## 1 Introduction  
Concrete is today the building material that is most commonly used worldwide. It is basically a combination of sand and cement, with the cement acting as a binder that glues the sand particles together. However, cement is not only expensive, but its production leads to emission of acidic carbon IV oxide into the atmosphere, which is one of the main greenhouse gases that can lead to global warming. Among the alternatives to cement that have been explored include geopolymer concrete, which has been found to be suitable in addition to being environmentally friendly. Moreover, the geopolymer has been found to process higher strength and deformation properties due to its more stable and denser aluminosilicate spatial microstructure [1, 2, 3, 4, 5]. Sand, on the other hand, is not always readily available, and can be expensive to purchase and transport, especially for people living in areas that are far away from where sand is found. Sand mining/harvesting is also one of the major causes of erosion of coastlines [6]. Earth (soil) is a natural material and is readily available on construction sites, in addition to being cheap and environmentally friendly. It has thus been utilised in the construction of walls of houses, more so in the developing countries. The simplest form of houses made of earth is called mud houses, whose walls (rammed earth walls) are simply constructed by mixing earth (a mixture of aggregates that include gravel, sand, silt, and a small amount of clay) and water, and then using this mixture to fill the spaces between flat panels, usually called formwork [7]. Earth building is a renewable and sustainable technology, and has become the means of providing safe, comfortable, and desirable homes [8]. However, the recent generation has developed negative views about earth-built houses, associating them with rural poverty, lack of exposure, and backwardness [9]. One of the main shortcomings of mud houses is their short lifespan. Normal earthen houses are unable to withstand harsh tropical rainy seasons. This is because the earthen walls soon start to develop cracks, which usually occurs because the earth particles are not sufficiently held together due to poor binding, as well as very low compaction pressure. However, a lot of research has been ongoing over the recent decades into the modifications of the earth blocks so as to improve their properties. These include altering the compaction pressure, as well as making them interlocking. The roughness of these walls has also undergone modifications, with the aim of making the walls smooth and thus, providing aesthetic value [10; 11; 12]. Figure 1 shows some of the modern earth blocks with aesthetic value, as well as better mechanical properties that are currently being used in the construction of walls of houses. In the search for alternative binders to cement, many materials have been investigated, including fly ash, rice husk, silica fume, eggshell, groundnut shell, sugarcane bagasse ash, cassava powder, waste glass, coal bottom ash, waste marble powder, and waste-derived ceramic concrete [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. However, large quantities of these materials that can be used in building reasonable-sized houses may not be easy to find. Water hyacinth (_Eichlhornia crassipes_) is an aquatic plant that is a nuisance. It causes many problems, including loss of biodiversity, water loss, effects on water quality, damage to infrastructure, and poor health of aquatic species. The hyacinth is freely available and is in large quantities in water bodies (freshwater lakes). Thus, finding its use in the construction of earth blocks serves as a way of utilising it. In the past, it has been used to make concrete in order to increase the material's permeability and tensile strength [24], as well as the partial replacement of cement in the form of water hyacinth ash (WHA) [16]. WHA consists mainly of calcium oxide (22.61%), potassium oxide (14.82%), magnesium oxide (14.01%), and silica (4.40%), with a specific gravity of 2.44.