Cement is one of the important components of concrete, and its quality will directly affect the quality of concrete, and then affect the quality of the whole project. How to correctly detect the quality of cement has become an important task for highway inspection departments.

This paper analyzes the problems and matters needing attention in the testing of physical indexes such as water consumption of cement standard consistency, cement setting time and cement stability.

Keywords: cement; Standard consistency water consumption; Cement coagulation; test

1 cement slurry mixing

The uniformity of cement slurry mixing directly affects the water consumption of standard consistency, setting time and stability determination.

Cement slurry mixing is related to the measurement of cement and water, cement slurry mixer, etc. Therefore, electronic balance, water dispenser, paste mixer and other instruments and equipment should be strictly controlled.

1. 1 water meter

According to the specification, the calibration value of the water meter is 0. 1mL, and the accuracy is 1%.

The reading should be based on the low meniscus.

Some laboratories don't pay enough attention to this regulation, and directly add water with a measuring cylinder. The dividing value of the measuring cylinder is 1mL, which can't meet the requirements of test accuracy at all, resulting in the error of adding water to standard consistency water.

1.2 electronic balance

The electronic balance shall meet the accuracy requirements, with the maximum range of 1000g and the inductance of 1g, and shall be checked regularly.

Call cement again to avoid measurement error.

Some laboratories use weighing method when adding water. They think that the accuracy of electronic balance is very high, and the amount of water added can be accurately controlled, but they ignore the influence of environmental temperature on the density of water. If weighing method is adopted, temperature correction must be carried out.

So as to ensure the accuracy of test data.

1.3 cement slurry mixer

The cement slurry mixer shall meet the requirements of JC/T729.

The working procedure of cement slurry mixer is: start mixer-low speed mixing 120s- stop15s-high speed mixing 120s and stop.

Before stirring the clean pulp, wet the inner wall of the stirring pot and the stirring blades with a wringing wet rag, but it can't contain clear water, and always keep the same humidity during repeated experiments. This is the key to adjust the amount of water added, and the water meter should add water accurately. It is difficult to control the amount of water added if the rag is wet and dry alternately.

First, carefully pour the measured mixing water into the pot from the pot mouth, and then carefully add the weighed 500g cement into the water with a feeder within 5~ 10s, so as to prevent the cement from splashing out or sticking to the inner wall and blades of the pot to form dry ash, which will affect the standard consistency of the paste.

Note that the order of the test is to add water first and then cement.

Scrape the cement on the pot wall and blade into the pot when stopping at 15s. Special reminder that the operation must be fast to prevent the scraper from scraping into the pot, and the mixer has started to stir at high speed, causing accidents.

2 Standard consistency of cement water consumption

The setting time of cement is determined by putting the pure slurry with standard consistency made of standard consistency water consumption into the conical test die.

The determination of water consumption of standard consistency is very important for the test of cement setting time and cement stability.

Different amounts of water have great influence on the setting time of cement. For the same cement, the more water is used, the longer the setting time is, and the less water is used, the shorter the setting time will be.

Therefore, the standard stipulates that the water consumption for setting time determination must meet the requirements of standard consistency water consumption to ensure that the water consumption of the same cement is basically the same.

There are two methods to determine the water consumption of standard consistency: standard method (bar test method) and substitution method (cone test method).

We usually use the test stick method.

Immediately after mixing, put the mixed cement slurry into the test mold, fill it at one time, and screw it in from outside to inside with a wet cement knife to make it dense. Then hold the glass plate together with the test mold, flatten it as much as possible, vibrate it gently for a few times to make the bubbles in the paste change from big to small, and then scrape off the excess cement slurry with a cement knife to make the scraping surface smooth and flush with the top edge of the test mold as much as possible.

After leveling, quickly move to the bottom of the test bar, so that the test bar can freely fall vertically and sink into the cement slurry. After the test bar stops sinking or releases for 30 seconds, record the sinking depth. The whole operation process should be completed within 65438 0.5 minutes after mixing.

