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At present, all over the country are actively promoting the use of dry mixed mortar, but there are also some problems in the production and use of dry mixed mortar。In this paper, several common quality problems are listed and analyzed to provide some references for the production, engineering application and popularization of dry-mixed mortar in the future。
Raw materials cement, aggregate, mineral admixtures, additives and so on are the basic components of dry mixed mortar。Each of their performance indicators directly affects the quality of dry mixed mortar。The supply department of dry mix mortar production enterprises should select qualified raw material suppliers according to the requirements of quality control, and establish and improve the qualified supplier files;The procurement contract should be reviewed to ensure that the raw materials meet the requirements, and various measures should be strengthened from the source to ensure the quality of dry mixed mortar。
Cement is the main cementing material in dry mixed mortar, which has great influence on dry mixed mortar. The key point of cement quality control is stability control。
In order to control the quality of dry mixed mortar and improve the production level, the following aspects should be done:
1. The cement used should comply with the provisions of the current national standard GB175-2007 "General Portland cement", and the cement strength grade should be selected according to the requirements of the mortar variety and strength grade。
2, it is appropriate to use bulk cement, as far as possible to use the same manufacturer, the same grade of cement, because the frequent replacement of cement varieties and grades is not conducive to quality control, mastery and use, is not conducive to the quality of dry mixed mortar stability, the quality is prone to fluctuations。
3. Use statistical methods to comprehensively evaluate the stability of cement, comprehensively evaluate the quality of cement, and determine the basis for the design and adjustment of the mix ratio of dry mixed mortar according to the actual statistical situation。
4、骨料
Dry mix mortar aggregate can choose natural sand or machine-made sand。The particle size, gradation of sand, clay or stone powder content and other harmful substances will have different levels of influence on the quality of dry mixed mortar。
The high clay content or stone powder content of sand, small fineness modulus or large size are easy to cause problems such as segregation, cracking and poor workability of dry mixed mortar。Therefore, it is helpful to enhance the quality stability of dry mixed mortar to screen and store the sand and use it together。The proportion of sand in dry mixed mortar accounts for 80%, it can be said that the quality of sand has a great impact on the quality of dry mixed mortar, in addition to the filling role, it can also improve the workability of mortar, workability, reduce the amount of cement, reduce the heat of hydration, reduce shrinkage, creep and improve wear resistance。
(1) Medium sand should be selected, and should comply with the provisions of the current industry standard JGJ52-2006 "Standard for the quality and inspection methods of sand and stone for ordinary concrete", and should all pass 4.75mm 的筛孔。天然砂的含泥量应小于 5.0%，泥块含量应小于 2.0%;The stone powder content of artificial sand should be determined according to the methylene blue MB value by test, and generally should not be more than 12%。
（2）砂含水率小于 0.5%, if the sand moisture content is too high, the mortar is easy to caking when stored for a long time, affecting the use of。The large particle size should meet the requirements of the corresponding mortar varieties, and the fineness modulus is controlled at 2.3～3.0, timely adjust the sand fineness modulus and other indicators according to customer requirements。
5、矿物掺合料
Scientific addition of mineral admixtures can not only improve the performance of dry mixed mortar, but also reduce the cost of dry mixed mortar and protect the environment。The commonly used mineral admixtures are fly ash, slag powder, natural zeolite powder, silica powder, etc., which should be selected reasonably according to the varieties and technical requirements of mortar。
(1) In order to ensure the quality stability of dry mixed mortar, try to choose a relatively fixed supply source and stable quality, large-scale mineral admixture manufacturers。
(2) The quality of mineral admixtures shall meet the relevant standards。
(3) Industrial solid waste such as steel slag and phosphorus slag can also be used as mineral admixtures in dry mixed mortar after grinding, but it should be used after testing according to the characteristics of different materials。
6、添加剂
The additives used by dry mixed mortar enterprises are mainly water-retaining thickening, anti-cracking and other materials, and according to the different needs of mortar performance, water reducing agent, early strength agent, antifreeze, waterproofing agent, retarding agent and air entraining agent will be added。They have small dosage, high price and large effect, which is particularly important for the quality control of additives, and must be strictly tested according to standards。
7、计量
计量包括配料和称量两个环节。According to the type of raw materials as much as possible to design a number of weighing devices to increase the flexibility of dosing, in strict accordance with the dry mixed mortar production mix ratio of dosing, must ensure the accuracy of the dosing, the measurement error is controlled within 2%。
