When preparing masonry mortar, the "Masonry Mortar Mix Proportion Design Regulations" (JGJ 98-2000) must be met, which clearly stipulates the technical indicators of mortar. In engineering practice, mortar strength is generally emphasized while mortar workability is ignored. In fact, mortar workability has the greatest impact on mortar application. The mortar is in a thin layer when working, and the operation is mainly manual. Only good workability can make the mortar uniform, smooth and transfer loads. When desert sand is used in engineering construction, it must first meet the workability requirements.
The fineness modulus of desert sand is 1.21, and it has a large specific surface area. It requires a large amount of cement slurry to wrap the sand surface to form a lubricating layer, so that the workability of the mortar mixture can be guaranteed. Therefore, the cement content in a single mortar is planned to be increased to 450kg.
The desert sand masonry mortar compound ratio and mixture properties are shown in Table 9.6.
Table 9.6 Desert sand masonry mortar mix proportions and mixture properties
9.1.3.1 Selection of admixtures in mortar
Masonry mortar for construction projects Most of them use cement mortar or cement-lime mortar, with lime paste as the admixture. However, in actual projects, it was found that the quality of lime paste was unstable, resulting in low strength of masonry mortar and easy cracks. Moreover, the price of lime powder was higher than that of fly ash. From the aspects of ensuring quality and reducing costs, lime paste was abandoned and replaced with pulverized coal. Ash is used as an admixture to prepare masonry mortar (Figure 9.1). Fly ash is a small particle with a spherical shape. Its microscopic state is glass body (Figure 9.2), and its main components are SiO2, Al2O3, and Fe2O3. When fly ash is used to replace cement of equal mass in cement mortar, the volume increases by about 30% compared to cement, which increases the volume ratio of slurry to sand. A large amount of slurry fills the gaps between sand particles, wrapping and Lubricating the sand particles reduces the frictional resistance between the sand particles, improves the fluidity of the mortar mixture, and also improves the water retention. The effect of adding fly ash on the strength of masonry mortar mainly depends on the potential pozzolanic activity of fly ash. This activity mainly refers to the glassy active silica and activated alumina in fly ash and the active alumina in cement mortar. Calcium hydroxide reacts to form hydraulic gelling substances such as calcium silicate hydrate and calcium aluminate hydrate. As the age of masonry mortar mixed with fly ash increases, the activity of fly ash will gradually develop, and the physical and mechanical properties of the hardened mortar in the later stage will be reliably guaranteed.
Figure 9.1 Mortar mixing process
Figure 9.2 Vitreous fly ash particles under an electron microscope
Cement fly ash mixed masonry mortar can satisfy both The construction and specification requirements of masonry mortar can also save a lot of lime for construction projects and help reduce costs. The application of fly ash as industrial waste in mortar complies with the national environmental protection policy and has significant economic and social benefits.
9.1.3.2 The impact of admixtures on the water consumption of mortar and its mechanism
Because the sand is fine, the water demand of mortar is too large, so the addition of water-reducing agent, air-entraining agent, After adding the composite admixture composed of water glass, the water consumption of the mortar is significantly reduced. When the consistency of the mortar mixture is basically the same, the water consumption is reduced by about 15%.
(1) Functional mechanism of polycarboxylic acid-based high-efficiency water-reducing agent
The water-reducing component of the composite admixture uses polycarboxylic acid-based high-efficiency water-reducing agent. The water-reducing and dispersing mechanism of the polycarboxylic acid-based high-efficiency water-reducing agent is based on electrostatic repulsion, steric hindrance (also known as steric effect, steric repulsion effect), and complexation. It also has hydration film lubrication and wetting effects, etc. The superposition of several effects results in the release of free water by destroying the flocculation structure, thereby dispersing the cement particles. The main substances that form the adsorption form of polycarboxylic acid-based water reducing agents on cement particles and dispersed water reducing agents are generally anionic surfactants. Their molecular structures contain many active groups that can be adsorbed on cement particles and their hydrates. , forming an adsorption layer with a certain thickness and a certain adsorption form, thus greatly changing the physical and chemical properties of the solid-liquid interface and the force between particles.
