than that of 2% nano-Fe 2 O 3 (3.09%)and 2% nano-TiO 2 (3.75%) [39,40].However, the improvement in water absorption tended to level off as the solution concentration and immersion time continued to increase.
When the solution concentration and immersion time reach a certain level, the effectiveness of improving water absorption will diminish.This could be attributed to the incorporation of nano-SiO 2 reacting with Ca (OH) 2 in PFA to produce C-S-H, thereby reducing the porosity of RA [41,42].However, the content of Ca (OH) 2 in PFA was limited and gradually depleted with the increase in solution concentration and immersion time.In the later stages, the main way of reducing water absorption of PFA was filling PFA pores with nano-SiO 2 .Moreover, the potential correlation between surface wettability measurements and water absorption rates in construction materials has been confirmed in recent research [43,44], which should be further investigated to better analyze changes in properties.

Apparent Density
The surface of PF is covered with a large amount of irregular and aged cement mortar.The higher the apparent density, the lower the proportion of attached mortar and the lower the porosity, typically resulting in higher quality [45].As shown in Figure 4a, the apparent density of RA increased with the increase in the concentration of NS solution and immersion time.The RCA was best treated when the concentration of NS solution was 3% and the immersion time was 72 h, with the apparent density increasing from an initial 2384 kg/m 3 to 2512 kg/m 3 .Under the same concentration, the increase in immersion time from 24 h to 48 h resulted in a much larger increase in apparent density than from 48 h to 72 h.Under the same immersion time, the increase in solution concentration from 1% to 2% showed a more significant effect on apparent density compared to the increase from 2% to 3%.
Buildings 2024, 14, x FOR PEER REVIEW 6 of 16 time.The RCA was best treated when the concentration of NS solution was 3% and the immersion time was 72 h, with the apparent density increasing from an initial 2384 kg/m 3 to 2512 kg/m 3 .Under the same concentration, the increase in immersion time from 24 h to 48 h resulted in a much larger increase in apparent density than from 48 h to 72 h.Under the same immersion time, the increase in solution concentration from 1% to 2% showed a more significant effect on apparent density compared to the increase from 2% to 3%.The increase in the apparent density of PFA was directly proportional to the increase in solution concentration and immersion time.However, when the solution concentration and immersion time reached 2% and 48 h, respectively, the increase in apparent density slowed down as the solution concentration and immersion time increased further.This might have been because PFA surfaces had numerous pores and voids, which were filled with NS and tended to saturation as immersion time increased.Therefore, increasing immersion time had no significant effect on increasing apparent density.Additionally, nanomaterials exhibit aggregation effects, and the continued increase in NS solution concentration might block the pores in RA, thereby restricting the filling effect of NS within the pores.Therefore, when selecting the concentration of NS solution, it is necessary to balance its effect on the filling of pores in RA with the aggregation effect.

Crushing Value
The crushing value of PFA was significantly improved by NS, which gradually increased with the increase in solution concentration and immersion time (Figure 4b).When the solution concentration was 2% and the immersion time was 48 h, the improvement was optimal with the crushing value decreasing from 24.1% to 19.1%.Under the same concentration conditions, the extension of immersion time from 24 h to 48 h revealed the most significant improvement in PFA.Furthermore, the crushing value began to rise when the immersion time increased to 72 h, and this phenomenon was also observed under different modified solution concentrations.The main reason for this phenomenon was that the improvement of the crushing value of PFA by NS mainly stems from its filling effect and volcanic ash effect.The NS could generate C-S-H through the hydration reaction with Ca (OH)2 in PFA, thereby helping to enhance the mechanical properties of cement mortar [46,47].However, the volcanic ash effect gradually slowed down the reinforcement effect on the aggregates as the residual Ca (OH)2 in PFA was consumed.NS played a dominant role in filling the pores in PFA, and the improvement effect on the crushing value of PFA also decreased gradually as the NS particles in the aggregate gaps reached saturation.The increase in the apparent density of PFA was directly proportional to the increase in solution concentration and immersion time.However, when the solution concentration and immersion time reached 2% and 48 h, respectively, the increase in apparent density slowed down as the solution concentration and immersion time increased further.This might have been because PFA surfaces had numerous pores and voids, which were filled with NS and tended to saturation as immersion time increased.Therefore, increasing immersion time had no significant effect on increasing apparent density.Additionally, nanomaterials exhibit aggregation effects, and the continued increase in NS solution concentration might block the pores in RA, thereby restricting the filling effect of NS within the pores.Therefore, when selecting the concentration of NS solution, it is necessary to balance its effect on the filling of pores in RA with the aggregation effect.

