Introduction

Ultra-high performance concrete (UHPC) is a novel category of concrete composite that has the potential to make a technical breakthrough in sustainable construction. Its importance lies in its ability to significantly reduce the environmental impact of construction while delivering exceptional strength and durability. By incorporating recycled materials and minimizing cement content, UHPC helps lower CO2 emissions and resource depletion. This eco-conscious approach not only contributes to a greener planet but also ensures that our infrastructure is built to last, promoting long-term sustainability in the construction industry. However, its adoption faces constraints, primarily related to higher production costs, the limited availability of eco-friendly materials, and the need for specialized expertise in its application. UHPC has a lot of advantages, such as good workability, flowability, durability, permeability, toughness, resistance against chloride diffusion, chemical resistance, freeze–thaw cycles, abrasion resistance, and a target compressive strength of more than 150 MPa [1,2,3,4]. The disadvantages of UHPC come from the typical constituents, notably a high proportion of Portland cement, in addition to high material costs. Cement manufacturing is well-known for accounting for about 7% of world CO2 emissions [5,6,7]. Enhancing cost-effective UHPC is vital for boosting the spread of these pioneering building materials. From this point of view, sustainable supplementary cementitious materials (SCMs), such as fly ash (FA), ground granulated blast furnace slag (GGBS), and silica fume (SF), began to appear to lessen cement composition without significantly decreasing UHPC mechanical strengths. In addition to these materials, nano-materials were mixed as a partial replacement of cement with different SCMs such as nano-silica, nano–titanium and nano-recycled glass powder to enhance the mechanical properties of concrete [8,9,10,11,12,13].

Several studies have enhanced the mechanical properties of UHPC using supplementary cementitious materials as a partial replacement of cement and filler materials as a partial replacement of sand in order to enhance the sustainability of concrete. SCMs enhance the properties of concrete in two ways: the first is caused by a reaction with cement hydration products, while the second is through increased particle packing efficiency. Various SCMs have already been investigated as powder substitutes in the formulation of reactive powder concrete [14]. Ngo et al. [15] designed mixtures with a water-to-binder ratio of 0.2, 25% SF of the total binder amount by mass, in addition to four ratios of GGBS of cement by mass, and concluded that replacing 15 or 30% of cement with GGBFS yields an improvement in mortar's strength and durability characteristics. Using SCMs in concrete mixtures boosts durability, lessens hydration heat, and boosts overall concrete characteristics [16, 17].

Recycling waste glass as a sustainable construction material is gaining popularity in the construction industry because it has the ability to reduce greenhouse emissions and associated environmental problems. As a result, researchers are concentrating on the production of concrete and cement mortar using waste glass as aggregate or as supplementary materials using normal curing and elevated temperatures. On the other side, the workability of concrete decreased as the partial replacement level of GLP, marble powder, and timber ash increased [18,19,20,21,22,23]. Jubeh et al. [24] used three ratios of GLP as a partial replacement of cement in addition to FA, SF, and styrene butadiene rubber and concluded that the compressive strength increases with increasing the ratio of GLP to achieve the highest compressive strength at 25% GLP. The incorporation of GLP as the precursor in alkali-activated materials (AAMs) resulted in enhancing the workability and an extension of the set times [25]. Almeshal et al. [26] concluded the effect of GLP with the aid of ammonium nitrate (NH4NO3) on the unit weight in addition to the strength of glass mortar, and the results showed that the maximum compressive strength (60 MPa at 60 days) was achieved at a 10% replacement level of cement by immersion in NH4NO3 solution. GLP has a notable impact on sulphate resistance, as 20% GLP replacement with cement led to improving the characteristics of the mortar at the 28th day in addition to enhancing the fresh properties of concrete [27,28,29]. Using SF with StF also plays an important role in enhancing the compressive strength and flexural behaviour of road pavement slabs [30]. Haido et al. [31] deduced that the recycled waste glass powder is a good alternative to the pozzolanic powder of silica fume.

