Cement additives are a foreign name. In China, cement additives and cement admixtures both refer to cement grinding aids. Cement additive is a chemical additive that improves the effect and performance of cement grinding. It can significantly increase the output of cement grinding stations and the strength of cement at all ages. Cement additives can greatly reduce the phenomenon of electrostatic adsorption of balls formed during the grinding process, and can reduce the tendency of ultrafine particles formed during the grinding process to coalesce again and reduce the phenomenon of over-grinding.
The role of cement additives
Cement additives can also significantly improve the fluidity of cement, increase the grinding efficiency of the mill and the powder selection efficiency of the powder separator, thereby reducing grinding energy consumption. As a chemical activator, cement additives can improve the distribution of cement particles and stimulate the hydration activity of each mixed material, thereby improving the early strength and later strength of cement.
Common cement additives are liquid and powder (solid), both of which can significantly increase mill output, improve product quality, or reduce grinding power consumption. The cement additives used in the wet grinding process are also called dispersants.
According to activity, it can be divided into active and inactive additives. The active ones mainly include volcanic ash, slag, fly ash, etc., and the inactive ones mainly include quartz sand, limestone, slow cooling slag, etc. Additives mainly used to adjust the hardening speed of cement include gypsum, bauxite, etc.
Construction application technology of cement additives
2.1.1 Raw material technical requirements Raw materials used in various cement commercial concrete structures of highway projects incorporating admixtures, namely cement, sand, stone, water, Unless there are special requirements, they should comply with the corresponding provisions of the current national and industry standards, specifications and regulations for raw materials of cement commercial concrete structures in highway projects.
2.1.2 Product access conditions Suppliers of various admixture products should provide sufficient test samples for indoor mix ratio and performance testing of cement commercial concrete to the (central) laboratory of the construction, construction and supervision unit In addition to quality re-inspection samples, the following documents should also be provided: (1) Quality instructions for each product, describing the main compounds of the product, the basic physical and mechanical properties of commercial cement concrete self-inspected by the manufacturer, and the recommended dosage range, etc.; (2) Product access certificate provided by provincial, ministerial, and prefecture-level quality inspection units or admixture inspection agencies, or performance reports of commercial concrete mixed with admixture cement; (3) New product identification certificates provided by provincial, ministerial, and prefecture-level departments or national, New product patent certificate provided by the Provincial Patent Office; (4) Product homogeneity test report provided by provincial, departmental, and department-level quality inspection units or admixture inspection agencies; (5) Product engineering application examples (if necessary); (6 ) Product supply guarantee document (large-scale projects).
When using admixtures on the pavements and bridge decks of highways, first-class highways, and second-class highways, extra-large, medium-sized bridges in various grades of highways, and the main structure of tunnels, inspection and inspection must be provided. The sample must be retested and must meet all the above requirements before it can be used.
2.1.3 Adaptability test For the types of admixtures used in various highway cement commercial concrete structure projects, the adaptability of the admixtures and cement must first be tested. Unsuitable admixtures must not be used, otherwise they need to be replaced. To adapt to the variety of admixtures or use cement that is compatible with the admixtures. The testing method is shown in Appendix D of the Guide. When the variety, strength, grade, and production plant of the cement in the commercial cement concrete used in the project changes, or the performance of the commercial cement concrete changes, the adaptability of the admixture to the cement should be re-tested.
2.1.4 When determining to use a certain admixture product in a technical and economic preferred project, construction raw materials should be used to make test pieces on the premise that the main purpose of using the admixture is met, and no less than 3 types of admixtures should be passed. Comparative tests on the performance of additive products, after the technical indicators meet the provisions of the corresponding types of additives in the guide, select the best or better ones. For economical reasons, the one with the lowest or lower price (unit cement dosage × actual dosage × unit price = admixture price) should be selected.
After comprehensive technical and economic comparison, admixture products were selected based on the principle of optimal performance-price ratio, and one product was determined as a use product and the other as a backup product through bidding.
2.1.5 Solution preparation Soluble admixtures should be prepared into a uniform solution 1 day in advance (special admixtures are allowed to be mixed in powder, but the stirring time should be appropriately extended), and the solution should be mixed through the mixer (floor) ) metering device is mixed into cement commercial concrete. When different types of admixtures are used in combination, their solubility and their impact on the performance of commercial cement concrete should be tested. They can only be used after meeting the requirements.