Crushing Value
The crushing value of PFA was significantly improved by NS, which gradually increased with the increase in solution concentration and immersion time (Figure 4b).When the solution concentration was 2% and the immersion time was 48 h, the improvement was optimal with the crushing value decreasing from 24.1% to 19.1%.Under the same concentration conditions, the extension of immersion time from 24 h to 48 h revealed the most significant improvement in PFA.Furthermore, the crushing value began to rise when the immersion time increased to 72 h, and this phenomenon was also observed under different modified solution concentrations.The main reason for this phenomenon was that the improvement of the crushing value of PFA by NS mainly stems from its filling effect and volcanic ash effect.The NS could generate C-S-H through the hydration reaction with Ca (OH) 2 in PFA, thereby helping to enhance the mechanical properties of cement mortar [46,47].However, the volcanic ash effect gradually slowed down the reinforcement effect on the aggregates as the residual Ca (OH) 2 in PFA was consumed.NS played a dominant role in filling the pores in PFA, and the improvement effect on the crushing value of PFA also decreased gradually as the NS particles in the aggregate gaps reached saturation.

Pore Structure
Figure 5 illustrates the cumulative pore volume curve and pore volume fraction of PFA after 48 h of immersion.The results indicated that the cumulative pore volume of untreated PFA was 0.0311 cm 3 /g, while the pore volume of PFA modified by NS was 0.0243 cm 3 /g, representing a reduction of 21.87%.This method could enhance the microstructure density, which was consistent with previous findings [48,49].Although PFA was pre-treated with different concentrations of NS solution, as well as subsequent pretreatment described in Section 2.2, the pore volumes were reduced to only 0.0271, 0.0252, and 0.0243 cm 3 /g.Compared to untreated PFA, the NS-modified PFA exhibited significant decreases of 12.86%, 18.9%, and 21.87%.As shown in Figure 5b, the mesopores of PFA decreased between 2 and 50 nm after NS modification, and the decrease in cumulative pore volume was associated with the fraction between 2 and 10 nm.However, the reduction in macropores (capillary pores > 50 nm) was not significant.There was a reduction in pore volume when the NS solution concentration increased to 3%, but the positive effect was not pronounced.The decrease in pore volume and changes in functional groups were mainly attributed to the formation of calcium carbonate and calcium bicarbonate due to the interaction between Ca and nano-SiO 2 [50,51].Changes in the mass of PFA after pretreatment might also have played a role, as the increased moisture content in Si-PFA-2 and Si-PFA-3 compared to other groups might have blocked some surface channels, reducing their pore-filling capacity [52].

Pore Structure
Figure 5 illustrates the cumulative pore volume curve and pore volume fraction of PFA after 48 h of immersion.The results indicated that the cumulative pore volume of untreated PFA was 0.0311 cm 3 /g, while the pore volume of PFA modified by NS was 0.0243 cm 3 /g, representing a reduction of 21.87%.This method could enhance the microstructure density, which was consistent with previous findings [48,49].Although PFA was pretreated with different concentrations of NS solution, as well as subsequent pretreatment described in Section 2.2, the pore volumes were reduced to only 0.0271, 0.0252, and 0.0243 cm 3 /g.Compared to untreated PFA, the NS-modified PFA exhibited significant decreases of 12.86%, 18.9%, and 21.87%.As shown in Figure 5b, the mesopores of PFA decreased between 2 and 50 nm after NS modification, and the decrease in cumulative pore volume was associated with the fraction between 2 and 10 nm.However, the reduction in macropores (capillary pores >50 nm) was not significant.There was a reduction in pore volume when the NS solution concentration increased to 3%, but the positive effect was not pronounced.The decrease in pore volume and changes in functional groups were mainly attributed to the formation of calcium carbonate and calcium bicarbonate due to the interaction between Ca and nano-SiO2 [50,51].Changes in the mass of PFA after pretreatment might also have played a role, as the increased moisture content in Si-PFA-2 and Si-PFA-3 compared to other groups might have blocked some surface channels, reducing their pore-filling capacity [52].

