This study represents an extension work to investigate the role of ultra fine sand (UFS) in enhancing the mechanical properties of fiber reinforced cementitious compounds. The micro-structural origins were identified by scanning electron microscope (SEM). About 50% of UFS had a diameter of less than 20？μm. Ordinary Portland Cement (OPC) was partially substituted by UFS at 3, 5, 7 and 10% by weight of binder. It was found that as UFS loadings increase, the flexural, compressive, and tensile strengths increased up to about 5% UFS loading by 12.9, 15.7 and 30.1%, respectively, thereafter, a decrease in these properties was observed. This can be attributed to the pozzolanic effect besides the filling effect of UFS resulting in enhancing the interfacial bonds between the sand grains and hydration products that makes the paste more homogeneous and dense. The effect of both short natural and artificial fiber loadings on the structural performance of compounds was also studied. Loadings of 2%, by weight, of short natural date palm leaves’ midribs fibers (DP) and artificial polypropylene fibers (PP) were added to the 5% UFS blended mix. An increase in both flexural and tensile strength was achieved, while a decrease in the compressive strength was observed. 1. Introduction The ultra fine sand (UFS) modifier is microscale crystalline silica which is an industrial waste available in Egypt [1]. This study represents the extension for the previous work that aimed to investigate the role of UFS in enhancing the mechanical and physical properties of cementitious pastes. Generally, the addition of very fine pozzolanic materials to cement paste leads to the formation of very fine hydration products, which in turn lead to a refinement of pores. Sand acts as filler, providing for an economical mix and controlling shrinkage. Either natural sand or processed sand may be used as filler. Gradation limits are given in ASTM C 144 [2]. Gradation can be easily and inexpensively altered by adding fine or coarse sands. Micro- or nanopozzolanic particles are incorporated as active substitutions to Ordinary Portland Cement (OPC) due to their reactivity with lime forming cementitious compounds with improved mechanical properties [3–7]. As they dispersed in OPC, they generate a large number of nucleation sites for the precipitation of the hydration products resulting in better distribution of fine pores which produces homogeneous and dense microstructure. In addition, denser packing within the cement results from the very fine grains, which reduces the wall effect in the transition
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