Natural pozzolans

In the study of the lime-natural pozzolan systems, lime-rich and lime-poor calcium silicate hydrates have been differentiated by The endothermal peak at 90°C was attributed to the lime-rich C-S-H phase, and that at 125°C, to the lime-poor C-S-H phase. An exothermic peak that appears in this system at about 850°C is caused by the crystallization of wollastonite. 

Conduction calorimetric investigations have been earried out on Portland cement in whieh natural pozzolans have been incorporated, It has been found that in the pozzolan-containing pastes, the dormant period is deereased and the normalized height of the seeond main peak is greater than that given by the reference cement. This indieates that the pozzolan aecelerates the hydration of alite. It is possible that the aceeleration effect is due to a eombination of faetors, viz., higher specifie surface area of the pozzolan, its chemical composition, the physical state of the surface, and the rate of release of the alkalis. 

The conduction calorimetric curve of cement with 45% diatomite shows that the pozzolan modifies considerably the heat generated in the samples (Fig. Not only does the main exothermic peak appear 

Thermal analytical techniques have been applied to investigate the causes leading to the deterioration of concrete subjected to various environmental factors. A mortar nearly two thousand years old obtained from El Tajan near Mexico City was analyzed by TG and electron microscopy. TG analysis was done on samples taken from different areas and depths. The loss of water below 100°Cwas caused by the adsorbed water from the volcanic tuff, while the endothermal effect at 700°C corresponded to the carbonated lime and carbonated silicates and aluminates derived from the pozzolan. The extent of the reaction of lime with pozzolan was computed to be 6.91%. 

The role of individual supplementary cementing materials and additives has been described in the previous sections. The effect of these materials, viz., silica fume, slags, rice husk ash, fly ash, metakaolinite, or natural pozzolans on the properties of concrete such as workability, setting, rate of hydration, strength development, heat, durability, etc., is known to 

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Naik, T., Ramme, B.W. and Tews, J.H. (1992). Pavement construction with high-volume fly. Fourth International Conference on The Use of Fly Ash Silica Fume, Slag and Natural Pozzolans in Concrete, Istanbul, Turkey, May 3-8, 65-80. 

Malhotra, V. M. 1986. Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete. American Concrete Institute Special Publication, SP-91. Detroit MI (Zvoli), Library of Congress 85-73563. 

ASTM 1994. ASTM C618 Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete. ASTM International. World Wide Web Address http// www.astm.org. 

Trass, or tras is a natural pozzolan occurring in Germany. 

ASTM (1994) Standard specification for fly ash and raw or calcined natural pozzolan for use as mineral admixture in Portland cement concrete. American Society for Testing and Materials, Annual Book of ASTM Standards, C618-92a, vol 04.02, West Conshohocken, PA, p 194... 

Nakamoto J, Togawa K,Miyagawa T, Fuji M (1998) Water permeability of high slag content concrete. In Malhotra VM (ed) Fly ash, silica fume, slag and natural pozzolans in concrete. American Concrete Institute, Bangkok, p 779... 

G. Fajardo, P. Valdez.J. Pacheco, Corrosion of steel rebar embedded in natural pozzolan based mortars exposed to chlorides, Constr. Build. Mater. 23 (2009) 768—774. 

However, apart from the cement binder (ASTM C 150 2012 or AASHTO M 85 2012), the hydraulic binders may be blended hydraulic cement (ASTM C 595 2013 or AASHTO M 240 2013), coal fly ash and raw or calcined natural pozzolan (ASTM C 618 2012 or AASHTO M 295 2011), ground granulated blast-furnace slag (ASTM C 989 2013 or AASHTO M 302 2013), silica fume (AASHTO M 307 2013) and lime (ASTM C 977 2010 or AASHTO M 216 2013). 

Natural pozzolans are usually volcanic in origin and the most widely used artificial pozzolan is the pulverized flyash (PFA) obtained from coal-fired power stations. Their composition varies widely but they are all silico-aluminate based. 

It is uncertain where it was first discovered that a combination of hydrated non-hydraulic lime and a pozzolan produces a hydraulic mixture (see also Pozzolanic reaction), but concrete made Irom such mixtures was first used by the Ancient Macedonians and three centuries later on a large scale by Roman engineers. They used both natural pozzolans (trass or pumice) and artificial pozzolans (groimd brick or pottery) in these concretes. Many excellent examples of structures made from these concretes are still standing, notably the huge... 

In the test, the mix proportions of different mixes (the ratio of water binder sand aggregate ) were almost the same with different dosages of fly ash, slag and natural pozzolan. From Fig.5, we can deduce some interesting observations ... 

Rodriguez-Camacho, Redz E., Uribe-Afif, R., Importance of using the natural pozzolans... 

ASTM C311-07 Standard Test Methods for Sampling and Testing Ply Ash or Natural Pozzolans for Use in Portland-Cement Concrete. 

C. Hubert, W. Wicker and D. Heidemann, Am. Conor. Inst., SP, 2001, SP-199(Seventh CAN-MET/ACl International Conference on Fly Ash, Silica Fnme, Slag, and Natural Pozzolans in Concrete, 2001, Volume 1), 83. 

ASTM C618, 2012. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. 

Mineral admixtures are finely divided materials that are added in relatively large amounts as replacement of cement and/or of fine aggregates in concrete. Some of them also possess self-cementitious properties in addition to being pozzolanic. A pozzolanic material is one that is able to react with calciinn hydroxide forming a cementitious material. Natural pozzolans such as volcanic tuffs, earths, trass, clays, and shales (raw or calcined), and industrial products such as fly ash, slag, silica fume, red mud, and rice husks are the main source of mineral admixtures. Extensive work has been carried out on fly ash, slag, and silica fume. 

Figure 28. Conduction calorimetric curves of tricalcium silicate treated with natural pozzolans.

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