Using agricultural waste in concrete has recently gained a lot of concentration in many countries. Alyami et al. [32] used local agricultural residue ash from rice husk (RHA), sugarcane leaf ash (SLA), and olive waste ash (OWA) as a partial replacement of 50% cement, and the results showed that UHPC could be prepared when replacing 50% of the OPC weight with (SLA 25% + RHA 25%) with high compressive, tensile, and flexural strength. Keratin fibres are also one of the newest waste products of the poultry industry that can be used to enhance the brittle behaviour of concrete [33]. Maglad et al. [34] used agricultural waste ash (AWA) as a partial substitute for cement in addition to sugarcane bagasse ash (SBA) and corn stalk ash (CSA) and concluded that producing UHSC with respective compressive and flexural strengths of more than 205 and 27 MPa is possible with these materials.

Currently, the optimization concept has begun to spread, which is the technique of obtaining the most beneficial outcome with the least amount of effort or with the most desired interests. In general, there is more than one efficient technique or design, so optimization objectives are met by selecting the best one. The design of experiments (DOE) is an important, complicated, and multifaceted technique for resolving complex and multifactor engineering problems that is driven by the desire to minimize the number of tests, costs, time, and physical activities. This includes RSM, prototypes, and models for managing continuous treatments when the goal is to achieve equilibrium or demonstrate the response [35]. The most widely utilized optimization methodology in RSM is central composite design (CCD). These strategies may be utilized to assess the findings from experiments using ANOVA as well as the importance of the variables in terms of their relationship to the response results [36]. The current UHPC mixture optimization efforts are primarily focused on lowering overall financial costs and enhancing fresh mixture properties while maintaining UHPC.

Research significance

Previous studies showed the defects of using a high proportion of ordinary Portland cement in UHPC, such as high costs and pollution. From this point of view, this research focused on producing sustainable eco-friendly UHPC using various SCMs and filler materials as a partial replacement of cement and sand, respectively. Supplementary cementitious materials SCMs can be categorized in this research into SF, FA, and GLP, meanwhile, filler materials can be categorized into GrP and LP. However, using trial batches for designing UHPC with numerous component materials is a difficult task. Response surface methodology (RSM) was used to design concrete mixtures instead of the conventional way in order to produce UHPC with optimal properties, low environmental impact, less effort, and low cost.

Experimental investigation

Materials

Ordinary Portland cement (OPC) CEM I 52.5N was used in this research with specifications according to ES 4756-1/2017 and BS EN 197-1/2017 [37, 38]. Supplementary cementitious materials (SCMs) such as silica fume (SF) with a particle size of 8 µm, confirming with ASTM C1240/2020 [39], glass powder (GLP) with a particle size less than 80 microns, and fly ash (FA) with a particle size 200 of µm were utilized. Filler materials such as granite powder (GrP) with particle size distribution in between 0.6 mm and 0.15 mm and white limestone powder (LP) with particle size less than 0.6 mm were utilized. Table 1 illustrates the physical in addition to chemical characteristics of the OPC, SF, FA, and GLP. Excellently classified natural river sand with a particle size less than 0.6 mm was used according to the Egyptian standards (E.S.S. 1109/2018) [40]. A polycarboxylate (PCE)-based high-range water-reducing admixture (HRWRA) conforming to BS EN 934-2 [41] with a 1.08 kg/litre density was utilized in the concrete mixes. Corrugated square steel fibres having a diameter of 1 mm and a length of 25.25 mm were used in the mixtures. The materials used to produce UHPC are shown in Fig. 1.

Table 1 The chemical and physical properties of the utilized cementitious materials
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Fig. 1
figure 1

The alternative materials used in the production of UHPC: a SF; b FA; c GrP; d LP; e GLP

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Mixtures proportions

Twenty-seven concrete mixes that included different supplementary materials were designed through central composite design (CCD) under response surface methodology (RSM) and tested to investigate the impact of SF, GLP in addition to FA as partial substitutes for cement, meanwhile, LP and GrP, as partial substitutes for sand on the compressive and flexural mechanics characteristics of UHPC. All the concrete mixtures were made using a 1100 kg/m3 content of cementitious materials (OPC, SF, FA and GLP). Table 2 shows the coded values of all variables that utilized in this research. SF, GLP, and FA, were used as partial substitutes for cement with ratios of (0–25%), (0–20%) and (0–30%), respectively. GrP and LP were utilized as partial substitutes for sand with ratios of (0–25%) and (0–50%), respectively. Table 3 illustrates the ratios of all mixtures. Steel fibres (StF) were utilized at a proportion of 2% (157 kg/m3) by the concrete volume in all mixtures. Mix 1 which contains the average value of each parameter was considered as the control mixture in this study.

