A certain project said: “The slump of the freshly mixed concrete was completely lost in 10 minutes, making it impossible to construct dozens of cubic meters of concrete. You, the admixture manufacturer, should come and make adjustments as soon as possible. You will be responsible for the economic losses incurred!” After the accident, we set out immediately, arrived at the scene, replaced the raw materials of the concrete one by one using the elimination method, observed the working performance of the concrete, and finally locked the cause on the washed sand.
◆Field testing
- Qualitative chemical testing of washed sand
Generally speaking, mud content is one of the main causes of concrete performance loss. Once concrete performance loss occurs suddenly, mud content is the first factor to be considered. However, when I tested the mud content of this batch of washed sand, I found that the mud content was only 1.2% and the mud block content was 0.3%, which fully met the national and industry standards for sand used in concrete mixing. After eliminating the mud content factor, I suspected that the sand contained a certain amount of flocculants, which caused the concrete performance loss too quickly. So I used my chemical knowledge to design a simple qualitative detection method to identify the presence of flocculants on the sand surface.
Test process:
Experimental steps:
(1)Soak the washed sand in distilled water, and use concentrated nitric acid to adjust the pH value of the sand-water system to 0 to ensure that the cations in the flocculant can stably exist in the solution;
(2)Filter the suspension of the sand-water system, and divide the filtered solution into three parts for inspection;
(3)Detect the change in the pH value of the solution, add sodium hydroxide solution to one of the solutions, and observe whether there is flocculent precipitation in the solution. Add silver nitrate solution to another solution and observe the changes in the solution. Add barium chloride solution to the last solution and observe the changes in the solution.
Test results:
It can be basically concluded that the solution contains a certain amount of aluminum ions.
Reason:
Experimental phenomenon:
When sodium hydroxide is added, as the pH value increases, flocculent precipitation gradually appears in the solution. As the pH value continues to increase, the flocculent precipitation gradually disappears, indicating that the solution contains amphoteric metal ions. Common amphoteric metal ions are only aluminum ions and zinc ions. Zinc ions are a retarder with a very strong retarding effect. No abnormal setting time was found in this batch of concrete, so the possibility of zinc ions can be basically ruled out.
When silver nitrate is added to the solution, a milky white precipitate appears in the solution, which basically indicates that the solution contains a certain amount of chloride ions. When barium chloride is added to the solution, there is no change in the solution, indicating that the solution does not contain sulfate ions.
Polyaluminium chloride is a water purification material and an excellent water treatment flocculant. Due to the bridging effect of hydroxide ions and the polymerization of multivalent anions, the inorganic polymer water treatment agent with a large molecular weight and a high charge has the functions of adsorption, coagulation, precipitation, etc. It has the advantages of wide adaptability to water areas, fast hydrolysis rate, strong adsorption capacity, large alum flowers, fast dense precipitation, low turbidity of effluent water, and good dehydration performance.
Test results:
After searching for relevant information, it was found that the flocculant that best matched the above experimental phenomena was polyaluminium chloride.
For sand and gravel supply companies, there are objective incentives to use polyaluminium chloride.
For sand and gravel supply companies, there are certain differences in the price of sand with different mud contents. In the process of sand washing, water is the most important factor in the cost of sand washing. If a simple and easy way can be adopted to recycle water, the cost of sand washing will be greatly reduced.
- Adverse effects of polyaluminium chloride on concrete performance.
The results show that from the perspective of concrete strength, trace amounts of polyaluminium chloride do not have a significant effect on the strength of concrete. Polyaluminium chloride has a greater impact on the working performance of concrete mixtures. A trace amount of polyaluminium chloride can cause a significant decrease in the initial slump, and the slump loss over time is even more serious.
- Reasons for rapid slump loss.
(1)The poor compatibility between concrete admixtures and cement causes the concrete slump to lose quickly;
(2)The amount of concrete admixtures is not enough, and the slow setting and plastic preservation effects are not ideal;
(3)The ambient temperature is high, and some admixtures fail at high temperatures; the water evaporates quickly, and the bubbles overflow, causing the fresh concrete to lose slump quickly;
(4)The initial concrete slump is too small, and the unit water consumption is too small;
(5)The construction site and the mixing station are not well coordinated, causing the tank truck to be pressed and jammed for too long, resulting in excessive concrete slump loss;
(6)The measurement error of the concrete mixing weighing system is large and unstable;
(7)The moisture content of coarse and fine aggregates changes;
(8)Cement is stored in mixed warehouses and used together;
(9)Is the cement just entered the barrel warehouse hot cement? If so, it is normal to lose 10CM in one hour;
(10)The compatibility between admixture and cement is very poor. You can do a pure slurry fluidity test. It is OK at 22-24cm, but you also need to look at the loss over time, half an hour or an hour. If the loss is too large, you need to find the admixture manufacturer to readjust their formula. Specifically, 300g of cement, 87g of water, and 1.8% of admixture or its recommended dosage;
(11)Weather factors also have a great impact. In this weather, a loss of 3cm in one hour is normal;
(12)There are many reasons for the rapid loss of slump, such as unqualified admixtures, unqualified cement stability, and too long transportation time… These are all reasons that affect the slump. The solution is very simple, just add water and water reducer.
◆Slump test
- Solutions
(1)Adjust the formula of concrete admixture to make it compatible with cement. Before construction, be sure to conduct a compatibility test between concrete admixtures and cement;
(2)Adjust the concrete mix ratio, increase or decrease the sand ratio and water consumption, and adjust the initial slump of concrete to more than 220mm;
(3)Add an appropriate amount of fly ash to replace part of the cement;
(4)Increase the amount of concrete admixtures appropriately, and adjust the retarding component in the admixture (especially when the temperature is much higher than the usual temperature);
(5)Prevent water from evaporating too quickly and bubbles from overflowing too quickly;
(6)Use slag cement or volcanic ash cement;
(7)Improve the water retention and cooling devices of concrete transport vehicles;
(8)The accuracy of the measuring equipment should meet the relevant regulations, and have a valid certificate issued by the statutory measurement department, strengthen self-inspection, and ensure accurate measurement;
(9)Strengthen the detection of aggregate moisture content, and adjust the mix ratio in time when it changes;
(10)The cement entering the warehouse should be stored and used according to the manufacturer, variety and grade;
(11)Post-addition of water reducer: that is, add water reducer after mixing sand, stone, cement and water. This method has a significant effect on inhibiting slump loss. This is mainly because after cement meets water, C3A and C4AF in cement can quickly generate calcium sulfonate in an environment with gypsum, and C3A and C4AF in the system are significantly reduced. At this time, when water reducer is added, the amount of water reducer absorbed and consumed by C3A and C4AF is significantly reduced;
(12)Retarder method: The theory of the effect of retarder on cement retardation includes adsorption theory, complex salt formation theory, precipitation theory and control of hydroxide crystal growth theory. Most organic retarders are surface active. They produce adsorption at the solid-liquid interface and change the surface properties of solid particles, that is, hydrophilicity. Due to adsorption, the hydroxyl groups in their molecules hinder the cement hydration process on the surface of cement particles, shielding the contact between crystals and changing the structure formation process;
(13)Adjust concrete admixtures: Use high molecular weight water reducers and use them in combination with appropriate water-retaining components. The free water content in concrete is increased without increasing the water consumption, which can alleviate slump loss.
Sand washed with flocculants is not a problem, but the residual flocculants in the washed sand are. So far, the only way to solve the problem of flocculants in sand is to add water reducing agent (the user increases the dosage, and the admixture company increases the mother liquor cost).