Long-term Properties of Aeolian Sand-magnesium Oxychloride Cement Composites and Its Engineering Application

In order to prepare a new material with long-term stable performance,low cost,easy construction,and ecological environmental protection,the influence of aeolian sand on the compressive and flexural strength as well as micro morphology and phase composition of magnesium oxychloride cement(MOC)was studied.The experimental results indicate that,with the increase of content of doping sand,the compressive strength and flexural strength of MOC decrease significantly.However,when the quality ratio of aeolian sand and light burned magnesia powder is 1:8,the performance meets the actual engineering needs.Namely,the compressive strength of MOC is not less than 18 MPa,and flexural strength is not less than 4 MPa.Meanwhile,within 12 months of age,the compressive strength and flexural strength are stable.There is no obvious change in phase composition,and its main phase is still 5·1·8 phase.Microscopic appearance changes from needle-like to gel-like shape.Based on engineering applications,it is found that when the novel sand-fixing material is used in the field for one year,its macroscopic feature is not damaged,compressive strength and flexural strength are also more stable,phase composition negligibly changes,and micro morphology has also been turned into be gellike shape.These further confirm the long-term stability and weather resistance of MOC doping aeolian sand,providing theoretical and technical support for the widely application of MOC in the field of sand fixation in the future.

Preparation and Performance of Chitosan/Citric Acid Modified Magnesium Oxychloride Cement

Citric acid(CA)and chitosan(CS)were employed to modify magnesium oxychloride cement(MOC).Multiscale measurements were implemented to study the properties of the modified MOC pastes.Results show that the addition of CA/CS significantly changes the content of each phase and the microstructure of phase 5.The single addition of CA can effectively increase the compressive strength of MOC after 7 d curing,while CS exerts no obvious effect on the compressive strength.As to the simultaneous addition of CA and CS,the compressive strength of MOC gradually decreases with the increasing content of CS.Interestingly,mixing CA and CS significantly enhances the water resistance of MOC and decreases the degradation rate of MOC in phosphate buffered solution,which can be ascribed to the low specific surface area of the plate-like crystals in the modified MOC and the reduction of pores in the structure.

Effects of H_3PO_4 and Ca(H_2PO_4)_2 on mechanical properties and water resistance of thermally decomposed magnesium oxychloride cement

The effects of H3PO4 and Ca（H2PO4）2 on compressive strength, water resistance, hydration process of thermally decomposed magnesium oxychloride cement （TDMOC） pastes were studied. The mineral composition, hydration products and hydration heat release were analyzed by XRD, FT-IR, SEM and TAM air isothermal calorimeter, etc. After being modified by H3PO4 and Ca（HzPO4）2, the properties of the TDMOC are improved obviously. The compressive strength increases from 14.8 MPa to 48.1 MPa and 37.1 MPa, respectively. The strength retention coefficient （Kn） increases from 0.38 to 0.99 and 0.94, respectively. The 24 h hydration heat release decreases by 10% and 4% and the time of hydration peak appearing is delayed from 1 h to about 10 h. The XRD, FT-IR and SEM results show that the main composition is 5Mg（OH）z＇MgCIz＇8H20 in the modified TDMOC pastes. The possible mechanism for the strength enhancement was discussed. The purposes are to extend the potential applications of the salt lake magnesium resources and to improve the mechanical properties of TDMOC.

Influence of MgO/MgCl_2 Molar Ratio on Phase Stability of Magnesium Oxychloride Cement

Formation, solution and phase change of hydration products in MgO-MgCl2-H2O system was studied with thermodynamics method, and resistance to water immersion and phase change of magnesium oxychloride cement with different MgO/MgCl2 molar ratio was experimented. The results show that pH value of immersion solution of cement paste has a remarkable influence on phase stability of hydration products. A higher pH value leads to a lower solubility and a better phase stability of hydration products. When the solution pH value is higher than 10.37, the precipitation of much Mg（OH）2 crystal induces a worse phase stability of hydration products. With the increasing MgO/MgCl2 molar ratio （lower than 6）, the more amount of MgO in the hydration products enhances the alkalinity of solution and the phase stability is improved. However, when the MgO/MgCl2 molar ratio is higher than 6 and the excessive MgO exsits in the hydration products, the cement paste may be damaged by the excessive crystallization stress of a great deal of Mg（OH）2 formation.