SEM
Figure 6 depicts the microstructure of the interfacial transition zone (ITZ) of NSmodified concrete observed under SEM.It was observed that the untreated PFA surface exhibited an irregular and disorganized morphology, containing components such as C-S-H, mono-carbonate, and CH crystals.Moreover, EDS analysis indicated that the main components were quartz and calcite.The loose particles in the NS-modified PFA were removed, and then a significant number of nanoparticles were generated, which could fill the pores in this area.It has been confirmed that the bond strength between aggregates and cement mortars is influenced by many factors, such as aggregate shape, cement content, mineral composition, and mineral admixtures [53][54][55].The mortar was covered with a large amount of interlocked C-S-H gel, resulting in a dense structure.The size of voids in the ITZ was reduced and filled with a substantial quantity of hydration products, thereby tightly connecting it with the mortar and forming a three-dimensional robust space.3.2.Microstructure Characterization of NS-Modified PFA 3.2.1.SEM Figure 6 depicts the microstructure of the interfacial transition zone (ITZ) of NSmodified concrete observed under SEM.It was observed that the untreated PFA surface exhibited an irregular and disorganized morphology, containing components such as C-S-H, mono-carbonate, and CH crystals.Moreover, EDS analysis indicated that the main components were quartz and calcite.The loose particles in the NS-modified PFA were removed, and then a significant number of nanoparticles were generated, which could fill the pores in this area.It has been confirmed that the bond strength between aggregates and cement mortars is influenced by many factors, such as aggregate shape, cement content, mineral composition, and mineral admixtures [53][54][55].The mortar was covered with a large amount of interlocked C-S-H gel, resulting in a dense structure.The size of voids in the ITZ was reduced and filled with a substantial quantity of hydration products, thereby tightly connecting it with the mortar and forming a three-dimensional robust space.
Compared to non-treated PFA, the surface morphology of PFA modified with 1% NS showed a significant increase in new carbonate formation.This formation was further enhanced in a 2% NS solution, resulting in well-distributed patches projected directly from the PFA surface.The texture of PFA became more fluctuant when the SiO 2 concentration increased to 3%, with carbonate stacking upon each other and forming complex intersecting rough segments, which was similar to the formation of magnesia calcite discussed by Dung and Unluer [56].The surface of PFA particles modified with NS solution could capture calcium ions, promoting cement hydration in weak areas.Additionally, phenolic hydroxyl groups complexed with metal ions such as Fe 3+ and Al 3+ in the cement slurry, which could form a metal grid structure to enhance the density and hardness of the ITZ [57,58].Compared to non-treated PFA, the surface morphology of PFA modified with 1% NS showed a significant increase in new carbonate formation.This formation was further enhanced in a 2% NS solution, resulting in well-distributed patches projected directly from the PFA surface.The texture of PFA became more fluctuant when the SiO2 concentration increased to 3%, with carbonate stacking upon each other and forming complex intersecting rough segments, which was similar to the formation of magnesia calcite discussed by Dung and Unluer [56].The surface of PFA particles modified with NS solution could capture calcium ions, promoting cement hydration in weak areas.Additionally, phenolic hydroxyl groups complexed with metal ions such as Fe 3+ and Al 3+ in the cement slurry, which could form a metal grid structure to enhance the density and hardness of the ITZ [57,58].

FTIR
Figure 7 shows the FTIR spectra of non-treated and NS-modified PFA, recorded in the range of 500 cm −1 to 4000 cm −1 .The major peaks observed in these spectra were centered around 1080 cm −1 , representing the asymmetric stretching of the carbonate C-O bond [59,60].In addition, C-H vibration peaks due to the presence of NS backbone were detected at 694 cm −1 and 776 cm −1 , while peaks at 3463 cm −1 were attributed to the O-H bend vibration of the secondary amine of NS [5].

