Because of Ethiopia's rapid urbanization, industrialization, and economic development, the building industry is expanding dramatically, resulting in a rise in the consumption of construction materials. Due to the fact that natural sand is not readily available in most places of Ethiopia, this has resulted in rapid depletion of natural sand and scarcity of sand of good quality. It must be carried over a considerable distance, or contractors will use whatever sand is available, which may or may not satisfy the required specifications 
Due to a wide range of unpredictability, cost, and quality issues, sand availability in the rising Ethiopian construction industry will be a concern . The building business, on the other hand, is one of the industries that contributes a significant amount of garbage to the environment. The amount of garbage generated by the building sector in developing countries is tremendous, and due to inadequate management procedures, these waste materials are deposited directly into landfills, causing contamination. However, by repurposing garbage for different purposes, the environmental impact can be decreased. Finding all options and prospects for minimizing the waste products of work areas must be a primary job if the environment is to be better preserved for sustainable growth .
In today's world, construction and demolition waste recycling is critical for long-term development. Because of the need for sustainability and the scarcity of natural resources, the United States, Japan, and parts of western regulations have considered the use of recycled and secondary aggregates, such as crushed concrete, asphalt, and industrial byproducts, and have put in place enough regulations to ensure that recycled and secondary aggregates account for more than 10% of total aggregate use. Because recycled materials take energy to manufacture, they may not necessarily be more sustainable. As a result, it may be necessary to assess the optimal Factory-made sand as a substitute for river sand has shown to be useful and is now widely used around the world. FMS is made by crushing larger quarry stones into a specific sand size, and its chemical and physical features, such as color, size, shape, and surface roughness, are dependent on the type of stone and its source. Manufactured sand has shown to be a viable alternative to natural sand, and it has become critical in meeting the technical, commercial, and environmental criteria of concrete manufacturing .
The uneven surface, irregular particle form, angular edges, increased roundness, length-width ratio, and mineralogy distinguish FMS from natural sand . As a result, it is critical to examine MS as an alternative material to Natural River sand in order to alleviate the problem of Natural River sand depletion and shortages in Ethiopia.
The most commonly used material for wall building in Ethiopia is HCB; nevertheless, it might be fractured on production or construction sites during loading and unloading, resulting in waste. These wastes are gathered in a site with varying sizes and quantities and disposed of in a landfill. Because of their abundance in both demolished and construction/production sites, recycling of wall materials (HCB wastes) is critical .
Due to the increasing demand for buildings and other infrastructure in Gondar City, high-quality sand is not always available and must be carried over vast distances. Increased demand for river sand results in a cost increase, which poses a problem to the business because it raises construction costs. Furthermore, river sand may be a concern not just in terms of cost, but also in terms of availability and quality. Because its resource will be diminished over time, the river sand's long-term viability is jeopardized. As a result, an alternative for HCB waste must be recycled. As a result, rather than creating aggregates from virgin sources, HCB wastes and FMS from local crusher facilities are more than adequate. As a result, the research's major goal is to evaluate the and cost of concrete produced by partially replacing river sand with a 1:1 mix of reused sand from demolition HCB wastes and factory-made sand in Gondar area.
1.1 Research Questions
What are the best sand replacement rates for achieving the desired concrete compressive strength using combination materials?
Which concrete, combined materials concrete or river sand concrete, is the most cost-effective?