Investigation properties of ultra-high performance concrete incorporating pond ash

: The study aims to substitute river sand used in ultra-high performance concrete (UHPC) with pond ash (PA), a waste by-product from the Sikka thermal power station in Gujarat, India, at replacement levels ranging from 0 to 20%. Also, 20% of the cement was replaced with ground granulated blast-furnace slag, which is a sustainable, eco-friendly material. As a result, this concrete is both environmentally and economically feasible. Experimental analysis evaluated the workability, compressive strength, split tensile strength, ﬂ exural strength, and microstructure of the UHPC mixtures. Incorporating 10% PA as a sand replacement enhanced the compressive strength, reaching 117 MPa at 90 days, as well as the ﬂ exural strength of 23 MPa and the split tensile strength of 14 MPa. The strength is positively impacted when 10% of the river sand is replaced with PA, while the strength of UHPC appears to be diminished if PA content is increased beyond 10% replacement of sand. Petrographic microscopy and X-ray di ﬀ raction were used to study the microstructure of UHPC made with PA. When PA was used instead of sand, the mortar mass solidi ﬁ ed and became denser, resulting in an improved microstructure of the UHPC with fewer surface cracks. With the inclusion of PA, the calcium silica hydrate gel content of the concrete increases, and enhanced performance of UHPC up to a certain amount of replacement has been observed.


Introduction
Ultra-high performance concrete (UHPC) is recognized for its exceptional durability and high performance.It has emerged as a promising material for enhancing the sustainability of various structural and infrastructural elements.Over the past two decades, there has been significant global interest in UHPC, with applications spanning bridges, building components, architectural features, precast concrete piles, repair and rehabilitation work, offshore and hydraulic structures, and thin-bonded overlays on deteriorated bridge decks [1,2].Among these, bridge and road construction are the most prevalent uses of UHPC [3,4].
A typical UHPC mixture includes binding materials, fine aggregates, superplasticizers (SPs), fibers, and controlled amounts of water.In both UHPC and conventional concrete, a dense particle skeleton is formed where the fine aggregate plays a critical role [5,6].The fine aggregate significantly influences the mechanical properties and durability of concrete.Historically, natural river sand (NRS) has been the primary fine aggregate used in UHPC [7].However, the unsustainable extraction of river sand poses serious environmental risks, including ecosystem disruption, riverbank erosion, and infrastructure instability [8].
With rapidly growing construction demands, India requires approximately 900 million tonnes of sand annually to sustain its development.This high demand for sand highlights the scarcity issue, as excessive sand mining can alter river channels, cause bank erosion, and disrupt aquatic habitats [9].The utilization of resources such as water, energy, and land plays a crucial role in climate change dynamics.Recognizing these challenges, the United Nations General Assembly established the Sustainable Development Goals in 2015 to promote global sustainability.The construction industry, being a significant consumer of non-renewable resources, must adopt more sustainable practices to mitigate environmental impacts.
Researchers are exploring the use of residual materials from thermal power plants, such as bottom ash, fly ash, and pond ash (PA), in cement, mortar, and concrete production to address waste disposal challenges [10,11].While substantial research exists on bottom ash and fly ash, there is limited information on PA utilization [12][13][14].Incorporating PA as a replacement material can help manage waste, reduce environmental impact, and address the scarcity of natural sand resources.Vast reserves of PA occupy significant land areas in India, and its use in construction could alleviate sand shortages [15,16].However, the presence of toxic components in ash ponds requires careful handling to prevent soil and water contamination as shown in Figure 1.
Extensive research has investigated the use of PA as a substitute for fine aggregate in conventional concrete, but its application in UHPC remains largely unexplored.This study aims to fill this gap by evaluating the potential of PA as a fine aggregate replacement in UHPC, presenting a novel avenue for research in sustainable construction materials.
River sand with a size range of 600-150 µ, OPC of 53 grade as per requirements specified by IS: 12269 [20] and microsilica with an extreme fineness and high amorphous silica concentration are all necessary ingredients in UHPC due to their filler and pozzolanic properties.The tensile strength of the brass-coated steel microfibers, which were 13 mm in long and 0.2 mm in diameter, was 2,500 MPa.Table 1 lists the chemical characteristics of the raw material used in UHPC, and Figure 3 shows the particle size distribution curve of different raw materials needed to produce UHPC.
The PA collected from the Sikka Thermal Power Plant was oven-dried before being used as a raw material.Figure 4 shows the X-ray diffraction (XRD) pattern of PA, which shows the peaks characteristic of quartz (SiO 2 )-Q, mullite (Al 6 Si 2 O 13 )-M, and iron oxides such as magnetite (Fe 3 O 4 )-Ma and hematite (Fe 2 O 3 )-H, both of which exist in crystalline form.Quartz and mullite are the most common phases and minerals found in ashes.The major peak is recognized as the most intense peak at 2θ = 27.78°due to quartz, the predominant mineral found in PA.Peaks near 2θ = 26.21°havebeen identified as mullite or aluminosilicate minerals, while those near 2θ = 17.2°have been identified as refractory mullite.The presence of heavy minerals such as magnetite and hematite is indicated by peaks near 2θ of 36.1°and34.3°(Table 3).Natural river sand (NRS); ground granulated blast-furnace slag (GGBS).

