In summer, adding a certain amount of retarder to concrete can improve the collapse resistance and quality. Regarding the impact of adding retarder in batches in summer on collapse retention, the editor of Anhui Yulong Materials Co., Ltd. will take you to understand it
1. Introduction
Influenced by the environmental protection situation, etc. Impact, the use of ready-mix concrete has become the mainstream concrete construction method. High fluidity and long-distance transportation are two major characteristics of modern concrete, and slump loss is the most common engineering problem. Adding water reducing agent and retarder when preparing concrete is the most common technical means to improve the fluidity of concrete and reduce slump loss. The main reasons for slump loss are the formation of hydration product network when the water-reducing agent interacts with cement particles, the consumption of water by the hydration reaction, and the encapsulation and consumption of the water-reducing agent by the hydration products. Studies have shown that the combined use of retarder and water-reducing agent has competitive adsorption of retarder and water-reducing agent, which can effectively improve the rheology of the slurry and has been widely used. Adding water-reducing agents at different times can effectively adjust the slump loss of concrete. Studies have shown that delayed addition of naphthalene-based water-reducing agents will have better dispersion effects. Polycarboxylic acid-based water-reducing agents have the largest adsorption capacity when mixed with water. , corresponding to the maximum fluidity of cement slurry. What this article needs to clarify is the effect of adding retarder in stages on the slump loss of concrete.
2. Test
2.1 Raw materials
2.1.1 Cement
P·O 42.5 cement, specific surface area 350 m 2 /kg.
2.1.2 Admixtures
Grade II fly ash, S95 mineral powder.
2.1.3 Sand and gravel
The sand is river sand, the fineness modulus is 2.4, the mud content is less than 1.0%, and the gravel is continuously graded from 5 to 31.5 mm.
2.1.4 Admixtures
Slump-preserving polycarboxylate water-reducing agent has a solid content of 40% and is liquid.
Retarders are sodium gluconate, sucrose, and maltodextrin. Its basic parameters are shown in Table 2.
2.2 Test method
2.2.1 Cement concrete test
The concrete test was carried out in accordance with GB/T50080-2002 "Standard for Test Methods for Performance of Ordinary Concrete Mixtures" , The fluidity test of cement slurry was carried out in accordance with GB/T8077-2012 "Test Method for Homogeneity of Concrete Admixtures".
2.2.2 XRD analysis
X-ray powder diffractometer (X-ray Diffractometer, referred to as XRD). XRD testing uses copper targets. Mix 100g cement and 50g water to make cement slurry. Take a certain amount of cement slurry into the sample tube at t =5 min, 30 min, 60 min, 90 min, and 120 min respectively, and add absolute ethanol to terminate hydration. Then it is dried and ground to prepare test samples, and a certain amount of cement is also taken as a blank sample.
2.2.3 Adsorption capacity
The adsorption capacity of polycarboxylate superplasticizer and retardant is measured using a total organic carbon adsorption instrument (TOC). Add 1g of cement to 20g of polycarboxylate water-reducing agent or retarder with a concentration of 0.2g/L, and then stir with a magnetic stirrer (TDL-80-2B type, rotation speed 3000 r/min) for 5 minutes and 30 min, 60 min, 120 min, and finally centrifuge for 5 min using a centrifuge (TDL-80-2B model, speed 2800 r/min). Use a syringe to take the supernatant of the centrifuge tube and filter it through a 0.22μm disposable organic filter. Use the filtrate as a sample for TOC testing.
3. Results and Discussion
3.1 Adsorption capacity
It is the change of the adsorption capacity of water reducing agent and retarder with time. From 0 min to 60 min, the adsorption amount of water-reducing agent or retardant on the surface of cement particles becomes larger and larger, and the adsorption amount will tend to level off after 60 min. This is because there is an adsorption equilibrium for the water-reducing agent or retarder adsorbed on the surface of the cement particles. The adsorption equilibrium point has not been reached before 60 minutes. Therefore, the water-reducing agent or retarder will continue to be adsorbed on the surface of the cement particles. After 60 minutes, the liquid phase As the concentration of the water-reducing agent decreases, the adsorption of the water-reducing agent or retarder on the surface of the cement particles tends to be saturated. Therefore, the adsorption amount of the water-reducing agent or retarder by the cement particles will gradually level off.
