P.K. Mehta

MECHANISM OF EXPANSION ASSOCIATED WITH ETTRINGITE FORMATION

Abstract Ettringite formation in portland cement concretes can be responsible for both deleterious and beneficial phenomena. Several hypotheses on the mechanism of expansion associated with ettringite formation are reviewed, and a new hypothesis is proposed. Experimental evidence is presented in support of the new hypothesis. It is shown that in the presence of lime the nature of ettringite formed is colloidal, and not long lath-like crystals. It is proposed that colloidal ettringite is able to attract a large number of water molecules which cause interparticle repulsion, thus causing an overall expansion of the system.

Mechanism of sulfate attack on portland cement concrete — Another look

Abstract Sulfate-generated deteriorations in normal portlant cement concretes include expansion, cracking, loss of strength and stiffness, and sometimes disintegration. The chemical phenomenon of ettringite formation as a result of reaction between sulfate water and hydration products of portland cement does not adequately explain all the physical manifestations of the sulfate attack. Furthermore, ettringite which causes expansions in some cases is apparently responsible for high strength in other cases. The published literature does not contain satisfactory explanations for this anomalous behavior of ettringite. In this paper, the author has attempted to provide answers to some of the questions.

Properties of Portland cement concrete containing fly ash and condensed silica-fume

Abstract Normal pozzolan additives, due to their low surface area and reactivity are not able to improve early strengths and durability of concrete. The problem can be solved by using a mixture of normal and highly reactive pozzolans, such as condensed silica-fume. Results of an investigation are reported here in which 30 percent portland cement in concrete was replaced by an equal volume of fly ash, condensed silica-fume, or a 50:50 mixture of the two. Sand-to-gravel proportions were adjusted to obtain workable concretes having the same water-cement ratio. As compared to the control concrete, the 7 and 28 days compressive strengths of the fly ash concretes were significantly lower, however, in the case of mixed-pozzolan addition, the 7-days strength was similar and the 28-days strength was higher. The differences in the pozzolanic activity of the additivies were confirmed by a parallel investigation involving determination of free lime and pore-size distribution of the cement pastes.

Studies on blended Portland cements containing Santorin earth

Abstract Although pozzolans, such as the Santorin earth, have been in use for over two thousand years for making cementitious products, the mechanism by which pozzolanic reactions contribute to the strength and chemical durability of mortars and concretes is not fully understood. Reported here are the results of an investigation in which portland pozzolan cements containing 10, 20, or 30 weight percent Santorin earth were used. Performance of the cements was evaluated with respect to strength development, drying shrinkage, sulfate resistance, and alkali-silica activity. The cement containing 20 percent pozzolan showed the highest compressive strength at 1 year, and the cements containing 20 or 30 percent pozzolan showed the least permeability and best resistance to sulfate attack. Microstructural investigations involving scanning electron microscopy, X-ray diffraction analysis, determination of free Ca(OH) 2 present, and pore-size distribution were conducted on hydrated cement pastes for the purpose of understanding the factors responsible for the observed behavior of the cements. From the results, it is concluded that the process of pore refinement associated with pozzolanic reactions plays an important part in enhancing the strength and chemical durability of portland pozzolan cements. It is suggested that the rate at which pore refinement occurs in a hydrating pozzolan cements is not only useful as a measure of the activity of the pozzolan presen, but also for a more reliable prediction of the performance characteristics of the cement.

INFLUENCE OF FLY ASH CHARACTERISTICS ON THE STRENGTH OF PORTLAND-FLY ASH MIXTURES

Abstract Modern thermal power plants are producing large amounts of fly ash that is generally quite suitable for use as a supplementary cementitious material in concrete. However, for this purpose the fly ash utilization in the United States continues to remain low, mainly on account of lack of quality control. This is because the current standards on fly ash do not contain specifications and test methods that are able to assess adequately the performance of a fly ash in concrete. Based on tests on 11 different fly ashes and direct determination of compressive strength of test mortars made with a fixed proportion of fly ash by weight of the cementitious materials, and a fixed ratio between water and the cementitious material, it seems that the calcium content and particle size distribution of the fly ash are the most important parameters governing the strength development rate in normally cured portland cement-fly ash mixtures.