Production of UHPC
The mixed proportion of UHPC generated in this investigation is summarized in Table 2.The combinations are identified using abbreviations based on the amount of PA in UHPC, as shown in Table 2.The conventional concrete, which was called UHPC0, containing cement, FA, and microsilica as binders, without any GGBS or PA.In contrast, UHPC5, UHPC10, UHPC15, and UHPC20 indicate that 5, 10, 15, and 20% of PA by weight were used to replace NRS, respectively, with 20% of the cement replaced by GGBS.When it came time to prepare the necessary number of specimens in accordance with the mix proportion, the various ingredients needed for the creation of UHPC were weighed and held separately [21,22].The components of UHPC were mixed using a planetary mixer.Figure 5 depicts the mixing order used for each mixed proportion of UHPC.The order is as follows: first, dry mixing was done for around 2 min with the following materials: cement, microsilica, fly ash, PA, GGBS, and NRS.After adding 50% of the water, the dry ingredients were slowly mixed for 3 min.
The SP was then added and properly blended with the remaining 50% water before being added to the mixture for the next 8 min [23].Steel fibers were added to the mixture once a thick cement matrix paste had formed, and mixing was continued for an additional three minutes to guarantee that the fibers got distributed uniformly throughout the concrete's mass [12].After being prepared, the mixture was transferred into the molds, which included prismatic beams (100 mm × 100 mm × 500 mm), cylinders (100 mm × 200 mm), and cubes (70.Steel fibers (SF); super plasticizer (SP).
Investigation properties of UHPC incorporating pond ash  3 procedure was completed, the casting process was finished in 10-15 min.The samples were then left in the molds at room temperature for 24 h.The samples were taken out of the molds after 24 h.According to the guidelines in IS-10262:2019 [24], the samples were cured in water for 7, 14, 28, and 90 days at room temperature.

Experimental methodology 2.2.1 Compressive strength
The most crucial characteristic of concrete elements is their compressive strength, and for this experimental study, three UHPC cubical samples of size 7.07 cm were created for each combination.The concrete sample must be poured into the cube molds in three layers.Each layer must be compacted by hand or by vibration.Each layer of concrete in the mold must be compacted with a tamping bar.The specimens were demolded after 24 h and then exposed to 28 days of curing in accordance with the requirements set forth in IS 516-1959 [25].The UHPC samples were placed on a compressive testing machine (CTM) with a 200-ton capacity until failure.At 7, 14, 28, and 90 days, each UHPC mix's three samples underwent testing.The compressive strength was determined using the three samples' average values [26,27].

Splitting tensile strength
Three samples per UHPC mix were cast as cylindrical specimens with a 100 mm dia.and 200 mm ht. to estimate the strength of concrete in tension.CTM tested the cylinders' splitting tensile strength at 7, 14, 28, and 90 days.Accomplishments and test results were controlled in accordance with IS 5816-1999 guidelines [28].

Flexural strength
The strength of the UHPC prismatic beam in flexure was tested on a 4-point flexural test apparatus using a 500-mmlong, 100-mm-squared cross-section cast prismatic beam with three samples per composition in accordance with IS 516:1959 [25].The slump cone test was used in this experimental study to highlight the fresh properties of all mixtures.For assessing the flowability of this type of concrete, which has a texture similar to mortar, slump flow is a suitable criterion.The conical cone had a height of 30 cm, a top dia. of 10 cm, and a bottom dia. of 20 cm.Fresh UHPC was first placed into a mold, after which the conical cone was elevated vertically.
The average of two parallel diameters across a concrete spread is referred to as a slump flow.

