At incipient red mud was collected as wet powder form from NALCO, Damonjodi, India. The as received power was dried under sunlight for 10 days to get moisture content up to 2% by weight. The red mud powder was ball milled to a specific particle size below 50 mm diameter. The composition of red mud used can be collected from [12] . The OPSC was procured from Sarang market as Konark cement, manufactured by Odisha cement limited, Rajgangpur, India.

The red mud and cement mixture was prepared separately by varying the weight % of red mud as 10, 20, 30, 40 and 50. Each mixture was thoroughly blended using an in house V-shaped blender, in order to get a uniform composite powder. Thereafter each composite powder was stirred with distilled water using a mixture (Guangzhou Xuzhong Machinery Company Limited). The water-binder ratio is maintained at a value of 0.4. The paste is poured into the cast iron mould to fill the entire volume. Casting was done by using a cast iron mould of size 8.5 × 5 × 4 cm³. Captured image of the die used for casting is shown in Figure 1. Then the die was mechanically vibrated

using external vibrator to achieve ultimate compaction of the paste. The die was removed immediately and composite bricks are allowed to harden for 2 days and then went for curing in normal water for 28 days at atmospheric condition.

Compressive strength is the most vital property to characterise the cement. The mortars were cut to cylindrical pieces having height to diameter ratio of 5.6. The specimen was compressed between the planets of the compression testing machine (Instron-1195) by application of load as per ASTM C39 standard. The results are shown in Figure 2, and compared with Indian standard IS 455 - 1989 designed for PSC. As normal PSC/OPSC bear a compressive strength of 33 MPa after 28 days of curing.

The results reveal an improved compressive strength up to 20% red mud reinforcement, might be due to improved hydration and structural properties approximating minimum air voids prevailing in the 20% RM composite.

Hardness of the sample was determined using Vickers hardness test equipment (LHV-1000B), applying 50 gram load with 10 seconds indentation time. The results are reported in Figure 3. It is observed that hardness is extreme for 10% red mud composite mortar. It is evident that the inclusion of RM to OPSC affects the rate at which it hydrates and changing its hardening properties.

Figure 4(a) shows the XRD pattern of OPSC after 28 days hydration. The XRDs were captured using a Philips X-ray diffractometer. Emphasis is given to analyse only the phase compositions. It was seen that the primary phases OPSC builds up after 28 days hydration are Calcite (CaCO₃), Portlandite (Ca(OH)₂), Calcium Silicate Hydrate (C-S-H) and minor phases are Larnite (2CaO. SiO₂), Calcium Aluminate (3CaO. Al₂O₃) and Monosulphate hydrate (3CaO. Al₂O₃. 3CaSO₄. 3H₂O). Portlandite and Calcium silicate hydrate are formed after water hydration. But rest of the phases are also seen in OPSC powder [13] . Figure 4(b) and Figure 4(c) reveals the XRD patterns of 20% RM + 80% OPSC and 40% RM + 60% OPSC composite mortars after 28 days hydration. Two new phases are found in the composite mortars as Hematite (Fe₂O₃) and Silicon dioxide (SiO₂), that red mud consistently poses. From the XRD analysis point of view; no new compound indicated, and there is no alternation in the phases by presence of RM particles was identified. Only revision of the relative intensities of the peaks was observed.

The IR characteristics of OPSC after 28 days hydration are identified and reported in Figure 5(a) using FTIR analyser (thermo Scientific^TM Nicolet^TM iS^TM 10) within the range of 500 - 4000 cm⁻¹. Table 1 reports the IR bands and their corresponding functional groups. Figure 5(b) reveals the IR spectra of 20% RM + 80% OPSC composite mortar succeeded by 28 days hydration, their IR frequencies and assigned groups are dispersed in Table 2. Additional H-OH stretch (bound water) was observed for 20% RM composite cement at 2923.76 and

1711.42 cm⁻¹ band. This might be due to higher rate of hydration with RM reinforcement. As the process of hydration is the progressive conversion of free (liquid) water in capillary pores in to bound water during the cement hydration. So it is a good approximation that inclusion of RM (up to 20%) to OPSC minimizes the free water and increases bulk liquid water and structural (chemically bound) water. The C-S-H gel phase is responsible to carry significant amount of free and adsorbed water in its gel pores, and holds more tightly bound water within its inter particle spaces catalysed by RM.

Morphology of composite mortars after 28 days hydration is captured using a SEM (Scanning Electron Microscope, JEOL; JSM-6480 LV). Microstructures are shown in Figure 6 and Figure 7. For 10% RM + 90% OPSC composite; distribution of RM particles is relatively low. Quite low numbers, small size air voids (pores) are visible. Minimum amount of air voids are observed for 10% - 20% RM reinforcement. Entrained air voids (porosity) are seen to be increasing for further addition of RM. The entrapped air bubbles are formed in during mixing of water-binder and play a dominant role for their effect on the strength of the concrete. More isolated and bigger globular voids (large capillary pores) are seen for 50% RM + 50% OPSC mortar and hence lowering the effectiveness. The inclusion of RM (˂20%) might be helping for increasing the C-S-H gel formation (including its internal gel pores) and thereby increasing the strength.