Table 2 Coded value of variables
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Table 3 Mixtures proportions of UHPC
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Sample preparation

To examine the impact of using various supplementary materials on the mechanical characteristics of UHPC, twenty-seven mixtures were prepared. All of the raw elements were combined in a 15-L mixer. Firstly, at a high speed of 280 rpm for 3 min, cement, SF, FA, GLP, GrP, LP, and fine sand were blended carefully. Then, the SP was premixed with water and added in two equal portions for 6 min at a low rpm of 140. When the ingredients became flowable, the steel fibres were gradually added to the mixer for 10 min in order to accomplish a constant dispersion. All samples were demoulded after one day and then cured in steam curing (100 °C) for another 24 h.

Tests

Compressive strength

Three ages 7, 28, and 90 days were used to assess the results of compressive strength tests. The compressive strength was determined conforming to ASTM C109/C109M-20b [42], and the mean values of three cubes with sizes 50 \(\times\) 50 \(\times\) 50 mm for each age were presented as experimental findings. Figure 2a shows the samples preparation and test performed for the compressive strength test.

Fig. 2
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Preparation of UHPC tests: a compressive specimens; b flexural specimens

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Flexural strength

A flexural strength test was performed at age 28 days. Three beams 40 \(\times\) 40 \(\times\) 160 mm were utilized to assess flexural strength at 28 days according to ASTM C293- 02 [43]. The load was centred in the beam on a flat and smooth face. Two pedestals at the beam's base were adjusted 25 mm from each edge. The loading speed was set at 0.2 mm/min. The average of three beams for each batch was calculated. Figure 2b shows the samples preparation and test performed for the flexural strength test.

Scanning electron microstructure

Scanning electron microscopy (SEM) is a useful method for analysing the morphological changes of UHPC paste hydration products. The samples were vacuumed and then coated with 10 nm carbon to prevent charges from accumulating during the imaging process. The samples were then carbon taped to a sample holder before being placed in the SEM inspection chamber.

Results, analysis, optimization, and prediction

The compressive and flexural strengths of UHPC mixes were analysed at a 95% confidence degree to determine the statistical significance of experimental constraints such as SF, GLP, FA, GrP, and LP. The effect of each variable at different ages was studied and analysed as follows:-

Compressive strength results

The results of compressive strength revealed that sustainable UHPC can be produced using supplementary cementitious materials (SCMs) as a partial replacement of cement, and filler materials as a partial replacement of sand. Figure 3 illustrates that the change in type and content of SCMs and filler materials resulted in a high significant effect in concrete compressive strength. The compressive strength of concrete mixtures ranged between (77–133 MPa), (102.6–175.8 MPa) and (103.8–180.2 MPa) at age 7, 28, and 90 days, respectively. The mixture containing 12.5% SF, 15% FA, and 10% GLP as partial replacements of cement, along with 12.5% GrP and 50% LP as partial replacements of fine aggregate, exhibited the highest compressive at all curing ages, and the compressive strength was 133, 175.8, 180.2 MPa at age 7, 28 and 90 days, respectively.

