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BIG ENCYCLOPEDIA FOR CHILDREN AND ADULTS
Why does cement harden? Detailed answer
Directory / Big encyclopedia. Questions for quiz and self-education Did you know? Why does cement harden? Cement is one of the most common materials in modern construction. By itself, it is a fine powder. But if it is mixed with water and allowed to harden, it, together with sand and gravel, turns into a solid, durable substance. Cement is the main constituent of mortar and concrete. Mortar is a mixture of cement, sand and water. Concrete is the same mixture, but with the addition of gravel or crushed stone. Modern cement is made by heating limestone and clay or slag to very high temperatures. This mixture is heated until large, sintered pieces are formed. They are called clinkers. Clinkers are then ground into powder. When water is added to cement powder, complex chemical reactions occur. As a result, a resistant artificial stone is formed, insoluble in water. What are these chemical reactions? What happens in order for cement to harden? Chemists do not have an exact answer to this question. The composition of cement includes four components. It is believed that each of these components, when water is added, turns into crystals. These crystals stick together and the cement hardens. The type of cement that hardens under water is called hydraulic cement. Surprisingly, the Romans discovered the process of obtaining hydraulic cement in the III-II century BC. e. They made such cement by mixing volcanic ash with lime. This discovery was one of the outstanding achievements of the Romans. Author: Likum A.
Why do some astronomers assume that the Sun is a double star? Research by paleontologists has established that over the past 250 million years, catastrophic climate changes have repeatedly repeated on our planet, leading to the extinction of large groups of living organisms. At the same time, these catastrophes occurred periodically with an interval of approximately 26 million years. The last such event occurred about 13,5 million years ago, and the era of dinosaur extinction clearly coincides with one of the peaks (65 million years ago), and the most powerful. The geological deposits belonging to this era are remarkable in that they are highly enriched in iridium: its content is a thousand times higher than normal. It has been suggested that iridium came to Earth as a result of the fall of an asteroid with a diameter of several kilometers. The power of the explosion that took place during the fall is estimated at 10 million megatons of TNT equivalent. This explosion was supposed to be accompanied by a strong dusting of the atmosphere, a decrease in the average temperature by several tens of degrees, hurricane-force winds and everything else that is predicted in well-known forecasts of the consequences of a global nuclear war. However, a collision with an asteroid is a random event, where does the periodicity come from? To explain the periodicity of such catastrophes, American scientists Davis, Hut and Muller in 1984 suggested that the Sun has a companion star that revolves around it with a period of about 26 million years. In the era of maximum approach to the Sun, this star, which received the sonorous name of Nemesis (the name of the ancient Greek goddess of retribution), invades the Oort cloud, bringing it, according to I. S. Shklovsky, "into a state of wild rage." Thousands of comets, which until then quietly moved along their near-solar orbits, under the influence of Nemesis rush towards the Sun. A number of cometary nuclei (several kilometers in size, differing from asteroids mainly in the presence of a large amount of ice) fall to Earth, causing the global catastrophes mentioned above. One of the authors of this hypothesis, Richard Muller, even published a book in 1988 called "Nemesis", the first chapter of which is called "Space Terrorist". Most likely, Nemesis is a red dwarf with a magnitude of 7 to 12. Almost all such stars are cataloged, but the distances to most of them have not yet been measured. It is likely that Nemesis, if it exists, can be seen with binoculars or a small telescope. The task of searching for Nemesis is to determine the coordinates of about 3 thousand candidate stars at intervals of a year and to identify among them a star with an anomalously large proper motion. This is a difficult matter, but not hopeless, and, according to the same I. S. Shklovsky, if successful, it will lead to one of the greatest discoveries in the history of science. However, Nemesis may turn out to be a black hole, but this is much less likely.
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