Study On The Perfermances And Microstructure Of Cement And Its Mortar Modified With HEMC

As an important modifying material of various dry-mixed mortars and with excellent capability of thickening performance of water, HEMC could not only prevent the moisture in the fresh cement-based materials to avoid excessive evaporation or absorption of drying grass-roots to ensure the cement hydrated adequately but also can improve the bonding properties of the cement mortar to adjust the rheological properties of cement mortar, cement mortar bond strength and green cement-based materials'capabilities of anti-bleeding, anti-erosion and anti-dispersion, and it can boost the shrinkage rate of the mortar to prevent the cracking of cement mortar, hollowing salt erosion and other phenomena as well. It plays an important role in guaranteeing the service function and lifespan of cement mortar and concrete.This article relied on the National Natural Science Foundation project "Hydration process and microstructure formation mechanism of thin-layer cellulose ether modified mortar"(50902107), comprehensively analyzed and researched on the rheological properties of cement mortar modified with HEMC and its work performance, mechanical properties, micro-structural features to analyze and grasp the variation pattern. The main conclusions were as follows:(1) Studying on HEMC's solubility properties and discussing the rheological properties and workability of slurry modified with HEMC to grasp HEMC's influences on the workability of modified slurry.Aqueous solution of HEMC was capable of inducing thermal gelation characteristics, and this process was reversible and only related to temperature; surface-treated HEMC could be quickly dispersed in water and delay in solubility; without surface treatment and directly added to the cold water, it could reunite and pack; HEMC's aqueous solutions were pseudoplastic fluids, the apparent viscosity declined with the rise of the solution's temperature, increased with increasing concentration of HEMC and the increase of HEMC degree of polymerization, and decreased with the increase in shear rate; high concentrations of HEMC aqueous solution have greater thixotropy; HEMC's aqueous solution was stable when its pH was3to10; high concentration of inorganic salt solution could cause the phenomenon of precipitating salt; grout modified with HEMC was also a pseudoplastic fluid, and its apparent viscosity increased several times with the increase of HEMC and decreased with the increasing shear rate; the pseudoplastic and thixotropy of modified slurry system were more sensitive in the low changes of shear rate; the amount of water for normal consistency of the slurry modified with HEMC increased with the HEMC's dosage and polymerization degrees; Used in low dosage (about0.02%to0.04%), the HEMC could improve the plasticity and workability of the cement paste; the retention of water boosted with the increasing HEMC's polymerization degrees. In addition, the slurry's sequence of retention of water with the same contents from high to low was:HEMC3> HEMC2> HEMC1; when the HEMC's content increased to0.4%, the retention rate of water could reach more than95%; HEMC had a significant retarding effect, and the setting time increased with increasing HEMC content and degree of polymerization; In experiment, the HEMC increased13.6%to153%the initial setting time than pure slurry while it lengthened15.9%to88.8%the final setting time.(2) Analyzing the compressive strength, flexural strength, bond strength of the mortar modified with HEMC, researching its influence on the mechanical properties of the modified mortar.Due to the air entrained by HEMC, leading to increasing porosity, the compressive strength of the modified slurry decreased with increasing HEMC content and degrees of polymerization; modified paste's rate of compressive strength to flexural strength decreased with the increasing content of HEMC; HEMC developed the flexibility and plasticity of the modified mortar and improved its bond strength between the modified cement paste and the interface of substrate material;(3) Using the optical microscopy, XRD, TG-DSC-DTG, FTIR, SEM and other microscopic testing methods to study on the micro-structural features of cement paste modified with HEMCHEMC used content from0%to0.4%, most of the holes of the modified slurry's surface,observed by optical microscopy, grew up from less than50μm to more than100μm; when the dosage was up to0.8%, most of the holes of the section in the modified slurry samples ranged among200-450μm, and the cement mortar modified with high polymerization degrees'HEMC could entrain more air; the modified sand slurry samples tested with XRD, TG-DSC-DTG analysis and FT-IR analysis, HEMC's involvement did not change the modified slurry's types of hydration products in each age period and did not generate a new phase, and the hydration products included ettringite, calcium hydroxide, CSH gel. HEMC could significantly delay the hydration products with the retarding effect; HEMC with different degrees of polymerization had little diversities on the hydration of cement, and HEMC hardly had any influences in the late period; hydrated for3d, it was observed in modified grout body that there was small number of clumps of fine cashmere-like fiber structure of C-S-H gel, shot outward collectively, and the ettringite size was coarse,short and in small number; hydrated for28d, the structure of the pure cement mud and the modified mortars had been very dense with complete development of the crystal. There were a large number of CSH gel and hexahedral Ca(OH)2crystals on the surface of solid across among the solid particles, appearing the reticular structure of mutual cross-linking,.Besides, within the scope of the view, few ettringite crystals were observed.