FTIR
Figure 7 shows the FTIR spectra of non-treated and NS-modified PFA, recorded in the range of 500 cm −1 to 4000 cm −1 .The major peaks observed in these spectra were centered around 1080 cm −1 , representing the asymmetric stretching of the carbonate C-O bond [59,60].In addition, C-H vibration peaks due to the presence of NS backbone were detected at 694 cm −1 and 776 cm −1 , while peaks at 3463 cm −1 were attributed to the O-H bend vibration of the secondary amine of NS [5].

XRD
Figure 8 shows the XRD patterns of non-treated and NS-modified PF.The analysis showed that the diffraction peak intensity of SiO2 in the modified PFA was higher than the non-treated PFA.Anorthite and quartz displayed prominent peaks in the natural aggregate (Control), but they were inactive.Moreover, the intensity of the C-H peaks was observed to decrease significantly, and a distinctive diffraction pattern of calcite was

XRD
Figure 8 shows the XRD patterns of non-treated and NS-modified PF.The analysis showed that the diffraction peak intensity of SiO 2 in the modified PFA was higher than the non-treated PFA.Anorthite and quartz displayed prominent peaks in the natural aggregate (Control), but they were inactive.Moreover, the intensity of the C-H peaks was observed to decrease significantly, and a distinctive diffraction pattern of calcite was noticed after the NS modification.The characteristic peaks of quartz remained predominant after modification with different NS concentrations [61].It was also observed that the calcite peaks increased with the increase in NS concentration up to 2%.Furthermore, due to the prolonged abandonment of public filler and its preparation and storage processes, CO 2 in the air reacts with Ca (OH) 2 and C-S-H in old mortar to form CaCO 3 .NS exhibits high pozzolanic properties and can react with Ca (OH) 2 to produce calcium silicate hydrate, reducing the size and content of Ca (OH) 2 crystals.The newly formed calcium silicate hydrate can fill the voids in the aggregate, thus significantly improving the macroscopic physical properties of recycled aggregates after modification.

XRD
Figure 8 shows the XRD patterns of non-treated and NS-modified PF.The analysis showed that the diffraction peak intensity of SiO2 in the modified PFA was higher than the non-treated PFA.Anorthite and quartz displayed prominent peaks in the natural aggregate (Control), but they were inactive.Moreover, the intensity of the C-H peaks was observed to decrease significantly, and a distinctive diffraction pattern of calcite was noticed after the NS modification.The characteristic peaks of quartz remained predominant after modification with different NS concentrations [61].It was also observed that the calcite peaks increased with the increase in NS concentration up to 2%.Furthermore, due to the prolonged abandonment of public filler and its preparation and storage processes, CO2 in the air reacts with Ca (OH)2 and C-S-H in old mortar to form CaCO3. NS exhibits high pozzolanic properties and can react with Ca (OH)2 to produce calcium silicate hydrate, reducing the size and content of Ca (OH)2 crystals.The newly formed calcium silicate hydrate can fill the voids in the aggregate, thus significantly improving the macroscopic physical properties of recycled aggregates after modification.In ten sity (a.u .)

2θ (degree)
Si-PFA-3  The effect of NS-modified PFA on the compressive strength of recycled mortar is shown in Figure 9.The average strength of the conventional mortar reached 41.5 MPa after 28 days of maintenance.When the natural sand was replaced by untreated PFA, the strength decreased to 32.3 MPa, which was 22.3% lower than that of the conventional mortar.This decrease was due to the defects in PFA reported in previous studies [62,63].After pretreating with NS, the strength of Si-PFA-1 increased to 34.7 MPa, which was 7.8% higher than the control group.When 2% NS-modified PFA was used instead of NA, the compressive strength was 39.1 MPa, showing an obvious increase of 21.9% compared to the control group and being close to the NA group.This enhancement in compressive strength of PFA modified with 2% NS (21.9%) was better than that of 2% nano-CaCO 3 (8.6%)and 2% nano-Fe 2 O 3 (5.7%)[64,65].The longer and higher the content of pre-immersed PFA in NS, the higher the compressive strength of the recycled mortar.However, the compressive strength of Si-PFA-3 was only 1.5% higher than that of Si-PFA-2, probably because the retardation effect of NS significantly reduced the hydration rate of cement.
The NS-modified PFA enhanced the strength of recycled mortar mainly due to the physical effect of the combination of dense microstructure and rough surface texture [66].NS particles adsorbed on the RCA surface and filled the internal microcracks to form a more compact and robust structure, which reacted with the Ca (OH) 2 crystals to form C-S-H. C-S-H strengthens the density and microstructure of the mortar and reinforces the interfacial properties to improve the compressive strength of the mortar [20].This was confirmed by the refinement of the pore structure and the reduction of water absorption, which also indicated that the denser structure could act as a stronger framework to resist compression when integrating into the interior of the mortar.
However, the compressive strength of Si-PFA-3 was only 1.5% higher than that of Si-PFA-2, probably because the retardation effect of NS significantly reduced the hydration rate of cement.
The NS-modified PFA enhanced the strength of recycled mortar mainly due to the physical effect of the combination of dense microstructure and rough surface texture [66].NS particles adsorbed on the RCA surface and filled the internal microcracks to form a more compact and robust structure, which reacted with the Ca (OH)2 crystals to form C-S-H. C-S-H strengthens the density and microstructure of the mortar and reinforces the interfacial properties to improve the compressive strength of the mortar [20].This was confirmed by the refinement of the pore structure and the reduction of water absorption, which also indicated that the denser structure could act as a stronger framework to resist compression when integrating into the interior of the mortar.

