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

In the application of sulphoaluminate cement in GRC, alkalinity control is the core technical point for ensuring the long-term durability of the material. Based on 28 years of engineering practice, Qinglong Group has found that although the alkalinity of sulphoaluminate cement itself (pH typically 10.5-11.5) is lower than that of ordinary Portland cement (pH 12.5-13.5), systematic control is still required in three aspects: "raw material selection, mix proportion design, and curing regime". In the Hainan International Convention and Exhibition Center project, by optimizing the alkalinity control of sulphoaluminate cement, our GRC roofing has maintained excellent performance after ten years of use in a high-temperature, high-humidity, and high-salt-spray environment, demonstrating the importance of scientific control.

Analysis of the Alkalinity Characteristics of Sulphoaluminate Cement

Sulphoaluminate cement takes anhydrous calcium sulphoaluminate as its main mineral component, and its hydration products contain a relatively low amount of calcium hydroxide, which is the fundamental reason for its relatively low alkalinity. Qinglong laboratory test data show that the liquid-phase pH of high-quality sulphoaluminate cement is usually around 11.0, 1-2 pH units lower than that of ordinary Portland cement. This mild alkaline environment significantly reduces the erosion of glass fibres, but it does not mean that the alkalinity issue can be completely ignored.

Synergistic Alkalinity Reduction Through Raw Materials

Qinglong further reduces alkalinity through the synergistic effect of the "cement-mineral admixture" system. We typically incorporate 15%-25% active mineral admixtures into sulphoaluminate cement, such as slag powder, fly ash, or silica fume. These active materials undergo secondary hydration reactions with the calcium hydroxide in the hydration products, which not only further lowers the alkalinity of the system but also refines the pore structure and improves compactness. In the Shenzhen Pingshan High-tech Zone Comprehensive Service Center project, this technology made the long-term strength development of GRC panels more stable.

Precise Control of Mix Proportion Design

The water-binder ratio is an important factor affecting alkalinity. Through extensive testing, Qinglong has determined the optimal water-binder ratio range to be 0.30-0.35. An excessively high water-binder ratio increases porosity and provides channels for alkaline ion migration, while an excessively low water-binder ratio impairs workability. At the same time, we strictly control the cement content at around 400 kg/m³, minimizing alkalinity while ensuring strength.

Optimization Strategies for the Curing Regime

Curing conditions affect the hydration process and the development of alkalinity. Qinglong adopts a two-stage curing regime of "low-temperature pre-curing + standard curing" to avoid the concentrated release of alkalinity caused by high-temperature curing. In the Nanning Wuxiang China Resources MixC One project, through precise control of curing temperature and humidity, we ensured that the alkalinity of GRC components decreased steadily while their strength developed, laying a solid foundation for long-term durability.