It is known that the use of recycled coarse aggregates (RCA) can raise a variety of problems, which are mainly due to the porosity of the old mortar contained in RCA. One of the simpler ways to solve these problems is the pre-wetting of RCA, which allows not only to minimize disadvantages but also to obtain the advantages associated with the effect of internal curing. Undoubtedly, the strongest positive effect of pre-wetted RCA is on the rheology of recycled concrete. But there are also possible positive effects of internal curing for strength and durability of blended cement concretes, which require longer curing times compared to normal Portland cement concrete. In this paper, we mostly study the influence of porous RCA on the rheology of cement paste, based on slag cement with a 75% slag content. For this purpose, the absorption properties of RCA of different sizes were studied. From this, mathematical dependences of the workability of cement systems on w/c and time could be obtained. These further underline the positive effect of pre-wetting of RCA with regard to retaining the workability of cementitious systems. This lays the basis for a broader study of pre-wetting RCA on the rheology of mixtures, strength, and durability to be covered in future publications.

This paper examines the influence of harvested fly ash on the properties of mortar and concrete. Class F and harvested fly ash were used at the substitution rate of 20% by weight of Portland cement. The investigated properties included heat release, consistency, setting time, compressive strength at different testing ages, absorption, the volume of permeable voids, surface resistivity, and drying shrinkage. The results revealed that the harvested fly ash produced the lowest released heat of hydration and longest setting times. Mixtures containing harvested fly ash displayed lower strength at all curing ages. Compared to traditional fly ash, harvested fly ash showed inferior transport properties for both absorption rate, permeable voids, and surface resistivity. Mixtures containing harvested fly ash showed comparable 120-day drying shrinkage when compared with the companion mortars made with traditional fly ash.

The Production of Portland cement used in concrete and the large amount of industrial waste generated worldwide represent critical environmental and economic issues. The reuse of bauxite residue generated during alumina production by Bayer’s process to replace Portland cement and produce sustainable and environmentally friendly geopolymer concrete is a promising solution. This paper presents the development and characterization of bauxite residue and class F fly ash-based geopolymer mortar and concrete. The parameters studied for the mixture proportions are the bauxite residue to class F fly ash ratio, the water-to-binder ratio, and the curing condition, in terms of duration and temperature. Then, the compressive strength of the geopolymer mortar and concrete is characterized with experimental tests. Results show that, with appropriate mixture proportions and curing conditions, a large amount of bauxite residue (up to 70%) can be used to replace fly ash and obtain geopolymer concrete with improved quality characteristics that meet the construction field’s sustainable development criteria.

The objective is to elucidate the effect of calcium nitrate CN on cement replaced with 50% blast furnace slag (ggbs). The influence of 2 and 4% CN accelerator on hydration development of these binders was investigated by isothermal calorimetry, X-ray diffraction, and thermogravimetry. The strength development of mortar with two different ggbs blended binders cured at 20°C was followed and the influence of CN was discussed. Concrete with 50% of a third ggbs, 0, 2, and 4% CN and w/c = 0.46 was cured at +5°C and compressive strength development was measured. CN led to lower mortar strength at 1 day at 20°C, but higher strength from 3 days onwards. Mortar with two different ggbs achieved the same strength at 7 and 28 days when added 4% CN. However, CN increased the 1-day strength of concrete cured at +5°C. Since ggbs is accelerated by high pH, the somewhat lower 1-day strength in mixes with CN is probably caused by the immediate precipitation of hydroxyl ions by calcium to portlandite leading to a lowering of the pH. CN led to more ettringite formation and AFm phases at 1-day sealed curing and the calcium hydroxide content was reduced. Reasons for calcium hydroxide reduction are discussed.

The present paper shows the study of a mixture design of the concrete used in the reinforced foundations of the bridge on the Straits of Messina in Italy. A cube compressive characteristic strength of 35 MPa (5,075 psi) is required for the foundation concrete. Due to the peculiar shape of the concrete foundations (completely embedded in the excavated ground), the damages caused by the thermal stress, the steel corrosion, and the alkali-silica reaction cannot be monitored and repaired. Therefore, a concrete structure must be designed without any damage for at least 200 years due to the very important role of this structure from a social point of view. In order to guarantee this long-term durability, there are two problems to be faced and solved: 1) the heat of cement hydration could cause cracks inside the foundation due to thermal gradients between the hotter nucleus of the massive structure and the colder peripheral areas; 2) the corrosion of the metallic reinforcements caused by the reaction between the metallic iron and the oxygen (O₂) present in the air to an extent of about 20%; 3) the alkali-silica reaction causing a local disruption in the concrete. All these problems can be solved using a blast-furnace slag cement such as CEM III B 32.5 R characterized by a very small heat of hydration and adopting a ground coarse aggregate with a maximum size as large as 32 mm (1.28 in): the choice of this aggregate produces a reduction in the amount of mixing water and then of the cement content and reduces the volume of the entrapped air at about 1.3% by concrete volume. This amount of O₂ would cause the corrosion of a negligible amount of iron corresponding to only 10 to 13 g (0.4 to 0.5 oz) of steel in 1 m³ (1.31 yd³) of concrete of each foundation. In order to prevent any ingress of air from the environment, the top of the foundation should be protected by self-compacting, self-compressing, and self-curing concrete.