The measuring device shall consist of weighing sensor and weighing display controller.When the measuring device is separated from the computer system, it should have the function of manual weighing.The measurement accuracy is not lower than the standard requirements of the national Ⅲ class scale。
8、配料装置
The batching device should be able to continuously measure various materials of mortar with different mix ratios, and should have the function of recording and storing the actual measurement results one by one, so as to facilitate quality reference and tracking。
9、混合
The efficiency of the mixer mainly refers to the mixing time and discharge time of the mixer。To achieve the required uniformity, the shorter the mixing time, the better;The unloading time affects the batch cycle time, and the unloading time should be as short as possible。The mixing time of large efficient mixers with a capacity greater than 3 cubic meters is generally 90 ~ 150s。There is a common misunderstanding about the performance of the mixer, that as long as the mixing time is longer, the uniformity will be higher, in fact, just the opposite, the improvement and maintenance of the uniformity, related to the model and structure of the mixer, and the mixing time has no linear relationship, but after a certain period of time shows a downward trend。
物料混合应注意以下两点：
(1) The temperature of various raw materials of dry mixed mortar should be lower than 65℃ before entering the mixer, and the filling coefficient of dry mixed mortar mixer should be controlled at 0.6～0.8 之间。
(2) At least the mixing time of the material should be based on the premise of ensuring the uniform mixing of the material, the unloading time of the discharge door of the mixer should not exceed 18s, and the residual amount of the mixer should not be greater than 0.10%。
9、出料袋装
Dry mixed mortar should be packaged by quantitative packaging scales, and dust collection equipment should be built in the packaging area to ensure a good environment。Bulk loading equipment is mainly used for loading dry mixed mortar into the transport tank truck. In non-working hours, the discharge port should be closed, the material guide pipe should be retracted, and the height must be raised above the tank car port to ensure the normal passage of the tank car.During working hours, the discharge port can be raised to the position of the receiving port of the tank car, and a certain margin is left。The margin here is mainly used to compensate the tank car due to the increase in the weight of the loading process caused by the drop of the feed port, if not timely compensation for the height of this part of the settlement will cause a series of problems such as powder overflow。
Due to the large distance between the discharge port and the receiving port of the tank car, the material has a certain height drop during the loading process, which is easy to generate dust, so it is necessary to install a dust collection system to protect the surrounding environment。
10、工艺流程
Natural sand drying (artificial sand mechanical crushing and processing)→ sand grading and screening → raw material inspection → raw material storage → computer ratio measurement feeding → mixer mixing → sampling inspection → packaging, storage and transportation。
11. Common quality influencing factors and analysis of segregation of dry mixed mortar
(1) The loading, feeding to the storage tank, and feeding process of the storage tank should be large and fast to avoid segregation;
(2) Select a special bulk dry mixed mortar transport vehicle, the dry mixed mortar in the tank should be stored as full as possible, uniform and smooth transportation to prevent segregation;
(3) Keep the amount of dry mixed mortar in the bulk moving silo at the construction site not less than 3 tons, so as to avoid segregation due to the dry mixed mortar in the moving silo during use, and the dry mixed mortar in the process of driving into the bulk moving silo, due to the large difference in cutting height, resulting in serious segregation。
12、干混砂浆本身质量问题
(1) The mud content or stone powder content of the sand is too large, or the sand is too fine to cause wall cracking, should control the mud content and fineness of the raw sand, and try to use medium sand。
(2) Adjust the mix ratio according to different temperatures, different material walls and other conditions。If the summer temperature is high, when plastering on the block wall, the water retention rate of plastering mortar should be larger, reaching 93% to 95%;When plastering porous bricks and red bricks, the water retention rate of plastering mortar should reach 89% ~ 93%。
13. Quality problems of dry mixed mortar caused by the construction unit
(1) The one-time plastering during construction is too thick, resulting in mortar cracking。Should be in accordance with the standard construction operation, a plaster should not be too thick。
(2) Plaster construction should not be carried out on the matrix or base with a lower strength level than the mortar. If necessary, the construction should be carried out in a layered transition way to avoid cracking of the plaster layer or empty shells。
(3) The junction of different materials should be treated with strengthened mesh to avoid wall cracking。
(4) Before plastering concrete walls, they must be treated with interfacial agents and kept moist。
