Performance Evaluation of Novel M25 Green Concrete with GGBS, DRS and RCA Material–A Study Employing Response Surface Methodology

Global push for sustainability is increasing rapidly and countries are trying their best to become self-sustainable. Achieving a closed loop system in an industrial production has become a major objective in order to achieve self-sustainability and thus reducing the carbon footprint. Key aspect of a closed loop system includes reusing the byproducts and waste materials for a more productive process rather than dumping up in a landll. In this study, waste and used materials like ground granulated blast slag (GGBS) up to 50%, dolomite rock sand (DRS) up to 30% and recycled coarse aggregate (RCA) up to 30% were chosen as replacement for cement, ne aggregate and coarse aggregate respectively for the making of novel M25 grade green concrete. Concrete properties viz., slump, compressive strength, split tensile strength and exural strength were tested and corresponding regression models were developed. The developed models were used to obtain optimum percentage addition of GGBS (> 20%), DRS (> 9%) and RCA (< 17%) for achieving M25 grade strength. Further an increase of GGBS to 50%, DRS to 30% and with elimination of RCA, the compressive strength increased by 59% (39.7 MPa), Split tensile increased by 23% (4.3 MPa) and exural strength increased by 19% (4.2 MPa) in comparison to M25 concrete. The study found that the waste materials can be sustainably used to prepare novel M25 green concrete which achieved higher strength than conventional concrete, thus reducing the cost of concrete and with reduced carbon footprint on the environment.


Introduction
Concrete, a composite material is used extensively for construction activities. Countries all over the world are on the surge of improving their infrastructure and heavily rely on concrete. This creates a huge demand of the raw materials used in concrete such as cement, crushed sand and natural aggregates (Soundara B  . Literature indicates that though the initial rate of strength gain of concrete with GGBS is comparatively less, it matches with the strength of a conventional concrete mix with OPC grade cement after 28 days (Phul et al 2019). In a study it was found that GGBS can be used up to 40% replacement of cement which improves the concrete strength.
The study also con rmed that the usage of GGBS in concrete improves its ow-ability in plastic stages (Mo et al 2015).
River sand, the commonly used ne aggregate is a non renewable resource but its demand is exponentially increasing and as a result causing serious environmental issues (Scrivener et al 2015).
Many countries have limited the use of river sand in construction and therefore the construction Prakash et al 2021) and dolomite rock sand (DRS). DRS is a nely powered crystalline silica and is a byproduct from cement manufacturing industries through the beati cation process of limestone using silica as raw material. From literature it was inferred that DRS can replace sand till 30%. When the % addition of DRS is greater than 30%, the amount of water available for hydration is reduced and results in shrinkage and reduction in strength (Chinnaraju et  Coarse aggregates have a major role on the strength property of concrete and take up a major part of the load that occurs on the concrete structures. Once the building is no longer serviceable, it is demolished and the wastes are disposed in land lls. According to a research work more than twenty thousand tons of C& D wastes (Construction and demolition) are piled up everyday (Al-Ansary 2013). Incorporating recycled coarse aggregate in concrete (RCA) helps in achieving closed loop system in the construction industry and reduces the demand on fresh coarse aggregates. Replacement of RCA up-to 30% does not affect the strength and durability of concrete. (Zheng et al 2018). Studies have shown that RCA is generally porous in nature and higher percentage replacement decreases the strength which can be attributed to the weak residual mortar layer, and also the characteristics of RCA obtained from the construction and demolition site. It was reported that concrete slump and strength decreased as more In this study, sustainable materials like ground granulated blast slag (GGBS), dolomite rock sand (DRS) and recycled coarse aggregate (RCA) were chosen as replacement for cement, ne and coarse aggregate respectively. Regression models was developed involving the use of above materials in concrete and their impact on the strength parameters viz., compressive, exure and split tensile was studied employing response surface methodology.

Materials
Cement (OPC 53 grade), crushed sand (< 4.75mm), coarse aggregate (20 mm), RCA (20 mm), were all procured from local suppliers. GGBS was procured from India Mart. DRS was obtained from ACC cement manufacturing industry, Coimbatore, India. The tests on the materials employed in this study were carried out as per BIS standards and their results are tabulated (Table 1). Table 1 Properties of materials used in the study.

Materials
Properties Results

Design Mix
Design mix for control sample corresponding to a compressive strength of 25 N/mm 2 , according to IS 10262:2019, and the mix was designed for 100 mm slump, with the use of OPC grade 53 cement. For replacement with various materials, the combined speci c gravity of the materials was found and the quantities of materials were expressed by absolute volume method. The water correction was calculated and the quantity of water added to the mix was corrected accordingly based on results of water absorption and the w/c ratio was chosen as 0.5. The quantities of materials used include cement = 396 kg/m 3 , FA = 718 kg/m 3 , CA = 1075 kg/m 3 and Water = 198 kg/m 3 and percentage replacements of materials were calculated accordingly.

Design of Experiment
Design of experiments employing RSM was chosen for this study. Replacement of different sustainable materials in concrete was performed and the performance evaluation of sustainable concrete was studied. The three factors chosen for replacement include GGBS % replacement by weight of cement (X 1 ) in the range of 0 to 50%, DRS % replacement by weight of ne aggregate (X 2 ) in the range of 0 to 30%, recycled coarse aggregate % replacement by weight of coarse aggregate (X 3 ) in the range of 0 to 30%.
MINITAB software was employed for generating experimental run orders and for further regression analysis. A total of 20 experiments were designed using MINITAB software and are presented (Table S1,

Regression analysis and modeling
The experimental results of various run order were inputted to MINITAB software and standard regression analysis was done for slump and other 3 strength parameters (Table S2 to S5 respectively in supplementary material). In this study, for all statistical analysis a con dence level of 95% was The model summary for the four properties of sustainable concrete is tabulated in Table 3. The S value which stands for standard error of regression indicates the closeness of the observed values to the regression line. R 2 is a statistical measure which ranges from 0 to 100 %, where 100 % indicates that the model explains all the variability of the response data. R 2 (pred) indicates the predicting capability of the model. In general, for all the models an R 2 value of > 93% (R 2 adj > 91%) was observed. This indicates that almost 90% of the variation of slump and the other strength parameters of sustainable concrete can be explained by the combination of the chosen three factors by the developed regression models. Further the higher R 2 indicates that the model obtained is a good predictor of the chosen response and it can be used for optimizing the different properties of sustainable concrete within the boundary conditions.