It is difficult to grasp the regulation law of water quantity in the operation of bar test method.

Pay attention to the following points when operating:

(1) When measuring the water consumption of standard consistency, add the mixing water at one time, then adjust the water quantity according to the sinking situation of the test bar and weigh the sample according to the experience and stir another pot, and so on until the sinking depth of the test bar is 6mm? 1mm, and be careful not to block the test rod and touch the Vicat instrument when sinking, so as to avoid errors caused by irregular operation.

(2) After the water consumption of standard consistency of cement is determined, the setting time cannot be directly measured by cement slurry molding. A pot of net slurry should be re-mixed according to the water consumption of standard consistency to determine the setting time, so as to avoid errors caused by too long operation time and gaps in the falling position of the standard consistency pointer.

(3) The determination of water consumption of standard consistency requires that the whole test operation be completed within 65438 0.5 minutes after mixing.

If the time exceeds 1.5min, the consistency of cement paste will increase due to hydration and water evaporation, and the standard water consumption will be greatly affected.

Therefore, the correct and skilled operation of testers is the key.

3 Determination of setting time of cement

The setting time of cement is very important to the quality and progress of engineering construction.

If the setting time of cement is too slow, the demoulding time will be prolonged due to the slow development of cement paste or concrete strength, which will seriously affect the project progress; Cement sets too fast, and the mixed cement slurry and concrete lose their fluidity or plasticity before transportation and pouring, which makes the pouring not go smoothly, or even

In the process of pouring and tamping, the paste structure without thixotropy is destroyed, which leads to the decline of concrete performance and affects the quality of the whole project.

Therefore, it is particularly important to measure the setting time of cement.

Control should be carried out from the following aspects:

3. 1 Control temperature and humidity.

The setting time of cement is greatly influenced by ambient temperature and humidity. Only under the specified temperature and humidity conditions can the determination of cement setting time be reproducible and comparable.

GB/T 1346-200 1 stipulates that the laboratory temperature is 20? 2℃, relative humidity greater than 50%, curing box temperature 20? 65438 0℃, relative humidity greater than 90%, the temperature of cement samples, mixing water, instruments and appliances should be consistent with the room temperature of the laboratory.

Because the higher the test temperature in the curing box, the faster the hydration speed of cement and the shorter the setting time; If the humidity is too small, the water evaporation of cement paste will be accelerated and the setting time will be shortened.

3.2 Do a good job of instrument inspection before the test.

The main instrument for measuring solidification time is Vicat instrument.

In order to ensure the metal rod of Vicat instrument to slide freely, during the initial setting time measurement, when the test rod touches the glass plate, set the pointer to zero, and when the test needle touches the glass plate, set Vicat instrument to zero.

When measuring multiple samples at the same time, the thickness of the glass bottom plate of each circular mold should be the same, so as to avoid the inconsistency of thickness affecting the zero point.

Adjustment, resulting in confusion of test data.

Some testers set the pointer to zero when the test strip touches the top edge of the test die, which is wrong, because the sample and the top edge of the test die are not necessarily on the same horizontal plane during the scraping process, resulting in test errors.

3.3 Determination of setting time

When determining the coagulation time, first check whether the test needle is bent or whether the surface is rusty. Bending or rusting test needles will increase the resistance to free fall, resulting in the illusion that the initial setting time is advanced.

When measuring the initial setting time, the metal column should be gently supported at the beginning to make it descend slowly to prevent the test needle from bending, but the result will be subject to free fall.

The impact point of the test needle should be away from the center of the circular die at a distance of 10mm from the inner wall of the circular die, and the impact point should be on the annular belt at a distance of 10 ~ 20 mm from the inner wall of the circular die to avoid over-dense pinholes.

Wipe the test needle with a damp cloth every time.

Pay attention to the first measurement from 30 minutes after the cement is completely added to the water, and test it every 5 minutes when the initial setting is about to happen.

