It is reported that the resources of river sand (river sand) in Zhejiang Province are close to exhaustion, and in order to protect the ecological environment of green mountains and rivers, mining has been banned everywhere. The river sand transported from other ports to Zhejiang Province is either expensive or fine sand or even extra-fine sand. Sea sand must be desalinated before it can be used, but the cost of desalination is very high, the treatment process has secondary pollution to the environment, and there are also cases of substandard treatment, so that some cities prohibit the use of sea sand, thus, the mining and use of machine-made sand is increasing.
At present, machine-made sand has been widely used in concrete, mortar, aerated wallboard and other building materials or products in Zhejiang Province. This paper discusses the production control and application technology of machine-made sand for ready-mixed mortar.
1. The current situation
of relevant standards for machine-made sand and its application in mortar. The national standard GB/T 14684 for Construction Sand only stipulates the general technical requirements for machine-made sand; The industry standard Standard for Quality and Inspection Methods of Sand and Stone for Ordinary Concrete (JGJ 52-2006) has not been issued due to the revision. The existing clauses are mainly for natural sand, and there is no technical clause for machine-made sand. The
national standard "Ready-mixed mortar" GB/T 25181-2019 does not involve the use of machine-made sand requirements.Although the Zhejiang provincial standard "Technical specification for application of ready-mixed mortar" DB33/T 1095-2013 involves the basic requirements for the use of machine-made sand, it does not stipulate that different types of mortar should limit the particle size of machine-made sand. The
industry standards of Specification for Mix Proportion Design of Masonry Mortar (JGJ/T 98-2010) and Technical Specification for Plastering Mortar (JGJ/T 220-2010) have not been revised, and they only aim at the mix proportion and requirements of natural sand mortar, and do not involve the mix proportion design clauses of machine-made sand mortar.
Therefore, the application requirements of machine-made sand in ready-mixed mortar are irregular or incomplete.
2. Production control
of machine-made sand 2.
Magmatic rocks are rocks formed by the extrusion of molten materials from the crust or cooling in the surface, such as granite, andesite, basalt and rhyolite. Among them, granite and basalt are often used as parent rocks of machine-made sand.
Sedimentary rocks are formed by a series of geological processes such as weathering, transportation, sedimentation and diagenesis, which are generally characterized by layered distribution structure, such as limestone, tuff, sandstone and shale. Among them, limestone, tuff and sandstone are often used as parent rocks of machine-made sand.
Metamorphic rocks are formed by metamorphism and mixing on the basis of existing rocks, and have the characteristics of both the above rocks and their own characteristics. Gneiss, amphibolite, granulite and marble are common. Among them, gneiss and marble are often used as parent rocks of machine-made sand.
(2) Distribution
of machine-made sandstone in Zhejiang Province Basalt deposits as magmatic rocks are mainly distributed in Shengxian, Xinchang, Tiantai, Ninghai, Sanmen and other places in Zhejiang Province. Granite deposits are mainly distributed in Shuilinkeng of Anji, Yujiashan of Shaoxing and Dalai of Wuyi. Limestone deposits
as sedimentary rocks are mainly distributed in Huzhou City, Hangzhou City, Jinhua City, Quzhou City and other areas west of Zhejiang-Jiangxi Railway and Zhuji City of Shaoxing City. Among them, Hangzhou area is distributed in Jiande, Tonglu, Fuyang and Linan, Huzhou area is distributed in the urban area, Changxing, Anji and Deqing, Jinhua area is distributed in Lanxi, Jindong District and Pujiang, Quzhou area is distributed in Jiangshan, Changshan, Kecheng District, Kaihua and Longyou, and Shaoxing area is distributed in Zhuji. There are tuff deposits in Ningbo area.
(3) Types
of machine-made sand At present, the main types of machine-made sand in Zhejiang Province are limestone sand, tuff sand, sandstone sand, basalt sand, a small amount of granite sand and marble sand, and a very small amount of river pebble or cobblestone broken sand. Among them, basalt sand has the functions of sound insulation and heat insulation.