The sulfonic acid group (-SO3H) and carboxyl group (-COOH) in the polycarboxylic acid-based high-performance water-reducing agent molecules provide adsorption points and electrostatic repulsion, allowing the water-reducing agent molecules to be directionally adsorbed on the surface of cement particles, and some polar groups Pointing to the liquid phase, the ionization effect of the hydrophilic groups causes the cement particles to be charged with the same electrical charge on the surface, forming an electric double layer. When the cement particles are close to each other and the electric layers overlap each other, electrostatic repulsion is generated between the cement particles. The flocculation structure of the cement particles disintegrates, the particles disperse each other, and the free water wrapped in the flocs is released, thereby effectively increasing the mixing rate. mobility of things. Polycarboxylate superplasticizer molecules contain a large number of polar groups, such as carboxyl groups (—COOH), hydroxyl groups (—OH), sulfonic acid groups (—SO3H), ether bonds (R—O—R′) and amino groups ( —NH2), etc. These polar groups have a strong hydrophilic effect and can associate with water molecules to form hydrogen bonds, improving the wettability of the surface of cement particles and allowing water to penetrate into the narrower pores between particles. middle. After the water-reducing agent molecules are adsorbed on the surface of the cement particles, a solvent hydration film with a certain mechanical strength is formed on the surface of the cement particles. The formation of the hydration film can destroy the flocculation structure of the cement particles, fully disperse the cement particles, and create obstacles. The effect of cohesion (Figure 9.3).
Figure 9.3 Schematic diagram of the action mechanism of water reducing agent
Figure 9.4 Tiny closed bubbles formed by the action of sodium dodecyl sulfonate
(2) Dodecane The mechanism of action of sodium dodecyl sulfonate air-entraining agent
The air-entraining component is sodium dodecyl sulfate, whose main component is surfactant. After sodium dodecyl sulfate is added to the mortar, due to The role of surfactant reduces the surface tension and surface energy of water, making it easy to generate bubbles when the mortar is stirred. At the same time, the surfactant is enriched and adsorbed on the surface of the bubbles and arranged in a directional manner to form a single-molecule adsorption film, making the liquid The membrane is mechanically strong and resistant to rupture. The role of sodium dodecyl sulfate, its tiny closed bubbles (Figure 9.4), plays the role of ball bearings in the mortar mixture, reducing the frictional resistance between sand particles during the flow of the mortar mixture, making the mortar mixture The fluidity is greatly improved, the water retention is good, and the construction operation is convenient.
(3) Water glass action mechanism
The water glass component uses water glass with a modulus of 2.4 and a solid content (mass ratio of solid phase to water) of 36%. The viscous state of the water glass solution itself can improve the water retention of the mortar mixture and reduce the bleeding phenomenon (Figure 9.5). Compared with methylcellulose, water glass generally does not reduce the compressive strength of mortar. Literature shows that a solid-liquid two-phase reaction occurs between solid fly ash particles and liquid water glass. When the fly ash and water glass solutions are mixed, the aluminosilicate glass phase depolymerizes under the action of the activator to form oligomerized [SiO4] and [AlO4]. Subsequently, the oligomeric [SiO4] and [AlO4] undergo polycondensation to form [Mx(AlO2)y(SiO2)z·nMOH·mH2O] colloid, which quickly precipitates on the surface of fly ash particles. And the unreacted fly ash particles are bonded together to eventually form a fly ash-based mineral polymer with a certain level of mechanical strength.
9.1.3.3 Comparative test of mortar delamination degree
The water retention of mortar refers to the ability of the mortar to retain water. Mortar with poor water retention is prone to bleeding and segregation, thus affecting its use. performance. The water retention of mortar is measured by the delamination degree (Figure 9.5). The steps are as follows:
Figure 9.5 Mortar delamination test
Figure 9.6 Mortar consistency test
1) Determine the consistency of the mortar mixture (Figure 9.6);
2) Put the mortar mixture into the layered cylinder at one time. When it is full, use a wooden hammer to tap around the container, such as When the mortar sinks below the mouth of the barrel, add it at any time, then scrape off the excess mortar and smooth it with a spatula;
3) After letting it stand for 30 minutes, remove the 200mm mortar from the upper section, and pour the remaining 100mm mortar into the Stir in the mixing pot for 2 minutes and then measure the consistency. The difference in consistency measured before and after is the layering value of the mortar.
