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A study on properties and application of calcium sulphoaluminate cement

1 Introduction

Calcium sulphoaluminate cement (CSA cement) was successfully developed by China Institute of Building and Materials Science in 1975 and won the second prize of national invention in 1982. It is made from bauxite,limestone and gypsum in a certain ratio and calcined at low temperature (1300~1350) °C to form clinker with calcium sulphoaluminate (C4A3S̅) and dicalcium silicate (C2S) as the main mineral phase.A hydraulic cementitious material could be made by co-grinding an appropriate amount of gypsum with short setting time, which is of high early strength and low alkalinity.When grinding calcium sulphoaluminate cement, some mixed materials (limestone, fly ash, etc.) are usually added as needed.With the deepening of research,calcium sulphoaluminate cement has been serialized according to the actual needs of the project. Currently,high-strength calcium sulphoaluminate cement,self-stressing calcium sulphoaluminate cement and fast-hardending calcium sulphoaluminate are produced,low alkalinity calcium sulphoaluminate cement and expanded calcium sulphoaluminate cement are applied on the market.

This paper briefly introduces the production,performance characteristics and engineering applications of calcium sulphoaluminate cement.

2 The production of calcium sulphoaluminate cement

Like the production of Portland cement,the production of calcium sulphoaluminate cement can also be summarized as"two mills and one bur".The key to the production of calcium sulphoaluminate cement is also the firing of clinker.

In the past ten years,with the emphasis on fuel economy, energy conservation and emission reduction in the cement industry,researchers at home and abroad have begun to pay attention to the application of industrial solid waste in the firing of calcium sulphoaluminate cement clinker.

Compared with Portland cement clinker, calcium sulphoaluminate cement clinker has some differences in raw materials,firing temperature and mineral formation.

The main raw materials used in the production of calcium sulphoaluminate cement clinker are calcareous raw materials,bauxite and gypsum.The technical requirements for raw materials are:Cao≥ 48% in calcareous raw materials (if less than 48%,it can be combined with Cao > 48% of calcareous raw materials),MgO<20%;Al2O3>50%,Fe2O3<20%,SiO2<20%,SO3>38% in bauxite;anhydrite is preferred for gypsum,and SO3>45%.In the process of producing calcium sulphoaluminate cement clinker,two parameters of aluminum to sulfur ratio (P) and alkalinity coefficient (CM) are mainly controlled.Among them, P is generally maintained between 3.5 and 4.0,and CM is generally maintained between 0.9 and 1.  If CM>1,a large amount of f-CaO or C3A, C12A7 and other minerals will be formed,which will affect the hydration speed,hydration product and morphology of the cement;if the CM is too small,the C2AS content in the clinker will increase,resulting inThe content of C4A3S̅ and β-C2S is reduced,which has an adverse effect on cement performance.If the gypsum content is insufficient, the Al2O3 will be excessive, and the C2AS content in the clinker will increase, leading to a decrease in early cement performance.

The production of calcium sulphoaluminate cement clinker is essentially half of the production of Portland cement, so calcium sulphoaluminate cement is considered a low carbon environmentally friendly cement.

3. Performance characteristics of calcium sulphoaluminate cement

3.1 Condensation is faster

According to the provisions of GB175-2007,the initial setting time of various types of Portland cement shall not be less than 45 minutes,and the final setting time shall not exceed 10 hours.The initial setting time of calcium sulphoaluminate cement is 30min~40min,and the final setting time is 55min~75min.

3.2 High early strength

Compared with ordinary Portland cement, the strength of calcium sulphoaluminate cement develops rapidly and has a very high 1d strength.The strength of 3d age can reach more than 85% of the strength of Portland cement 28d.As shown in Table 1,since calcium sulphoaluminate cement contains a certain amount of C2S in clinker,its late strength will increase slowly, and there will be no late strength shrinkage exhibited by some fast-setting,fast-hardening and early-strength cement.

3.3 A certain rate of free expansion

The ettringite formed by the hydration of calcium sulphoaluminate cement produces volume expansion in crystal growth,and thus the calcium sulphoaluminate cement has a certain degree of free expansion.It can be seen from Table 2 that in the case of a lime-sand ratio (C/S) of 1:0.5,the free expansion ratio of the low-alkali calcium sulphoaluminate cement 28d is 0.00% to 0.10%,and the fast hard calcium sulphoaluminate cement  The free expansion rate of 28d is 0.00%~0.07%[12].It can be seen that although the calcium sulphoaluminate cement has a certain degree of expansion,the expansion ratio is low and the application is very safe.

