International Concrete Abstracts Portal

In the presence of silica fume (SF), the hydration of high alumina cement (HAC) produces hexagonal hydrates (CaH10 and C2 AH8 ) which do not convert into the cubic hydrate (C3 AH6 ) and therefore the related strength loss does not occur: this is due to the formation of C2 ASH8 which blocks the conversion process. However, due to the presence of SF, the required mixing water significantly increases. Therefore, an effective water reducing admixture is needed to compensate for the presence of SF and to allow the mixture to attain the same strength level of pure HAC at early and later ages. Two alternative admixtures - sodium tripolyphosphate (TPP) and/or acrylic polymer (AP), instead of other traditional superplasticizers - were studied as water reducers for the blended HAC-SF binder. Both were very effective in reducing the amount of mixing water. However, in the presence of TPP there was a quick fluidity loss after about 45 min. This was a sort of flash set of the binder followed by a sudden heat development. Due to the restrained thermal expansion of the hotter nucleus with respect to the colder surface areas, microcracks formed. Upon contact with liquid water microcraks changed to macrocraks. This severe distress caused a strength failure of the specimens. This change was produced by the pressure exerted by crystal growth of hexagonal hydrates in the water-filled microcraks. By reducing the fluidity loss - for instance by using AP instead of TPP - the heat development became more gradual and therefore microcraks as well as macrocracks disappeared. Consequently any strength loss was removed in the blended HAC-SF binder.

This paper reports the properties of cement paste, mortar and fresh and hardened concrete using superplasticizers with electric repulsion, steric barrier and those possessing both dispersing mechanisms and their interaction with cementitious materials having pozzolanic properties and those without pozzolanic properties prepared by different mixing methods. It was found that there is an a optimum W/C for the primary water (Wl/C) where bleeding is minimum and thus dispersion state is optimum. The fresh properties of concrete prepared by double mixing method are different from those by conventional mixing method (Single mixing). Also, when superplasticizers are added in primary water (WI), fresh properties of concrete are different from those when added in secondary water (WZ). The effect of dosing method of superplasticizers and replacement of cement with different cementitious materials, varies greatly with the type of superplasticizers.

Calcium nitrate (CN) has been increasingly used as a chloride free set accelerator in later years. Mature cement pastes and concrete have been subjected to microstructure investigations in order to identify possible long term changes when a high dosage of CN was added to the fresh mixes. The following changes were found for 2 year old cement pastes (w/c = 0.50) based on ordinary Portland cement (OPC) and sulphate resistant Portland cement (SRPC) when 5.26 % CN was added: 1) The degree of hydration was only marginally lower. 2) The amount of calcium hydroxide (CH) was significantly lowered (= 10 %) in the case of OPC and unaltered for SRPC. 3) The amount of chemically bound water in both pastes was increased. 4) The average length of the polysilicate anions in the amorphous CSH-gel was prolonged (17% for OPC and 5 % for SRPC). 5) The porosity of the OPC paste was increased (+7%) and inhomogeneously distributed, while it was decreased (-2%) for the SRPC paste. 6) The morphology of calcium hydroxide (CH) in the pastes was changed from being evenly distributed to be gathered in larger (~50um diameter) clusters. This phenomenon may be explained by restricted diffusion due to the high Ca2+ concentration supplied by CN. This latter effect was also observed for a plain concrete (w/c = 0.57) based on high strength Portland cement (HSPC) and 3.86 % CN.

When the historical buildings of the Republic of Venice were erected all the construction materials and the corresponding techniques were always carefully selected. Even at that time architects were aware of the importance of the durability of buildings in a very hostile environment such as that existing in Venice characterized by permanent humid air and capillary rise of salty water from the foundations. In particular cementitious materials were adopted in agreement with the empirical rules of the Romans’ experience and adapted for the particular Venetian environment. The present paper examines two specific cementitious materials which became very popular throughout the world: the stucco plaster and the terrazzo concrete. At the time of the Republic of Venice, chemical admixtures were not available. Therefore, masonry artisans and architects developed a special know-how to manufacture durable materials. This was based on the use of mineral admixtures and natural substances, the invention of innovative binders and the development of special application techniques. After the advent of portland cement and especially of chemical admixtures, modem stucco plaster and terrazzo concrete can be produced with different (not necessarily better) properties and at higher rates of productivity. The present paper examines the microstructural aspect, the composition, the performance, and the manufacturing process of the original and modem materials.

Anti-washout admixtures are used to enhance stability of cement-based systems, such as in the case of concrete intended for underwater repair of marine structures, self-leveling and segregation-free concrete for above-water placement, and for shotcrete. Mixtures modified with an anti-washout admixture can also incorporate silica fume or fly ash to enhance fresh and hardened properties. Anti-washout admixtures are relatively new, and there is a lack of systematic data comparing the efficiency, advantages, and limitations of using them in concrete, especially in silica fume or fly ash concrete. A laboratory investigation was undertaken to evaluate the effect of cementitious material combinations on fresh and hardened properties of fluid concrete made with three commonly used anti-washout admixtures. Each anti-washout admixture was used at two concentrations corresponding to relatively low and medium dosages that are typically used in concrete. The anti-washout admixtures included a powder-based product, a liquid-based cellulosic material, and a liquid-based microbial polysaccharide. The mixtures were prepared with 100% of a Type 10 cement, a blended silica fume ‘cement containing 8% silica fume, as well as a Class F fly ash used at 20% replacement. All mixtures were made with a 0.41 water-to-cementitious materials ratio and had initial slump values of 230 + 5 mm (9.1 + 0.2 in.). The influence of fly ash and silica fume incorporation on slump retention, washout resistance, bleeding, segregation, surface settlement, setting time, compressive and splitting-tensile strength developments was evaluated for the fluid mixtures. The paper summarizes the results of the experimental study that involved the comparison of data obtained from 21 concrete mixtures.