The adsorption amount of water reducing agent on the surface of cement particles is much greater than the adsorption amount of retarder, and different retarders have different adsorption capacities on the surface of cement particles. It can be seen that sodium gluconate has different adsorption capacity on the surface of cement particles. The adsorption speed and adsorption amount on the particle surface are the most significant, followed by maltodextrin and sucrose the worst. Plank [8] and others pointed out that when water-reducing agent and citric acid or water-reducing agent and tartaric acid are mixed into the cement slurry, the two will compete with each other and adsorb on the surface of the cement particles. Therefore, the water-reducing agent and the retarder should be added at the same time. In the cement slurry, the water-reducing agent and the retarder will produce a competitive adsorption phenomenon. The adsorption amount of the water-reducing agent and the retarder on the surface of the cement particles depends on the size of their adsorption capacity. The adsorption of the retarder on the surface of the cement particles. It can be inferred that the competitive adsorption capacity of different retarder and water reducing agent is ranked as follows: sodium gluconate > maltodextrin > sucrose.
3.2 Early hydration analysis
XRD patterns of cement slurry hydrated for 5 min, 30 min, 60 min, 90 min, and 120 min. In the XRD pattern, there are obvious AFt characteristic diffraction peaks in the hydrated samples hydrated for 5 min, 30 min, 60 min, 90 min, and 120 min, but the CH characteristic diffraction peak is not significant. This shows that a small amount of AFt was generated during cement hydration for 5 min, 30 min, 60 min, 90 min, and 120 min, but a large amount of CH hydration products were not generated. Therefore, it can be inferred that the hydration reaction and the formation of hydration products before 120 min have little impact on the fluidity of the slurry, and the adsorption of the water-reducing agentPerformance is the main factor affecting liquidity.
3.3 Fluidity of cement slurry
The fluidity of cement slurry is given by the effect of retarder on the binary system of water reducing agent and retarder under the condition of batch addition. influence the rules. The dosage of retarder and water-reducing agent are both 0.2%, and the admixtures are added in two ways: one-time addition or first adding 80% and then adding 20%, at t = 0 min, 60 min, 90 min, respectively. Measure the fluidity of cement paste at 120 minutes and record them as A1, B1, C1, D1 (PC), A2, B2, C2, D2 (PC+ GS), A3, B3, C3, D3 (PC + MA), A4 , B4, C4, D4 (PC + SU).
When the binary system of water-reducing agent and retarder is added at one time (100% in 0 min, A1 ~ A4), there is an obvious loss in fluidity of the cement slurry over time, while adding it in batches (Incorporate 80% at 0 min, then add the remaining 20% at 60 min (B1 ~ B4, 90 min (C1 ~ C4), and 120 min (D1 ~ D4))), the fluidity of the cement slurry at 90 min, 120 min When the binary system of polycarboxylate superplasticizer and sodium gluconate with a mass percentage ratio of 1:1 is added at one time, the fluidity of the clean slurry is 230 mm in 90 minutes and 205 mm in 120 minutes. When the initial addition is 80% and the second addition is 20% in 60 minutes, the fluidity of the clean slurry is 280mm in 90 minutes and 240mm in 120 minutes. When 20% is added twice in 90 minutes, the fluidity of the clean slurry is 287mm in 90 minutes and 260mm in 120 minutes. When 20% is added twice in 120 minutes, the fluidity of the clean slurry is 245mm in 120 minutes.
From the above analysis, it can be seen that whether it is a polycarboxylate water-reducing agent system or a polycarboxylic acid water-reducing agent and retarder binary system, add it in stages (add 80% first, then Adding 20%) can effectively improve the fluidity loss of the slurry over time.
From the analysis of hydration products, ζ-potential, and adsorption, it can be seen that there are less hydration products generated in the early stage of hydration, and the adsorption of admixtures on the particle surface is the main factor affecting the fluidity of the slurry. When an admixture is added at one time, more admixture may be consumed due to the hydration reaction or the wrapping of the hydration product, which reduces the active ingredients of the admixture in the liquid phase and breaks the balance between adsorption and consumption of the admixture, resulting in flow. However, under the condition of divided addition, although the initial addition amount is reduced, a certain amount of additives can be added during the second addition to increase the concentration of liquid phase additives. This part of the additives avoids the consumption of early hydration reactions. Coated with hydration products, it can be quickly adsorbed on the surface of cement particles and produce a good dispersion effect, thereby achieving the purpose of increasing fluidity.