3.4 Low liquid alkalinity
The PH of the silicate cement hydration liquid phase is about 13,and the PH of the calcium sulphoaluminate cement hydration liquid phase is 11.5~12.0.  Low alkali cement is suitable as a cementing material for glass fiber composite materials,especially in the calcium sulphoaluminate cement with a certain proportion of mixed materials,such as limestone,fly ash,slag powder, etc.,which can further reduce the alkalinity and become the ideal glass cementitious material for fiber products.

3.5 Good resistance to freezing and thawing
The good frost resistance is also one of the excellent properties of calcium sulphoaluminate cement.Because it becomes hard fast,the early freezing has little effect on its strength.Ordinary Portland cement is frozen at an early stage,and its strength loss is about 50% when the temperature returns to be normal,while calcium sulphoaluminate cement only loses about 9%.Calcium sulphoaluminate cement is used in low temperature environment (0~10) °C,its early strength is 5~8 times that of ordinary silicate cement;It can be used in low temperature environment of (-20~0) °C,adding a small amount of antifreeze,keep the mold temperature above 5 °C,and the strength of concrete (3~7) days can reach 70%~80% of the design strength.

4 The application of calcium sulphoaluminate cement
4.1 calcium sulphoaluminate cement used as a quick repair material
A large number of experimental studies have shown that fast-hardening early-strong calcium sulphoaluminate cement is a relatively advanced hydraulic cementing material for emergency repair and construction in China.In road repair works,road concrete prepared with calcium sulphoaluminate cement and early strength agent does not require special maintenance.After 8 hours, the strength can reach 80% of the design strength,and the design strength can be achieved in 12 hours.At the same time,calcium sulphoaluminate cement has strong cohesiveness and weak self-expansion,which can effectively prevent concrete shrinkage.High penetration resistance can prevent surface water from seeping and damaging the road base layer.Therefore,calcium sulphoaluminate cement is an ideal road repair and repair engineering materials.

4.2 calcium sulphoaluminate cement used as grouting material
Many new materials have been invested in the construction of high-speed railways in China,especially grouting materials,which have been widely used in the anchoring of rail and steel structure anchor bolts, the reinforcement of road and bridge engineering and the reinforcement of concrete structures.On the basis of fast-hard calcium sulphoaluminate cement,Yan Xiangjun studied a new grouting material with good fluidity,controllable condensing time,early strength and gradual improvement of late strength,which is of great significance for the improvement of on-site grouting technology.  

4.3 Application of calcium sulphoaluminate cement in corrosion resistance engineering
Calcium sulphoaluminate cements have excellent resistance to chloride and sulfate attack, and even uncured calcium sulphoaluminate cement mortar specimens have significant resistance to sulfate attack.The sections of the South Xinjiang Railway and the Qingxin Railway that contain high concentrations of chloride, magnesium and sulphate groundwater, use more than a dozen bridges of quick-hard calcium sulphoaluminate cement field experiments and construction.

4.4 Application of calcium sulphoaluminate cement in the production of GRC related products
GRC products refer to products made of calcium sulphoaluminate cement and alkali-resistant glass fiber.They have excellent properties such as light weight, high strength and durability,and have been widely used in China.Calcium sulphoaluminate cement has low alkalinity,and when combined with glass fiber to prepare glass fiber reinforced cement (GRC) products,it can slow the corrosion of glass fiber and greatly improve the durability of GRC products19.At present,there are many kinds of GRC products,mainly including GRC inner and outer partition wall panels,GRC building exterior decorative members and artworks,GRC insulation boards and foam materials.

4.5 Application of calcium sulphoaluminate cement in other projects
Sulfurate cement has a series of excellent properties due to its high early strength,strong resistance to permeation,good freeze-thaw resistance,long-term stability,corrosion resistance,short setting time,low alkalinity and low free expansion rate.In addition to being widely used in the above projects,it is also widely used in the following engineering fields[20]:
(1) Spray anchor support and reinforcement works;
(2) Hydraulic and offshore engineering;
(3) underground works;
(4) Anti-alkali aggregate reaction project;
(5)Preparation of concrete precast members and high-strength prestressed concrete pipe piles.

5 Outlook
Calcium sulphoaluminate cement has special properties such as fast hardening, early strength,crack resistance and corrosion resistance.The CO₂ emitted during production is less than that of Portland cement. Under the current background of energy saving and emission reduction, its development is in line with national industrial policies.On the road of the implementation of the"the Belt and Road"initiative, in order to meet the needs of some special projects,calcium sulphoaluminate cement is welcoming a broad application prospect as a special cement.As Professor Karen Scrivener made predictions,calcium sulphoaluminate cement may not replace ordinary Portland cement on a large scale,but it may provide ideas for the harmonious development of cement and resources and the environment.
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