A novel binder system for cementbased composites depending upon the strength and durability characteristics is introduced in this study. The possibility of calcined red mud cement pastes with and without colloidal nanosilica (CNS) over Ordinary Portland Cement paste (OPC) at three W/B ratios (0.3, 0.4, 0.5) is evaluated. The optimum percentage of cement replacement by red mud (15%) was selected from compressive strength values of different cement replacements (5%, 10%, 15%, and 20%). Colloidal nanosilica (CNS) was added at 0.5%, 1%, 1.5%, and 2 % to the selected red mud cement paste. Water absorption, sorptivity, resistance to sulfate attack, and resistance to acid attack tests were conducted for optimum red mud cement paste with and without CNS. The experimental results are explained based on tortuosity with empirical formulas and mathematical models of pore network distribution. The tortuosity is directly proportional to the interconnectivity of the pores. The mixes with 15% calcined red mud and 1.5% CNS replacement performed better strength and durability at all W/B ratios. The mix (R15NS1.5) with minimum tortuosity value results in the higher overall performance of the paste. The mixes with a 0.3 W/B ratio give highperformance cement paste compared to higher W/B ratios
En este estudio se presenta un nuevo sistema aglomerante para compuestos en base cemento que depende de las características de resistencia y durabilidad. Se evalúa la posibilidad de incluir pastas de cemento de lodo rojo calcinado con y sin nanosílice coloidal (CNS) en pastas de cemento Portland ordinario (OPC) en tres relaciones a/b (0,3, 0,4, 0,5). El porcentaje óptimo de reemplazo de cemento por lodo rojo (15 %) se seleccionó de los valores de resistencia a la compresión obtenidos entre los diferentes reemplazos de cemento considerados (5 %, 10 %, 15 % y 20 %). Se añadió nanosílice coloidal (CNS) al 0,5 %, 1 %, 1,5 % y 2 % a la pasta de cemento de lodo rojo seleccionada. Se realizaron pruebas de absorción de agua, sorción, resistencia al ataque de sulfatos y resistencia al ataque de ácidos para obtener una pasta de cemento de lodo rojo óptima con y sin CNS. Los resultados experimentales se explican basados en parámetros de tortuosidad con fórmulas empíricas y modelos matemáticos de distribución de redes de poros. La tortuosidad es directamente proporcional a la interconectividad de los poros. Las mezclas con 15% de lodo rojo calcinado y 1,5% de reemplazo de CNS presentaron mejor resistencia y durabilidad en todas las relaciones a/b. En la mezcla (R15NS1.5) con un valor mínimo de tortuosidad se obtiene el mayor rendimiento general de la pasta. Las mezclas con una relación a/b de 0,3 generan una pasta de cemento de alto rendimiento en comparación con relaciones a/b más altas.
Red mud is a waste produced in the aluminum manufacturing industry. One tonne of aluminum produces approximately 23 tonnes of red mud (
The effective disposal of red mud is done by disposing it into seawater after neutralization and dry and wet stack piling on land. A large amount of alkaline solution, and the leaching of toxic and radioactive elements from the deposit, can lead to the disintegration of the balance of the aquatic environment (
India is the 3^{rd} largest producer of primary aluminum and the 7^{th} largest bauxite reserve globally. According to the Ministry of Mines, in India, growth in aluminum production is 3.3 MMT in 201718, 5.3 MMT in 202021, and is expected to grow by 7.2 MMT in the next five years (
The chemical composition of red mud is similar to cement (
The chemical compound proportion of red mud is sound enough to produce an excellent cementitious product. However, high porosity and water demand limit the utilization of red mud concrete as structural elements. Therefore, the current applications of red mud concrete are limited to the construction of pavements and subgrade materials (
Nowadays, nanosilica is introduced in cement composites for better strength and durability (
In this investigation, the introduction of CNS to red mud cement pastes and the effect on the mechanical and durability properties are considered. Strength and durability studies were conducted for red mud blended OPC with and without the addition of CNS. The cement paste is developed by analyzing the porosity and tortuosity of the paste matrix. Pore radius from the capillary action was estimated, and the fluid ingression concerning tortuosity in red mud cement paste with CNS was examined. The results were analyzed and explained with the mathematical models.