Slump
Using Eq. (1) pareto chart (Fig. 1) and the contour plots (Fig. 2a -2c) were generated for slump. It was found that the use of GGBS and DRS increases the workability and RCA reduces the same. It can be inferred from pareto chart of slump that RCA (factor C) is a signi cant parameter which in uences the slump model followed by GGBS and DRS replacements. Further the combination of DRS and RCA plays a positive role in slump. In general, it was observed that there is an increase in workability in DRS incorporated concrete up to 35%, for 30% DRS replacement which is due to the ball bearing effect imparted by it. (Fig. 2c). With an increase in RCA, the slump falls below 80 mm, which is due to the porous nature of the aggregates. It was observed that RCA replacement up to 15% can produce a workable slump and further increase, drastically reduces the slump. Further, the combination of RCA (up to 15%) and DRS (up to 30%) positively in uences the slump. (Fig. 1). Slump(m

Compressive strength
Using Eq. (2) pareto chart (Fig S1, Fig S1 to S6) and the contour plots (Fig S2a -S2c) were generated for compressive strength. In general, use of GGBS and DRS increases the compressive strength and RCA reduces the same. Referring Fig S1, in uence of RCA in the obtained model is major followed by GGBS and DRS. The contour plots show the variation of materials used and their impact on the compressive strength. It was inferred that a compressive strength of 25 MPa (as that of control specimen) could be achieved with GGBS replacement of 20% and DRS replacement of 15%, making the concrete more economical using sustainable materials. Further increase of GGBS to 50% and DRS to 30% increases the compressive strength by 59% and a maximum compressive strength of 39.7 MPa was achieved (Fig S2).
High strength obtained is due to the micro ller effect of DRS. Further addition of GGBS to the mix makes it stronger than conventional concrete (Hawileh et al 2017). If required to make the mix more economical, incorporation of RCA can also be considered for structures which might requires a less compressive strength like paver blocks and in other secondary construction activities.

Split Tensile strength:
Using Eq. (3) pareto chart ( Fig S3) and the contour plots (Fig S4a -S4c) were generated for split tensile strength. Similar to other strength parameter, use of GGBS and DRS increases the tensile strength and RCA reduces the same. In uence of RCA in the obtained model is major followed by GGBS and DRS. A split tensile strength of 3.5 MPa could be achieved with GGBS replacement of 25% and DRS replacement of 17% when compared to conventional concrete and further increase of GGBS to 50% and DRS to 30% increases the split tensile strength by 23% to 4.3 MPa (Fig S4).

Flexural Strength
Using Eq. (4) pareto chart (Fig S5) and the contour plots (Fig S6 a -S6c) were generated for exural strength. In general, use of GGBS and DRS increases the exural strength and RCA reduces the same.
In uence of RCA in the obtained model is major followed by DRS and GGBS. It was found that a exural strength of 3.5 MPa could be achieved with GGBS replacement of 30% and DRS replacement of 25% when compared to conventional concrete and further increase of GGBS to 50% and DRS to 30% increases the exural strength by 19% and a maximum exural strength of 4.2 MPa was achieved (Fig S6).

Optimization
The optimisation of the factors was performed employing the response optimiser tool of MINITAB.
Optimum condition considering the slump 80mm, compressive strength 25 N/mm 2 , split strength 3.5 N/mm 2 and exural strength 3.5 N/mm 2 were obtained (Fig. 3). From Fig. 3, the optimal composite desirability (D) was obtained by taking the average of the individual composite desirability of the chosen factors. The value of D varies from 0 to 1, where the value closer to 1 is taken as the most feasible process. It can be inferred that in order to achieve an M25 grade concrete the required quantities of GGBS, DRS and RCA replacements are 20.09 %, 9.68 % and 17.24 % by weight respectively.
RCA was found to have a negative effect on the strength parameters. The decrease of strength due to increase of RCA is due to its porous nature and its low crushing strength (Chakradhara Rao et al 2018).
Complete elimination of RCA replacement resulted in increase in strength parameters. With zero replacement of RCA, and GGBS replacement of 50%, and DRS 30% replacement, it was found that maximum compressive (39.7 N/mm 2 ), tensile (4.3 N/mm 2 ) and exural strength (4.2 N/mm 2 ) was achieved. Micro lling of GGBS and DRS, which leads to effective lling up of the voids in the concrete leads to an increased strength. The performance of concrete exceeds expectations for a mix including 50% GGBS and 30% DRS and 0% RCA by weight. Experiments for validation were performed under the optimum condition and the various parameters were tested (Table 4). It is observed that the difference between predicted value and observed values of slump, compressive, exural strength and split tensile strength is observed to be < 10%, thereby proving the correctness of the model. emissions and also make way for applications in mass concreting. The results obtained in this study illustrate the possibility of using by products from industries as potential ingredients in making a novel M25 grade green concrete which exhibits higher strength than the conventional concrete. This study helps to achieve a more sustainable construction practices at reduced cost, without compromising its strength parameters Declarations Figure 1 Pareto Chart of the standardised effects(slump).