Microstructure analysis
Utilizing petrographic imaging analysis, the UHPC's microstructure is examined.Small pieces of prismatic specimens are used to make the slender pieces of the hardened UHPC mix samples, which are then examined under an optical microscope.When 5×, 10×, and 20× lenses are put together, it is easier and more accurate to figure out the exact shapes of the final mix [29].

Compressive strength
The results of determining the compressive strength of UHPC specimens with various replacement percentages of PA by weight of NRS -0, 5, 10, 15, and 20%, and the variation in compressive strength is shown in Figure 6.
PA affects the strength of UHPC when sand is replaced with it.The 10% replacement of PA in UHPC after 28 days of curing results in the greatest compressive strength improvement.Other curing days' increases in strength showed a similar pattern.For concrete mixtures up to 10% (UHPC10) cured for 7, 14, 28, and 90 days, an improvement in strength was seen with increasing PA content.The hydration products combine with the integrated supplemental cementitious materials (SCMs), resulting in a newly formed, stronger gel.The whole microstructure, particularly interfacial transition zone (ITZ), had been saturated with this stronger gel, which clearly showed UHPC's dense and impermeable structure.These SCMs enhance the compressive properties of UHPC due to their pozzolanic reaction with Ca(OH) 2 [30].
The strength of the concrete mixtures decreases beyond 10% replacement content.Due to a rise in PA content, more porous particles in the mortar mass did not react with Ca (OH) 2 to form C-S-H gel after 10% replacement.As a result, strength drops after UHPC10.
In general, it can be said that PA is marginally good for enhancing UHPC's compressive strength [15].PA is therefore an acceptable choice for the aggregate in UHPC from the perspective of its mechanical properties [9,31].

Split tensile strength
Split tensile strength serves as an indirect indicator of the tensile strength of concrete.The conventional UHPC (UHPC0) cylinder's average split tensile strengths at 7, 14, 28, and 90 days are 8, 11, 13, and 15 MPa, respectively.When compared to conventional concrete, concrete that contains PA in various Investigation properties of UHPC incorporating pond ash  5 percentages has lower split tensile strength values.Figure 7 illustrates the change in tensile strength [32].At all stages of curing, the inclusion of PA decreased the split tensile strength of UHPC.However, the split tensile strength of the UHPC mixture adding 10% PA, or UHPC10, was comparable to that of the controlled UHPC, i.e., UHPC0.At 7, 14, 28, and 90 days, the UHPC10 mixture's split tensile strength decreased by 12.5, 22.7, 7.69, and 6.66%, respectively.Concrete mixtures UHPC5, UHPC15, and UHPC20 showed lower split tensile strength at 7 days of curing age compared to conventional UHPC, or UHPC0, of 18.75, 31.2, and 37.5%, respectively.Similar to the conventional UHPC, or UHPC0, these UHPC mixtures at 28 days showed lower split tensile strengths of 15.38, 19.23, and 30.76%, respectively.The split tensile strength of these UHPC combinations treated with PA decreased by 20, 13.33, and 33.33%, respectively, after curing for 90 days.PA may have a poorer binding with cement paste than sand, which results in a weaker microstructure of the concrete and a drop in the split tensile strength of UHPC.Cracks appear early in the loading process because of a weak connection between the cement paste and the PA particles.All of these elements together lead to a reduction in split tensile strength.The heterogeneous discontinuity of the cementitious matrix with the weaker transition zone increases as PA concentration rises, and the split tensile strength of concrete further declines [33].

