1. What Cement Is and How It Differs from Concrete
Cement is a gray powder made primarily from limestone. Concrete is a different material, created by mixing cement with water, sand, and gravel. [1]
The production process has three main steps. First, limestone is mixed with clay and crushed into fine particles (raw material process). Next, it is burned in a large rotating kiln (burning process), creating hard lumps called "clinker." Finally, gypsum is added to the clinker, and it is ground into powder (finishing process). [1]
The temperature inside the kiln during burning is about 1,450°C. [1]
2. How Many Raw Materials Go into Making 1 Ton of Cement
In the 2024 fiscal year, the raw materials used to make 1 ton of cement totaled 1,554 kilograms. This included 1,198 kg of limestone types, 213 kg of clay types, 71 kg of silica materials, 31 kg of iron materials, and 40 kg of gypsum. The figures in the table exclude moisture. [2]
Since 1 ton of cement weighs 1,000 kg, there is a difference of about 0.5 tons between the input and output. This gap is thought to be because carbon dioxide and other gases escape from the limestone during burning. [2]
Limestone types make up about 77% of the total raw materials (1,198 divided by 1,554). According to the Cement Association, this limestone is 100% domestic in Japan. [1]
3. How Much Waste and By-Products the Cement Industry Uses Now Compared with 1990
In the 2024 fiscal year, the cement industry used 478 kilograms of waste and by-products per ton of cement produced. This was calculated by dividing the total usage of 21,867 thousand tons by the production volume of 45,748 thousand tons. In the 1990 fiscal year, this figure was 251 kilograms, meaning it has increased about 1.9 times over approximately 34 years. [2]
The most common materials are, in descending order of quantity: coal ash (6,188 thousand tons), blast furnace slag (5,110 thousand tons), sludge (2,627 thousand tons), by-product gypsum (1,740 thousand tons), and incineration ash and dust (1,324 thousand tons). These are not just household garbage but mostly materials generated by industries. [2]
The sources are clear: coal ash and by-product gypsum come from thermal power plants; blast furnace slag from steel mills; sludge from sewage treatment plants and chemical factories; incineration ash from waste incinerators; construction soil and wood chips from construction sites; and waste tires from auto repair shops (based on 2017 data). [3]
These materials are used in two ways: as a substitute for raw materials or as a substitute for thermal energy. [3]
4. Three Reasons Cement Kilns Can Accept Waste
Cement factories can use waste for three main reasons. First is composition: they select materials that can substitute for cement raw materials after checking their chemical makeup. Second is heat: waste can serve as a thermal energy source. Third is temperature: high-heat burning decomposes organic matter and odor-causing substances. [4]
Furthermore, waste often contains many of the main components of cement (calcium oxide, silicon dioxide, aluminum oxide, and iron oxide). Because these are incorporated into the clinker, no new waste is created. [2]
The amount of waste that ends up in final disposal during manufacturing is 5.1 grams per ton of cement (2024). Since 1 ton is 1 million grams, this is a ratio of about 0.0005%. It is not that ash is zero, but that the amount is extremely small. [2]
5. How Cement Factories Extend Landfill Life and Handle Disaster Debris
In the 2023 fiscal year, the cement industry accepted 22.58 million tons of waste and by-products. This accounts for about 11% of the total waste and by-products recycled within Japan. [5]
The Cement Association estimates that this extends the life of domestic final disposal sites (landfills) by about 11 years. They compare this volume to about 14 Tokyo Domes. Note that this is the Association’s estimate, and the value changes if the acceptance volume changes by fiscal year. [5]
It is also indicated that about 1 million tons of disaster waste from the Great East Japan Earthquake was processed. [5]
6. The Origin of Portland Cement: the 1824 Patent and Its Arrival in Japan
The Portland cement used today traces its origin to October 21, 1824, when Joseph Aspdin, a bricklayer in Leeds, England, took out a patent (British Patent No. 5022). The name comes from the fact that the color of the hardened material resembled limestone from the Isle of Portland. In Japan, it is still called "Portland" because the technology arrived via Germany. Detailed differences in manufacturing methods vary by source, so they are not listed here. [6]
It is said that imported Portland cement may have first been used in construction in Japan at the Nagasaki Iron Works in 1861 (the first year of the Bunkyū era). At that time, imported cement cost 50 to 60 yen per ton. Pacific Cement estimates that this was equivalent to about 500,000 yen per ton in today’s money, based on the price of rice at the time. This 500,000 yen figure is an estimate based on rice prices and is not definitive. [7]
The modern cement that accepts 478 kg of waste per ton is positioned very differently from the cement of that era. [2]
7. How Japan First Made Portland Cement in 1875 After Failures
The first production of Portland cement in Japan was successfully achieved on May 19, 1875 (Meiji 8), at a government-run factory in Fukagawa Kiyozumi-cho, Tokyo, by Saburo Utsunomiya. It is reported that there were about 1.5 years of repeated failures before a small amount of cement was completed. At the time, it was written with the characters "Setsu Men Toku" (phonetic approximation). [7]
The first private cement company was the "Cement Manufacturing Company," established in 1881 (Meiji 14) by Junhachi Kasai in Yamaguchi Prefecture (now Onoda). Production began in 1883. The location was chosen because coal, limestone, and clay were easily available, and it was convenient for transporting products. [7]
Even so, during Meiji 10–19 (1877–1886), the production volume of the government-run Fukagawa factory covered only about 40% of domestic demand. The amount of cement needed was increasing to build railways, ports, and government buildings. [7]
8. Why Using Waste Does Not Easily Reduce CO2 from Cement Making
The total CO2 emissions from the cement industry were about 33 million tons in the 2023 fiscal year. About 60% of this comes from carbon dioxide released when limestone decomposes due to heat (process-derived), and about 40% comes from energy sources like fuel. Limestone is a substance called CaCO₃, which splits into CaO (calcium oxide) and CO₂ when burned. [5]
Waste contains CaO and other substances that have already completed decomposition. Therefore, replacing some natural limestone with waste can reduce this 60% portion. However, the reduction effect at present is said to be extremely small. [5]
Using waste reduces landfill use. However, the CO2 problem is separate and remains as long as limestone is burned. How to address this challenge is currently under discussion. [5]
9. Looking at Concrete in Your Town and Picturing the Cement Inside
When walking through town, look at the concrete on building walls, roads, and river embankments from a distance. You may see small grains or holes on the surface. The gray part is powder made by burning limestone and other materials, which has hardened after mixing with water. [1]
We have discussed how current cement is about half waste and by-products per ton. However, you cannot tell just by looking when the cement in the wall in front of you was made.
If you are interested in the numbers, you can check the latest usage of waste and by-products on the Cement Association’s website. The values change as new fiscal years are added. [2]
For safety, be careful of cars and bicycles when observing. Do not enter construction sites. Do not touch the concrete; just look. [1]