Influence of calcium sulfoaluminate cement on early-age properties and microstructure of Portland cement with hydroxypropyl methyl cellulose and superplasticizer

In order to improve the construction efficiency of thin layer masonry mortar (TLMM) even to realize mechanized construction, it is necessary to maintain suitable workability and at the same time get early-strength. To improve the workability of TLMM cellulose ether and superplasticizer were usually used. Calcium sulfoaluminate cement (CSA) was used as mineral accelerator in the paper to make the mortar get early-strength. Therefore, the effect of CSA on the early-age properties and microstructure of Portland cement (PC) with hydroxypropyl methyl cellulose (HPMC) and polycarboxylic acid superplasticizer (PA) was investigated. The X-ray diffraction (XRD), thermogravimetry (TG), isothermal calorimetry, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to clarify the impacts. Results showed that the setting time of the PC with HPMC-PA was accelerated with the increase of CSA replacement rate (CRR). CRR also had a significant impact on the development of compressive strength, especially when CRR was over 5.0% by weight, the compressive strength of samples were obtained after curing for 6 h. Moreover, the microstructure of the sample changed with CRR. With the increase of CRR, the content of ettringite (AFt) was increased and its morphology became shorter and thicker, the hydration of aluminate (C(3)A) and tricalcium silicate (C3S) in PC was delayed and the amount of portlandite was decreased, and the pores with diameter over 20 nm were increased. In addition, a new discovery was that C(3)A in PC and ye'elimite (C(4)A(3)(S) over bar) in CSA were competitive with each other in reaction with gypsum to form AFt, and C(4)A(3)(S) over bar preferentially reacted.