Fig. 3
figure 3

Compressive strength results after 7, 28, and 90 days

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The results illustrated that the change in SF content has a higher effect on compressive strength of concrete. When comparing mixes 1, 3, and 10, it was found that the optimum ratio of SF as an alternative to cement which achieved the highest compressive strength was 12.5% as shown in Fig. 3. This is attributed to the pozzolanic reaction and pore size refinement mechanism. It can be realized that the higher the silica fume percentage ratio (more than 12.5%) as an alternative to cement, the lower the compressive strength after all ages, as displayed in Fig. 3, and this is agreed with Kessal et al. and Wang et al. [44, 45]. Bhanja and Sengupta [46], and Benaicha et al. [47] reported that the loosening effect associated with relatively high SF content leads to the entrapment of air bubbles and eventually lower compressive strength. FA has an important role in increasing concrete strength because of pore structure densification. The effect of FA on compressive strength appears clearly when comparing mixes 1, 2, and 11, it was found that the increase in FA content resulted in increasing in compressive strength and the optimum ratio to achieve high compressive strength (165.8 MPa at age 90 days) was 30% as an alternative to cement as shown in Fig. 3. These finding was in agreement with Escalante and Sharp [48], Xi et al. [49], and Bahedh and Jaafar [50]. The particles of FA tend to fill the paste's microspores, reducing the size of the hydrates that can form (a process known as the filling effect) in addition to densification of the pore structure, thereby restricting the size of the particles formed by the hydrates that were formed [20, 51, 52]. As a result, the improved mechanical properties are most likely the result of the combined benefits of enhanced cement hydration and the FA's pozzolanic reactivity. Also, the results revealed that the use of 20% GLP as a partial replacement for cement achieved the highest strength (126.5, 170.2 and 172 MPa) at age 7, 28, and 90 days, respectively, compared to other ratios of GLP, this effect is clearly visible when comparing mixes 1, 8, and 9. This increasing is attributed to the pozzolanic activity, which is due to an increasing amount of amorphous material [53].

LP is considered one of the most effective filler materials in UHPC. LP was used in ratios between 0 and 50% as a partial replacement for sand. The results illustrated that the increase in LP content as a partial replacement of sand resulted in increase in concrete compressive strength, this effect is clearly visible when comparing mixes 1, 4, and 6. This may be due to the improve in the microstructure packing density of concrete [52]. LP has an effective role in enhancing the secondary pozzolanic hydration and thus producing denser structure, and this is attributed to supplementary pozzolanic hydration is more intense than C3S/C2S hydration, which can increase the potential for longer strength enhancement [54]. Also, comparing the results of mixes 1, 5, and 7 illustrated that the optimum ratio of GrP as a partial substitute for sand was 12.5%. This result may be due to the activated pozzolanic reaction of GrP; these results were in agreement with Zhang et al. [55].

The contour plots can be used to determine the optimal content of variables to achieve the desirable properties of responses. A sample of cases was selected to be drawn and studied. Figure 4 shows that the desirable compressive strength (130–140) and flexural strength (25–28) MPa at age 28 days could be achieved using SF and GLP in the range of (10–15) and (17–19) %, respectively. Response surface plots in Fig. 5 illustrate the variations of compressive strength at ages 7, 28, and 90 days with SF and LP. High results of compressive strength at 7, 28, and 90 days were achieved with low ratios of FA and high ratios of LP, as shown in Fig. 5 (a, b and c). On the other hand, various combinations of SF, GLP, and LP content cause a notable impact on compressive strength at 7, 28, and 90 days, as shown in Fig. 6. Low ratio of SF and high ratio of LP cause high results of compressive strength at 7 days, as shown in Fig. 6a. Meanwhile, a high ratio of GLP led to increasing compressive strength at 28 days, as illustrated in Fig. 6b. Figure 7 shows the choice of evaluating the best variable values to achieve the desired response. For example, the target of the responses is 25 MPa ƒFlex28 and 125 MPa ƒC28days. The suggested mixture is composed of 23.98% SF, 15.35% GLP, 2.77% GrP, 12.29% FA, and 18.16% LP dosage. Generally, it is possible to produce sustainable UHPC with a compressive strength of 175.8 MPa at 28 days using available supplementary cementitious materials (SCMs) as a partial replacement of cement and filler materials as a partial replacement of sand. The measured responses, compressive strength (ƒC7days, ƒc28days, and ƒc90 days) were assigned as dependent variables in this analysis, while the experimental conditions were assigned as independent factors. Minitab software was used to conduct the regression analysis. Tables 4, 5 and 6 show the connection among independent responses and variables, the significance of every parameter, each variable's interaction and quadratic influence, and the likeness and proportion between R2 and R2 (adj.) that showed a perfect match for the suggested models. Equations (1–3) show the formula for models.

