Evidence for Minimal Pozzolanic Reaction in a Fly Ash Cement During the Period of Major Strength Development
- PDF / 1,996,923 Bytes
- 10 Pages / 420.48 x 639 pts Page_size
- 37 Downloads / 166 Views
EVIDENCE FOR MINIMAL POZZOLANIC REACTION IN A FLY ASH CEMENT DURING THE PERIOD OF MAJOR STRENGTH DEVELOPMENT SIDNEY DIAMOND, QIZHONG SHENG and JAN OLEK Purdue University, School of Civil Engineering, West Lafayette, IN 47907, USA Received 21 December, 1988; refereed ABSTRACT
Strengths developed in fly ash concretes usually equal or exceed that of similar plain concrete after a few months, with much of the response usually attributed to "pozzolanic" reaction between ash and secondary calcium hydroxide (CH). The CH contents of pastes made with five different fly ashes were determined by DTA for periods up to six months. The CH contents found did not decrease notably over the period, and were substantially identical to that expected for plain cement pastes diluted with the same amount of inert material as the amount of fly ash used. Scanning electron microscope examination of the pastes showed only minimal evidence of reaction even up to 1 year of age, although many fly ash grains were in intimate contact with CH. Non-evaporable water contents of the fly ash pastes were substantially higher than expected at each age, suggesting that the fly ash promoted more complete cement hydration or that the hydration products formed bound substantially greater amounts of water than plain cement paste ordinarily does. INTRODUCTION
The use of fly ash as a partial replacement for cement in concrete has become routine practice in recent years; indeed, in some areas it is now difficult to obtain concrete without fly ash. In part this wide acceptance has been based on demonstrated performance of fly ash concrete in terms of strength gain. Despite substantially reduced cement contents, fly ash bearing concretes commonly reach the strengths of reference concretes made with the same materials after several weeks, and by a few months are often substantially stronger than similar plain concretes. Incidental illustrations of this general tendency were provided in- the present symposium by Patel and Pratt [1] and by Day [2]; a striking illustration was provided by Dhir [3] as his Figure 7.21 The extra strength developed by fly ash concretes during the period starting after the first month or so is almost universally attributed to pozzolanic reaction, i.e. reaction between the fly ash and calcium hydroxide (CH), which is assumed to generate extra C-S-H gel hydration product. In an attempt to assess whether pozzolanic reaction does in fact explain the augmented strength gain of fly ash concretes during this period, we examined cement and cement-fly ash pastes periodically over a six-month period of hydration. Specifically, we determined the residual content of calcium hydroxide (CH) and the non-evaporable water content at intervals during this period. The pastes were additionally examined by scanning electron microscopy, but only after 1 year of hydration.
EXPERIMENTAL PROCEDURES Pastes Studied
Portland and Portland-fly ash cement pastes were prepared by standard mixing methods (ASTM C-305), placed in plastic ointment jars and sealed for ro
Data Loading...
