Calcareous Aggregates Research Articles

Transient heating effect on high strength concrete

This study shows some differences in the properties and the behaviour at high temperature of two concretes (ordinary and high strength) made with the same calcareous aggregates. During heating tests at 1 °C min −1, cylindrical samples of diameter 160 × 320 mm of high strength concrete, more dense, may explode in a critical temperature zone between 250 and 300 °C. Differences in behaviour between OC and HSC appeared at high temperatures: there were disparities especially in thermo-hydric transfer, porosity and thermal stability. The dense microstructure of high strength concrete was found to slow up the escape of vaporized water.

Proposal for limitation and control of fines in calcareous sands based upon their influence in some concrete properties

The development and final conclusions of a research programme which has gone on for four years are explained here. The products of Basque Country quarries have been analysed, studying also the influence of calcareous and clay fines in mortars, as well as in crushed calcareous aggregate concretes, this being the objective of the present article. 42 different concrete mixes in duplicate, with the same workability, have been made, taking the contents of fines and clay in sand and the cement content as variables. This process has involved handling a total of 14 tonnes of aggregate and the manufacturing of more than 900 standardized cylinders. It is concluded that up to 15% of fines in calcareous and ground sand are admissible, provided that the proportion of clay fines is controlled by means of a sand equivalent sight test and, even better, by the methylene blue test method. This research work has been the basis for the new text of the corresponding paragraphs in the recent Spanish Concrete Standard EH-88, but could be, a reference for others with a similar type of aggregate.

Alkali-silica reaction (ASR) in carbonate rocks

The employment of non-dolomitic calcareous aggregates in the composition of concrete may serve to cause degradation if the mother rock contains sufficiently disordered silicates in the form of finely dispersed microinclusions. This paper reports the observation of fissures due to the formation of gel as a result of ASR in aggregates with less than 4% of microinclusions and even when this value was less than 1%. It was also found that: i) the formation of cracks is closely dependent on the degree of disorder of the silicate phase; ii) even in extremely compact aggregates, the residual porosity is enough to ensure penetration of the alkaline cement solution so that it reaches and attacks the silicate microinclusions. It also appeared that the gel thus formed exerted a breaking action on the aggregate, whose extent depended on the shape of the pores and the presence of narrowings between intercommunicating pores (even of large volume).

Evolution microstructurale des composites du systeme ciment alumineux-granulat calcaire. II. Influence sur la porosite et les resistances mecaniques

The analysis of the evolution of mechanical strengths of different composite materials based on aluminous cement has allowed the authors to point out two phenomena: 1. 1. a noticeable improvement in strength by the use of calcareous aggregates or fillers due to the formation of a transitional layer rich in hydrated monocarboaluminate which grows and develops at the aggregate (or fillers) - cement matrix interface, 2. 2. a direct connection between mechanical strengths and a porosity of the material, named “critical porosity”, which corresponds, in the present case, to pores with diameter higher than 10,000 angstroms.

Evolution microstructurale des composites du systeme ciment alumineux-granulat calcaire. I. Mode de propagation de la fissure

The transitional layer, formed at the interface between a calcareous aggregate and an aluminous cement paste, changes versus the hydration time and leads to a zone rich in hydrated carboaluminate, which is formed by epitaxic growth on calcium carbonate. This phenomenon is responsible for a noticeable change in the crack propagation process in the solid. In the system calcium aluminate - siliceous aggregates, the process is always intergranular. In the case of aluminate - calcareous aggregate it is intergranular at the beginning of hydration and becomes transgranular after some months. Then the crack propagation concerns the carbonate grain itself but neither the surrounding zone nor the interface aggregate - transitional zone.

Calcium hydrocarboaluminate, carbonate, alumina gel and hydrated aluminates solubility diagram calculated in equilibrium with CO 2g and with Na aq+ ions

The solubility in lime water of calcium hydrocarboaluminate and carbonate is studied to account for the formation of calcium hydrocarbonate from calcareous aggregates in aluminous and Portland cements. We present a thermodynamic study of the solubility curves of these compounds, calculated as the projections of tridimensional equilibrium curves vs. P(CO 2). We show that simultaneously the projections of the alumina gel and calcium hydrated aluminates solubility curves undergo shifts that we determine vs. P(CO 2) and possibly vs. Na aq + ions concentration.

Sedimentation of the northwestern Gulf of Mexico

Six years of intensive study of the shallow water sediments of the northwestern Gulf of Mexico have indicated that there are many criteria by which ancient sediments deposited under similar conditions can be recognized. Thus the presence of echinoid fragments and of glauconite-filled Foraminifera tests favors marine shelf deposition over bay deposits; calcareous aggregates and grains of gypsum suggest high salinity bay deposits; abundance of wood fibers, high mica content, ferruginous aggregates, and well laminated sediments all are suggestive of deposition near river mouths; greater roundness distinguishes dunes from adjacent beach sands; and coarser grain size distinguishes beach sands from shallow shelf sands in the vicinity.