Retarders are very important during the production of cement-based materials. The delay in setting might be helpful in avoiding negative phenomena related to the long-term transport of the fresh concrete mix, unforeseen breaks in the transport, or laying of concrete. These admixtures prevent the local temperature rise of the concrete, and thus the formation of cracks and also extent the workability. Set-retarders provide a correct development of the microstructure and the undisturbed setting and hardening of cement which lead to higher strengths of cement-based materials. An investigation of the cement mortar with potassium methylsiliconate (MESI) applied as set-retarding admixture was carried out. Siliconates are a highly alkaline water solution of methylsiloxane resin in the potassium or sodium hydroxide. The study involved the cement paste and mortar with three dosages (1%, 2%, and 3% per cement mass) of organosilicon admixture. So far, the siliconates were not applied as admixtures for cement mortar or concrete. The mortar specimens were tested for compressive strength after 1, 2, 7, and 28 days and frost resistance after 25 freeze-thaw cycles. Moreover, the impact of the methylsiliconate admixture on the hydration (by isothermal calorimetry) and setting time of the ordinary Portland cement was also studied.

The long-term stability of aluminum metal in binders intended for concrete has been studied. Aluminum was cast in paste where 55% cement was replaced by calcined smectitic clay. After 7 days of curing several samples were submersed in either distilled water or in a 6% NaCl solution. After 1 year of submersion, the microstructure of samples submitted to both exposure conditions was analyzed. Chloride corresponding to 1.5-2% Cl- of cement mass had reached the Al-bar. The surface of the Ø 10 mm aluminum bar had corroded to a depth of less than 10μm, irrespectively if it was submerged in water or in 6% NaCl. There was no sign of intergranular corrosion. The hydrated binder consisted of amorphous calcium silicate hydrate gel and crystalline layered double hydroxides of hydrocalumite-type. When cured in water, the crystals were a mix of hemi- and mono-carboaluminate, but when stored in 6% NaCl, the carbonate/hydroxide was partly replaced by chloride. There was a dense binder zone formed around the ≈ 15 μm oxide layer of the aluminum bar richer than the overall binder in aluminum. Some of the alumina formed on the metal surface may have been partly dissolved by alkalis and precipitated/reacted with the nearest binder region and densified it and thus preventing further reaction.

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.

Due to the high carbon dioxide emissions linked to concrete production and a rapidly increasing demand for cementitious materials, particularly in the global South, it is inevitable to use cement in concrete more efficiently. This requires chemical admixtures to enhance the overall performance of the binder and to cope with the negative rheological influences of supplementary cementitious materials that are used to replace ordinary Portland cement. However, particularly in the growing economies of the Southern hemisphere, where a massive part of the future construction activities will take place, the supply chains for performance-enhancing chemical admixtures are often poor, and local production facilities are lacking today. This paper presents case studies of polysaccharide-based alternative admixtures such as acacia gum, cassava starch, and the gum of the bark of Triumfetta pendrata A. Rich, which can be used effectively as superplasticizer, robustness enhancer, and thixotropy incorporating agent, respectively. Their modes of operation are discussed based on their spread flow, zeta potentials, and hydrodynamic diameters in the presence and absence of calcium ions.

Polycarboxylate superplasticizers are increasingly being used in nuclear power plant construction including the dry shielding concrete containment, which directly surrounds the reactor pressure vessel. The problems of their thermal and radiation stability are brought to the fore. Tests were performed based on the simultaneous thermal analysis of admixtures based on HPEG-type polycarboxylate ethers. Scanning calorimetry (DSC) and thermogravimetry (TG) measurements were employed in this study. Both measurements were performed on a Netzsch STA 429 CD Simultaneous Thermal Analysis Apparatus. For analysis of degradation products, Netzsch aQMS 403 C quadrupole mass-spectrometer was used that allows performing analysis (IC curves) of thermal degradation products within the range from 1 to 121 atom-charge units. The aluminum oxide tablets impregnated with superplasticizers were used. The comprehensive analysis of superplasticizers has proven their quite high thermal stability. During heating up to 250°С and thermal degradation of polycarboxylate ethers, there is no emission of explosive and toxic gases.