Hybrid cement (H-CEMENT) is an innovative cement of the producer from Slovakia. H-CEMENT is suitable for the production of ready-mixed concrete of compressive strength classes up to C 30/37 (4350/5370 psi) along with shrinkage-reducing and alkali-aggregate reaction-mitigating properties. The results of 5-years of exposure of H-CEMENT mortar in an aggressive sulfate solution are compared with two reference cement mortars made either with CEM I or sulfate-resistant CEM I SR 0. Sulfate resistance of H-CEMENT was evaluated in the regularly-renewed aggressive 5% solution by none-destructive tests (dynamic modulus of elasticity and length changes), destructive tests (flexural and compressive strength), microstructure studies (XRD, TG-DTA and SEM), wet chemical analyses (mainly the estimation of SO₃ content), and pore structure technique (MIP). The results give evidence of the same high sulfate resistance for H-CEMENT as that for CEM I SR 0 with C₃A = 0.

In order to improve compressive strength and the durability of concrete, such as, alkali-aggregate reaction resistance, chloride ion permeation resistance, carbonation resistance, and freezing and thawing resistance, a new type of combined cementitious materials was used to make the concrete. One part of the cementitious materials was high early strength Portland cement (similar to ASTM type III Portland cement), which had more than 63 mass% C3S and hydrated quickly to generate calcium hydroxide to accelerate pozzolanic reaction. Another part of the cementitious materials was fine blast furnace slag powders which had more than 6000 cm²/g Blaine specific surface area to get faster hydration with the calcium hydroxide. And other part of the cementitious materials was fly ash which had high specific surface area and low ignition loss to get faster pozzolanic reaction. According to the results of tests in this research, it is clear that the compressive strength of the concrete made with the combined cementitious materials is near that of the concrete made with the high early strength Portland cement only. However, the alkali-aggregate reaction in the concrete made with the combined cementitious materials is much lower than that of the concrete made with the high early Portland cement, and/or mixed with the fine blast furnace slag powders or fly ash respectively. It is also confirmed that chloride ion permeation resistance, carbonation resistance, and freezing and thawing resistance of the concrete made with the combined cementitious materials are improved considerably.

Rogun Dam is an embankment dam in construction on the Vakhsh River, Southern Tajikistan. The Hydro construction preparatory period was started in 1976. By 1993, the upstream cofferdam height reached 40 m (131 ft) and 21 km (68 898 ft) of tunnels had been driven, and the spaces of the powerhouse hall and the transformer station had been mined. After collapse of the Soviet Union the Hydro construction was suspended. Resumption of construction had required expert investigations including analyses of the earlier built structures. As established, many reinforced concrete components were damaged in underground hydraulic structures. The phase and element composition of concrete core specimens was studied. The analysis data have shown that the structure of sample cores comprises chalcomorphite and thaumasite that gives evidence of the sulfate corrosion process. At the same time, photomicrographs of thin and polished sections obtained using optical and electron scanning microscopes show signs of alkali-aggregate reactions (AAR). AAR is also evidenced by the presence of typical cracks directed radially from aggregate grains deep into the mortar matrix as well as the cracks penetrating the aggregate particle itself. The cracks are filled with the gel-like reaction product. With consideration of the results obtained the recommended commercial concrete compositions will be optimized and studied in detail to avoid the degradation processes of concrete in the future damages hereafter

The continuous consumption of natural aggregate in concrete production is steadily straining such natural resources. The need for a more sustainable solution has led to a serious consideration of using recycled concrete aggregate as a replacement to natural aggregate. Studies in the literature were conducted to determine the properties of recycled aggregate concrete. For non-structural applications, recycled aggregate concrete has been widely accepted in several countries mostly in Japan. However, further studies on its durability are still required. The lack of comprehensive standard guidelines for mixture proportion as well as the limited studies on its durability has hindered its wider implementation especially in structural applications. The studies performed on the durability of recycled aggregate concrete has been surveyed and compiled in this paper. The paper addresses issues pertaining to durability including the mixture design, permeability and water absorption, the resistance to alkali-silica reactions, reinforcement corrosion, abrasion, freeze-thaw resistance, and sulfate attack. Generally, a coarse aggregate replacement ratio of 20% to 50% did not negatively affect the performance of recycled aggregate concrete in many cases. The literature overview showed that recycled aggregate concrete performs satisfactorily under various conditions and has a comparable durability to natural aggregate concrete if designed properly.

Mortar bars made with silica glass aggregate were tested at 23°C (73°F) to evaluate the applicability of a previously proposed chemical model for the alkali silica reaction (ASR). The model, based on tests at 80°C (176°F), proposes that ASR gel does not form until portlandite (CH) in the hydrated paste is locally depleted and the calcium silicate hydrate (C-S-H) has been locally converted to a more highly polymerized and lower Ca/Si form. SEM-EDX, XRD, and ²⁹Si NMR spectroscopy of the 23°C (73°F) mortars show that the same chemical processes operate at both temperatures. At 23°C (73°F) and up to 60 days, only a small amount (~1%) of ASR gel forms and is confined to cracks entirely within the aggregate grains, but this small amount of gel containing Na, K, and Ca is sufficient to cause substantial expansion. There is no large-scale depletion of CH or increase in the C-S-H polymerization in the paste due to the small amount of gel formed and its confinement in the aggregate grains. Local reduction in both the amount of CH and the Ca/Si ratio of C-S-H in the paste is observed near places where gel-filled cracks in the aggregate contact paste, consistent with the proposed chemical model.