Capillary Water Absorption
Figure 10 shows the variation curves of the capillary water absorption mass (CWAM) of recycled mortar with time.The CWAM of each group increased with water absorption time, and the CWAM of the Control group increased significantly within 8 d than that of the NA group.This might have been because more cracks were generated in PFA during crushing and the interface between the old and the new cement mortar was weak and porous, and these pores and the microcracks formed a connecting network, which accelerated the water transmission.The CWAM of the PFA group was significantly reduced after modification with the NS solution, and the results were in good agreement with the pore characterization test.The total porosity of the modified PFA mortar was reduced, while the nano-particles produced by the reaction of SiO2 with CH and calcite

Capillary Water Absorption
Figure 10 shows the variation curves of the capillary water absorption mass (CWAM) of recycled mortar with time.The CWAM of each group increased with water absorption time, and the CWAM of the Control group increased significantly within 8 d than that of the NA group.This might have been because more cracks were generated in PFA during crushing and the interface between the old and the new cement mortar was weak and porous, and these pores and the microcracks formed a connecting network, which accelerated the water transmission.The CWAM of the PFA group was significantly reduced after modification with the NS solution, and the results were in good agreement with the pore characterization test.The total porosity of the modified PFA mortar was reduced, while the nano-particles produced by the reaction of SiO 2 with CH and calcite covered the pore surface, reduced the pore diameter, and decreased the water transport channel and rate.At the water absorption time of 831 s 1/2 , the CWAMs of each group were 14.5 g (NA), 65.7 g (Control), 59.1 g (Si-PFA-1), 29.1 g (Si-PFA-2), and 26.2 g (Si-PFA-3).The gross absorption masses of the NS-modified PFA were reduced by 11.26%, 56.3%, and 60.7%, indicating a better waterproof effect of NS-modified PFA.The water absorption process for the recycled mortar group mixed with modified PFA under capillary action gradually tended to level off as the specimens were dry internally and moisture primarily entered the specimens through capillary suction.Subsequently, as the moisture content inside the specimens gradually increased, capillary suction decreased and eventually stabilized under the combined effects of gravity.covered the pore surface, reduced the pore diameter, and decreased the water transport channel and rate.At the water absorption time of 831 s 1/2 , the CWAMs of each group were 14.5 g (NA), 65.7 g (Control), 59.1 g (Si-PFA-1), 29.1 g (Si-PFA-2), and 26.2 g (Si-PFA-3).
The gross absorption masses of the NS-modified PFA were reduced by 11.26%, 56.3%, and 60.7%, indicating a better waterproof effect of NS-modified PFA.The water absorption process for the recycled mortar group mixed with modified PFA under capillary action gradually tended to level off as the specimens were dry internally and moisture primarily entered the specimens through capillary suction.Subsequently, as the moisture content inside the specimens gradually increased, capillary suction decreased and eventually stabilized under the combined effects of gravity.

