Abstract
The demand for partial replacement and complementary products of cement to enhance the properties of concrete and reduce the environmental pollution from the cement industry is recently increasing. Nano cotton stalk ash (NCSA) and palm leaf ash (PLA) are agricultural residues resulting from the burning of cotton stalk and palm leaves used as biomass. This investigation aims to study the effect of using NCSA and PLA as partial replacements to cement in ultrahigh-performance concrete (UHPC). As partial replacements for cement mass, 0%, 2.5%, 5, 7.5% and 10% of NCSA and 10%, 20% and 30% of PLA were adopted to produce the UHPC. Sixteen mixtures were prepared to evaluate the new and mechanical properties of UHPC containing the NCSA and the PLA. In addition, NCSA and PLA morphologies were evaluated using a scanning electron microscope, and thermogravimetric analysis with differential thermal analysis and X-ray diffraction was applied to assess ash characteristics. Results of the microstructure tests showed that heat treatment of the ash improved its properties by removing carbon and unburned organic matter with a slight change in the mineral composition of the PLA. The increase in the cement replacement rates by ashes led to a decrease in the slump flow, whilst the largest slump flow reduction was recorded in the mix containing 30% of PLA and 10% of NCSA as a replacement of Portland cement by 14.5% mm compared with the reference mix. The application of replacement rates by 2.5% and 5% of NCSA and 20% of PLA by cement mass to produce UHPC respectively realised the highest compressive strengths of 161.7 and 170.1 MPa at 90 days. Moreover, the mixture containing 20% of PLA with 5% of NCSA achieved the highest values of splitting tensile strength, flexural strength and elastic modulus.