## 1 Introduction  
NS is manufactured using thermo-decomposition, sol-gel of silica fume and vapor-phase reaction. It is used in several applications such as ceramics, chemicals/catalysis analysis, and biological science [1]. NS is synthesized using chemicals that control the shape, size, and purity of the materials. RHA is an agricultural by-product that contains a high amount of silica. In recent years, the construction industry has attempted to incorporate NS and RHA in the production of concrete to reduce the cost of the structure [2]. Conventional concrete is mainly composed of cement, coarse aggregate, fine aggregate, and an adequate amount of water. The rapid growth in the construction industry and the vast formation of waste have pushed the development of sustainable products using RHA, micronized biomass silica, ground granulated blast-furnace slag, crumbed rubber, NS, different fibers, superplasticizers, and different admixtures, among others, in conjunction with concrete [3]. In general, the use of admixtures, minerals, or artificial waste materials can provide sustainable concrete with moderate cost and several benefits in terms of avoiding environmental pollution by effectively disposing of waste or by-products, which could pollute lands, water, and air to a large extent [4]. In this investigation, RHA and NS are considered waste, which could potentially be used to partially replace fine aggregate to improve concrete strength. After a comprehensive survey, it was observed that the addition of RHA to concrete or mortar increased their durability, compressive strength, workability and flexural strength [5]. It was also found that incorporating NS with RHA dramatically decreases the compressive strength at an early age but increases at a later age. NS increases the heat of hydration of concrete or mortar and hardens the cement paste by forming calcium silicate hydrate (C-S-H). The optimum use of RHA with NS enhances the chloride permeability, electrical resistivity, and capillary absorption in concrete mixes or mortar [6]. The colloidal NS decreases the workability of concrete but increases its compressive and tensile strength compared to the nominal mix. It was also found that a high dosage of RHA decreases the strength with the addition of NS in concrete. Ultrafine RHA of particle size between 5 and 95 \(\upmu\)m increases the strength but reduces water absorption capacity in concrete [7]. NS distributes uniformly in concrete with higher water permeability resistance, which is identified from the microstructural analyses [8]. An optimum dose of 30% of RHA unfavorably influences concrete workability, strength, and water permeability [9]. The use of ultrafine RHA enhances the compactness of concrete. An optimum dosage increased the workability of concrete with RHA and its compressive strength was improved by adding NS [10]. The 2D X-ray image processing techniques are performed using a linear traverse for predicting air-voids. The high accuracy of 3D is used to predict the distribution of air-voids in the hardened concrete. Thus, the pore microstructure of concrete without damage was investigated by means of a non-destructive method. We note that X-ray imaging techniques have been previously used to characterize pore microstructure and compressive strength of cement-based materials and organic foams [11; 12]. However, there is a lack of investigating the effect of NS and RHA on the workability, mechanical properties, and pore microstructure of concrete. In particular, this study focuses on the determination of the optimum dosage of RHA and NS. We also compare the properties of conventional concrete with RHA and NS concrete. Our main interest is to develop sustainable concrete with the use of RHA and NS. Therefore, the investigation of the influence of NS and RHA on concrete, with the aid of image processing, will help to understand their combined properties and microstructure.