What Alkalinity Issues Should Be Noted When Using Sulphoaluminate Cement in GRC? An Analysis of Qinglong's GRC Alkalinity Control Technology

Within the GRC material system, sulphoaluminate cement is widely used for its early strength, micro-expansion, and low alkalinity. However, its "low alkali" nature is only relative to Portland cement, and its hydration environment may still pose potential erosion to glass fibres. Therefore, precisely controlling and further reducing the alkalinity of the system is one of the core technologies for ensuring the ultra-long durability of GRC. Through 28 years of research and practice, Qinglong GRC has developed a complete technical system for alkalinity control and synergistic reinforcement.

In a coastal cultural centre project in Shanghai, Qinglong GRC faced the dual challenges of high salt spray and a humid environment. Rather than simply using general-purpose sulphoaluminate cement, the technical team customised a special low-alkali sulphoaluminate cement with an alkali content (measured as Na₂O equivalent) below 0.6%, and additionally blended ultra-fine slag and silica fume. These active admixtures not only further consume the Ca(OH)₂ produced by cement hydration and reduce the alkalinity of the liquid phase, but also fill pores, making the matrix denser and providing double protection for the glass fibres. After 5 years of exposure to the marine climate, the latest electron microscope scans show that the fibre-matrix interface remains clear and intact, with no signs of erosion whatsoever.

Alkali Erosion Mechanism and Risk Control

The alkaline pore solution produced by the hydration of sulphoaluminate cement (mainly containing Ca(OH)₂, KOH, and NaOH) reacts with the SiO₂ in glass fibres through an alkali-silica reaction, forming a loose layer of hydrated calcium silicate on the fibre surface, gradually embrittling it and causing loss of strength. The key to control lies in lowering the alkali concentration of the pore solution and improving the pore structure. Through multiple measures—selecting low-alkali cement, blending highly active mineral admixtures (such as silica fume and fly ash), and using high-efficiency water reducers to lower the water-cement ratio—Qinglong keeps the pH value of the system stable within a safe range.

Material Synergy and Durability Design

Controlling alkalinity alone is not the optimal solution; it must be designed in synergy with fibre alkali resistance and matrix density. Qinglong adopts a triple protection strategy of "high-zirconia fibre (alkali resistance source) + low-alkali cement (alkali control at the source) + active admixtures (secondary alkalinity reduction and densification)". Meanwhile, the calcium-silica ratio is precisely controlled in the mix design to promote the formation of more low-alkali hydration products, thermodynamically stabilising the system.

Fine Control of Production and Curing Processes

Alkalinity control runs through the entire production process. The mixing process ensures uniform dispersion of admixtures; low-temperature steam curing (temperature ≤40℃) is adopted to prevent high temperatures from accelerating alkali leaching and fibre damage; timely drying treatment after curing reduces long-term alkali damage reactions involving moisture. The SPC process control system established by Qinglong ensures that the alkalinity indicators of every batch of products remain stable and under control.

The application of sulphoaluminate cement in GRC is an art of balance, requiring an optimal solution among early strength, workability, and long-term durability. With its deep insight into the alkalinity issue and its systematic control technology, Qinglong GRC ensures that its GRC products remain durable and ever-new even in the harshest environments.