Proportion and application of HPMC in machine sprayed mortar

HPMC, also known as hydroxypropyl methyl cellulose, cellulose ether, is the selection of highly pure cotton cellulose as raw material, treated with lye at 35-40℃ for half an hour, pressed, crushed cellulose, appropriate aging at 35℃, so that the average polymerization degree of alkali fiber in the required range. Put the alkali fiber into the etherification kettle, add propylene oxide and methane chloride successively, etherize at 50-80℃ for 5h, the highest pressure is about 1.8mpa. Then add proper amount of hydrochloric acid and oxalic acid washing materials in 90℃ hot water to expand the volume, and then dehydrate by centrifuge. Finally, after repeated washing to neutral.

In recent years, with the establishment of large dry-mixed mortar plants throughout the country, the quality and stability of mortar has been guaranteed. Dry mortar is, however, produced by the factory processing, from raw materials, the price must higher than the mixing, if continue to adopt the way of artificial plaster, relative to the site mixed mortar no competitive advantage, if some countries' ban now 'policy, the new dry mortar plant is difficult to make a living, eventually go bust. What's more, many first-tier cities such as Beijing, Guangzhou, Shenzhen and other places have a shortage of migrant workers, the labor cost of construction is getting higher and higher, prompting the perfect combination of mechanized construction and dry mixed mortar.

Brief introduction of the HPMC performance of machine sprayed mortar

Compared with the traditional site mixing mortar, the biggest difference of machine spraying mortar is the introduction of HPMC hydroxypropyl methyl cellulose ether and a series of admixtures can optimize the performance of mortar, so that the workability of the new mixing mortar is good, high water retention rate, long distance, high height after pumping still has good performance. Its biggest advantage lies in the construction efficiency is high, the quality of the mortar after molding is good, because the mortar has a large initial speed when spraying, so it can have a relatively strong grasping adhesion with the matrix, can effectively reduce the occurrence of hollow drum cracking phenomenon.

Through continuous tests, it is found that when the mortar is sprayed by the compounding machine, the machine-made sand with the maximum particle size of 2.5mm and the content of stone powder less than 12% and reasonable gradation is adopted, or the river sand with the maximum particle size of 4.75mm and the content of mud less than 5% will not cause pipe blockage. When the water retention rate of the new mixing mortar is controlled above 95%, the consistency value is controlled at about 90mm, and the loss of 2h consistency is controlled within 10mm, the mortar has good pumping and spraying performance. Moreover, the mortar after molding has smooth and smooth appearance, uniform and rich slurry, and no flow hanging or hollow drum cracking phenomenon.

Discussion on compound additive of machine sprayed mortar

The construction process of spraying mortar mainly includes mixing slurry, pumping and spraying. Under the premise of reasonable formula and qualified raw material quality, the main function of compound additive for machine-shotcrete mortar is to optimize the quality of newly mixed mortar and improve the pumping performance of mortar. Therefore, the general machine spray mortar compound additive is composed of water retention agent and pumping agent. Hydroxypropyl methyl cellulose ether is an excellent water retaining agent, which can not only increase the viscosity of mortar, but also significantly improve the fluidity of mortar under the condition of the same consistency value, and reduce the occurrence of segregation and bleeding phenomenon. Pumping agent is generally composed of air entraining agent and water reducing agent. In the mixing process of new mixing mortar, a large number of tiny bubbles are introduced to form ball effect, which can reduce the friction resistance between aggregate particles and improve the pumping performance of mortar. In the spraying process of machine spraying mortar, due to the rotation of the screw pump caused by the micro vibration easy to stratify the mortar in the hopper, resulting in the upper consistency of small value, the lower consistency of large value, the machine is easy to lead to blocked pipe, and after forming the quality of the mortar is not uniform easy to appear dry shrinkage cracking. Therefore, in the design of jet mortar composite additives, should be appropriate to add some stabilizer, slow down the layered phenomenon of mortar.

When the staff is doing the machine spraying mortar experiment, the compound additive dosage is 0.08%, and the final mortar has good workability, excellent pumping performance, no flow hanging phenomenon in the process of spraying, and the maximum thickness of one spraying can reach 25px.