$$\begin{aligned} f_{C7days} \left( {{\text{MPa}}} \right) & = {149}.{7 }{-}{ 4}.{7}0{\text{SF}} + {2}.{\text{63G}}_{{\text{L}}} {\text{P}} - {1}.{\text{95G}}_{{\text{r}}} {\text{P}} - {1}.{\text{43LP}} \\ & \quad + 0.0{\text{913SF}}*{\text{LP}} + 0.0{799} {\text{G}}_{{\text{r}}} {\text{P}}*{\text{LP}} \\ \end{aligned}$$
(1)
$$\begin{aligned} f_{c28days} \left( {{\text{MPa}}} \right) & = {222}.{2} - 0.{\text{14G}}_{{\text{L}}} {\text{P}} - 0.{\text{34FA}} - {1}.{\text{57G}}_{{\text{r}}} {\text{P}} - {1}.0{\text{2LP}} \\ & \quad - 0.0{\text{55SF}}*{\text{G}}_{{\text{r}}} {\text{P}} - 0.{197} {\text{G}}_{{\text{L}}} {\text{P}}*{\text{FA}} + 0.0{74}0 {\text{G}}_{{\text{r}}} {\text{P}}*{\text{LP}} \\ \end{aligned}$$
(2)
$$\begin{aligned} f_{c90days} \left( {{\text{MPa}}} \right) & = {23}0.{8 } - {7}.0{\text{1SF}} - 0.{\text{14G}}_{{\text{L}}} {\text{P}} - 0.{\text{34FA}} \\ & \quad - {1}.0{\text{2LP}} + 0.{\text{183SF}}*{\text{G}}_{{\text{L}}} {\text{P}} + 0.0{74}0{\text{SF}}*{\text{LP}} \\ & \quad + 0.{\text{175SF}}*{\text{FA}} - 0.{\text{197G}}_{{\text{L}}} {\text{P}}*{\text{FA}} \\ \end{aligned}$$
(3)
Fig. 4
figure 4

The overlaid contour of compressive strength ƒc28days vs ƒflex28days

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Fig. 5
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The surface plot of compressive strength (Fc) vs SF and LP during a ƒC7days, b ƒC28days, c ƒC90days

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Fig. 6
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The contour of compressive strength (ƒC) vs different independent variables ratio during a ƒC7days, b ƒC28days, c ƒC90days

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Fig. 7
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Optimum variables content to achieve the target values of responses ƒc28 days and ƒFlex28 days

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Table 4 Variance analysis of the 7-day compressive strength quadratic model
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Table 5 Variance analysis of the 28-day compressive strength quadratic model
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Table 6 Variance analysis of the 90-day compressive strength quadratic model
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Flexural strength results

The flexural strength test has a significant role in producing UHPC with acceptable mechanical properties. It was carried out on 27 concrete mixtures over 28 days. Figure 8 illustrates that the change in type and content of SCMs and filler materials that were used as partial replacement for cement and sand, respectively, resulted in a high significant effect in concrete flexural strength. The flexural strength of concrete mixtures ranged between (18.1–38.5 MPa) at age 28 days. The mixture composed of 12.5% SF, 15% FA, and 10% GLP as partial replacements for cement, along with 12.5% GrP and 50% LP as partial replacements for fine aggregate achieved the highest flexural strength at age 28 days. FA has a notable effect on increasing concrete flexural strength. When comparing mixes 1, 2, and 11, it was found that using FA with high levels (15–30%) as a partial substitute for cement resulted in significantly increase in concrete flexural strength, these results is in agreement with Naik et al. [55]. The UHPC mix with 30% FA had the highest peak load and toughness, which is attributed to comparable activation energy linked with temperature for cementitious material hydration. The boost in bonding force between the fibres and the matrix can be attributed to the improvement in flexural behaviour. Flexural strength was reduced when an excessive amount of fly ash was applied. This could be related to a change in the Ca/Si ratio and, ultimately, the reactivity of the binder system [56]. The same effect of SF on compressive strength was observed on flexural strength, the optimum ratio of SF which achieved the highest flexural strength was 12.5% as an alternative to cement. As shown in Fig. 8 GLP has an effective role in enhancing the flexural strength of UHPC. By comparing the results of mixes 1, 8, and 9, it was found that as the same results of compressive strength, using 20% GLP as a partial substitute for cement resulted in the highest increase in flexural strength findings. GLP milled to micro-scale undergoes a low pozzolanic reaction and acts as a catalyst, accelerating the dissolution of clinker phases and forming low basicity calcium silicate hydrate (C–S–H). These reactions have a positive effect on the mechanical and microstructural properties of UHPC. GLP acts more as a chemical activator than pozzolanic material; however, it possesses both characteristics, thus increasing the mechanical properties of UHPC [57].

