Portland cement hydration in the vicinity of electrically polarized conductive surfaces

Hardening of Portland cement-based materials in vicinity of electrically conductive surfaces, especially when the surfaces are electrically or galvanically polarized, can lead to both morphological and chemical changes in cement close to the surfaces due to combined electrochemical and electrophysical processes. Cement hydration products close to graphite and steel surfaces being positively (anode) and negatively (cathode) electrically polarized (direct current) were studied. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy were used to compare structure and atomic composition of cement hydration products on cathode, anode and a reference surface with no electrical polarization. The application of direct current (DC) potential in aqueous Portland G cement dispersion significantly affects cement hydration products close to cathode and anode and different products were found at the anode compared to the cathode surfaces. At the graphite anode, calcium sulphate crystals along with calcium hydroxide were most abundant, while the graphite cathode was mainly covered with calcium hydroxide. The calcium hydroxide carbonated upon exposure to air during drying. When steel electrodes where used, the most significant adsorption occurred at the anode, in contrast to graphite where the largest amount of the adsorbed material was found on the cathode. The observed differences were explained in view of electrophysical (electrophoresis, electroosmosis) and electrochemical (reduction and oxidation) processes occurring at electrode surfaces upon application of DC current. The knowledge gained in this work is important for engineering of electrically conductive cement nano-composites where typically the contact surface of an electrically conductive filler and a cementitious matrix is high.publishedVersio

Long term field study into the durability of silica fume concrete in a marine tidal zone

In 1983, samples of reinforced concrete made from a range of concrete mixtures containing varying amounts of CEM I and silica fume were placed in a marine tidal zone at Trondheim, Norway. Examination and testing of the samples occurred periodically during the initial 21.5 years of exposure. In 2014, after 31 years exposure, the field site closed and the samples examined for a final time. This paper presents the results of compressive strength, electrical resistivity and chloride ingress tests on cores taken from the 31-year-old samples. The durability performance of the silica fume mixtures was seen to be significantly better than the CEM I mixtures. The uniqueness of this research is being able to draw upon 31 years of empirical real-world data

Alkali metal distribution in composite cement pastes and its relation to accelerated ASR tests

Accelerated testing of alkali silica reaction (ASR) in concrete at elevated temperatures of 38 and 60 °C has an unknown impact on the alkali metal distribution in the cement paste. This paper investigates how the alkali metals released from hydrating Portland cement, limestone, and SCMs distribute between non-reactive and unreacted phases, C-A-S-H, and the pore solution. The SCMs investigated were fly ash and a volcanic pozzolan. The hydrate assemblage and pore solution of cement pastes cured at 20, 38 and 60 °C were analysed and related to the expansion of concrete prisms. There is little difference in alkali metal distribution at 20 and 38 °C, whereas curing at 60 °C has a large impact for the SCM containing blends. At alkali metal concentrations in the pore solution below 0.5 mol/L (Na + K) expansion of concrete was suppressed. Pore solution analysis could be used to screen new SCMs for ASR mitigation.publishedVersio

Effect of leaching on the composition of hydration phases during chloride exposure of mortar

Mortar specimens were exposed to either a 3% NaCl solution or a 3% NaCl+KOH solution for up to 180 days. Exposure to the NaCl solution provoked much more leaching than the NaCl+KOH exposure. Leaching strongly impacted the chloride ingress profiles. The extended leaching led to a maximum total chloride content almost three times higher and a deeper chloride penetration than exposure with limited leaching after 180 days. The higher maximum chloride content seems to be linked to the enhanced binding capacity of the C-S-H and AFm phases upon moderate leaching as determined by SEM-EDS. The total chloride profile appears to be governed by multi-ion transport and the interaction of chloride with the hydration phases. Service life prediction and performance testing both rely on total chloride profiles and therefore ought to take these interactions into account.publishedVersio

Low water permeability through hydrophobicity : COIN Project P1 advanced cementing materials : SP 1.5 F Low porosity / permeability

The durability and aesthetic appearance of concrete may be improved by the addition of hydrophobizing agents as a consequence of reduced water permeability. Hydrophobizing agents lead to less water absorption at the same time as they let water vapour out. This may lead to a dryer interior over time and thereby reduced rate of detrimental reactions needing liquid water as reaction medium. The ingress of water born aggressives like chlorides will be reduced (in particular in marine splash zones), but also corrosion rates may be decreased. Carbonation rates may, however, be somewhat increased. Vegetable oils seem to be the most cost-effective hydrophobizing agents as good effects may be achieved by additions of only 0.5 % of the cement mass. Furthermore, the cheapest and most available vegetable oil based on rapeseed is among the most effective tested. It is recommende to continue research on the effect of rapeseed oil as a concrete admixture, also at dosages above 1.5 %. The research should focus on rapeseed oil as hydrophobizing agent, but also on its effect on other interesting concrete properties for COIN; like hardening retarder, shrinkage reducing agents, electrical resistivity and pH reduction

Acid Activation of Natural Rocks - A Case Study with Norwegian Anorthosite and Phosphoric Acid

Norway is rich in natural resources, especially different types of rocks that could potentially serve as raw material for alternative binder concepts like alkali activated binders or magnesium phosphate and magnesium hydroxy carbonate cements. Another less studied concept, as discussed in this paper, is acid activation of rocks to produce alternative binders. Although these systems cannot compete yet with ordinary Portland cement, they might be applicable for certain specialised areas and thus help to reduce cumulated CO2 emissions by decreasing the total amount of cement used. Alternative binder systems based on Norwegian rocks as raw material show potential for further research and potential implementation of new specialised products for the domestic market. To demonstrate the potential of acid activation, first results of a case study on anorthosite activated by phosphoric acid are presented. Results show that it was possible to produce a binder with 28-day compressive strength of about 20 MPa, by mixing warm phosphoric acid with finely ground Norwegian anorthosite. The paper presents preliminary results and further optimization, and research is necessary to produce higher strength and understand the hardening mechanisms of the acid activated anorthosite binder.publishedVersion10201956

Bauxite Residue as Supplementary Cementitious Material – Efforts to Reduce the Amount of Soluble Sodium

This study investigates the feasibility of using bauxite residue (BR) as supplementary cementitious material (SCM) for the cement and concrete industry. It is shown from pastes of BR and calcium hydroxide, that BR is highly pozzolanic in nature. The early hydration of cement pastes with BR is accelerated while long-term strength is reduced probably due to the alkaline nature of BR. To be used as cement replacement material in concrete, attempts have been made to reduce the alkali content of BR, in particular to reduce the chance of alkali-aggregate reactions. Co-calcination of BR with kaolin or washing and cooking of BR with calcium hydroxide or calcium hydroxide and gypsum resulted in considerable reduction of alkali content; up to 75%. At the same time the reactivity of the BR was reduced but still being higher compared to fly ash already used in the cement industrypublishedVersio