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This paper reports the effects of key factors on the properties of geopolymer concrete mixtures containing 100, 90, and 85 mass% of natural pozzolan (NP) as primary and 0, 10, and 15 mass% of Portland cement as secondary cementitious materials. Two types of alkaline activators (NaOH and KOH), and three different sodium silicate (SS)/(NaOH or KOH) ratios of 1, 2, and 4 (by mass) were used. NaOH performed better than KOH in the development of compressive strength. While the strength increased with increase of silicate/alkali hydroxide ratio, a decrease in the strength was observed at a higher NP content. Results of the tensile strength, modulus of elasticity, shrinkage, thermal resistance, water absorption, acid resistance and SEM analysis tests conducted on selected mixtures also confirmed the beneficial effects of using a lower NP content activated by NaOH. An optimum mixture of geopolymer concrete, having a NP content of 85% and SS/NaOH ratio of 4.0, is recommended for the construction of structural members requiring high-performance concrete. [ABSTRACT FROM AUTHOR]

In the present study, ultra-high performance concrete (UHPC) mixtures reinforced with varying dosage of steel fibers were considered for studying the effects of exposure duration and fiber content on mechanical properties of the mixtures subjected to a sustained pre-spalling elevated temperature of 300 °C for different durations. Test results showed an increase in compressive strength and modulus of toughness even after exposing to elevated temperature for 5 h. However, modulus of elasticity and flexural strength decreased significantly with increase in exposure duration. The experimental data were statistically analyzed using analysis of variance (ANOVA) method to examine the effects of fiber content and exposure duration on the mechanical properties. Empirical equations were obtained with excellent degrees of fit correlating the mechanical properties of UHPC mixtures with fiber content and exposure duration. [ABSTRACT FROM AUTHOR]

Highlights • Use of natural pozzolan as partial replacement of cement and microsilica in UHPC mixtures. • No negative impact on the key properties of UHPC due to addition of natural pozzolan. • Significant saving of microsilica and cement due to use of natural pozzolan. Abstract Ultra-high performance concrete (UHPC), comprising of high amounts of cement and microsilica as binder and micro filler, is a new generation of concrete possessing highly dense microstructure with excellent mechanical properties, durability characteristics and ductility (fracture toughness). However, the use of high amounts of cement and microsilica makes UHPC costly and environment-unfriendly. In the present study, an effort is made to study the effects on properties of UHPC mixtures due to incorporation of natural pozzolan (NP) as a partial replacement of cement and microsilica. Targeting a significant reduction in consumption of cement and microsilica without compromising the required workability, six UHPC mixtures were considered. The first mixture was taken as control mixture without replacement of cement and microsilica by NP. In the second and third mixtures, 25 and 50% microsilica was replaced by NP, respectively. In fourth, fifth and sixth mixtures, 10, 20 and 30% of cement was replaced by NP, respectively. Specimens were moist-cured for 14 days before exposing them to air in the laboratory conditions. All the six UHPC mixtures were tested to determine the mechanical properties (compressive and tensile strengths, modulus of elasticity, and fracture toughness), drying shrinkage, and chloride permeability. Test results indicate that the replacement of cement (up to 30%) and microsilica (up to 50%) by NP did not significantly affect the fresh and hardened properties of UHPC mixtures confirming the potential of using NP for producing cost-effective and environment-friendly UHPC mixtures. [ABSTRACT FROM AUTHOR]