Encapsulated admixtures in concrete have emerged as a prospective material to be used in future constructions. The core material is temporarily protected by the shell material, which also prevents the core from reacting with the concrete environment. The shell can be made of a pH-sensitive material that can deliver the core material based on the pH of environment rather than through mechanical rupture. The initial high alkaline nature of the concrete (pH 13.5) during the hydration and subsequent pH reducing conditions (by the environmental factors) can be used to design the delivery time of the core material. The coatings can be used to deliver the core material either within a short period or after a long period. The delivery of the core material at a desirable time can be achieved by the selection of a suitable coating agent.

Palm oil fuel ash (POFA) is a by-product procured from the palm oil mill through the incineration of empty fruit bunches, mesocarp fibers, and shells so as to produce electricity. POFA was considerably used as a cementitous supplement in various types of concrete, bricks, blocks, mortar, and grout due to its pozzolanic content. However, using raw POFA as cementitious replacement caused a distinct deterioration on the properties of the hardened mixture. Therefore, various treatment methodologies were adopted to enhance the properties of POFA to improve the mechanical properties of the hardened mixture. This study reviews the treatment approaches performed on POFA and their effects on the physical, chemical, and microstructural properties of POFA. It was documented that grinding POFA increased its fineness and decreased the voids and porosity of the mixture. However, the optimum use of grounded POFA was ranged 5 to 25% by weight of cement. On the other hand, thermal treatment of POFA exhibited a substantial improvement on the physical, chemical, and morphological properties of POFA; consequently, the hardened properties were dramatically developed. Thermal-treated POFA could be used as binder supplement up to 70% by weight of cement, whereby environmental pollution was dropped and sustainability was achieved. It was concluded that the higher fineness of POFA contributed to a significant pozzolanic reaction and thus promoted better performance in the hardened matrix. However, future detections should address the leaching behavior of POFA and the leaching performance of the hardened mixture incorporating POFA. Besides, the durability of specimens containing POFA as binder supplement should be well covered in the prospectus research.

Advanced scanning electron microscopy methods were used to characterize the microstructure of cement paste containing entrained air voids. Environmental electron microscopy allowed the imaging of the cement paste without exposing it to high vacuum, therefore significantly eliminating the risk of shrinkage cracks. The fine details of the air-void shell were resolved using field emission microscopy. Finally, ice crystals inside the air-void were imaged using low-temperature scanning electron microscopy. These microscopy studies showed the presence of a transition layer surrounding the air void, which can have a significant effect on the ice formation inside the air voids. The microscopic analysis seems to confirm that the entrained air voids also cause cryo-suction by driving the liquid water from the cement paste during freezing. Both observations are included in a mathematical model that extends the original Powers’ model for freezing of concrete.

In this paper, the morphology of ice as it appears in the entrained air voids of hydrated portland cement paste is examined in detail. The main experimental technique employed in this work is the low-temperature scanning electron microscope, which allows for the imaging of frozen, hydrated, cement-paste specimens. Four main classes of cement-paste specimens were examined. First, normal cement-paste specimens are examined when frozen after 4 days of moist curing. Herein, discrete ice crystals were seen growing from the air void wall, making an average contact angle of 91 degrees with the air void wall. When bulk ice was observed in the air voids of these specimens, it appeared to be smooth in texture and without striations. Second, cement-paste specimens made with CaCl2 dissolved in the mixing water were examined to determine the effect of deicing salts used on concrete surfaces. For these specimens, striations were seen in the bulk ice found in the air voids, assumed to be eutectic lines that form when the discrete crystals grow together and coalesce. The third type of specimen is early hydration specimens, where normal cement-paste specimens were frozen after 4 h of hydration. Herein, ice crystals were seen to grow outside of the air void shell in the air void transition zone. These crystals pushed the air void shell out of their way, indicating the relative weakness and impermeability of the air void shell at 4 h of age, compared with the mature air voids in the first two sections. The last type of specimen is normal cement paste subjected to freezing-and-thawing cycles. After three freezing-and-thawing cycles, large hydration products were observed in the air voids; these hydrates could be significant enough in volume to affect the frost-protecting capacity of the air voids.

Lightweight aggregates are produced from a wide variety of raw materials, and their production conditions may also vary. Therefore, the characteristics of lightweight aggregates may vary within wide limits. For high-strength concrete, the characteristics of the aggregate are more important for the concrete properties than for low- to medium-strength concretes. For production of high-strength lightweight concrete, it is urgent, therefore, to provide information on the characteristics of the aggregate. In this paper, an investigation of some high-strength lightweight aggregates available on the European market is reported. For the various aggregates, particle shape, surface texture, and pore structure varied within wide limits. Most of the pores in the aggregate were open, and thus susceptible to water absorption. The pore sizes also varied considerably. The shapes of the pores were rather irregular. In all the aggregates, there were some relatively large voids and fissures that made the aggregate weak and friable. Some of the aggregates had a distinct dense outer shell. The particle density varied from 1.07 to 1.54 g/cm3. This variation was related to characteristic differences in macroscopic and microscopic pore structure. After the first 30 min, the water absorption varied from 8 to 13 percent by weight, but most of the water absorption took place within the first 2 min. Within the first 30 min, more than half of the 24-hr water absorption was observed. For cut particles of some aggregates with a dense surface layer, the water absorption within the first 30 min was approximately 30 percent higher than that of whole particles of the same specific surface. 113-391