Effects of Citric Acid on Hydration Process and Mechanical Properties of Thermal Decomposed Magnesium Oxychloride Cement

In order to make full use of salt lake magnesium resources and improve the strength of the thermal decomposed magnesium oxychloride cement （TDMOC）, the effects of citric acid on the hydration process and mechanical properties of TDMOC was studied. The hydration heat release at initial 24 h and strengths at 3, 7, and 28 days of TDMOC specimens were conducted. The hydration products and paste microstructure were analyzed by XRD, FT-IR and SEM, respectively. The results showed that citric acid can not only reduce the 24 h hydration heat release and delay the occurring time of second peak of TDMOC, but also produce more 5Mg（OH）z.MgC12.SH20 and less Mg（OH）2 in hydration process of TDMOC. More perfect and slender crystals were observed in the microstructure of the TDMOC pastes with citric acid. The results demonstrated that citric acid as an additive of TDMOC can decrease the hydration heat release and increase the compressive strength and flexural strength of TDMOC. The possible mechanism for the strength enhancement was discussed.

Influence of Fly Ash and Silica Fume on Water-resistant Property of Magnesium Oxychloride Cement

By incorporation of fly ash or silica fume into magnesium oxychloride （MOC） cement, a high water resistance material can be formed for successful industrial applications. The influences of fly ash and silica fume on water-resistant property were investigated by SEM and EDS. It is found that the incorporation of fly ash or silica fume can improve the water-resistance of the MOC. The improvement of the water resistance of the MOC incorporated with fly ash or silica fume may be attributed to the alumino-silicate 5·1·8 gel or silicate 5·1·8 gel.

Characterizing properties of magnesium oxychloride cement concrete pavement

The performance of magnesium oxychloride cement concrete(MOCC)in road engineering in the arid region in northwest China was investigated over a two-year period.Two categories of MOCC pavement,light-burnt magnesia concrete road(Road-L)and dolomite concrete road(Road-D),were prepared with light-burnt magnesia and a mixture of light-burnt magnesia and caustic dolomite(1:3 by mass),respectively.Variations in the properties of the MOCC pavement,such as compressive and flexural strength,mineralogical phase,and microstructure,after being exposed to two rainy seasons in the field were monitored.The compressive strength of the cored samples were conducted after being aged for 28 d,and the compressive and flexural strength were tested at ages of 1,2,3,28,90,180,270,360 and 720 d.The mineralogical phase and microstructure of the pavement were also analyzed by X-ray diffraction(XRD)and scanning electron microscopy(SEM).The results demonstrate that MOCC pavement obtained desirable compressive and flexural strengths after curing for 3 d for Road-L and 28 d for Road-D.Both of the compressive and flexural strength of Road-L and Road-D decreased slightly after experiencing two rainy seasons,with the major hydration products being 5Mg(OH)2 MgCl28H2O(Phase 5)and 3Mg(OH)2 MgCl28H2O(Phase 3).The decomposition of Phase 5 is mainly responsible for reducing the mechanical strength of the MOCC pavement.

Microstructure and Properties of Cement Foams Prepared by Magnesium Oxychloride Cement

Microstructural features including pore size distribution, cell walls and phase compositions of magnesium oxychloride cement foams（MOCF） with various MgO powders and water mixture ratios were studied. Their infl uences on compressive strength, water absorption and resistance of MOCF were also discussed in detail. The experimental results indicated that moderate and slight excess MgO powders（MgO/MgCl2 molar ratios from 5.1 to 7） were beneficial to the formation of excellent microstructure of MOCF, but increasing water contents（H2O/MgO mass ratios from 0.9 to 1.29） might result in opposite conclusions. The microstructure of MOCF produced with moderate and slight excess MgO powders could enhance the compressive strength, while serious excess MgO powders addition（MgO/MgCl2 molar ratios = 9） would destroy the cell wall structures, and therefore decrease the strength of the system. Although MOCF produced with excess MgO powders could decrease the water absorption, its softening coefficient was lower than that of the material produced with moderate MgO powders. This might be due to the instability of phase 5, the volume expansion and cracking of cell walls as immersed the sample into water.

Frost Resistance of Magnesium Oxychloride Cement Mortar Added with Highland Barley Straw Ash