(5) When indoor plastering is carried out in the season of high temperature, windy and dry air, the doors and Windows should be closed, and the outdoor plastering should take sunshade and windproof measures。
(6) Timely moisturizing maintenance after mortar condensation to avoid mortar cracking。
14, often encountered problems in engineering applications and cause analysis
（1）砂浆使用时感觉较粗
For dry mixed mortar produced by inferior artificial sand, the large particle size of sand is generally less than 3.5mm，远远小于标准规定的4.75mm, the construction workers feel the mortar is still coarse, in fact, the feeling is thicker mainly because the workability of the mortar is poor, the construction is not good, it appears that there are more large particles, the main reason is that the particle grading of artificial sand is not good, often the composition of sand is "more than two ends, less in the middle", and the sand in the middle area needs to be added。
（2）抹灰砂浆开裂、空鼓
Mortar cracking can be divided into plastic cracking and dry shrinkage cracking。Plastic cracking refers to the cracking of the mortar before hardening or hardening process, which generally occurs in the early hardening of the mortar, the crack is generally relatively thick, the crack length is short;Dry shrinkage cracking refers to the cracking of mortar after hardening, which generally occurs in the late hardening of mortar, and the crack is characterized by fine and long。
Plastic cracking is mainly due to the shrinkage stress caused by rapid water dispersion in the plastic state shortly after mortar plastering. When the shrinkage stress is greater than the adhesive strength of the mortar itself, cracks occur on the surface。It is often related to the material and temperature of the mortar, the ambient temperature and the demeanor。The larger the amount of cement, the smaller the sand fineness modulus, the higher the mud content, the greater the water consumption, the worse the water retention of mortar, the more prone to plastic cracking of mortar。Dry shrinkage cracking is mainly due to the volume shrinkage caused by large amount of mortar cement and high strength.Late construction maintenance is not in place;The drying shrinkage value of mortar admixture or additive is large;The wall itself is cracked;Interface handling is improper;The strength grade of mortar is misused or misused;The elastic modulus difference between the substrate and mortar is too large。
（3）结块、成团现象原因
The sand moisture content in the raw materials of dry-mixed mortar production enterprises does not meet the standard requirements;Mortar mixing time is too short, mixing is not uniform;The construction enterprise failed to clean up the dry mixed mortar silo and agitator in time according to the construction requirements。
（4）砂浆抹面不久出现气泡
① The adaptability of additives and cement is not good, resulting in the reaction of bubbles。
(2) The porosity is too high because the sand fineness modulus is too small or the particle grading is not good。
（5）砂浆凝结时间不稳定
Due to external temperature, substrate differences, etc. :
(1) High construction environment temperature, low water retention rate of mortar and low dosage of retarder shorten the setting time of mortar。
(2) The decrease of the construction environment temperature and the high content of retarder lead to the extension of the mortar setting time。It is necessary to strengthen the site inspection, timely understand the construction information, and timely adjust the dosage of retarder and the water retention level of mortar。
（6）出现异常不凝结
The problem of additive measurement leads to excessive additive incorporation;Mortar mixing water is too large, resulting in mortar segregation, consistency is too large, do not set。
（7）抹面硬化后出现表面掉砂
The main reason is that the fineness modulus of sand used in dry mixed mortar is too low, the mud content exceeds the standard, and the proportion of cementing material is small, which leads to some sand surfacing and sand formation。
(8) The surface of the mortar is powdery and peeling
Because the sand used in the production of dry mixed mortar is too fine, the content of stone powder is large and the amount of admixture is too large, the calendering causes some powder to float and gather on the surface, so that the surface strength is low and the powder is peeled。
(9) The plastering surface is easy to fall and cannot stick
Due to the poor workability of mortar, the bonding force is too low;The construction party once plastering is too thick, plastering time interval is too short;The substrate interface is handled improperly。The production mix ratio of dry mixed mortar can be adjusted to enhance the adhesion of mortar.It is recommended to layer plaster during construction, the total thickness should not exceed 20mm, pay attention to each process time;Do a good job of basic wall interface processing。
（10）抹面后不好收光
The workability of dry mixed mortar is not good because of the unreasonable sand gradation。
The use of dry mixed mortar in engineering construction will play an important role in energy saving and emission reduction, reducing environmental pollution and improving the quality of construction projects。The factors affecting the quality of dry mixed mortar are complex, so the quality management and control of production, transportation and construction must be strengthened to ensure the quality of dry mixed mortar products and construction quality。
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