Preparation And Properties Of Porous Concrete With Alkali-activated Cementitious Materials

As a low carbon and green cementitious material,alkali-activated cementitious material has the characteristics of fast hardening,high strength,good frost resistance,high temperature and acid corrosion resistance,while porous concrete,as an ecological concrete,it has multiple environmental benefits in flood control and drainage,sound absorption and noise reduction,sewage removal and water purification,planting and afforestation,which is of special significance for the construction of"sponge cities",the combination of the two has good development potential and broad application prospects.In this paper,the following aspects of research work were carried out:（1）The fluidity and mechanical properties of alkali activated slag/fly ash paste and the interfacial adhesion between alkali activated slag/fly ash paste and limestone aggregate were tested.The effects of water binder ratio,Na₂O content,sodium silicate modulus and fly ash content on the above properties were studied;（2）The mechanical properties,water permeability and pore alkalinity of alkali activated slag/fly ash porous concrete were tested.The effects of water binder ratio,sodium silicate activator content and modulus,fly ash content and aggregate size on the above properties were studied and analyzed;（3）The durability of alkali activated slag/fly ash porous concrete was studied;（4）The composition and microstructure of hydration products of alkali activated slag/fly ash cementitious materials were analyzed.The main conclusions are as follows:when the content of sodium silicate activator increases from 3%to 6%,the compressive strength,fluidity and interfacial bond strength of alkali activated slag/fly ash paste increase;when the modulus of sodium silicate increases from 1.0 to 1.6,the compressive strength,fluidity and interfacial bond strength of alkali activated slag/fly ash paste increase first and then decrease When the content of fly ash increases from 0%to 40%,the compressive strength of alkali activated slag/fly ash paste decreases,the fluidity increases,and the interfacial bonding strength between the paste and limestone aggregate increases.The results show that the water binder ratio of alkali activated slag/fly ash porous concrete should be controlled between 0.32～0.34,and the fluidity of cement paste should be controlled between 130～160mm;the compressive strength of alkali activated slag/fly ash porous concrete can reach 33mpa and the permeability coefficient can reach 3.4mm/s in 28d under the optimal mix proportion（target porosity 15%,Na₂O content 6%,water glass modulus 1.2,fly ash content 20%）;Under the condition of the same target porosity,the strength of alkali activated slag/fly ash porous concrete prepared with different particle size of limestone aggregate from high to low is 4.75～9.5mm,2.36～4.75mm,9.5～16mm;the p H value of alkali activated slag/fly ash porous concrete pore solution is 11.27～11.72 when the content of sodium silicate is 4%～6%and the modulus is 1.0～1.6 The p H value of pore solution of alkali activated slag/fly ash porous concrete increases.Under the alternate action of drying and wetting,the compressive strength of alkali activated slag/fly ash porous concrete increases with the increase of drying and wetting cycles.After 30 drying and wetting cycles,the compressive strength of porous concrete begins to decline.Under indoor drying conditions,the more times of drying and wetting cycles,the greater the strength loss rate of porous concrete;under freeze-thaw cycles,the strength loss rate of alkali activated slag/fly ash porous concrete after 25 freeze-thaw cycles is less than20%,and the mass loss rate is less than 2%,which meets the requirements of CJJ/T135-2009frost resistance index.And with the increase of fly ash content,the strength of porous concrete increases,both degree loss rate and quality loss rate are increasing.Alkali activated slag/fly ash cementitious materials contain calcite（Ca CO₃）,quartz（Si O₂）and hydrotalcite(Mg₆Al₂（OH）₁₆CO₃·4H₂O),mullite and other minerals.Hydrated products C-（A）-S-H and N-A-S-H gel mostly appear in amorphous state.