The influence of different retarder on the fluidity of cement slurry in the binary system of water reducing agent and retarder under the condition of secondary addition of 90 min. It can be seen that the effects of the three retarders on the fluidity of the cement slurry in the binary system of water reducer and retarder are quite different. Sodium gluconate can significantly maintain the fluidity of the cement slurry for 120 min. The fluidity at 120 min is 250 mm; maltodextrin is second, with a fluidity of 240 mm at 120 min; sugar is the worst, with a fluidity of 220 mm at 120 min. The ability to influence the fluidity of cement slurry is: sodium gluconate > malto paste Essence>Sucrose.
3.4 Concrete slump and slump loss
Changes in concrete slump and expansion by different retarder and water-reducing agent binary systems under the condition of batch addition . The concrete test first prepared a binary system of water reducer and retarder according to 200 g polycarboxylate water reducer, 200 g retarder and 780 g water, and then added the binary system of water reducer and retarder in batches. in concrete. For the first time at t =0 min, add 80% of the binary system of water reducing agent and retarder to the concrete to measure the slump and expansion of the concrete. Then put the concrete into the basin and cover it. At t =60 min , 90 min, and 120 min respectively, add the remaining 20% of the water-reducing agent and retarder binary system to measure the slump and expansion of the concrete. At the same time, the best binary system of water reducing agent and retardant was selected and added to the concrete at one time to measure the slump and expansion of the concrete at t = 60 min, 90 min, and 120 min for comparison.
For the same binary system of retarder and water-reducing agent, there is a big difference in the impact of one-time addition and two-time addition (add 80% first, then add 20%) on the slump of concrete. When the binary system of sodium gluconate and superplasticizer is added at one time, the initial slump is relatively large, and then the slump decreases sharply with time. After 90 minutes, there is basically no slump. When 80% of the binary system is initially added, The initial slump is not much different from the one-time addition. After the second addition of 20% sodium gluconate and water-reducing agent binary system in 60 minutes, the slump reaches 210 mm and the expansion is 540 mm; the second addition in 90 minutes After adding 20% sodium gluconate and water-reducing agent binary system, the slump reaches 220 mm and the expansion reaches 540mm; after adding 20% sodium gluconate and water-reducing agent binary system for the second time in 120 minutes, the slump reaches is 200mm, and the expansion is 395 mm. Obviously, secondary addition can effectively improve the slump loss of concrete over time.
Comparative analysis of the three retardants, we can see that sodium gluconate and polycarboxylate water-reducing agent have the best matching under the condition of batch addition; taking 60 minutes as an example, the initial When 80% of the admixture is added and 20% of the admixture is added for 60 minutes, the slump of the PC + GS binary system in 60 minutes is 210 mm and the expansion is 540 mm; the slump of the PC + MA binary system in 60 minutes is 200 mm, the expansion is 425 mm; the slump of the PC + GS binary system in 60 minutes is 200mm, and the expansion is 430 mm. Therefore, different retarders have different effects on the slump of concrete in the binary system of water reducer and retarder under the condition of divided addition. Sodium gluconate can significantly maintain the slump of concrete, followed by maltodextrin and sucrose. The effect is the worst, which is consistent with the fluidity test of clean slurry.
IV. Conclusion
(1) The adsorption capacities of different retarder on the surface of cement particles are as follows: sodium gluconate > maltodextrin > sucrose; under the condition of adding in batches, the retarder can The influence of coagulant on the fluidity of cement slurry in the binary system of water-reducing agent and retarder is as follows: sodium gluconate>maltodextrin>sucrose;
(2) Add water-reducing agent in batches The binary system with retarder (adding 80% first and then 20%) can significantly improve the fluidity of cement slurry and maintain the slump of concrete. Among them, the binary system of sodium gluconate and water-reducing agent has the best effect. good.
IV. Conclusion
(1) The adsorption capacities of different retarder on the surface of cement particles are as follows: sodium gluconate > maltodextrin > sucrose; under the condition of adding in batches, the retarder can The influence of coagulant on the fluidity of cement slurry in the binary system of water-reducing agent and retarder is as follows: sodium gluconate>maltodextrin>sucrose;
(2) Add water-reducing agent in batches The binary system with retarder (adding 80% first and then 20%) can significantly improve the fluidity of cement slurry and maintain the slump of concrete. Among them, the binary system of sodium gluconate and water-reducing agent has the best effect. good.
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