Hydration of cement and pore structure of concrete cured in tropical environment

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Abstract

This paper presents the results of an experimental study on the effect of curing on the degree of cement hydration and capillary porosity of concrete in a tropical environment. It provides basic information for the estimation of w/c of hardened concrete using petrographic methods. Concrete with w/c ratios of 0.30 to 0.70 with an increment of 0.05 was investigated. The concrete was cured at 20 and 35 °C and exposed to various durations of moist curing. The results indicated that the concrete cured at 20 °C water for 28 days had a higher degree of cement hydration and lower capillary porosity than did the concrete cured in water of the same temperature for 7 days followed by exposure to outdoor air for 21 days, but had an opposite trend compared to the concrete cured in 35 °C water for 7 days followed by exposure to outdoor air. However, the differences on the degree of cement hydration and capillary porosity for the concrete cured in these different conditions were not significant.

Introduction

Petrography has been used for research and forensic investigation of concrete in Europe and North America for many years. Most of the information available is based on cement pastes and concretes cured at a standard temperature of ∼20 °C. However, many South East Asian countries are located near the equator, and the temperature and humidity are relatively high. For example, the temperature in Singapore ranges from ∼23 °C at night to ∼33 °C during the day, and the relative humidity (RH) ranges from ∼65% to 95%, with a daily average of 84% throughout the year. As the weather conditions of the South East Asian countries are quite different from those from Europe and North America, study is needed to assess if the petrographic methods can be used to determine the water-to-cement ratio (w/c) and cement content of concrete cured in tropical environment, and if any adjustment is needed.

In the petrographic methods developed [1], [2], [3], [4], [5], [6], the estimations of w/c using fluorescent thin section and cement content of hardened concrete are essentially based on the capillary porosity, degree of cement hydration, quantity of unhydrated cement particles, density and volume of cement paste, or a combination of the above. Since the capillary porosity, degree of cement hydration, and quantity of unhydrated cement clinker are affected by the curing conditions substantially, the effect of curing on these parameters must be known to estimate the w/c and cement content accurately.

This paper presents the results of an experimental study on the effect of curing on the degree of cement hydration and capillary porosity of concrete. Concrete with w/c ratios of 0.30 to 0.70 with an increment of 0.05 was investigated. The concrete was cured at 20 and 35 °C and exposed to various durations of moist curing. Compressive strength, degree of cement hydration, capillary porosity, and pore size distribution were determined. The results will be used for the estimation of w/c and cement content of hardened concrete by petrographic methods, which are reported elsewhere [7].

Section snippets

Materials

Ordinary Portland cement (ASTM Type I), natural sand, and crushed granite with maximum size of 20 mm were used for all concrete mixes. Chemical admixtures were also used, and the information is given together with the concrete mix proportions (Table 1).

Concrete mix proportions and curing conditions

Concrete with w/c ratios of 0.30 to 0.70 with an increment of 0.05 was prepared and cured at 35 °C and 20 °C, respectively. Cubes of 100 mm and prisms of 100×100×500 mm were cast from each concrete mix. The mix proportion and properties of the

Degree of cement hydration and compressive strength of concrete

The compressive strength of the concrete and the corresponding degree of cement hydration are shown in Table 2, Table 3. As expected, the degree of cement hydration increased with time and temperature. The results also show that after water curing at 20 °C for 28 days, the degree of cement hydration increased somewhat if the concrete was continuously cured in water for another 28 days, but did not change significantly if the concrete was left in air outside the lab for the same period.

The

Conclusions

Based on the limited results available, the following conclusions may be drawn:

  • (1)

    The degree of cement hydration increased with time and temperature, but the increase beyond 28 days was not significant.

  • (2)

    The concrete cured at 20 °C water for 28 days had a higher degree of cement hydration and lower capillary porosity than the concrete cured in water of the same temperature for 7 days followed by exposure to outdoor air for 21 days, but had opposite trend compared to the concrete cured in 35 °C water

Acknowledgements

The authors would like to thank Lawrence R. Roberts, Derek R. Brown, Ara A. Jeknavorian, Souri S. Lee, Steven Loh, and Jiabiao Jiang of W.R. Grace and Dr. Ulla H. Jakobsen of Concrete Experts International for their valuable comments of the paper.

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