Flexural strength
Comparing PA in various proportions to conventional UHPC, the flexural strength has decreased at 7, 14, 28, and 90 days.The conventional UHPC (UHPC0) prismatic beam's average flexural strengths are 11, 15, 23, and 26 MPa, respectively.In comparison to conventional concrete, concrete that contains different percentages of PA has lower values of flexural strength.Figure 8 depicts the variation in flexural strength.At all stages of curing, adding PA decreased the flexural strength of UHPC.The flexural strength decreases with increasing PA content due to the combined effects of flocculation and the toughening effect of steel fiber.PA causes flocculation, which resists compressive forces better than tensile or flexural stresses.Flocs increase with PA concentration, lowering tensile, and flexural strengths.However, a UHPC mixture that contained 10% PA, or UHPC10, showed flexural strength that was comparable to conventional UHPC, or UHPC0.At 7, 14, 28, and 90 days, the UHPC10 mixture's flexural strength had decreased by 27.27, 33.33, 13.04, and 11.53%, respectively.Concrete mixtures UHPC5, UHPC15, and UHPC20 all showed reduced flexural strengths at 7 days of curing age compared to conventional UHPC, or UHPC0, of 18.18, 40.9, and 36.36%,respectively.Similar to the conventional UHPC, or UHPC0, at 14 days, these UHPC combinations showed decreased flexural strengths of 13.33, 6.66, and 26.66%, respectively.These PA-modified UHPC mixtures lost 17.39, 21.73, and 30.43% of their flexural strength after 28 days of curing, respectively.The loss in flexural strength of these UHPC mixtures treated with PA after 90 days of curing age was 15.38, 19.23, and 30.76%, respectively.

Workability
The quantity of fines and the characteristics of the fine aggregate used in the concrete determine its desired workability.NRS is made up of compact particles that lose their shape and smoothen as a result of weathering.Mica and other weak minerals are taken out of it.PA particles, on the other hand, are angular, rough in texture, and porous.Comparing NRS to PA, the PA contains a greater amount of particles that are smaller than 75 µm.As a result, when PA is used in place of natural sand in concrete, the number of fines and irregularly shaped, rough-textured, and porous particles increases, increasing the friction between the particles.Fresh concrete's flow properties are hindered by the increased interparticle friction.Additionally, because PA has a significantly higher water absorption ratio than natural sand, a portion of the water is absorbed internally by the porous PA particles, reducing the amount of water that can be used to lubricate the particles to obtain the appropriate workability.As a result, using PA in place of sand affects the workability of concrete at a certain water cement ratio.According to the published research results, using PA instead of sand increases the water requirement for the same slump of concrete.The slump values of various concrete mixtures along with the fluctuation in slump flow are shown in Figure 9.

Microstructure analysis
Utilizing a petrographic image analysis, the microstructure of the hardened paste and the ITZ between the aggregate and the cement paste were examined to comprehend the strength mechanism of UHPC.The fractured specimen of UHPC was cut up into small pieces to provide samples for analysis.
The binder granules were near one another because of the exceptionally low water/binder ratio, causing very little porosity.The UHPC matrix is extremely dense and primarily made up of hydration products that resemble a gels that have been created by a diffusion process.A homogenous morphology of CSH gel makes up the majority of the image in Figure 10 and is the principal hydration product.However, there are also a few crystals of calcium hydroxide, i.e., Ca(OH) 2 , and calcium alumino silicate hydrates, i.e., CASH [34].Investigation properties of UHPC incorporating pond ash  7 Figure 10 illustrates the exceptionally dense microstructure of UHPC with 10% PA replacement.The low water-to-binder ratio results in a compact arrangement of cement particles, which, combined with the pozzolanic activity of GGBS and microsilica, minimizes voids and enhances the material's density.The primary hydration product, calcium silicate hydrate (C-S-H) gel, dominates the microstructure, contributing to the material's strength.The homogenous morphology of the C-S-H gel indicates effective pozzolanic reactions and improved mechanical properties [17].
The development of the microstructure post-hydration is shown in Figure 10.
(i) About 70% of the solids volume and the source of the material's improved strength, calcium silicate hydrates, i.e., CSH gel make up the majority of the UHPC sample.(ii) About 20% of the solids' volume is made up of hexagonal-shaped calcium hydroxide crystals, i.e., Ca(OH) 2 .
(iii) The prismatic crystals known as calcium aluminosilicate hydrates, i.e., CASH, have needle-like shapes.CASH makes up around 8% of the solids volume of hydrated concrete.
Different steel fibers found in UHPC are depicted in Figure 11 with red boundaries.Figure 11 highlights the distribution of steel fibers within the UHPC matrix.The strong bond between the steel fibers and the cement paste enhances the tensile and flexural properties of the concrete.The well-distributed fibers act as crack arrestors, preventing the propagation of microcracks and enhancing the durability of UHPC.
Figure 12 shows the ITZ structure, which is critical for the composite action between the paste and the aggregate.The UHPC with 10% PA exhibits a denser ITZ with fewer microcracks compared to higher replacement levels.This denser ITZ is attributed to the optimal filler effect of PA, which improves the packing density and reduces the porosity at the interface.The well-distributed fibers act as crack arrestors, preventing the propagation of microcracks and enhancing the durability of UHPC, while the interface between steel fiber and cement paste is represented in Figure 12 with a blue boundary.Steel fiber-matrix bond qualities have a significant impact on the mechanical properties of steel fiber-reinforced concrete.The tension is transferred between the fiber-matrix phases through the fibermatrix bond.The main element influencing bond strength is the fiber-matrix interface characteristic (fiber-matrix transition zone).
The decrease in ITZ cracks indicates a tighter link between the cement matrix and aggregate at the interface, which increases interface strength and raises compressive strength.When PA replacement was at 10%, the initial ITZ crack was practically nonexistent.When PA replacement reached 20%, the UHPC fluidity increased quickly, the bonding power at interfaces between the cement paste and the aggregate reduced, and some cracks started to appear in the ITZ, which causes slight drop in compressive strength [35].
The change in hydration gel formation in the UHPC at day 28 is depicted in Figure 13(a)-(e) and it compares the hydration gel formation at different PA replacement levels.The C-S-H gel is indicated by a defined area with a green boundary.The maximum surface area of the C-S-H gel is observed at 10% PA replacement (Figure 13c), indicating the optimal pozzolanic activity and enhanced microstructural integrity.In contrast, higher replacement levels (Figure 13d and e) show a decrease in C-S-H gel formation, leading to increased porosity and reduced mechanical strength.The microstructural analysis confirms that the incorporation of 10% PA in UHPC not only improves the material's density and reduces porosity but also enhances the bond strength at the ITZ.These microstructural improvements directly contribute to the observed enhancements in compressive, tensile, and flexural properties, validating the optimal replacement level of PA in UHPC.