Fig. 8
figure 8

Flexural strength results at 28 days

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The use of GrP and LP as a partial replacement of fine aggregate has a remarkable influence on raising the flexural strength, as illustrated in Fig. 9. GrP was used with ratios from 0 to 25% as a partial substitute for sand, and the optimum ratio of GrP which achieve the highest flexural strength was 12.5%, this can be seen when comparing mixes 1, 5, and 7. Adding GrP resulted in a change in hydration products and pore structures; thus, a high ratio of GrP resulted in high findings of flexural strength [58]. LP was used with ratios from 0 to 50% as a partial substitute for sand. By comparing the results of mixes 1, 4, and 6, it was found that LP with ratio 50% as a partial substitute for sand exhibited the greatest flexural strength findings, as shown in Fig. 9. When using LP as a partial or full mass substitute for concrete, grinded LP was proposed as a possible additive to help to lessen voids. LP also has also a noticeable impact on the composite's fresh properties, such as mixing time and workability [59]. Wang et al. [60] reported that LP altered the hydration products of cementitious materials and enhanced the mechanical properties of UHPC.

Fig. 9
figure 9

The contour of flexural strength (ƒflex28 days) vs different independent variables ratio

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Response surface plot in Fig. 10 depicts the variations of flexural strength at age 28 days SF, GLP, LP, and FA. First, it is clear that a rise in SF and decrease in GLP decrease the flexural strength at age 28 days as shown in Fig. 10a, while an increase in LP and decrease in FA increase the flexural strength at age 28 days, as illustrated in Fig. 10b. The measured response, flexural strength (ƒFlex28days) was assigned as a dependent variable in this analysis, while the experimental conditions were assigned as independent factors. Minitab software was used to conduct the regression analysis. Table 7 shows the connection among independent responses and variables, the significance of every parameter, each variable's interaction and quadratic influence, and the likeness and proportion between R2 and R2 (adj.) that showed a perfect match for the suggested model. Equation 4 shows the formula for models.

$$\begin{aligned} f_{Flex \, 28days} \left( {{\text{MPa}}} \right) & = {37}.{72 } - {1}.{1}0{5} {\text{SF}} + 0.{33}0 {\text{G}}_{{\text{r}}} {\text{P}} - 0.{381} {\text{FA}} + 0.{147} {\text{LP}} \\ & \quad + 0.0{264} {\text{SF}}*{\text{G}}_{{\text{r}}} {\text{P}} + 0.0{137} {\text{SF}}*{\text{FA}} + 0.0{165} {\text{SF}}*{\text{LP}} \\ & \quad - 0.0{215} {\text{G}}_{{\text{r}}} {\text{P}}*{\text{LP}} + 0.00{153} {\text{FA}}*{\text{LP}} \\ \end{aligned}$$
(4)
Fig. 10
figure 10

The surface plot of flexural strength (ƒflex28 days) vs different independent variables ratio

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Table 7 Variance analysis for flexural strength at 28 days
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Scanning electron microstructure (SEM)

SEM analyses were utilized to assess the density of the morphology and microstructure of the consolidated concrete in addition to showing how supplementary materials influenced the mechanical properties of UHPC, as well as how microstructural elucidation revealed pozzolanic interactions. Figure 11 illustrates the SEM micrographs of (a) the control mixture (Mix No.1), (b) the mixture containing 50% LP, (c) the mixture containing 30% FA, and (d) the mixture containing 20% GLP. Results clarified in Fig. 11 showed some morphological properties of the microstructure of mixtures. The differences in capillary pores and their arrangements in the control mixture and mixtures with supplementary materials are clearly shown in Fig. 11. As shown in Fig. 11, the voids are obviously shown by adding a high ratio of FA, SF, and GLP as a partial alternative to cement and a low ratio of LP and GrP as a partial alternative to sand, as shown in Fig. 11a. Meanwhile, with a high ratio of limestone powder (50%), the voids approximately aren’t shown in the SEM test. In comparison between Fig. 11a and b, adding a high amount of LP resulted in a lower amount of unhydrated cement. The limestone particles are also surrounded by the outer product C–S–H, indicating that the hydration components have been deposited uniformly throughout. This is supported by the fact that C–S–H nucleation is favoured on parts of limestone's surface [61]. The additional C–S–H comes from the pozzolanic interaction between the supplementary materials and the hydration products generated by the cement hydration. The calcium enrichment of the cementitious matrices with FA, GLP, and LP causes the decalcification of the C–S–H structure, refinement of the pore structure, and reduction of microcracks, thus producing a denser concrete microstructure, as illustrated in Fig. 11b, c, d, and this is in agreement with Nistratov et al. [62]. SEM analysis revealed that the incorporation of supplementary materials such as FA has an obvious effect on reducing the voids in the microstructure, and this is attributed to improving the microstructure and ITZ of concrete containing FA. The dense matrix was attributed to the possible filling effect, pozzolanic phenomena, and the synergic reaction of FA. This is consistent with [63, 64].