2. The moisture content of crushed stone should be controlled in the dry sand making process. 1
. The sand making link is mainly as follows: firstly, the jaw crusher is used to pre-crush the parent rock for the first time (coarse crushing) by extrusion, so as to reduce the diameter of the parent rock and reduce the production pressure of the counterattack crusher; Then a counterattack crusher is used for carrying out secondary crushing (intermediate crushing) on the master batch in a riprap impact mode, so that the aggregate micro-cracks generated by the crushing are further developed, and finally the aggregate is crushed again from the micro-cracks to complete the secondary crushing and the primary shaping of the aggregate; Finally, the vertical shaft impact crusher is used to crush the aggregate for the third time (fine crushing) and shape the aggregate for the second time in the way of "stone beating stone" or "stone beating iron". The vertical shaft impact crusher generally has the function of shaping. After shaping, the particles are regular, round, cubic and have certain edges and corners. The mutual locking force between the particles increases and the displacement between the materials decreases. Most of the sand production lines of ready-mixed mortar enterprises
in Zhejiang Province are mainly dry crushing of purchased gravel. That is to say, the vertical shaft impact crusher is used to crush the aggregate for the third time (fine crushing) and the second shaping in the way of "stone beating stone" or "stone beating iron". In the process of sand
making, it is also necessary to consider the step topography of the first and second crushing sites, the design of power lines, the soil removal device, and the control of stone powder by the air supply volume of the fan during dry production.
2.3 Machine-made sand sieving and sifting
The reasonable sifting of machine-made sand is a sand silo, and the maximum particle size of sand should not be greater than 2.36 mm, which is mainly used for the preparation of masonry mortar; A coarse sand bin with sand grain size ranging from 2.36 mm to 1.18 mm, and a fine sand bin with sand grain size ranging from 1.18 mm to 0.075 mm, which are mainly used for the matching of plastering mortar and ground mortar, and the sand in the fine sand bin can also be used for the preparation of special mortar; In a stone powder bin, the particle size is less than
0.2.4. The quality problem
of machine-made sand (1) The gradation of machine-made sand often presents the characteristics of "more at both ends and less in the middle", in which the sieve residue of particles with particle size greater than 2.36 mm is as high as 40%. The particle size is less than 0.
(4) The stone powder in the machine-made sand is easy to accumulate on the inner wall of the silo. Therefore, the sand silos, coarse sand silos and fine sand silos should not be emptied, and feeding should be carried out when two-thirds of the silo capacity is used. Otherwise, once the stone powder on the inner wall of the accumulation silo collapses, the stone powder content in the machine-made sand will increase sharply, and the quality of the prepared mortar will be difficult to control.
3. Research on
application technology of machine-made sand 3.
See Table 1 and Figure 1 for the mix proportion and test results of machine-made sand mortar calculated according to the Specification for Mix Proportion Design of Masonry Mortar JGJ/T 98-2010.
Table 1: Test results
of mortar prepared according to Specification for Mix Proportion Design of Masonry Mortar JGJ/T 98-2010 
the characteristic coefficient of machine-made sand mortar is being tested and studied by the Zhejiang Provincial Standard "Specification for Mix Design of Ready-mixed Mortar" edited by Zhejiang Institute of Architectural Design and Research. The characteristic coefficient of mortar in the regression equation of the curve is α = 1.56, β = -8.98, The correlation coefficient is 0.
The verification test has been carried out by Zhejiang Jianke New Materials Development Co., Ltd., Zhejiang Zhaoshan Building Materials Technology Co., Ltd., Zhejiang Huazheng Testing Co., Ltd., Hangzhou Jiumangu Building Materials Co., Ltd., Jiande Xinneng New Building Materials Co., Ltd., Zhejiang Yifu New Building Materials Co., Ltd., Wenzhou Construction Quality Supervision Research Institute Co., Ltd. and other units carried out the test according to the three conditions of powder 300kg/m3, 330kg/m3 and 350kg/m3, totaling 504 groups of test blocks. Among them, Zhejiang Zhaoshan Building Material Technology Co., Ltd. calculates the mortar mix ratio according to the loose packing density of the company's machine-made sand. The 28-day strength of the P · O42.5 ordinary Portland cement used is 48.