In order to study the influence of highland barley straw ash (HBSA) prepared under certain conditions on the durability of magnesium oxychloride cement mortar (MOCM) under freeze-thaw damage,rapid freeze-thaw cycle tests were carried out firstly.The relative mass evaluation parameters and the relative compressive strength evaluation parameters,which represent the degradation of freeze-thaw resistance,were used as the indices to study the degradation rule of MOCM.Secondly,nuclear magnetic resonance (NMR) tests were carried out on MOCM under different freeze-thaw cycles to analyze the pore diameter changes in the freeze-thaw process.The microstructure of MOCM was tested by Fourier transform infrared spectroscopy (FTIR),X-ray diffraction (XRD) and scanning electron microscopy (SEM),and then the effect mechanism of HBSA on the anti-freezing performance of MOCM was revealed.Finally,the two-parameter Weibull distribution function was used to analyze the reliability of durability degradation of MOCM added with HBSA under freeze-thaw cycles.The specific conclusions are as follows:With the increase of HBSA's addition,the freeze-thaw resistance of MOCM increase firstly and then decrease.When the addition of HBSA is 10%,the decay rate of relative mass evaluation parameters and relative compressive strength evaluation parameters is the slowest,and the frost resistance is the best.The proportion of harmful pores and more harmful pores in MOCM added with 10% HBSA decreases by 25.11% and 21.34%,compared with that without HBSA before and after freeze-thaw cycles.A lot of magnesium silicate hydrate (M-S-H) gels are generated in MOCM with HBSA content of 10%,which fills part of the pores,so that the proportion of harmful pores and more harmful pores is the lowest.The Weibull function can be effectively applied to the reliability analysis of the freeze-thaw cycle of MOCM added with HBSA,and the theoretical results are in good agreement with the experimental results.

Properties and printability evaluation of three-dimensional printing magnesium oxychloride cement by fully utilizing aeolian sand

Three-dimensional concrete printing(3DCP)is increasingly being applied in harsh environments and isolated regions.However,the effective utilization of aeolian sand(AS)resources and by-products derived from arid zones for 3DCP is yet to be fully realized.This study developed a three-dimensional(3D)printing composite using AS and magnesium oxychloride cement(MOC)from local materials.The effects of the mole ratio of MgO/MgCl_(2)and sand/binder(S/B)ratio on the mechanical properties such as water resistance,drying shrinkage strain,rheology,and printability,were investigated systematically.The results indicated that the optimal mole ratio of MgO/MgCl_(2),was 8,which yielded the desired mechanical performance and water resistance.Furthermore,the S/B ratio can be increased to three within the desired printability to increase the AS utilization rate.The rheological recovery and buildability of the 3D-printed MOC with AS were verified.These findings provide a promising strategy for construction in remote deserts.

THE PROPERTIES OF SILICA FUME-MAGNESIUM OXYCHLORIDE CEMENT MATERIALS

The properties of a new magnesium Oxychloride cement (MOC) material formed by silica fume uniformly mix in MOC paste was presents. The influence of silica fume on the water resistance and compressive strength of MOC paste was invesigated in this study. It is shown that when 30 weight percent of silica fume is added to the MOC paste, a high strength and water resisting new material with 112MPa compressive strength and 1 00 water resisting coefficient could by obtained.

Effects of EVA Latex on the Properties of Glass-fiber/Magnesium-oxychloride Cement Composites

The effects of Ethylene-Vinyl Acetate copolymer （EVA） latex as an additive or a glass fiber surface modifier on the properties of Glass-Fiber （ GF ）/ Magnesium Oxychloride Cement （MOC） composites was studied. The mechanical properties, water resistance aud aging resistance of the cured GF/ MOC composites were estimated and chemical ingredients analysis and morphological study of the GF/ MOC composites were also performed. It is found that EVA added to the MOC matrix could substantially improve the interfacial adhesion, water resistance aud aging resistance of GF/ MOC composites. EVA treatment on glass fibers resulted in decreasing initial flexural strength of GF/ MOC composites while enhancing the soft coefficients. In addition, the drying time and dilution of the EVA treatment on glass fibers also had an otwioas effect on the properties of GF/ MOC composites. However, excessive EVA interfered with the growth of the 5 Mg（ OH）2· MgCl2 ·8H2O crystal and the properties of GF / MOC composites.

Effect of Mixing on Properties and Microstructure of Magnesium Oxychloride Cement

This paper investigated the effect of mixing on the properties of magnesium oxychloride cement, such as apparent viscosity, setting time, compressive strength, and specific strength as well. The phase composition and crystallographic structure of the cement were determined by X-ray diffraction and scanning electron microscopy. The results indicate that the increases of stirring rate and mixing time change apparent viscosity distinctly, shorten setting time and enhance mechanical property. Magnesium oxychloride cement after mixing for 20 min exhibits the optimal mechanical performance. The effect of stirring rate on the phase composition of magnesium oxychloride cement dominates over the effect of mixing time. The increases of stirring rate and mixing time can increase the dispersion state and accelerate the neutralization, thus promoting the strength of magnesium oxychloride cement due to the formation of stable 5·1·8 phase, needle-like crystals and continuous crystalline structure in the whole cement matrix. The properties and microstructure of magnesium oxychloride cement at a stirring rate of 280 rpm are better than those at 140 rpm.