Electric Flux
Figure 11 shows the results of the electrical flux test of recycled mortar with various PFA.The recycled mortar modified by different concentrations of NS solution significantly improved the resistance to chloride ion migration.Moreover, the electric fluxes of Si-PFA-1, Si-PFA-2, and Si-PFA-3 groups were reduced by 13.5%, 15.1%, and 15.5%, respectively, compared to the Control group.This might be attributed to the fact that the nano-particles produced by the reaction of NS and CH and calcite covered the pore surface, which reduces the pore diameter and decreases the chloride ion transport pathways [67].Meanwhile, the NS-modified PFA bonded better with the new mortar and the ITZ became more denser, which was conducive to the improvement of the anti-chloride ion permeation performance.

Electric Flux
Figure 11 shows the results of the electrical flux test of recycled mortar with various PFA.The recycled mortar modified by different concentrations of NS solution significantly improved the resistance to chloride ion migration.Moreover, the electric fluxes of Si-PFA-1, Si-PFA-2, and Si-PFA-3 groups were reduced by 13.5%, 15.1%, and 15.5%, respectively, compared to the Control group.This might be attributed to the fact that the nano-particles produced by the reaction of NS and CH and calcite covered the pore surface, which reduces the pore diameter and decreases the chloride ion transport pathways [67].Meanwhile, the NSmodified PFA bonded better with the new mortar and the ITZ became more denser, which was conducive to the improvement of the anti-chloride ion permeation performance.

Reinforcement Mechanism of RM Prepared by NS-Modified PFA
The reinforcement mechanism of RM prepared by NS-modified PFA is illustrated in Figure 12.The old mortar attached to the surface of PFA increased the interfacial barrier effect, which made it difficult for the freshly mixed cement particles to fill the space near the surface of the aggregate and form defect-enriched multiple interfaces [67].Nano-SiO 2 modification can improve the microstructure of the PFA surface by filling some gaps and cracks, significantly reducing surface voids and cracks [47,57].When the recycled mortar is configured, the modified PFA comes into contact with the new cement paste, and part of the calcium carbonate on its surface dissolves, slowly releasing CO 3 2− and reacting with the aluminate ions in the cement paste to generate C 3 A•CaCO 3 •11H 2 O, resulting in the continuous erosion of CaCO 3 on the PFA surface [20,58].Although it will improve the bonding effect on the PFA surface, the inhomogeneous reaction will cause the enrichment of defects at the interface.The volcanic ash effect of nano-SiO 2 enables it to react with Ca (OH) 2 adhered on the PFA surface to generate C-S-H, which improves the interfacial bonding effect of PFA [49].Moreover, the nano-SiO 2 adsorbed on the PFA surface can be dispersed into the freshly mixed cement slurry, which improves the internal structure of PFA, and results in a significant improvement of its mechanical and anti-erosion properties [68,69].

Reinforcement Mechanism of RM Prepared by NS-Modified PFA
The reinforcement mechanism of RM prepared by NS-modified PFA is illustrated in Figure 12.The old mortar attached to the surface of PFA increased the interfacial barrier effect, which made it difficult for the freshly mixed cement particles to fill the space near the surface of the aggregate and form defect-enriched multiple interfaces [67].Nano-SiO2 modification can improve the microstructure of the PFA surface by filling some gaps and cracks, significantly reducing surface voids and cracks [47,57].When the recycled mortar is configured, the modified PFA comes into contact with the new cement paste, and part of the calcium carbonate on its surface dissolves, slowly releasing CO3 2− and reacting with the aluminate ions in the cement paste to generate C3A•CaCO3•11H2O, resulting in the continuous erosion of CaCO3 on the PFA surface [20,58].Although it will improve the bonding effect on the PFA surface, the inhomogeneous reaction will cause the enrichment of defects at the interface.The volcanic ash effect of nano-SiO2 enables it to react with Ca (OH)2 adhered on the PFA surface to generate C-S-H, which improves the interfacial bonding effect of PFA [49].Moreover, the nano-SiO2 adsorbed on the PFA surface can be dispersed into the freshly mixed cement slurry, which improves the internal structure of PFA, and results in a significant improvement of its mechanical and anti-erosion properties [68,69].

