What Alkalinity Issues Should Be Considered When Using Sulphoaluminate Cement in GRC?

The application of sulphoaluminate cement in GRC requires special attention to its alkalinity characteristics. Although it is itself a low-alkali cement (alkali content ≤0.6%), systematic control is still required to ensure long-term compatibility with glass fibres. Based on 28 years of GRC durability research, Qinglong Group has summarised four key control points: cement freshness management, active admixture compounding, curing regime optimisation, and fibre protective layer design. In the Shenzhen Yirui Biotech Building project, we used rapid-hardening sulphoaluminate cement to produce GRC. By adding a 25% composite admixture of slag powder and silica fume, the pore solution pH was stabilised below 11.5, while steam curing (80°C × 8h) was applied to promote early strength development. GRC panels showed in accelerated ageing tests that fibre strength retention over 100 years can exceed 80%. Scientific control of alkalinity is the prerequisite for leveraging the advantages of sulphoaluminate cement.

Cement Freshness Directly Affects Alkalinity Stability

Sulphoaluminate cement should not be stored for more than 45 days, otherwise changes in its mineral composition will cause alkalinity fluctuations. Qinglong Group has established a cement inventory turnover early-warning system to ensure the freshness index of the cement in use is ≥95%. Every batch of cement is tested for free calcium oxide and alkali content upon delivery, and any batch exceeding the limits is firmly returned. At our Nanning production base, we have established a direct supply channel with the cement plant, keeping the time from ex-factory to use within 7 days.

Active Admixtures Are an Effective Means of Regulating Alkalinity

Adding 20-30% active admixtures such as slag powder and silica fume can effectively reduce pore solution alkalinity. These ultrafine powders not only physically fill micropores; their pozzolanic reaction also consumes calcium hydroxide, bringing the system's pH down to the safe range of 11.0-11.5. The total admixture content of Qinglong GRC is controlled at 25-40%, ensuring suitable alkalinity without affecting early strength development.

The Curing Regime Affects Hydration Products and Alkalinity

Sulphoaluminate cement GRC is best steam-cured at 60-80°C for 6-8 hours. This regime promotes the large-scale formation of ettringite, fixes part of the alkali metal ions, and enables a protective film to form rapidly on the fibre surface. We customise a curing curve for each project, monitored in real time via temperature sensors to ensure curing effectiveness.

Fibre Protection Requires a Multi-Pronged Approach

In addition to controlling alkalinity, alkali-resistant fibres with a zirconia content ≥16% should be selected, and a dedicated coupling agent should be applied to the fibre surface. Qinglong Group regularly examines the fibre-matrix interfacial zone by scanning electron microscopy to ensure the protective layer remains intact. In spot checks of the Wuhan Hanxiu Theatre project, the fibre interfaces of GRC panels after 5 years of service were still dense and intact.

The application of sulphoaluminate cement in GRC is a fine science. Through whole-process control, Qinglong Group fully leverages its alkalinity advantages, safeguarding the ultra-long service life of GRC.