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The presence of chloride ions is one of the most widespread causes of corrosion initiation in reinforcing steel in concrete. Trace chlorides present in cementitious materials or admixtures typically result in very low fresh chloride contents in normal-strength concrete that do not present a danger of corrosion. UHPC mixture designs, however, use much higher dosages of cementitious materials and admixtures that can result in non-negligible total fresh chloride contents. These high chloride values are likely to occur more frequently in the future as more UHPC mixtures are made with locally available materials and alternative cementitious materials and may result in concrete mixtures failing to meet specifications for fresh chloride content limits that are based on mixture proportions used in normal-strength concrete mixtures. UHPC and normal concrete samples were made without fibers and with increasing levels of internally admixed chlorides for four different levels of strength to determine chloride thresholds for internally added chlorides. The chloride threshold for fresh concrete was measured using a slightly modified version of the accelerated test EN 480-14. The water-soluble and acid-soluble chloride ion content of UHPC mixtures tested were measured according to ASTM C1218 and Florida Method FM 5-516 to determine the bound chlorides and fresh chloride limits for corrosion. The results demonstrate that the UHPC had ~ 25% higher chloride threshold than the control mixture when measured as an absolute content per unit volume of concrete. When the UHPC chloride content is normalized by mass of cementitious material, it was found that the amount needed to initiate corrosion may be lower than fresh chloride limits given in ACI-318 and ACI 222. Therefore, the ACI-318 water-soluble chloride limits as a % by mass of cementitious materials were found to be non-conservative for the two of the UHPC mixtures tested and should be re-examined for UHPC.

The addition of supplementary cementitious materials (SCMs) to low-carbon concrete mixtures has been investigated in recent years as part of the sustainability of the concrete sector. Recently, most traditional SCMs, such as fly ash and blast furnace slags, have become unavailable in several developed countries, mostly due to environmental restrictions. Consequently, several new by-products from fast-growing sectors are being considered as potential replacements for traditional SCMs. However, the durability of these new by-products in low-carbon concrete has not been thoroughly explored. As a result, this paper presents the first part of a project related to an extensive experimental characterization, in which low-carbon concrete with high compactness, paste optimization, and partial cement replacement by the addition of waste by-products from the agricultural, metallurgical, paper, and glass industries is studied. Alternative SCMs including rice husk ash, biomass fly ash, rock wool residues, or waste foundry sand are incorporated into corresponding mortar matrices and the results concerning the mechanical properties (flexural and compressive strength) and durability (capillary water absorption, surface electrical resistivity, and carbonation resistance) are presented and analyzed. The outcomes indicate that it is possible to reduce the Portland cement content without compromising the mechanical and durability properties of the concrete.

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.

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.

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.