, machine-made sand mortar is adjusted
according to the characteristic coefficient of machine-made sand mortar α = 1.56, β = -8.
(1) The adjustment of the maximum particle size of machine-made sand, the content of stone powder in machine-made sand and the amount of water-retaining and thickening material The maximum particle size of machine-made sand should not be greater than 2. The amount of water-retaining and thickening material should be determined according to the requirement of water retention rate of 88% ~ 92%. On the one hand, too high water retention rate will increase the cost of mortar. On the other hand, it will reduce the later strength of mortar. The stone powder content in the machine-made sand used for plastering mortar should not be more than 5% of the total amount of machine-made sand.The study shows that the probability of mortar shrinkage cracking increases when the content of stone powder exceeds 5%.
(2) Adjustment
of mortar mix proportion in summer and winter In summer construction, when cellulose ether is used as a water-retaining and thickening material, the gel temperature of cellulose ether should not be lower than 70 ℃; when the atmospheric temperature is higher than 30 ℃, the mortar can be mixed with retarder. Lignosulfonate, gluconate, citric acid, etc. Should be selected as retarders. Powdered retarder shall be used for dry-mixed mortar. The dosage of retarder shall be determined according to the mortar test.
In winter, when the atmospheric temperature is lower than 5 ℃, the mortar can be mixed with early strength agent or antifreeze. Calcium formate should be used as early strength agent, calcium nitrate should be used as antifreeze, and urea or antifreeze containing chloride ions should not be used.
3. The main reason is that all the production enterprises use machine-made sand and stone powder to prepare mortar. Machine-made sand has many edges and corners, and the friction coefficient between materials is large; the shape of stone powder is polygonal and not round, which will also increase the friction between materials, and the mutual adsorption of stone powder is strong. These factors will cause the prepared mortar to fall unsmoothly. The best way to reduce the friction is to add 10 kg ~ 20 kg fly ash per ton of mortar. Fly ash has "morphological effect". Under the microscope, it shows that the fly ash contains more than 70% of glass beads, the surface is smooth, the lubrication between materials is increased, the friction is reduced, the adsorption is reduced, and the mortar will fall smoothly.
(2) Machine-made sand mortar has rough
handfeel. If machine-made sand is produced according to the national standard GB/T 14684 -2022, the maximum particle size of sand is 4. Therefore, it is necessary to limit the maximum particle size of machine-made sand. It is suggested that it should not exceed 2.
(3) Machine-made sand plastering mortar is easy to crack
. Compared with the mortar prepared with natural sand, machine-made sand plastering mortar has large shrinkage and is easy to crack. The main reason is that the water demand of stone powder in the mortar is large and the shrinkage increases; the second reason is that the machine-made sand particles contain micro-cracks, which will crack slowly under the stress caused by external temperature; in addition, the mortar loses water too quickly after construction. Therefore, it is necessary to limit the amount of stone powder, add LD shrinkage-reducing and thickening powder, maintain the plastering layer for more than 7 days, and control the strength of plastering mortar (generally M5 for interior walls and M10 or M15 for exterior walls) to reduce mortar cracking.
(4) The strength of machine-made sand mortar is low
, the fineness modulus of machine-made sand is high, the particles are coarse, and the gap between sand particles is large; the gradation of machine-made sand is poor, and it is difficult to meet the requirements of Zone 2 specified in the national standard GB/T 14684 for Construction Sand. Therefore, the strength of the prepared mortar is naturally lower than that of the natural sand mortar with the same proportion, so it is necessary to adjust the gradation of fine aggregate by matching machine-made sand with different particle sizes or adding a certain proportion of natural sand, so as to improve the strength of mortar under the same cementitious material. Table 3 shows the screening results of machine-made sand produced by a production enterprise, which fails to meet the requirements of Zone 2 sand.
< IMG SRC = "Shaping technology of machine-made sand particles with https://img7.ccement. but different rock crushing, digital intelligent management technology of machine-made sand production line, mix proportion design of machine-made sand and stone powder with different rock crushing in mortar, guarantee measures for construction and maintenance of machine-made sand mortar, etc., All of them need to be studied and explored in depth.
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