When the test needle sinks to 4? When the lmm is repeated, the initial setting is reached with the same conclusion. The standard requires that when the initial setting time comes, the round die should be turned over in time to determine the final setting time. When turning, you should pay attention to the operation skills. First, put a glass plate on the test mold, turn it over together with the glass plate, and pull out the original glass plate evenly and forcefully along the cement direction, taking care not to damage the test mold body.

Test the final setting with a final setting needle with a circular attachment. Before each measurement, make sure that the vent hole of the annular accessory is not blocked, and there is no cement slurry in the groove between the annular ring and the test needle.

When approaching the final setting, the test shall be conducted every 15 minutes, and the test shall be repeated when the test needle sinks into the test body for 0.5 mm, that is, the ring cannot leave traces on the test body at the beginning, and the final setting will be achieved when the two conclusions are the same.

When testing the initial setting and final setting time, we should pay attention to the sudden relaxation of the test needle, so that the test needle can sink vertically and freely into the cement slurry, and the Vicat instrument can be prevented from shaking.

4 Volume stability of cement

The volume stability of cement is one of the important indexes to evaluate the quality of cement, and it is also a necessary condition to ensure the quality of cement products and concrete.

The cement with poor stability will expand and crack the hardened cement, reduce its strength and even cause serious engineering accidents.

There are two methods to determine the stability of cement: test cake method (alternative method) and Reyes method (standard method). In case of dispute, Reyes clamp method shall prevail.

We usually use the pliers method.

Attention should be paid to the following points in operation:

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There is another case: the knife used for tamping is too wide, which will open the clamp when tamping down, and bring out some cement slurry when pulling up, which is not easy to compact and also affects the accuracy of the test results.

Therefore, the width of the knife should generally not be greater than10 mm.

(2) Boiling, that is, adjusting the water level in the boiling box, and keeping the sample in the whole boiling process.

Remove the glass plate of Raytheon fixture and the test piece. First, measure the distance a from the tip of the needle of Raytheon pliers, accurate to 0.5mm, and put the specimen on the support plate in the boiling box, with the needle facing up and the specimens not crossing each other. And then, 30 minutes later? Heat to boiling within 5 min and keep boiling for 3h? 5 min, don't add test water halfway to avoid high temperature burns.

(3) Results After boiling, take out the specimen and measure the distance c from the tip of Reykjavik's pliers to the nearest 0.5 mm According to the standard, when the average value of the distance (C-A) increased after boiling of two specimens is not more than 5.0mm, the cement stability is considered as qualified. When the difference between the (C-A) values of the two specimens exceeds 4.0 mm, the test shall be repeated immediately with the same cement.

5 concluding remarks

This paper expounds the test links such as cement slurry mixing, water consumption measurement of standard consistency, cement setting time measurement, cement stability measurement, etc., and emphatically discusses the influence of factors such as matters needing attention, test environment, instruments and equipment during the test operation, and puts forward some suggestions on how to improve the cement test ability.

refer to

[1]JTG E30-2005, Test Code for Cement and Cement Concrete in Highway Engineering.

[2]JC/T 727- 1996, Standard Consistency and Setting Time Tester of Cement Physical Testing Instrument.

[3]JC/T 729- 1996, cement slurry mixer for cement physical inspection instrument.

Zhongyi Sun, Wang Jianhua. Testing engineer handbook of highway engineering [M]. Beijing: People Communications Publishing House, 2009.

Physical indexes and test methods of general portland cement II

The physical indexes and testing methods of ordinary portland cement are introduced.

Keywords: solidification time; Stability; Strength; elegant

Ordinary portland cement is divided into portland cement, ordinary portland cement, slag portland cement, pozzolanic portland cement, fly ash portland cement and composite portland cement according to the variety and content of mixed materials.

The strength grade of portland cement is divided into six strength grades: 42.5, 42.5R, 52.5, 52.5R, 62.5R and 62.5r.

The strength grade of ordinary portland cement is divided into four strength grades: 42.5, 42.5R, 52.5r and 52.5 r.