In this study, OPC 53 grade cement with a surface area of 330 m^{2}/kg and a mean particle size of about 10µm was used. Highly alkaline bauxite residue (red mud) from HINDALCO Belgaum was used to prepare laboratoryscale binding material rich in hematite and alumina. The amount of and present in the raw sample is significantly less. Therefore, to improve the percentage, the red mud was calcined at 700˚C. The chemical composition of raw red mud and calcined red mud is given in
Compounds  % by mass (Raw red mud)  % by mass (Red mud calcined at 700˚C) 

Al_{2}O_{3}  17.8  30.4 
Fe _{2}O_{3}  45  22.0 
SiO_{2}  8  21.8 
TiO_{2}  10  3.28 
Na_{2}O  4  11.6 
CaO  2  5.6 
The powder XRD and FTIR of calcined red mud are shown in
The FTIR spectra of phase transformation of red mud calcined at 700^{0}C are defined in
Colloidal nanosilica (Levasil CB8) contains 50% by weight solids and has a specific gravity of 1.03. The average particle size and specific surface area of nanosilica are 20nm and 86 m^{2}/g. The specimens underwent some characteristic studies to analyze the crystallinity of the structure, pore structure, and hydration products. A polycarboxylic ether (PCE) based superplasticizer (SP) was used to acquire a target spread of 14 to 15cm in the minislump cone test at a 0.3 W/B ratio.
Mix designation  Cement  Red mud  Nanosilica  SP dosage for W/B0.3(% by weight of binder) 

R0NS0  100      0.125 
R5NS0  95  5    0.375 
R10NS0  90  10    0.375 
R15NS0  85  15    0.625 
R20NS0  80  20    0.875 
R15NS0.5  84.5  15  0.5  0.500 
R15NS1  84  15  1  0.375 
R15NS1.5  83.5  15  1.5  0.375 
R15NS2  83  15  2  0.250 
The compressive strength test of cubes cured for 28day was conducted as per IS 5162018 (
The effect of water absorption, bulk density, and permeable pore space of red mudcement pastes on the addition of CNS was studied as per ASTM C64213 (
Standard sodium sulfate solution was prepared as per the ASTM C1012 (ASTM 2012b). The cubes of dimensions (50 × 50 × 50 mm) reached a curing age of 28day, were weighed, and successively cured in a sodium sulfate solution. The effect of sulfate attack on the strength of red mud cement paste with and without CNS was analyzed by conducting the compressive strength test for 28, and 56day sulfate cured specimens. The optical microscopic images did visual inspection of sulfate attack in the specimens. Specimens were cured in 0.5 mol/ L HNO_{3} with a pH ≤ 3 as per ASTM C1898. The specimens were tested for 28day and 56day acid curing.
Compressive strength was analyzed at 3, 7, and 28days, red mudcement pastes (W/B ratio of 0.30, 0.40, and 0.50). The compressive strength was decreased with the increase in red mud content. All the values obtained are less than the reference paste. The reduction of strength with the addition of red mud is mainly due to the increased porosity and the loose structure (
a) 0.3 W/B ratio b) 0.4 W/B ratio c) 0.5 W/B ratio
The addition of CNS to R15NS0 from 0.5% to 1.5% enhances the strength gain of the red mudcement paste for all W/B ratios. The change in compressive strength is not significant at the early curing ages (3 and 7 days). For instance, at a 0.3 W/B ratio, an increment in compressive strength from 0 to 1.2% concerning R0NS0 is observed in red mud cement mixes with CNS. However, with a cement paste of 28day curing, an explicit increment of compressive strength was observed with the CNS addition compared to the red mud cement paste without CNS (R15NS0). A maximum of 37%, 5%, and 35% strength increment at 0.3, 0.4, and 0.5 W/B ratios were recorded for R15NS1.5 than R15NS0 (
The water absorption test determined the water absorption, bulk density, and permeable pore space of 28day cured red mud cement paste with and without CNS (
The sorptivity results for the paste of 28day curing are analyzed. The coefficient of sorptivity values for different W/B ratios are illustrated in
(a) and coefficient of secondary sorptivity (mm/s^{1/2}) (b) of three W/B ratios 0.3, 0.4, 0.5
The coefficient of initial sorptivity of R0NS0 shows a similar trend as in water absorption results. The primary and secondary sorptivity showed a lesser value than all other mixes. The addition of red mud increased the pore volume considerably. Therefore, the addition of CNS at a low percentage is insufficient to overcome the water ingress through the interconnected pore network. However, minimum primary (0.03, 0.68, 1.38) and secondary sorptivity (0.02, 0.13, 0.15) were found in R15NS1.5 for 0.3, 0.4, and 0.5 W/B ratios due to CSH gel formation and expansion to the capillary pores and the blockage of water transportation. The further addition of CNS to 2% slightly enhanced the sorptivity value to R15NS1.5 because of the fluid pressure developed in the pore solution and capillary pores by the excess fluid present in the CNS suspension.
