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Date palm fiber (DPF) is one of the abundant solid waste materials in the agriculture sector in Saudi Arabia, and it is gaining great attraction due to its advantages compared to synthetic and other natural fibers. For proper utilization of DPF in cementitious composites, its performance under high temperatures needs to be understood. This is because DPF is a cellulose-based agricultural fiber material and is expected to degrade when subjected to high temperatures. This will cause a significant loss in strength and structural integrity of the composites. The use of Pozzolanic materials has been reported to reduce the loss in mechanical properties of cementitious composites under high temperatures. With powdered activated carbon (PAC) being a low-cost material compared to other Pozzolanic materials, this study utilized PAC as an additive to the DPF-reinforced concrete to mitigate its loss in mechanical strength when exposed to elevated temperature. The experiment was designed using response surface methodology (RSM), which was used to construct mathematical models for estimating the strengths of the concrete exposed to high temperatures. The DPF was added at proportions of 1%, 2%, and 3% by weight of cement. Similarly, the PAC was added at 1%, 2%, and 3% by weight of cement to the concrete. The concrete was subjected to elevated temperatures of 300 °C, 600 °C, and 900 °C for a 2 h exposure period. The degradation of the concrete in terms of mass loss and the compressive strength of the concrete after heating were measured. DPF in the concrete led to an escalation in weight loss and reduction in strength, which was more pronounced at a temperature of 600 °C and above. The addition of PAC resulted in an enhancement in the strengths of the concrete containing up to 2% DPF at 300 °C, while at 600 °C the improvement was minimal. The models developed for predicting the mass loss and strengths of the DPF-reinforced concrete under high temperatures were statistically significant with a high correlation degree. Based on the optimization results, DPF-reinforced concrete produced with 1% DPF, and 2.27% PAC as additives and subjected to a temperature of 300 °C for 2 h yielded the lowest mass loss of 2.05%, highest residual compressive strength and relative strength of 45.85 MPa and 106.7% respectively.

Calcium carbide residue (CCR) is the end-product of production of acetylene gas for the applications such as welding, lighting, ripening of fruits, and cutting of metals. Due to its high pH value, disposing of CCR as a landfill increases the alkalinity of the environment. Therefore, due to its high calcium content, CCR is mostly blended with other pozzolanic materials, together with activators as binders in the cement matrix. In this study, cement was partially substituted using CCR at 0%, 7.5%, 15%, 22.5% and 30% by weight replacement, and nano silica (NS) was utilized as an additive by weight of binder materials at 0%, 1%, 2%, 3% and 4%. The properties considered were the slump, the compressive strength, the flexural strength, the splitting tensile strength, the modulus of elasticity, and the water absorption capacity. The microstructural properties of the concrete were also examined through FESEM and XRD analysis. The results showed that both CCR and NS increase the concrete’s water demand, hence reducing its workability. Mixes containing up to 15% CCR only showed improved mechanical properties. The combination of CCR and NS significantly improved the mechanical properties and decreased the concrete’s water absorption through improved pozzolanic reactivity as verified by the FESEM and XRD results. Furthermore, the microstructure of the concrete was explored, and the pores were refined by the pozzolanic reaction products. The optimum mix combination was obtained by replacing 15% cement using CCR and the addition of 2% NS by weight of cementitious materials. Therefore, using a hybrid of CCR and NS in concrete will result in reduction of cement utilization in concrete, leading to improved environmental sustainability and economy.

Alkali-activated fly ash (AAFA) is usually cured at elevated temperature, which limits its wide field applications. However, several studies have shown that properties of AAFA cured at ambient temperature can be significantly improved by incorporating different cementitious additives. This research investigated the impact of substituting a small portion of fly ash with different potential cementitious additives such as slag, metakaolin, and silica fume on various properties of AAFA mixtures at ambient curing conditions. Toward that end, several experimental observations including workability, geopolymerization heat released, compressive strength, Young’s modulus, electrical resistivity, chloride permeability, and pore structure distribution were made. The effects of alkaline solution concentration combined with the different chemical compositions on mechanical properties and permeability characteristics were also investigated. An ordinary Portland cement was also included in the testing program as a comparative reference. Test results showed that the AAFA mixtures exhibited lower heat as compared to ordinary Portland cement mixture, and the results of heat evolution were in line with early-age strength development. The inclusion of metakaolin resulted in a significant improvement in the early strength and stiffness. The inclusion of slag led to the improvement of the mechanical and permeability properties of AAFA cured at ambient temperature, and the effect of sodium silicate (SS) to sodium hydroxide (SH) solution ratio was found to be more profound when the slag was used as an additive. Incorporating silica fume in the AAFA mixtures decreased the mechanical properties and increased the volumetric percentage of large capillary pores.

Approaches to improve the early strength of geopolymer mortar were studied in order to apply it to a fast-tack concrete pavement repair. The effects of modifying the molar ratio of silicon dioxide/sodium oxide (SiO2/Na2O) of the alkaline solution in a conventional geopolymer mixture and the addition of calcium aluminate cement and slag were studied. The early strength measured in the experiments was at curing times of 8 and 24 h, which are typically construction controlling times for pavement repair. It was found that a molar ratio of silicon dioxide/sodium oxide of 1·0 gave the shortest curing time and the highest early strength after 24 h. Adding calcium aluminate cement was found to reduce the curing time, but decrease the early strength. Slag substitution helped workability, but reduced the early strength at 8 and 24 h. Durability was also tested for the purpose of finding an economical pavement repair solution. A freeze–thaw durability experiment of the suggested metakaolin-based geopolymer concrete was conducted. It was found that a mix with a silicon dioxide/sodium oxide molar ratio of 1·0 produced higher durability than a mix with a silicon dioxide/sodium oxide molar ratio of 1·4. The geopolymer concrete mix with slag showed less weight loss than the other mixes.