Among them, the consistency test operation procedures are as follows:
a. Clean the container and test cone, and lightly wipe the sliding rod with lubricating oil so that it can slide freely;
b. Mix the mortar Put the contents into the container at one time, so that the mortar surface is about 10mm lower than the container mouth. Use a tamper to tamp from the center of the container to the edge 25 times, then gently shake the container 5 to 6 times to make the mortar surface smooth, and then place the container on the base of the consistency meter;
c. Unscrew the brake screw of the test cone sliding rod, move the slide rod downward, and when the tip of the test cone is in contact with the mortar surface, tighten the brake screw to make the rack The lower end of the side rod just touches the upper end of the sliding rod, and align the pointer with the zero point;
d. Unscrew the brake screw and count the time. After 10 seconds, fix the screw immediately and bring the lower end of the rack measuring rod into contact with the sliding rod. At the upper end of the rod, read the sinking depth from the dial, which is the consistency value of the mortar.
Through the test, it was found that the degree of delamination of the mortar mixture without composite admixtures was 40mm, which did not meet the requirement of "the delamination degree should not be greater than 30mm" in the technical regulations for masonry mortar. The stratification degree of the mortar mixture mixed with composite admixtures is reduced and basically meets the requirements. Add a certain amount of fly ash to the H3 mortar mix ratio (the equivalent replacement rate of fly ash is 11%, that is, 50kg of fly ash replaces the same amount of cement, and an additional 50kg of fly ash is added to improve the mix of the mortar mixture. workability), the degree of delamination of the mortar mixture is further reduced, the water retention is improved, and the on-site workability of the masonry mortar is improved.
9.1.3.4 Changes in the wet apparent density of mortar
Since the air-entraining component in the composite admixture increases the air content of the mortar, the mortar mixture will have the same quality The volume increases, so the wet apparent density of the mortar mixture mixed with composite admixtures will inevitably decrease. In order to determine the impact of the incorporation of composite admixtures on the wet apparent density of mortar, a wet apparent density test was conducted. The main steps are as follows:
1) First measure the consistency of the mixed mortar. When the mortar consistency When the thickness is greater than 50mm, the tamping method is used. When the mortar consistency is not greater than 50mm, the vibration method is used.
2) Weigh the capacity cylinder, accurate to 5g. Put the funnel on the volumetric cylinder, fill the volumetric cylinder with mortar mixture and make it slightly surplus. In the tamping method, fill the capacity cylinder with the mortar mixture at one time to make it slightly surplus. Use the tamping rod to tamping evenly 25 times. If the mortar sinks below the mouth of the cylinder during the tamping process, add mortar at any time and tap again. Beat 5 to 6 times; in the vibration method, fill the capacity cylinder with the mortar mixture once and vibrate it on the vibration table together with the funnel for 10 seconds. During the vibration process, if the mortar sinks below the mouth of the cylinder, add mortar at any time.
3) After tamping or vibrating, scrape off the excess mortar mixture at the mouth of the barrel to make the surface smooth, wipe the outer wall of the volume cylinder, and weigh the total weight of the mortar and volume cylinder, accurate to 5g.
According to the "Masonry Mortar Mix Proportion Design Regulations" (JGJ 98-2000), the density of the mixed mortar mixture should not be less than 1800kg/m3, which is not a mandatory requirement. The test shows that the density of Mu Us desert sand mixed mortar mixture is above 1740kg/m3, so it does not affect the performance of the mortar.
Under the premise of adding fly ash and composite admixtures, the cement dosage is controlled at the level of 450kg, and the workability of the H3 mix ratio meets the specification requirements. After comprehensive consideration of the strength evaluation, it is found that the strength is biased. Low (Table 9.7, Table 9.8). After analyzing the reason, it may be caused by the low strength caused by the weathering and deliquescence of 1# cement. P.C32.5 grade composite Portland cement (i.e. 2# cement) was repurchased for the second batch of tests.
Table 9.7 Mortar strength growth
Table 9.8 Mortar strength grade evaluation
Note: The strength conversion coefficient is 1.35.