Microstructure of High C3A Portland Slag Cement Pastes,Modified with Accelerating Admixtures for Concrete

摘要

Modern concrete technology includes mineral additives and chemical admixtures usage.It is caused by their beneficial influence on properties of concrete mix and hardened concrete.Accelerating admixtures for concrete are commonly used for shortening of time demanded for demoulding and repeat use of forms in precast facilities.They allow to conduct works during low-temperature season.Main advantage of accelerating admixtures is enhancement of early strength of concrete.Alas they may cause decrease of long-term strength and durability of concrete or increase its shrinkage One of the most popular mineral additives is ground granulated blast furnace slag (GGBFS).It is non-clinker main constituent of CEM II,CEM III and CEM V.GGBFS may be also used as additive with latent hydraulic properties for concrete.GGBFS as constituent of concrete increases consistency,long-term strength and durability,and decreases hydration heat evolution.Early compressive strength of concrete with GGBFS is lower than for Portland cement concrete.Accelerating admixtures and ground granulated blast furnace slag show advantages and disadvantages that can be equalized.In early terms calcium nitrate and crystal seeds enhanced compressive strength.Their efficiency is similar.Cement kiln dust also caused increase of compressive strength but not as much as former ones.Sodium hydroxide caused great increase of compressive strength after 12 hours but not in longer terms.In case of cements rich with C3A the compressive strength in early stage of hardening is shaped by C-S-H phase and well-developed ettringite crystal skeleton.In spite of minor differences in non-modified and calcium nitrate modified cement pastes microstructure,the compressive strength of calcium nitrate modified mortars is significantly greater in comparison to non-modified ones.The greatest compressive strength was achieved by mortar modified with crystal seeds.Responsible for this increase is more well-developed C-S-H phase.Mortars modified with sodium hydroxide are weaker after 2 days of curing in comparison to non-modified mortar.It is caused by sparse ettringite crystal skeleton.Microstructures of non-modified and modified with cement kiln dust (CKD) cement pastes are similar.It is connected with similarity of chemical composition of CKD and Portland clinker.The compressive strength of CKD modified mortars is slightly greater than non-modified one.