The strength grades of slag portland cement, pozzolanic portland cement, fly ash portland cement and composite portland cement are divided into six strength grades: 32.5, 32.5R, 42.5, 42.5R, 52.5R and 52.5 r.

The technical requirements of ordinary portland cement include chemical indexes and physical indexes.

Chemical indexes include insoluble matter, loss on ignition, sulfur trioxide, magnesium oxide and chloride ion.

Physical indicators include setting time, stability, strength and fineness.

1 setting time

The initial setting time of portland cement is less than 45min, and the final setting time is not more than 390min.

The initial setting time of ordinary portland cement, slag portland cement, pozzolanic portland cement, fly ash portland cement and composite portland cement is not less than 45min, and the final setting time is not more than 600min.

Determination of initial setting time, mixing of cement slurry: using cement? The slurry mixer stirs, first wipe the mixer and stirring blades with a wet cloth, then pour the stirred water into the stirring pot, and then carefully add the weighed 500g cement into the water within 5s- 10s to prevent water and cement. Out; When stirring, put the pot on the pot seat of the mixer, raise it to the stirring position, start the mixer, stir at low speed 120s, and stop 15s. At the same time, scrape the cement slurry on the blade and the wall of the pot to the middle of the pot, then stir at high speed 120s, and stop.

Immediately after mixing, put the mixed cement slurry into the test mold that has been placed on the glass floor, insert it with a knife, vibrate it gently for several times, scrape off the excess cement slurry and smooth it, then quickly move the test mold and floor to the Vicat instrument, place it in the center below the test bar, lower the test bar until it touches the surface of the cement slurry, tighten the screw 1s-2s, and then suddenly loosen it, so that the test bar can freely sink into the cement slurry vertically.

Record the distance between the test bar and the bottom plate when the test bar stops penetrating or is released for 30 seconds, and wipe it immediately after the test bar is lifted; The whole operation should be completed within 65438 0.5 minutes after stirring.

Sink the test bar into clean mud, 6mm away from the bottom plate? 1 mm cement slurry is of standard consistency.

Mixed water is the standard consistency water consumption of cement, calculated as a percentage of cement quality.

Make a paste with standard consistency with standard consistency water consumption, fill the test mold at one time, shake it several times, add water and put it into a moisture curing box for curing until 30min, and make the first measurement. The test needle of Vicat tester touches the surface of cement paste, and suddenly relaxes after tightening the screw 1s-2s, or observe the pointer reading after the test needle stops sinking for 30s.

When it is close to the initial setting, measure it every 5 minutes.

When the test needle sinks to 4mm from the bottom plate? 1mm, the initial setting state of cement is reached, and the initial setting time of cement is the time when all cement is added into water to reach the initial setting state.

Determination of final setting time: the large diameter end is upward, and then put it into a moisturizing curing box to continue curing. When the final setting time is near, it shall be measured every 15 minutes. When the test needle sinks into the test body for 0.5 mm, and the annular attachment can't leave traces on the test body at first, the cement reaches the final setting state.

The time from the complete addition of cement to the final setting is the final setting time. Min? Express delivery.

2 stability

Each sample should form two test pieces, and each lightning test clip should be equipped with two glass plates with a mass of about 75-85. The inner surfaces of all glass plates and lightning test clips in contact with cement slurry should be lightly coated with oil.

Place the clamp on the lightly oiled glass plate, and immediately fill the prepared standard consistency paste with the clamp at one time. When pouring paste, gently hold the clamp with one hand, scrape it with a knife about 10mm wide with the other hand, then smooth it, cover it with a slightly oiled glass plate, and immediately move the specimen to a moisture curing box for 24 hours. Two hours.

Adjust the water level of the boiling tank to ensure that the whole boiling process can exceed the specimen, without adding test water in the middle, and at the same time ensure 30min? Rise to boiling in 5 minutes.