At a 0.3 W/B ratio, minimum waterpermeable pores and capillary pores existed because of the dense structure provided by hydration. Hence, the primary and secondary sorptivity have similar values for the consecutive mixes. Different sorptivity values were observed at 0.4 and 0.5 W/B ratios in each mix. Highly interconnected capillary pores were inferred for R15NS0 from the results. The trend obtained from the experimental observation contradicts the previous study, which explained the effect of CNS addition in the cement matrix. Du and Pang (2019) stated that the interconnectivity of the porous network was reduced due to the nanoparticle addition in the cement matrix (
Cement paste with 28days of water curing was immersed in a 5% sodium sulfate solution prepared as per ASTM C1012 (ASTM 2012b). The specimens were tested for compressive strength after a specific immersion period of 28day and 56day in a sodium sulfate solution. Here, the variation in compressive strength was compared and analyzed with the 28day compressive strength of the same mix. The change in compressive strength and weight behavior of specimens due to sulfate attack at three W/B ratios are in
a) 0.3 W/B ratio, c) 0.4 W/B ratio, e) 0.5 W/B ratio; The loss in compressive strength (%) for b) 0.3 W/B ratio, d) 0.4 W/B ratio, f) 0.5 W/B ratio
It is noteworthy that at all W/B ratios, the control cement paste reported less resistance to sulfate attack. The compressive strength is highly affected due to the concentrated calcium hydroxide and sulfate ion reaction to produce gypsum, or the reaction between calcium aluminate hydrates and gypsum to form ettringite (
The red mud cement paste mixes with and without CNS showed relatively lesser weight gain due to salt deposition and loss in compressive strength at all W/B ratios. At a 0.3 W/B ratio, the maximum and minimum percentage of weight change (7.9% and 0.9%) and loss in compressive strength (13.7% and 0.1%) were recorded in R0NS0 and R15NS2, respectively (
The addition of red mud decreased the loss in compressive strength due to the reduced calcium hydroxide content. The CNS particles occupied the capillary pores and blocked the fluid ingress through the pore network. Therefore, the weight gain and loss in compressive strength decreased significantly, and the specimen attained stability in the sulfate solution. The highest resistance to sulfate attack is observed in R15NS2 at 0.3 W/B ratios.
Specimens were cured in 0.5mol/L solution with a pH≤3. Highly soluble calcium nitrate (Ca (NO_{3})_{2}) salts were formed at the surface of the hardened cement paste. The low pH of the surrounding acid solution changed to 10.6, which is equivalent to the highly alkaline unaffected red mud cement paste. The weight loss due to acid attack is given in
A maximum percentage loss in weight was observed in R0NS0 as 0.8%, 2.13%, and 3.46% at 0.3, 0.4, and 0.5 W/B ratios respectively. At all W/B ratios, the addition of CNS decreased the loss in the density of the specimens, and R15NS1.5 exhibited high stability and resistance to the acid attack. The minimum loss in weight at 0.3, 0.4, and 0.5 W/B ratios was recorded in R15NS1.5 as 0%, 0.05%, and 0.09% (
a) 0.3 b) 0.4 c) 0.5 W/B ratios
White color patches (White ring) appeared due to the formation of heavy silica precipitates by CNS, which is visible in
The primary purpose of this study is to correlate the experimental results of red mud blended OPC with the addition of CNS by the pore structure distribution and tortuosity (τ) with mathematical relations. The parameters are chosen from the previous studies and derived as per the requirement of the current investigation.
Here
Vv. is the water permeable pore volume from the experimental results, and Vs. is the total volume of the specimen.
The relation of compressive strength with the porosity can be explained by the tortuosity (τ) of the pore network. The relation of τ with the compressive strength of different mixes is illustrated in
Here α is the aspect ratio of the pore. Considering Powers equation (
Here ρ is the density, and µ is the coefficient of viscosity of water. The relationship of compressive strength with τ is given in
Very close values of the percentage of waterpermeable voids and the water absorption rate were obtained for the mixes with 0.3 W/B. The experimental result of water absorption does not follow any particular patterns with increased porosity. As the W/B ratio extends to 0.4 and 0.5, the relation between the parameters, as mentioned earlier, showed an apparent direct relationship with the addition of CNS. The best interaction of the water absorption with porosity (higher value for
The tortuosity of the paste matrix and the interconnectivity explain the variation in the sorptivity results. The porosity and pore radius calculated from the experimental data can express the coefficient of tortuosity for each mix. The calculated tortuosity value from the waterpermeable porosity and pore radius can describe the variation in water ingression better than the cumulative pore volume for porous materials like red mud.
