Dolomitic filler in self-compacting concrete: a review

The utilization of fine powders as fillers in self-compacting concrete (SCC) application is widespread, particularly in Europe. The incorporation of these fillers to attain the self-compatibility properties of SCC seems to be cheaper than the use of chemical admixtures. Among the wide range of potential fillers, dolomitic powders, particularly generated as by-products from quarry’s processing, are locally available and can be used to produce SCC. Few studies have shown that dolomitic powders can be incorporated in the SCC’s mix design, resulting in acceptable fresh and hardened properties of SCC. The particle size distribution and fineness of the dolomitic powder as well as the level of addition are the key factors affecting those properties. The influence of the chemical nature of the dolomitic powder on the properties of SCC, particularly the durability (e.g. alkali-carbonate reaction), is yet to be investigated. Furthermore, more efforts are still required to investigate the use of dolomitic by-products in the production of SCC.

Inhibition of the Alkali-Carbonate Reaction Using Fly Ash and the Underlying Mechanism

In this paper, fly ash is used to inhibit the alkali-carbonate reaction (ACR). The experimental results suggest that when the alkali equivalent (equivalent Na2Oeq) of the cement is 1.0%, the adding of 30% fly ash can significantly inhibit the expansion in low-reactivity aggregates. For moderately reactive aggregates, the expansion rate can also be reduced by adding 30% of fly ash. According to a polarizing microscope analysis, the cracks are expansion cracks mainly due to the ACR. The main mechanisms of fly ash inhibiting the ACR are that it refines the pore structure of the cement paste, and that the alkali migration rate in the curing solution to the interior of the concrete microbars is reduced. As the content of fly ash increases, the concentrations of K+ and Na+ and the pH value in the pore solution gradually decrease. This makes the ACR in the rocks slower, such that the cracks are reduced, and the expansion due to the ACR is inhibited.

Suzuki–Miyaura Cross-Couplings under Acidic Conditions

Suzuki–Miyaura reactions with Pd(PPh3)4 have been carried out using lithium N-phenylsydnone-4-carboxylate as additive, which gave best yields at pH 5.7 in a mixture of acetic acid, water, and sodium carbonate. Reaction parameters such as the Pd source, the solvent, reaction time and temperature, acid, base and carboxylate have been varied and some representative examples of the Suzuki–Miyaura reaction have been examined.

Alkali Aggregate reaction in Dam Structures – a Review

Alkali – Aggregate Reaction is an unwanted reaction which occurs in concrete, mainly the area which is subjected to more moisture content. It occurs over time, between cement paste and silica. This in turn alters the expansion of the aggregate and often in an unpredictable way, which will result in loss of strength of concrete and also a complete failure. Hydro structures, mainly Dams store water, presence of moisture can cause such problem, comparatively more than the other structures. Normally, most of the dams had been constructed several years back and they still exist. The design was based on the environmental conditions prevailed at that period. But now the fast changing environmental aspects and industrial growth and technological development apart from global warming has severe impact on the life and performance of the dams. Dam Structures cannot be easily replaced, and the swelling can block spillway gates or turbine operations. The durability of dams has the impact on human life, society and the environment. To enhance the life and durability of the dams or hydro structures, several studies were made. There are two forms of Alkali Aggregate Reactions available, Alkali Silica Reaction and Alkali Carbonate Reaction and this paper reviews Alkali Aggregate reaction within concrete construction.

The Expansion Cracks of Dolomitic Aggregates Cured in TMAH Solution Caused by Alkali–Carbonate Reaction

In this study, concrete microbars and rock prisms made of dolomitic aggregates were cured in a 1-mol/L tetramethylammonium hydroxide (TMAH) solution at 80 °C to avoid the effect of alkali–silica reaction (ASR) on expansion. The expansion of specimens was only caused by the alkali–carbonate reaction (ACR). The reason that self-made cement was used in this work was to ensure that the Mg2+ contained in the brucite originated only from dolomite. Expansion of concrete microbars and rock prisms was measured, the expansion cracks were systematically observed by orthogonal polarizing microscopy, and the products of ACR were analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results showed that the dolomite crystals in the dolomitic aggregates reacted with the TMAH solution and resulted in ACR, which formed calcite and brucite and led to cracking of the specimens. The source of the expansion was the dolomite crystals of the dolomite enrichment area. Expansion cracks either extended inside the rock or into the cement phase and eventually disappeared. The alkali–carbonate reaction significantly contributed to the expansion of dolomitic aggregates cured in TMAH solution at a later curing age.

Effects of curing condition and particle size of aggregate on the expansion and microstructure of dolomitic aggregates cured in TMAH solution

In this paper, the modified microbars prepared by dolomitic aggregates with three kinds of particle size and self-made cement without K + and Na + were cured in 1 and 2 N tetramethyl ammonium hydroxide (TMAH) solution at 20°C, 60°C and 80°C, respectively. TMAH was used as curing solution to exclude the expansion contribution of alkali–silica reaction. Effects of the concentration of TMAH solution, curing temperature and aggregate grain size on the expansion of dolomitic aggregates were systematically investigated to determine the expansion characteristics only caused by alkali–carbonate reaction (ACR). Expansion of modified microbars cured in TMAH solution was measured. The porosities of original and reacted aggregates were also measured. Microstructural studies were carried out by scanning electron microscopy (SEM) and thermo-gravimetric (TG) analysis. The results showed that the aggregate grain size and curing temperature can influence the expansion of modified microbars significantly. When the modified microbars prepared by aggregates with 2.5–5 mm grain size and cured in 1 N TMAH solution at 80°C, the samples exhibited obvious expansion only caused by ACR, which is beneficial to detect the ACR reactivity of dolomitic rocks exclusively in concrete engineering. Based on the pore structure analysis, there was a slight increase (13%) in porosities of aggregates cured for 140 days at 80°C. Rod-like brucite crystals formed in the process of ACR were also found in TG analysis and SEM images.