Influence Of Cellulose Ethers On The Hydration And Performance Of Calcium Sulphoaluminate Cement

Calcium sulphoaluminate(CSA)cement is deemed to be the third series of cement.It has the futures of fast setting and high early strength which enable it widely used in fast repair or construction,construction in winter,harbor projects,prefabricated articles etc..Cellulose ethers(CE)are one of the most important additives in modifying cement-based materials due to their good water retention and thickening effect.Although the research on the application of CE in Portland cement-based materials has been relatively rich,there are very limited existing studies on their application in CSA cement-based materials,especially lack of understanding on the influence of CE structural parameters.With the above background,it is targeted to narrow the gap in this research.Six grades of CE under three chemical categories were selected for the study.These CE include one hydroxyethyl cellulose(HEC),two hydroxyethyl methyl cellulose(HEMC)with different substitution degrees,one modified HEMC and two hydroxypropyl methylcellulose(HPMC)with different substitution degrees.Their influence at 0.3%-0.9% dosages on the properties of CSA cement mortar was evaluated at fixed mortar flow or water to cement ratio.The hydration process and the pore structure of CE modified CSA cements were studied by calorimetry,ultrasonic test,scanning electron microscope(SEM),X-ray diffraction(XRD),thermogravimetry(TG)and mercury intrusion porosimetry(MIP).In this study,the influence of CE structure on the properties of fresh and hardened CSA cement mortar is revealed.Their influence on the setting and hydration process together with pore structure change of CSA cement is also clearly demonstrated.A novel model is proposed to illustrate how CE change the early-stage hydration process.The relationship between application properties of modified mortar and CE structural parameters is established through a statistical model.Key findings of the study are as below.The basic properties of these CE and their solutions were characterized.HEC has obviously larger particle size than the rest.It is the most hydrophilic with high surface tension.Highly-substituted HPMC is the least hydrophilic and its solution has the lowest surface tension.The increase of CE concentration exerts less impact on surface tension than CE structure.The effects of CE on the properties of fresh and hardened mortar are revealed.At fixed water to cement ratio,the addition of CE leads to an increase in the water retention rate,air content and evaporation rate of water within 24 h,while reducing the flow,consistency value and wet density of mortar.The structure and dosage changes of CE have negligible effect on water retention rate,but significantly affect other properties.Higher CE dosage tends to have more pronounced effects on these properties,but the degree of the effects varies for different properties.The effects of CE with different structures on the flow and consistency value are obviously different.The wet density is positively related to the surface tension of CE solution in a nonlinear manner.Lower wet density favors a higher water loss rate within 24 h.The plastic shrinkage of modified mortar decreases,and the decrease becomes less when lowly-substituted HEMC/HPMC are added.The increase of CE dosage also lessen the decrease.After hardening,the compressive strength and flexural strength of the modified mortar suffer great loss,and the compressive strength to flexural strength ratio is significantly reduced.The higher the CE dosage or the degree of substitution,the more pronounced the effects.The results of compressive and flexural strength are positively correlated with wet density.The tensile adhesion strength of modified mortar is improved.It is driven by the roughness of substrate,wettability and strength of mortar.The optimal choice of CE varies with different substrates.The capillary water absorption and drying shrinkage of modified mortar may increase or decrease depending on CE structures.CE affect the capillary water absorption by changing the pore structure.The accumulation of pores within the diameter range of 5 μm-50 μm to a certain extent gives a negative impact on the results.The addition of lowlysubstituted HEMC/HPMC is beneficial to obtaining lower drying shrinkage,and the effect of HEC and highly-substituted HPMC is more influenced by their dosage rates.At fixed flow,the water demands of modified mortar increase almost linearly with the increase of CE dosage.Higher water loading has significant negative impacts on the compressive and flexural strength,capillary water absorption and drying shrinkage.The compressive to flexural strength ratio is further reduced.The addition of modified HEMC leads to an additional increase in water demand,but it narrows the performance gap of modified mortar between at high and low CE dosage.Recommendations are made on the choice of CE in modifying CSA cement based on the above findings.HEC is recommended for scenarios that require high compressive/flexural strength.Lowly-substituted HEMC/HPMC(especially the former)are preferred for scenarios that need more balanced performance.The addition rate of CE is not supposed to be higher than 0.6% to cement.The influence of CE on the setting and hydration of CSA cement is revealed.The pore structure changes of modified mortar impacted by CE are also disclosed.The setting time of modified mortar is prolonged.The distance among cement particles and the particle size of CE powder may act as key contributing factors.There exist two exothermal peaks in 24-hour hydration process.The addition of CE advances the second exothermal peak where the structure and dosage of CE play important roles.The total hydration heat within 24 h is slightly impacted.The hydration degree of HEC-modified cement at low dosage is slightly promoted.Others are either not influenced or become slightly lower.The morphology of hydrates changes with the change of CE structure.AFt grains identified in HEC-modified cement are the largest and most complete.Two functioning mechanisms are found when CE exert influence on the hydration of CSA cement at early stage.Firstly,CE forms film in matrix which acts as a medium for easier water migration to remained unhydrated cements.This makes the second exothermal peak take place earlier.Secondly,the connectivity of pores in matrix is changed by the addition of CE.When the pores are highly connected,it becomes easier for free water to migrate to unhydrated cement particles which accelerates the hydration.The latter acts a supplement mechanism to the former.With the addition of CE,the porosity together with the median(by volume)and average pore diameter of modified mortar increase significantly.The pore size distribution changes from single-peak to multi-peak model.The change of CE structure exerts more pronounced impacts on the pore structure than their dosage change.A statistical model is built with the application properties of modified mortar as dependent factors,using the substitution degree and dosage of CE as independent factors.The model shows that the interaction between methyl and hydroxyalkyl groups makes the performance of HEC and HEMC/HPMC different.In the presence of methyl group,the wet density,compressive strength and flexural strength of mortar are affected by the content of hydroxyalkyl groups and CE dosage;In the case of high hydroxyalkyl content,methyl group has a significant impact on wet density and compressive strength;Dry shrinkage is mainly affected by methyl and hydroxyalkyl groups.The model can well explain and predict the impacts of the changes in these CE parameters.