Conclusions
This research investigated the enhancement of the surface morphology and microstructure of construction and demolition waste (PF) through the modification with nano-SiO2 (NS), as well as the preparation of fully recycled fine aggregate mortar.The findings were summarized based on aspects including the pore structure of PF, surface

Conclusions
This research investigated the enhancement of the surface morphology and microstructure of construction and demolition waste (PF) through the modification with nano-SiO 2 (NS), as well as the preparation of fully recycled fine aggregate mortar.The findings were summarized based on aspects including the pore structure of PF, surface morphology, phase analysis, mineral composition, compressive strength of recycled mortar, capillary water absorption, and electrical flux.
The microstructure of PFA became denser, and the total pore volume and water content were effectively minimized through the impregnation and air-drying effect of NS.When NS concentration was 2% and soaking time was 48 h, the water absorption rate of PFA decreased by 28.6%, apparent density increased by 5.4%, and crushing value reduced by 5.1%.This was primarily due to the highly active NS particles generating more C-S-H and filling internal voids of the mortar, thereby increasing resistance to water molecule penetration.
The compressive strength of fine aggregate mortar increased from 32.3 MPa to 38.52 MPa with 2% NS modification.This enhancement was attributed to the densification of pore structure and roughening of surface texture.However, the aggregation of nano-particles in the mortar intensified as the NS content increased, resulting in a negative impact on compressive strength after the hardening of the mixture.
The porosity of PFA decreased after being modified with NS solutions, leading to a denser interface between new and old mortar.This reduction in porosity resulted in decreased pathways of water, chloride ions, and other substances, thereby reducing the transportation rate and slowing down the infiltration and erosion of external water and harmful substances.Compared to the control group, the capillary water absorption and electrical flux test results of the Si-PFA-2 decreased by 56.3% and 15.1%, respectively.
The above results indicated that NS could be a promising material for improving the microstructure and surface morphology of PF.The application of NS-modified PFA in specific engineering projects could potentially offer significant advantages, including the creation of environmentally friendly building materials that sequester carbon, reducing the amount of waste sent to landfills, enhancing the load-bearing capacity of these materials, and promoting the principles of a circular economy.This application of treated recycled demolition waste is expected to be used in various fields such as pavement construction and landscaping.Furthermore, it is indeed necessary to explore in greater depth the long-term durability of NS-modified PFA under various environmental conditions.Specifically, the stability and durability of materials in harsh environments such as freeze-thaw cycles and chemical attacks are crucial, as they directly impact the reliability and service life of the material in practical engineering applications.Therefore, subsequent tests should include evaluations for freeze-thaw resistance, chemical resistance, and other relevant durability tests.
The main research objective of this article is to utilize NS to enhance the comprehensive performance of PFA, thereby optimizing the best technical solution.By modifying PFA with NS, it is possible to improve its mechanical properties and expand its application scenarios and pathways.Additionally, enhancing the mechanical properties of PFA allows for reduced cement usage in concrete mix design, thereby reducing carbon emissions.Despite the increase in unit cost associated with NS modification, the overall cost of preparing concrete using modified PFA is reduced when considering comprehensive expenses.Subsequent research will delve deeper into these aspects.

Figure 2 .
Figure 2. Flow of raw material processing and specimen preparation.

Figure 2 .
Figure 2. Flow of raw material processing and specimen preparation.

Figure 8 .
Figure 8. XRD patterns of non-treated and NS-modified PFA.

Figure 9 .
Figure 9. Compressive strength of recycled mortar with various PFA.

Figure 9 .
Figure 9. Compressive strength of recycled mortar with various PFA.

Figure 10 .Figure 10 .
Figure 10.Capillary water absorption of recycled mortar with various PFA.3.3.3.Electric Flux Figure 11 shows the results of the electrical flux test of recycled mortar with various PFA.The recycled mortar modified by different concentrations of NS solution significantly improved the resistance to chloride ion migration.Moreover, the electric fluxes of Si-PFA-1, Si-PFA-2, and Si-PFA-3 groups were reduced by 13.5%, 15.1%, and 15.5%, respectively,

Figure 10 .
Figure 10.Capillary water absorption of recycled mortar with various PFA.

Figure 11 .Figure 11 .
Figure 11.The electric flux of recycled mortar with various PFA.

Figure 12 .
Figure 12.Illustration of RM prepared by NS-modified PFA.

Figure 12 .
Figure 12.Illustration of RM prepared by NS-modified PFA.