In this work, models for predicting six structural characteristics and cost of sand-quarry dust blocks were developed. Three model equations namely Scheffe’s simplex lattice design (pseudo component), Scheffe’s simplex lattice design (component proportion) and Osadebe’s model were developed for each property. The properties investigated were Compressive strength, Water absorption and Split tensile strength. The others are Static modulus of elasticity, Flexural strength and Shear strength. The models were fitted to data obtained from tests on various mixes of 140 sand-quarry dust hollow blocks of 450 x 225 x 225 mm, 66 beams of 600 x 150 x150mm and 66 cylinder specimens of 150mm diameter and 300mm long. Adequacy of the models were confirmed using F statistic and normal probability plot. Computer programs, were developed to determine the responses to a given mix and the mixes that give a desired response value. The effect of the partial replacement of sand with quarry dust on the characteristics of the blocks was also studied. Component interactions was studied using Cox response trace plots. Comparisons between the experimental and model predicted results and between the models were made. The percentage difference between the experimental and model predicted values were all below 5% for all the models and responses. The analyses also show that there is no significant statistical difference between the models. The minimum and maximum values of compressive strength predictable by Scheffe’s pseudo component model are respectively 2.74 and 5.22Nmm-2. The corresponding values for the water absorption of the blocks are 3.21 and 7.84%. For the Scheffe’s component proportion model, the predictable compressive strength values range from 2.77 to 5.23Nmm-2. The corresponding range for water absorption is 3.20 to 7.84%. The minimum and maximum flexural strength predictable by the Scheffe’s pseudo component model are 2.40 and 4.34Nmm-2 respectively. The corresponding values for the Split tensile strength are 2.24 and 3.33Nmm-2. For Scheffe’s component proportion model, the corresponding values are 2.45 and 4.35Nmm-2 for the flexural strength and 2.27 and 3.33Nmm-2 for the split tensile strength. Analyses of the pseudo component models show that there is binary synergy between sand and quarry dust for all the properties. Other binary combinations anti synergistic effects. Cement and water has the greatest effect on the properties. The structural properties of the blocks improved when 10 to 40% of the sand was partially replaced with quarry dust. The optimum replacement was at 40% with an increase in compressive strength of 27%. A list of 117 mixes that meet NIS 87: (2004) recommended minimum compressive strength of 3.45Nmm-2 for load bearing sandcrete blocks was established. It is recommended that the inclusion of quarry dust in sandcrete block production be encouraged especially in areas where quality sand for sandcrete block production is scarce and expensive.
Walls are the basic element in the construction of most buildings. They are often required to be load bearing, especially in low rise buildings (1-2 upper floors). Sandcrete blocks are the most commonly used unit in wall construction in modern Nigeria and, indeed, the entire West Africa. The use of laterite and other forms of walling units, for the construction of modern residential buildings have not made much progress when compared to the use of sandcrete blocks. The same can also be said of bricks. The major advantage of sandcrete blocks is the ease of production and laying of the blocks The structural properties of blocks of interest include compressive strength, flexural strength, water absorption, modulus of elasticity, shear modulus and split tensile strength. Others are density, fire resistance, durability and thermal conductivity. These properties are dependent to a very large extent on the relative proportions of the constituents and the method of production process.
Sandcrete blocks are traditionally made of cement, natural river sand and water. The constituents are mixed and placed in a mould which is removed immediately after compaction and leveling of the top. The newly produced blocks are self-supporting; hence they are often referred to as zero slump concrete. Individual blocks are joined together, after curing, to form walls using cement-sand mortar. It is often the practice to partially replace the sand portion with other materials such as laterite, coarse aggregate or quarry dust.