RESEARCH ON THE RHEOLOGY OF MAGNESIUM OXYCHLORIDE CEMENT Ⅱ—STUDY OF INFLUENTI AL FACTORS ON RHEOLOGICAL PROPERTIES

The influences of solution cement ratio (Wn/C), MgO content of the magnesia, specific gravity of the MgCl 2 solutions and mixing regime on the rheological properties of magnesium oxychloride cement (MOC) pastes were studied with a rotating viscometer. It was found that different influential factors cause difference rheological properties of the MOC pastes.

RESEARCH ON THE RHEOLOGY OF MAGNESIUM OXYCHLORIDE CEMENT Ⅰ—RHEOLOGICAL BEHAVIOR

The rheology of magnesium oxychloride cement (MOC) was presented. The rheological properties and thixotropic behaviors of the MOC pastes were determined with a rotating viscometer. The results have showed significant difference between the MOC and portland cement in terms of their rheological behaviors. The yield stress and plastic viscosity of the MOC pastes are much larger than those of portland cement pastes. Therefore, the MOC has a better cohesive property.

Effect of the Proportion of Bamboo Scraps on the Properties of Bamboo Scraps/Magnesium Oxychloride Composites

This study was designed to solve the problem of large waste volume from bamboo processing residues in recent years.Using magnesium oxychloride(MO)cementitious material as the main material and bamboo residue(BR)as the reinforcing material,a BR/MO composite material was prepared.The effects of BR amount on the molding properties,mechanical strength,and water resistance of BR/MO composites were examined and discussed.Scanning electron microscopy(SEM),X-ray diffractometry(XRD),and thermogravimetric analysis were used to characterize composite microscopic morphology,crystalline structure,and heat resistance.The results showed that,when the BR content was 1.00%(by wt),the flowability of MO paste was beneficial to composite molding.Composite mechanical properties and water resistance were greatly affected by BR addition.When the BR content was 1.00%,composite compressive and bending strengths and softening coefficient all reached maximum values.Meanwhile,increases in water absorption by 24 h and decreases of contact angle were small.These results suggested that,when the BR content was 1.00%,composite mechanical properties and water resistance were the best and the mechanical strength also improved with extended composite storage time.SEM analysis indicated that BR played the role of a reinforcing phase in MO matrices.However,when the BR content exceeded 1.00%,interfacial bonding between BR and MO became less.XRD analysis showed that,with 1.00%BR content,composites showed more 5-phase crystals with high strength.This further explained the reason why this composite’s mechanical properties were the best and the heat resistance not deteriorated due to BR,which was easily decomposed.

Consolidation Mechanism of Chloride Ion in Soda Residue from Ammonia Soda Process Method

Soda residue-magnesium oxychloride cement is prepared with soda residue from ammonia soda process method,magnesium oxide and magnesium sulfate heptahydrate as main raw materials,and its consolidation mechanism of chloride ion is studied.The results show that the hydration products of soda residue-magnesium oxychloride cement are mainly 5-phase,gypsum and brucite,which exist in the matrix in needle rod shape,long plate shape and hexagonal plate shape,respectively.When the molar ratio of MgO/MgCl_(2) is 8:1,the concentration of MgSO_(4) is 29%,and the mass ratio of soda residue:magnesium oxide:magnesium sulfate heptahydrate is 45.8:36.4:17.8.The chloride ion consolidation effect of the sample is the best,and the chloride ion consolidation content of the 7 d sample is about 93%.The chloride ion consolidation content of the 28 d sample is about 96%.

Composites Based on GM(1,1)-Markov in the Salt Spray Corrosion Environment

This study was designed to solve the problem of magnesium hazards due to potash extraction in the salt lake region.Using basalt fiber(BF)as the reinforcement material and magnesium oxychloride cement(MOC)as the gelling material,a BF/MOC composite material was prepared.Firstly,the effect of BF addition content on the basic mechanical properties of the composites was investigated.Then,through the salt spray corrosion test,the durability damage deterioration evaluation analysis was carried out from both macroscopic and microscopic aspects using mass change,relative dynamic modulus of elasticity(RDME)change,SEM analysis and FT-IR analysis.Finally,a GM(1,1)-Markov model was established to predict the durability life of composite materials by using durability evaluation indicators.The results show that:when the BF content is 0.10%(by volumetric content),the composites have the best mechanical properties and resistance to salt spray corrosion.However,when the volume of BF content exceeds 0.10%,a large number of magnesium salt crystallization products are observed from the microscopic point of view,and the corrosion of the main strength phase of MOC is more serious.The prediction results of the GM(1,1)-Markov model are highly identical with the raw data.In addition,using the change of RDME as a predictor,RDME is more sensitive to environmental factor compared to the change of mass.Predictions using the change of RDME as a threshold indicate that MOC-BF0.10 has the longest durability life,which is 836 days.The model is important to promote the application of MOC composites in the salt lake region and to promote the healthy development of green building materials.