Fig. 11
figure 11

SEM image of UHPC matrixes with a Mix No.1, b mixture containing 50% LP, c mixture containing 30% FA and d mixture containing 20% GLP

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Conclusion

In the context of this study, the investigation centred on the feasibility of incorporating supplementary cementitious materials (SCMs) such as SF, FA, and GLP as partial substitutes for cement in conjunction with filler materials like LP and GrP as partial replacements for sand in the formulation of ultra-high performance concrete (UHPC). To comprehensively assess the impact of these supplementary and filler materials on both the compressive strength and flexural properties of environmentally sustainable UHPC, a systematic analysis was performed on twenty-seven unique concrete mixtures. Drawing insights from the outcomes of various tests, the following conclusions were emerged:

  • It is possible to manufacture sustainable eco-friendly UHPC with minimal environmental impact and impressive mechanical properties by incorporating SF, GLP, and FA as partial replacements for cement, alongside LP and GrP as partial alternatives for sand.

  • The compressive and flexural strength noticeably declined when using low content of LP, SF, and GLP, while high content of GrP and FA had a similar detrimental effect.

  • As the replacement rate of LP for sand increased, there was a corresponding increase in both the compressive and flexural strength of the concrete. When the substitution rate reached 50% LP, impressive compressive and flexural strength values of 175.8 and 38.5 MPa, respectively, were achieved at age 28 days.

  • The use of substantial amounts of FA up to 30% in place of cement resulted in a more cost-effective and efficient UHPC. The compressive strength and flexural strength both reached approximately 162.5 MPa and 32.3 MPa, respectively, at age 28 days of curing.

  • The utilization of SF and GLP as a partial replacement of cement resulted in significant increase in concrete compressive and flexural strength, and the optimum replacement ratio was 12.5% and 20%, respectively.

  • By utilizing a binder mixture consisting of 62.5% cement, 12.5% SF, 15% FA, and 10% GLP, along with a fine aggregate blend comprising 37.5% sand, 50% LP, and 12.5% GrP, the highest compressive strength was achieved, reaching 133, 175.8, and 180.2 MPa at age 7, 28, and 90 days, respectively.

  • Response surface methodology (RSM) offers a valuable tool for anticipating the optimal connections between UHPC components and their outcomes, as well as for estimating the ideal variable values required to achieve the desired results. The predictive models represented by Eqs. (1) to (4) can be employed to forecast the compressive strength at 7, 28, and 90 days, as well as the flexural strength at 28 days.

  • The findings highlighted a noteworthy correlation between input parameters and output responses when predicting the characteristics of UHPC mixtures. It is anticipated that employing a binder composition consisting of 62.5% cement, 12.5% SF, 15% FA, and 10% GLP, in conjunction with a fine aggregate blend comprising 37.5% sand, 50% LP, and 12.5% GrP, will result in the production of UHPC mixes exhibiting outstanding compressive and flexural strength properties.

  • The SEM results revealed that the higher proportion of LP, FA, and GLP contributed to a clear reduction in voids and enhanced concrete microstructure.

Recommendations for future research

To obtain a deeper knowledge through the research society's collaborative efforts, we recommend that future investigations discover more supplementary materials that may still be necessary and need to be addressed in further research to be used in UHPC to enhance its mechanical properties, in addition to supplying sufficient data on all materials and having sufficient testing to explain this effect.