Conclusions
From the aforementioned findings, it can be inferred that UHPC can be produced using standard technology with high binder content, several additions of fly ash, microsilica, GGBS, and PA, an exceptionally low W/B ratio, and a high standard SP.The UHPC10 performed brilliantly in our study, with a slump of 456 mm and the highest compressive strengths of 98 MPa at 28 days and 117 MPa at 90 days.The bulk density of 2,450 kg/m 3 and the specific density of 2,600 kg/m 3 for the UHPC hardened concrete were obtained.For the preparation to be effective, it is found that a water binder ratio of 0.21, 1,200 kg/m 3 of cementitious material, made up of 50% cement, 17% fly ash, 13% GGBS, 12% microsilica, and 8% PA, and an acceptable dosage of SP to guarantee optimal fluidity are necessary.
The following inferences are drawn from the findings of the fractional replacement of cement with GGBS and the 0-20% replacement of river sand with PA: 1.When compared to conventional UHPC, the UHPC incorporating GGBS and PA demonstrated enhanced mechanical properties.2. Specifically, replacing up to 20% of cement with GGBS and up to 10% of river sand with PA resulted in significant strength improvements.The UHPC with 10% PA replacement achieved a compressive strength of 117 MPa at 90 days, compared to 108 MPa for conventional UHPC.The optimal replacement level for PA was identified as 10%, beyond which the strength gains diminished.3. The workability, indicated by slump flow, showed acceptable values around 450 mm, with the UHPC containing 10% PA exhibiting a slump flow of 456 mm.

Microstructural analysis reveals that incorporating 10%
PA in UHPC enhances material density, reduces porosity, and strengthens the ITZ bond.These improvements directly contribute to increased compressive, tensile, and flexural properties, confirming the optimal replacement level.5.These findings suggest that moderate amounts of GGBS and PA can be effectively substituted in UHPC to enhance its strength and sustainability without compromising its performance.

Figure 1 :
Figure 1: PA sample collected from thermal power plant.

Figure 3 :
Figure 3: Particle size distribution curve of different raw materials.

Figure 6 :
Figure 6: Compressive strength of UHPC cubes with different PA content.

Figure 7 :Figure 8 :
Figure 7: Split tensile strength of UHPC cylinders with different PA content.

Figure 9 :
Figure 9: Variation of slump flow of UHPC mixes with different PA contents.

Table 1 :
Chemical properties of raw materials used in UHPC at room temperature (25°C)

Table 3 :
XRD pattern peaks and mineral phases of pond ash at room temperature (25°C)