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Properties of geopolymer concrete using metakaolin (MK) as the aluminosilicate source and recycled concrete aggregate (RCA) as partial replacement of natural aggregate are presented in this paper. The effects of sodium silicate (SS) to sodium hydroxide (SH) ratios, and RCA in different percentages on the mechanical and durability properties of geopolymer concrete were determined. Microstructural changes of geopolymers as a result of using RCA were evaluated via scanning electron microscopic (SEM) images. Test results showed that compressive strength of geopolymer concrete improved with increasing the SS/SH ratio. Although the use of RCA reduced compressive strength by up to 28%, the strength was still high enough for structural applications.In addition, increasing the SS/SH ratio reduced the chloride ion permeability and absorption of geopolymer concrete. The morphology results showed that the deboning width at the interfacial transition zone (ITZ) between RCA and binder decreased with the increase of SS/SH ratio. The polymer products in the proximity of adequate monomer ratios became more uniform and

Concrete shrinkage and volume reduction happens due to the loss of moisture, which eventually results in cracks and more concrete deformation. In this study, the effect of polypropylene (PP), steel, glass, basalt, and polyolefin fibers on compressive and flexural strength, drying shrinkage, and cracking potential, using the ring test at early ages of high-strength concrete mixtures, was investigated. The restrained shrinkage test was performed on concrete ring specimens according to the ASTM C1581 standard. The crack width and age of restrained shrinkage cracking were the main parameters studied in this research. The results indicated that the addition of fiber increases the compressive strength by 16%, 20%, and 3% at the age of 3, 7, and 28 days, respectively, and increases the flexural toughness index up to 7.7 times. Steel and glass fibers had a better performance in flexural strength, but relatively poor action in the velocity reduction and cracking time of the restrained shrinkage. Additionally, cracks in all concrete ring specimens except for the polypropylene-containing mixture, was developed to a full depth crack. The mixture with polypropylene fiber indicated a reduction in crack width up to 62% and an increasing age cracking up to 84%.

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The effect of fly ash (10, 20 and 30%) and silica fume (5 and 10%) as a substitute for cement in binary and ternary mixtures on the properties of fresh and hardened concrete are investigated. To determine mechanical properties, the compressive strength, splitting tensile strength and modulus of elasticity tests are used. Also, water penetration depth, water sorptivity, specific electrical resistivity, rapid chloride permeability and rapid chloride migration tests are applied to evaluate concrete durability. To examine the pore structure and transition zone morphology of concrete specimens, mercury intrusion porosimetry and scanning electron microscopy are used, respectively. The fresh concrete results show that the presence of silica fume in binary and ternary mixtures reduces workability and air content, but fly ash increases them. Adding silica fume to mixtures containing fly ash increases the mechanical strength at the ages of 7, 28 and 90 d. In addition to increasing the quality of the aggregate–cement matrix bond and improving the pore structure, the presence of supplementary cementitious materials has a considerably positive effect on the transport properties of concrete at all ages.