Fiber-cement compositions modified by flocculating additives
Chemical additives in the production of cement-fiber products are used to improve the properties of cement-fiber suspension and freshly formed sheets, as well as to accelerate the process of filtration and sedimentation of the solid phase in recuperators. To improve the filtration properties, flocculating additives are added to the composition of cement-fiber suspensions. The purpose of research is to establish the influence of the degree of ionic charge and the molecular weight of polyacrylamide (PAA) on the kinetics of cement hydration, the rate of precipitation of cement-fiber suspension and the strength of cement-fiber plates. According to the results of the first stage of the performed experimental studies, it was established that the deposition efficiency increases with an increase in the degree of ionic charge of PAA. At the second stage, it is determined that the amount of PAA from 0.05-0.15% by weight of cement slightly increases the normal density (NG) of the cement paste and, depending on the content, has approximately the same effect on its setting time. The beginning of the setting occurs at 54-64 minutes later, and the end of the setting at 16-31 minutes before the composition without additives. At the third stage, the kinetics of cement hydration with the studied additives of PAA was studied. The process of cement hydration was studied by systematically determining a quantitative indicator of the kinetics of heat release and contraction of the Portland cement system and subsequent analysis and change it over time. The achievement of the temperature maximum on the curves of hydration of Portland cement with the content of PAA up to 0.1% indicates the intensification of the hydration process in the initial period of hardening. The increase in the amount of PAA in the composition of more than 0.1% leads to a slower process of cement hydration and a decrease in the temperature maximum. Also, the specific heat of Portland cement in all periods of hydration increases with increasing content of PAA to 0.1% and decreases with a further increase in its dosage. In this regard, the optimal content is 0.5-0.1% for all brands of PAA. The growth of activity and the value of specific contraction of Portland cement contraction curves increases with the content of PAA to 0.1%. The increase in the number of PAA in the composition of more than 0.1% leads to a decrease in activity in the value of specific contraction. Introduction to the composition of more than 0.10% of PAA hinders the filtration ability of the cement-fibrous mass during pressing due to the high water-holding capacity of the composition. At the next stage, it is noted that the greatest increase in strength is achieved when using anionic PAA Besfloc K4046, which has a high degree of ionic charge. The introduction of this additive in the cement-fiber mixture in the amount of 0.05-0.1% increases the bending strength by 10-15%.