| The offshore and coastal structure engineering will be seeing an increase in number,and at the same time,the existing engineering are suffering from increasingly serious corrosion.Calcium sulfoaluminate cements exhibit good resistance against freezing-thawing and chemical corrosion caused by seawater,as well as excellent cementitious characters,making them suitable for the construction and repairing in offshore and coastal structure engineering.However,the current scientific theories of calcium sulfoaluminate cements are limited.One of the most important things is that the microstructure and cementitious mechanism of aluminum hydroxide phase(AH3)are not adequately investigated,while AH3 phase is an important hydration product in calcium sulfoaluminate cements,and makes an important contribution to the strength of calcium sulfoaluminate cements and concretes.In this work,the microstructure of AH3 phase is investigated in detail using a multitechnique approach,the microstructure of AH3 phase is controlled by pH and temperature,and the cementitious mechanism of AH3 phase and the source of strength of calcium sulfoaluminate cements are eatablished,which are favorable to better improve the performances of CSA cements.The main conclusions can be drawn as follows:The microstructure of AH3 phase formed during the hydration of calcium sulfoaluminate cements is investigated in detail.Various AH3 phases are synthesized from their chemical constituents using sol-gel processing and compared with the AH3 phase formed during the hydration of st-ye'elimite(i.e.,the main mineral in calcium sulfoaluminate cements).The final AH3 phase formed during the hydration of st-ye'elimite is partly crystalline with a microcrystalline structure,and not amorphous.Entropy,free energy changes in thermodynamics,and irreversible and oriented aggregation mechanism may be the main reasons for the existence of microcrystalline AH3 phase.Bedides,the similarities and differences of the microstructure of the final AH3 phase formed during the hydration of three structural modifications of ye'elimite(namely,ss-ye'elimite,Sr-ye'elimite,and Ba-ye'elimite)are investigated.Firstly,the formed amount of AH3 phase in three structural modifications of ye'elimite is different at an early vs late stage and is always lower than that in st-ye'elimite.Secondly,the AH3 phase in three structural modifications of ye'elimite exhibits a slightly different composition that consists of traces of Ca,Fe,and Sr in the corresponding ye'elimite pastes although all AH3 phase have an agglomerated lamellar morphology.Finally,the AH3 phase exhibits a different final value of log KsO in the structural modifications of ye'elimite pastes although the solutions are almost at equilibrium with respect to microcrystalline AH3,and the AH3 phases formed from ss-and Sr-ye'elimite pastes are nanocrystalline,whereas the AH3 phase formed in Ba-ye'elimite pastes is microcrystalline.In addition,this work also focuses on the microstructure of AH3 phase in two main hydration product systems in calcium sulfoaluminate cements,i.e.,the AFm-AH3 system and the AFt-AH3 system.It is seen that AH3 phase in both AFm-AH3 system and AFt-AH3 system has a gibbsite-like structure presented in a microcrystalline or nanocrystalline structure,but the microstructure of AH3 phase in both system is different.The AH3 phase in AFt-AH3 system has a small particle size,and the ion activity product of AH3 phase in AFt-AH3 system is higher than that in AFm-AH3 system,indicating the AH3 phase in AFt-AH3 system has a smaller crystal size.And the AH3 phase in AFt-AH3 system has an average crystallite size close to 20 nm using a multitechnique fitting method.The microstructural evolution of AH3 phase under pH and temperature control was studied.Different alkaline and temperature conditions are used to control the microstructure of AH3 phase,the microstructural evolution of AH3 phase is found,and the relationship between microstructure and macroscopic performance is established.On the one hand,as alkali concentration increases,the crystallinity of AH3 phase improves.In the water or low alkaline conditions,the AH3 phase has a microcrystalline or nanocrystalline structure.The average crystallite size of the AH3 phase is about 8-9 nm.The particle size distribution of the AH3 phase mostly ranges from 110 nm to 210 nm.In high alkaline conditions,the AH3 phase presents a well-crystalline nature.The average crystallite size of the AH3 phase increases up to about 30-40 nm.The particle size distribution of the AH3 phase is mostly 800-1400 nm or 1000-1500 nm.On the other hand,as temperature increases,the crystalline state of AH3 phase formed in ss-ye'elimite pastes remains a microcrystalline structure;however,the crystallinity of AH3 phase formed in ss-ye'elimite pastes improvs from microcrystalline to well-crystalline structure.In addition,it is the first time using TEM technology to directly envidence that AH3 phase has a microcrystalline structure and grow into a single crystal at 80?.The cementitious mechanism of AH3 phase is studied.The radio of st-ye'elimite,gypsum and calcium hydroxide could determine the AFt to AH3 ratios in the final hydration products,which is confirmed by corresponding thermodynamic calculation and experiments.The mechanical performance of hardened cement pastes with different AFt to AH3 ratios is investigated,and XRD,TG-DTG,SEM and MIP measurements are used to analyse the effect mechanism of AH3 phase.It is seen that owing to its microcrystalline nature,AH3 phase has great influence on the mechanical strength and fills empty spaces thus improving the compacity of the cement pastes,and the compressive strength improves with an increase of the content of AH3 phase.The content of AH3 phase decreases from 16.8 wt%to 2.3 wt%,the corresponding compressive strength decreases from 22.6 MPa to 9.5 MPa,the porosity increases from 24%to 32%,the median pore diameter increases from 1 38.1 nm to 1439.4 nm,and the pore size increases from 28.9 nm to 326.2 nm.The effect mechanism of AH3 phase is optimized by combining with hydration degree,and the source of strength of calcium sulfoaluminate cements is eatablished.It is seen that when 27.32 wt%gypsum is added,the flesh cement paste shows the good yield stress and plastic viscosity;in addition,the harden calcium sulfoaluminate cement paste has the highest compressive strength and the best the compacity and pore structure,and the relationship between the hydration degree and AH3 content reaches the best match point. |
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