Detailed description:

Principles of Xi'an Experimental Instruments

The properties of cement include fineness, water consumption for standard consistency, setting time, stability, and strength of cement binder.

The thickness of cement particles has a significant impact on the properties of cement. The particle size of cement particles is generally in the range of 7-200 μ m. The finer the particles, the faster and more complete the hydration, and the higher the early and later strength. According to national standards, the fineness of cement can be tested using sieve analysis and specific surface area method. The fineness test uses a test sieve with a mesh diameter of 80 μ m.

The setting time is divided into initial setting and final setting. The initial setting is the time required for cement to be mixed with water until the standard consistency slurry begins to lose its plasticity; The final setting is the time required for cement to be mixed with water until the standard consistency slurry completely loses its plasticity and begins to produce strength. According to national standards, the setting time of cement is determined by measuring the setting time of cement paste with a standard viscosity using a cement paste setting time tester under specified temperature and humidity conditions. The standard for Portland cement stipulates that the initial setting time shall not be earlier than 45 minutes and the final setting time shall not be later than 6.5 hours

Poor volume stability refers to the phenomenon of uneven volume changes occurring after cement has hardened. Poor volume stability is generally due to excessive free calcium oxide content in the clinker. Or it may be caused by excessive free magnesium oxide contained in the clinker or excessive gypsum added. According to national standards, the stability of cement is tested by boiling method. The boiling method accelerates the maturation of calcium oxide, so only the poor cement stability caused by free calcium oxide can be checked. However, free magnesium oxide only accelerates its maturation under steam pressure, and the hazards of gypsum can only be detected in room temperature water for a long time, both of which are not convenient for rapid testing. Therefore, national standards stipulate that the content of free magnesium oxide in cement clinker shall not exceed 5.0%, and the content of sulfur trioxide in cement shall not exceed 3.5%, in order to control the volume stability of cement.

The significance and purpose of Xi'an experimental instruments

Cement is in powder form and, when mixed with water, can undergo physical and chemical reactions to transform from a plastic slurry into a hard stone like body, and can bond granular materials into a whole. Therefore, cement is a good mineral cementitious material. Given the widespread use of cement in engineering, mastering the properties of various types of cement is of great significance. This experimental method is applicable to Portland cement, ordinary Portland cement, slag Portland cement, volcanic ash Portland cement, and fly ash Portland cement.

The purpose of this experiment is to enable students to understand the national standards for cement, learn the operation methods for cement quality inspection and the production of strength specimens, and master the practical engineering significance of various technical indicators of cement quality.

Experimental methods and steps

(1) Negative pressure screening method

Before the sieve analysis test, the negative pressure sieve should be placed on the sieve seat, covered with a sieve cover, powered on, the control system checked, and the negative pressure adjusted to the range of 4000-6000Pa.

Weigh 25g of the sample, place it in a clean negative pressure sieve, cover it with a sieve cover, place it on the sieve seat, and start the sieve analyzer to continuously sieve for 2 minutes. During this period, if any sample adheres to the sieve cover, gently tap it to make the sample fall. After sieving, weigh the residue with a balance.

When the working negative pressure is less than 4000Pa, the cement inside the vacuum cleaner should be cleaned to restore the negative pressure to normal.

Experimental results:

The percentage of cement sample residue is calculated according to the following formula:

F - Sieve residue percentage of cement sample,%;

Rs - Quality of cement residue after sieving, g；

W - Quality of cement sample, g。

Calculate the result to 0.1%.

Clean the test sieve.

(2) Standard viscosity water consumption test

The water consumption for standard consistency can be determined by either adjusting the water consumption or keeping the water consumption constant. In case of dispute, the method of adjusting the water consumption shall prevail.

Before the experiment, it is necessary to check that the metal rod of the instrument can slide freely; When the cone is lowered to the top of the mold, the pointer should be aligned with the zero point of the scale; The mixer should operate normally.

Before mixing the cement slurry, wipe the mixing pot and mixing blades with a damp cotton cloth, and pour 500g of cement sample weighed into the mixing pot. When mixing, first place the mixing pot on the mixing pot seat, raise it to the mixing position, start the machine, and slowly add mixing water. Slowly mix for 120 seconds, stop mixing for 15 seconds, then quickly mix for 120 seconds before stopping.

When using the method of adjusting the water volume, the mixing water volume should be based on experience. When using the method of constant water volume, the mixing water volume should be 142.5ml, with the water volume accurate to 0.5ml. After mixing, immediately put the mixed slurry into the cone mold, use a small knife to compact it, vibrate it several times, scrape off excess slurry, smooth it out, and quickly place it in a fixed position under the test cone. Lower the test cone to the surface of the slurry and tighten the screw, then suddenly relax it, allowing the test cone to sink freely into the slurry. When the test cone stops sinking, record the sinking depth of the test cone. The entire operation should be completed within 1.5 minutes after mixing.

When using the method of adjusting water volume for measurement, the standard consistency of the slurry is the slurry when the sinking depth of the test cone is 28 ± 2mm. The mixing water amount is the standard consistency water amount (P) of the cement, expressed as a percentage of the cement mass. If the sinking depth exceeds the range, another sample must be weighed, the water volume adjusted, and the test repeated until it reaches 28 ± 2mm.

When measuring with the constant water volume method, according to the measured sinking depth S (mm) of the test cone, follow the formula (or the corresponding scale on the instrument) below

Calculate the water consumption P (%) for standard consistency.

P=33.4－0.185S

Note: When the sinking depth of the test cone is less than 13mm, the method of adjusting the water volume should be used for measurement.

(3) Determination of Condensation Time

Before measurement, place the circular mold on the glass plate, apply a slight layer of engine oil on the inside, adjust the setting time tester so that when the test needle contacts the glass plate, align the pointer with the zero point of the ruler.

Weigh 500g of cement sample and prepare a standard consistency slurry using the method of measuring standard consistency with water. Immediately load the slurry into a circular mold, shake it several times, scrape it flat, and then place it in a wet curing box. Record the time when water is added as the starting point for the setting time.

Determination of setting time: The test piece is cured in a wet curing box for 30 minutes after adding water. When measuring, take out the round mold from the maintenance box and place it under the test needle, so that the test needle contacts the clean slurry surface. Tighten the screw for 1-2 seconds and then suddenly relax. The test needle sinks vertically and freely into the clean slurry, and observe the pointer reading when the test needle stops sinking. When the test needle sinks to a distance of 2-3mm from the bottom plate, the cement reaches its initial setting state; When the sinking does not exceed 1-0.5mm, the cement reaches the final setting state. The time from adding water to the initial setting and final setting states of the cement are respectively the initial setting time and final setting time, expressed in hours (h) and minutes (min). When measuring, attention should be paid to gently supporting the metal rod during the initial measurement operation to prevent the test needle from hitting and bending. However, the result should be based on free fall, and the position where the test needle penetrates should be at least 10mm away from the inner wall of the circular mold during the entire testing process. When approaching the initial setting, it should be measured every 5 minutes, and when approaching the final setting, it should be measured every 15 minutes. When reaching the initial or final setting state, it should be repeated immediately. Only when the two results are the same can it be determined that it has reached the initial or final setting state. During each measurement, the test needle must not fall into the original needle hole. After each test, the test needle must be wiped clean and the mold must be placed back in the curing box. Throughout the entire measurement process, the mold must be protected from vibration.