Scanning electron micrographic studies of ettringite formation

Abstract The morphology of ettringite, formed by various reactions (C3A + gypsum, C3A + anhydrite, CA + magnesium sulfate, in the presence and absence of calcium hydroxide) was studied by scanning electron microscopy. Methods included immersion of solid samples into saturated solutions of the other reactant, and paste hydration. It is concluded that slender needles and spherulites are formed only if sufficient space is available; alternately (e.g. in pastes of lower water-to-solid ratios) ettringite occurs as short, prismatic crystals. Ettringite is formed in C3A-gypsum pastes near the surface of C3A grains by a through-solution mechanism; a topochemical mechanism can be definitely excluded. Results are significant for a better understanding of mechanism of expansion and set retardation by gypsum.

Properties of fresh concrete containing large amounts of fly ash

Abstract The effect of replacing 35 to 50 percent of cement by fly ash on workability, water requirement, bleeding, and setting time of lean concrete mixtures was investigated, using two ASTM Class F and two ASTM Class C fly ashes. Workability of all concrete mixtures containing fly ash was found to be better than that of the control mixtures (without fly ash). The water requirement for obtaining the designated slump (2 in., 5cm) of all concrete mixtures containing fly ash was reduced by 5 to 10 percent. The rate and volume of the bleeding water was either higher or about the same compared with the control mixture, depending on the type of fly ash and the mix proportions. Setting time was delayed for both fly ash types and at all levels of fly ash substitution compared with the control mixture; initial setting time was delayed from 20 min up to 4 hrs and 20 min, and the final setting time from 1 hour up to 5 hrs and 15 min, depending on the type and the amount of fly ash used.

Studies on chemical resistance of low water/cement ratio concretes

Abstract Solutions containing mineral acids, and certain organic acids and salts are highly corrosive to portland cement concrete. Since permeability is the key factor governing the rate of deterioration, it is customary to use a low water-cement ratio in making concretes or concrete overlays required to resist corrosive action of aggressive chemical solutions. Pozzolanic admixtures are often used to provide additional protection against acidic attack. Highly reactive pozzolanic admixtures, such as condensed silica fume, which rapidly react with calcium hydroxide and reduce both the alkalinity and permeability of concrete are now being used for improving durability. During the last two decades, latex admixtures have also found widespread application. The polymeric constituents of a latex seem to coat the alkaline hydration products of portland cement, thus protecting them from attack by aggressive solutions. An experimental study was undertaken to evaluate the relative chemical resistance of low water-cement ratio concretes, containing either a styrene-butadiene latex or a silica fume admixture, to the following solutions; 1% HCl, 1% H 2 SO 4 , 1% lactic acid, 5% acetic acid, 5% ammonium sulfate, and 5% sodium sulfate. Time taken to register 25 percent weight loss by fully submerged concrete specimens was used as a criterion for failure. From the data it appears that, except for the ammonium sulfate solution, the concrete containing the silica fume generally showed better resistance to chemical attack than other concrete types.

Interaction between carbonate rock and cement paste

Abstract A reaction in the transition zone between the portland cement paste and the carbonate rocks is generally attributed to the formation of calcium carboaluminate hydrates. This investigation shows that a reaction product is formed even when an alite cement containing no aluminates is used. From the study of the transition zone between cement paste and carbonate rock using X-ray analysis and scanning electron microscopy, it is concluded that a hydrated calcium carbonate-calcium hydroxide compound is formed at the interface. The substitution of the large and highly oriented crystals of calcium hydroxide by the compound having smaller crystals appears to be responsible for the strenghening of the transition zone.

Properties of alite cements

Abstract For carrying out a comprehensive investigation on physical and mechanical properties of alite mortars and concretes, large quantities of monoclinic alite were produced at the University of California at Berkeley. Laboratory-size specimens were employed to determine strength, drying shrinkage, and sulfate resisting characteristics of mortars and concretes made with alite cements. Small amounts of gypsum (3%) addition accelerated the setting and hardening of the alite cements, however, large amounts (6%) resulted in strength deterioration. Drying shrinkage of alite concretes was significantly lower than portland cement concretes of the same fineness. Long term sulfate immersion of concrete specimens made with an alite cement caused serious spalling and strength loss.