Mixes with a high W/B ratio explicitly reduced water conductivity due to the capillary action. The pore radius has a spectacular influence on the water permeability. In a study conducted by Yang et al. (2019), the relationship between volumetric flow rate is defined in Equation [
Here,
Applying the capillary pressure in Equation [
The initial boundary conditions h=0, t=0:
Here,
considering the mass, volume and density equation
Where A  surface area in contact with water, h= capillary rise (Equation [
Substituting and rearranging:
rpore radius, Sinitial sorptivity, p water permeable porosity, k proportionality constant.
For further studies from the sorptivity graph has been taken into consideration. The sorption of the hardened paste structure depends upon the pore network distribution, which is explained based on the tortuosity of the pore structure. The given equation is extracted from the previous study to elucidate the connection between pore diameter and tortuosity (
The tortuosity from the pore diameter was calculated from the experimental data (Equation [
The tortuosity is extracted from Equation [
Tortuosity  

Mix designation  28day curing in water  28day curing in 5% solution  56day curing in 5% solution 


R0NS0  1.9  1.7  1.8 
R15NS0  2.8  2.8  2.8 
R15NS0.5  1.8  1.8  2.0 
R15NS1  1.7  1.6  1.7 
R15NS1.5  1.6  1.6  1.7 
R15NS2  1.4  1.4  1.4 


R0NS0  2.7  2.9  3.9 
R15NS0  3.6  3.7  3.7 
R15NS0.5  3.3  3.1  3.4 
R15NS1  2.8  2.0  2.5 
R15NS1.5  2.1  1.5  1.9 
R15NS2  2.6  1.4  1.8 


R0NS0  2.8  2.6  2.4 
R15NS0  2.5  2.6  2.7 
R15NS0.5  2.5  2.4  2.3 
R15NS1  2.5  2.4  2.4 
R15NS1.5  1.7  1.6  1.7 
R15NS2  1.2  1.2  1.2 
A higher value of tortuosity shows reduced stability of the specimens in the sulfate solution. The addition of red mud increased the τ value concerning R0NS0. The addition of CNS reduced the τ value, and the minimum was observed in R15NS2 for all W/B ratios. Here minimum τ indicates the higher stability in sulfate solution, which can be related to the experimental results (
The following conclusions are drawn based on the experimental studies and results.
Cement replacement with calcined red mud at 700˚C reduced the compressive strength of cement paste. An optimum of 15% cement replacement by red mud was obtained from compressive strength results considering the reference mix’s minimum percentage reduction in strength.
The CNS addition to the optimum red mud cement paste enhanced the strength and durability properties. 1.5% addition of CNS with 15% red mud cement paste performed better in strength and durability.
Maximum and minimum percentage water absorption and volume of permeable pores were identified in R15NS0 and R15NS1.5, respectively. Primary and secondary sorptivity are directly proportional, and bulk density is inversely proportional to the water absorption and permeable pore volume at all W/B ratios.
The addition of red mud and CNS enhanced resistance to sulfate and acid attack of cement paste. R15NS2 showed the highest resistance to sulfate attack by exhibiting minimum weight change due to salt deposition and loss in compressive strength. Whereas R15NS1.5 showed better resistance to acid attack with minimum spalling and cracking. The visual examination with light optical microscopic images of eroded specimens defined the deformations in strength and density. High deposits of salt, mid cracks, and formations of chemical reactions were identified, indicating the paste’s instability in an aggressive chemical environment.
Minimum mean pore radius was observed in the dense microstructure at a low W/B ratio (0.3), giving lower porosity and tortuosity in the highperformance mixture. Whereas mixes with W/B ratio > 0.4 gave the maximum tortuosity value indicating a more interconnected pore network and thereby poor performance in strength and durability compared to lower W/B ratios.
Maximum strength and durability values were observed in mixes at 0.3 W/B ratios in R15NS1.5. However, in the same mix at a 0.4 W/B ratio, the strength and durability values are near the conventional mixes.
This work was ﬁnancially supported with a seed grant by the Vellore Institute of Technology, Vellore (Seed no.SG20210204). The authors gratefully acknowledge the testing labs of Vellore Institute of Technology, Vellore, for supplying facilities to carry out this research work. We are also grateful to Dr. Sushant Kishore, Assistant professor, School of Social Science and Languages, Vellore Institute of Technology, Vellore, for proof reading and improved the manuscript significantly.
Conceptualization: T. Shanmugapriya. Data curation: K. Athira. Formal analysis: T. Shanmugapriya. Funding acquisition: T. Shanmugapriya. Investigation: T. Shanmugapriya. Methodology: K. Athira. Project administration: K. Athira. Resources: K. Athira. Supervision: T. Shanmugapriya. Validation: K. Athira. Visualization: K. Athira, T. Shanmugapriya. Writing, original draft: K. Athira. Writing, review & editing: T. Shanmugapriya.