Remove the glass plate and sample. First, measure the distance (a) between the tips of Reynolds fixture, accurate to 0.5mm, and put the sample into the sample holder in the boiling water tank for 30min? Heat to boiling and constant boiling within 5min minutes? 180min？ Five minutes.

After boiling, drain the water and cool it to room temperature.

Measure the distance (c) from the tip of Reynolds clamp pointer to 0.5mm. When the average value of (C-A) is not more than 5.0min, it is considered that the cement stability is qualified. When the difference between two test pieces (C-A) exceeds 4.0 mm, the test shall be conducted again with the same sample immediately.

If so, it is considered that the stability of cement is unqualified.

3 strength

The temperature of the sample forming laboratory should be kept at 20℃? 2℃, the relative temperature should be not less than 50%.

Should the temperature of curing box or fog room for mold curing be kept at 20℃? 65438 0℃, the relative temperature should be not less than 90%.

The water temperature of the specimen curing pool should be 20℃? 1℃.

Cement 450g? 2g, standard sand 1350g? 5g, soil and water conservation 225ml? 1 ml.

Add water to the pot, then add cement, put the pot on the fixed frame of the planetary cement mortar mixer and rise to a fixed position.

Start the machine immediately, stir at low speed for 30s, and then add sand evenly at the beginning of the second 30s.

Stir again at high speed for 30 seconds.

Stop for 90s, and then stir at high speed for 60s.

Mortar is formed immediately after preparation.

Fix the empty test mold and mold sleeve on the tamping table, and load the sand into the test mold in two layers. When installing the first layer, put about 300g of rubber sand in each groove, use a large seeder to stand vertically at the top of the formwork sleeve, level the material layer back and forth along each groove, and then tamp it for 60 times.

Then put the second layer of mortar, smooth it with a small seeder, and tamp it again for 60 times.

Remove the mould sleeve, take the test mould out of the vibration table, stand at one end of the top of the test mould at an angle of about 90℃ with a metal ruler, and then slowly move to the other end along the length direction of the test mould with the action of a transverse saw, scrape off the rubber sand outside the test mould at one time, and grind the surface of the test mould body with the same ruler in a nearly horizontal state.

Put it on the horizontal shelf of fog room or wet box for maintenance.

The age of the specimen is calculated from the time when the cement is added with water to start the test, and the strength tests of different ages are carried out in the following time.

24h？ 15min, 48h? 30 minutes, 72 hours? 45min，7d？ 2h，& gt28d？ The eighth hour

4 fineness

The fineness of portland cement and ordinary portland cement is expressed by specific surface area, and its specific surface area is not less than 300m? 2/kg； The fineness of slag portland cement, pozzolanic portland cement, fly ash portland cement and composite portland cement is expressed by sieve residue, and the sieve residue of 80μ m square hole sieve is not more than 10% or 45μ m square hole sieve residue is not more than 30%. The test screen used before the test should be kept clean, and the negative pressure screen and manual screen should be kept dry.

Negative pressure sieve analysis method: during the test, 25g samples are taken for 80um sieve analysis test, and 10g samples are weighed for 45um sieve analysis test.

Before the sieve analysis test, put the negative pressure sieve on the sieve seat, cover the sieve cover, turn on the power supply, check the control system, and adjust the negative pressure to 4000Pa―6000Pa.

Accurately weigh the sample to 0.0 1g, put it in a clean negative pressure sieve, put it on the sieve seat, cover the sieve cover, turn on the power supply, start the sieve analyzer, and sieve continuously for 2 minutes. In the meantime, if any sample is attached to the screen cover, tap the screen cover to make the sample fall.

After screening, weigh all residues with a balance.

For other powdery materials, or using a square hole sieve other than 45-80 for sieve analysis test, the specifications of the sieve, sample weighing, sieve analysis time and other related parameters should be indicated.

The test screen must be kept clean at all times, and the screen holes should be clear. Use 10 times before cleaning.

Special cleaning agent should be used when cleaning metal frame screen and copper wire mesh screen, and weak acid should not be used for soaking.